design manual for sedimentation control through sedimentation
TRANSCRIPT
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TABLE OF CONTENTS CHAPTER I
Page
I INTRODUCTION
1.1 Thepurpose. . . . . . . . . . . . . . . . . . . . . . . 1.1 1.2 Application of Sedimentation Ponds . . . . . . . . . . . 1.1 1.3 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 1.4 Design Manual U s e . . . . . . . . . . . . . . . . . . . . 1.3
I. INTRODUCTION
1.1 The Purpose
The need f o r control of sediment eroded from areas disturbed by coal
mining operations has been w e l l documented. Presently, several erosion and
sedimentation control measures are avai lable t o the operator.
various methods, sedimentation ponds have been the mos t widely used and are
required by federal regulations. Sedimentation ponds are typica l ly the last
treatment measure applied before runoff leaves the permit area. Therefore, it
is paramount t h a t sedimentation ponds be designed, constructed, and maintained
t o provide sediment removal to meet regulatory e f f luent l imi ta t ions and main-
t a i n the hydrologic balance.
Of these
Previously, federal and state regulations have required design of sedi-
( 1 ) to provide a spec i f ic s torage mentation ponds f o r two general criteria:
capacity based on the amount of disturbed area and ( 2 ) provide a required
storage capacity t o r e t a in the runoff from a design precipitation event f o r a
spec i f ied period of time.
ponds designed t o meet the above criteria do not necessarily meet applicable
e f f luen t l imitat ions. This inconsistency is addressed by the regulat ions
Recent s tudies have shown that the sedimentation
', curren t ly published by the Office of Surface Mining (OSM), whereby seaimen-
t a t i o n ponds are required to meet ef f luent l imitat ions and the se lec t ion of
sedimentation pond design criteria such as storage volume, pond geometry, and
detention time is l e f t to the design engineer. Thus, t he design of sedimen-
t a t i o n ponds should be based on the pond's a b i l i t y to achieve specific
effluent limitations.
\
1.2 Application of Seamentation Ponds
As stated previously, sedimentation ponds are the last treatment measure
applied before the runoff leaves the permit area. However, it should be
understood t h a t sedimentation ponds are not t he only means of sediment and
erosion control , but simply an i n t e g r a l part of an overa l l plan.
a complete sediment and erosion cont ro l plan before, during, and a f t e r mining
operations based on sound engineering knowledge is necessary to minimize
p o t e n t i a l environmental damage from surface mining activities. Further, it is
e s s e n t i a l that the designer r ea l i ze t h a t t he drainage basin i n t h e permit area
is only one part of a larger, mre complex drainage system. The drainage
The need f o r
.. .. . . 1. . . . . . . .-. .. .
1.2
network i n the pepnit area i n t e r a c t s w i t h other parts of the l a rge r drainage
system i n a complex fashion.
established a state of balance or quasi-equilibrium. The mining opera t ion , or
any other large-scale disturbance, w i l l a f f e c t this balance or equilibrium and
can r e s u l t i n dynamic responses through the system. The &signer must
recognize this s i t u a t i o n i n order to restore the disturbed topography and
drainage t o a condi t ion where it w i l l again properly function as part of the
larger system.
Over time this complicated system has
Sedimentation ponds as referred to in this manual are used f o r the re-
moval of sediment due to erosion from dis turbed areas during the a c t i v e mining
phase and during the reclamation phase u n t i l adequate revegetation has been
e s t ab l i shed . Sedimentation ponds are used i n a l l OSM regions, w i t h a l l types
of d n i n g methods, on n a t u r a l drainageways and in conjunction with d ivers ions .
The major c o n t r o l l i n g f a c t o r in the app l i ca t ion of sedimentation ponds is
topography of the s p e c i f i c site.
normally -associated with eas t e rn mines in the Appalachian Hountain range,
l imited mining is conducted on steep sloped t e r r a i n i n the Rocky Mountain sta-
tes. There are also r o l l i n g and f l a t t e r r a i n areas i n southeastern parts of
Although mining in steep sloped t e r r a i n is
. . t he United States. Therefore, techniques f o r app l i ca t ion and design of sedi- \
mentation ponds cannot be spec i f i ed by region, but are very dependent on the
topography of the si te being analyzed.
1.3 SCOP
The procedures presented h this manual are based on a comprehensive
literature review and assessment of the best technology cu r ren t ly available.
Se lec t ion criteria f o r i nc lus ion in t h e design manual for t h e range of design
methodologies a v a i l a b l e included consideration of the phys ica l environment of
surface mine opera t ions , cu r ren t design procedures employed, the problems with
e x i s t i n g sedimentation ponds, and the l e v e l of effort requi red t o provide
compliance with e f f l u e n t l imi t a t ions . Modeling methods f o r design of sedimen-
t a t i o n ponds are considered state-of-the-art procedures. However, based on
the capabilities and p resen t procedures used by most opera tors , modeling is not included i n the manual.
i nc lud ing some methods i n comwn use, are presented i n t h i s manual.
I n con t r a s t , many of t h e s impl i f i ed procedures,
1.3
This manual addresses all aspects of the pond that affect the removal of
suspended solids including, but not limited t o 8 type of mining, topography,
location soil types8 pond geometry, inlet and outlet control, and maintanence.
No attempt is made to present information on structural design.
To help meet the needs of designers and operators, contacts were made
with appropriate agencies in states where significant active mining operations
occur. E’urther, contacts were made with operators to develop a background of
their capabilities, problems in sedimentation pond performance, innovative
techniques, and present design procedures. This information provided insight
for development of a useable design manual.
1.4 Design Manual Use
The methodologies and considerations in design of sedimentation ponds
have been presented to provide the designer or operator with an understanding
of the processes involved to remove suspended solids and what effect6 these
processes have. In Chapter 11, preliminary considerations of watershed
characteristics and sources of sediment are discussed. In Chapter 111, com-
putational methods for water routing and removal efficiency are presented
\ along with a discussion on the Characteristics of sediment removal to meet . . effluent limitations. This chapter contains the data requirements and the
methodologies that are used to design a sedimentation pond.
discussion in this chapter is that pertaining to sediment data, specifically
the particle size distribution.
tions is greatly dependent on the particle size distribution. great care should be taken to develop an accurate representative size distri-
An important
The design of ponds to meet effluent limita-
Therefore,
bution. Chapter N presents modifications that
performance of the sedimentation pond. Chapter
sediment removal. Maintenance of sedimentation
enough. Lack of pond maintenance is one of the
the per-
can be made to improve the
V deals with maintenance and
ponds cannot be emphasized
major problems in
fonnance of existing sedimentation ponds and the development of a maintenance
program is a significant part of pond design.
sections are interrelated in the design process.
Chapter V I presents how these
To bring the information and methodology together, the final chapter pre-
sents the procedural steps for design along with a comprehensive design
1.4
example. Users of the design manual are encouraged to carefully review the
example presented in Chapter VI to better understand the design methodology.
With a l i t t l e practice, the complete design process w i l l become familiar and
8 traight f orward.
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TABLE OF CONTENTS CHAPTER I f
Page
I1 . PRELIMINARY CONSIDERATIONS FOR APPLICATION AND USE OF SEDIMENTATION PONDS FOR SURFACE MINING
2.1 Off ice of Surface Mining Regulations and Environmental Pro tec t ion Agency Water Quality Standards . . . . . . . . . . . . . . . . .
2.2 Watershed Charac te r i s t i c s . . . . . . . . . . . . . . . . 2.2.1 Climatology .................... 2.2.2 Geology . . . . . . . . . . . . . . . . . . . . . 2.2.3 soils . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Vegetation . . . . . . . . . . . . . . . . . . . . 2.2.5 Topography . . . . . . . . . . . . . . . . . . . . 2.2.6 Iiydr010gy
2.3 Location of Major Sources of Sediment . . . . . . . . . . 2.3.1 Haul and Access mads . . . . . . . . . . . . . . 2.3.2 Areas of Active Mining . . . . . . . . . . . . . . . 2.3.3 Areas Being Cleared for f i n i n g Activit ies . . . . 2.3.4 Areas in Process of Reclamation . . . . . . . . .
2.4 Typ es and Applications of Sedimentation Ponds . . . . . . 2.4.1 Excavated Sedimentation Ponds . . . . . . . . . . 2.4.2 Embankment and Combination npbankment/
Excavated Sedimentation Ponds . . . . . . . . . . 2.4.3 Sedimentation Pond Spillway Type . . . . . . . . . 2.4.4 Multiple Pond Systems . . . . . . . . . . . . . . 2.4.5 Physical/Chemical Treatment Ponds . . . . . . . . 2.4.6 Dry Basin versus Permanent Pool . . . . . . . . .
2.5 Summary of Preliminary Considerations . . . . . . . . . .
2.1 2.2
2.2 2.3 2.3 2.4 2.4 2.6
2.6
2.7 2.7 2.8 2.8
2.9
2.9
2.10 2.11 2.12 2.12 2.13
2.14
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LIST OF FIGURES C E U P E R I1
Page
Figure 2.1. Example of r i l l and gully erosion . . . . . . . . 2.5
. ..
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11. PRELIMINARY CONSIDERATI013S FOR APPLICATION A?$D USE OF SEDIMENTATION PONDS FOR SURFACE MINING
Off ice of Surf ace Mining Regulations and Environmental Pro tec t ion Agency Water Qua l i t y Standards
Design criteria f o r s e m n t a t i o n ponds are es t ab l i shed through f e d e r a l
2.1
and state regulations. OSM sedimentation pond design criteria are intended t o
prevent, to t h e ex ten t possible, a d d i t i o n a l con t r ibu t ions of suspended solids
t o stream f l o w or runoff outside t h e permit area and t o achieve appl icable
federal and state w a t e r q u a l i t y standards.
dards include e f f l u e n t l imi t a t ion guide l ines f o r c o a l mining po in t sources
e s t a b l i s h e d by t h e Environmental Pro tec t ion Agency (EPA). Therefore, the
requirements of the EPA are d i r e c t l y t i e d t o the performance standards for the
design criteria of sedimentation ponds.
These d n h m water q u a l i t y stan-
OSM cons iders sedimentation ponds i n conjunction with a l t e r n a t i v e c o n t r o l
measures as t h e best practical technology (BPT) c u r r e n t l y ava i l ab le , as
e s t a b l i s h e d by t h e EPA, for the con t ro l of sediment.
s p e c i f i c a l l y defined by OSM t o include any barrier, -8 or excavated
depression which slows water runoff allowing sediment to settle.
A sedimentation pond is
',
0% sedimentation pond design criteria must enable compliance with the 5 . . \ e f f l u e n t l imi t a t ions , water q u a l i t y standards, and t h e s a f e t y requirements of
state and f e d e r a l governments. The determination of sedimentation pond design
cri teria is l e f t up to the operators and t h e r e g i s t e r e d p ro fes s iona l engineers
designing the ponds and reviewed by the regula tory a u t h o r i t i e s .
Currently, t h e requirements of the EPA e f f l u e n t Umi ta t ions are the
result of t h e Federal Water Pol lu t ion Control A c t Amendments of 1972. I n 8um-
mary, p o i n t source water discharges are required to meet e f f l u e n t l i m i t a t i o n s
r e q u i r i n g t h e app l i ca t ion BPT cur ren t ly available by' Ju ly l, 1977, - and best -.'
available technology (BAT) economically achievable by Ju ly I, 1983. The C l e a n
Water A c t of 1977 rev ised t h e control program for nontoxic po l lu t an t s .
detailed discussion of appl icable e f f l u e n t l imi t a t ions , see Section 3.3.
Suggested design criteria f o r sedimentation ponds are based on t h e c u r r e n t
e f f l u e n t lFmits found in many states.
vary; however, t h e s ta te agency requirements must be taken i n t o account by the
design engineer.
For
E x a c t requirements f o r each agency
2.2
2.2 Watershed Characteristics
The location, design, construction, and methods of treatment used i n
sedimentation ponds a re dependent on the character is t ics of the watershed
where mining a c t i v i t y takes place.
logy, geology, s o i l s , . vegetation, topography, and hydrology of the watershed
w i l l help f a c i l i t a t e the design of e f f i c i e n t sedimentation ponds t h a t m e e t
water quality standards and regulations. These charac te r i s t ics do vary on a
watershed-by-watershed basis; proper consideration of this must be made i n
order to design sedimentation ponds properly.
Preliminary consideration fo r the climato-
2.2.1 Clfmatology
Climatological elements in sedimentation pond design include temperature
and precipi ta t ion.
be t r e a t e d as w e l l as how ef fec t ive ly it is treated i n a pond.
ambient temperature changes the viscosi ty of water which changes the s e t t l i n g
ve loc i ty of sediment particles. Consequently, the detention t i m e required t o
settle out a waterborne particle of a given size changes. Ambient temperature
a l s o influences the magnitude and seasonal dis t r ibut ion of runoff.
I n general, temperature can a f f ec t the amount of runoff to
Variation in
. . \ Prec ip i ta t ion i n the form of snow causes l i t t l e or no erosion. However, snow
melting i n the Spring on p a r t i a l l y frozen ground is a source fo r higher rates
of runoff and erosion. I f low temperatures exist fo r a s ign i f icant period of
time, ice formations in sedimentation ponds may be a problem. I f not taken
i n t o consideration i n pond design, ice formations may cause f a i l u r e of embank-
ments o r damage energy d iss ipa tors such as baff les or riprap. L o w tem- peratures f o r a s igni f icant period of t i m e may disrupt, damage, or halt
chemical treatment processes that may be used as part of water treatment i n
sedimentation ponds.
Prec ip i ta t ion is the mos t important element of climate in determining the
rate of erosion. The magnitude of annual precipi ta t ion, i n addition t o the
frequency, in tens i ty , duration, and seasonal dis t r ibut ion of prec ip i ta t ion
events a f f e c t the magnitude of runoff and erosion. Precipi ta t ion charac-
teristics a l so e f f e c t the type of o u t l e t and operation of a sedimentation
pond.
from an event.
have s u f f i c i e n t l y settled t o meet w a t e r qual i ty standards.
For example, gated o u t l e t s a r e normally used t o store a l l of the runoff
The gates are opened t o discharge the runoff after sediments
This would be
2.3
d i f f i c u l t to design f o r a pond on a watershed i n t h e eas t e rn region of the
United States where annual p r e c i p i t a t i o n is much greater and precipitation
even t s that occur are c h a r a c t e r i s t i c a l l y of a longer duration, l o w e r inten-
s i t y , and amre frequent. Gated o u t l e t s are much more suitable f o r ponds i n
the western reg ion where annual p r e c i p i t a t i o n and water y i e l d are much lower
and p r e c i p i t a t i o n events are of high i n t e n s i t y but for a r e l a t i v e l y s h o r t
dura t ion .
Seasonal d i s t r i b u t i o n of p r e c i p i t a t i o n r e s u l t s i n a seasonal d i s t r i b u t i o n
The eros ion p o t e n t i a l is greater during the high of annual e ros ion p o t e n t i a l .
p r e c i p i t a t i o n period such as Spring than it is during Winter. This is impor-
t a n t in es t ima t ing t h e y i e l d and concentration of seUiment i n t h e runoff. For
f u r t h e r cons idera t ion of seasonal e ros ion p o t e n t i a l refer to %=face Water
Hydrology and Sedimentology Manual" (OSM, 1982). -- - --
2.2.2 Geology
The geology of a watershed interacts somewhat with a l l hydrologic charac-
teristics of t h e watershed. The geology has a s i g n i f i c a n t e f f e c t on the
topography. Topography is a major cons idera t ion in the loca t ion and shape of
. . a sedimentation pond. \
~n important cons idera t ion of the geology is t o eva lua te the d i f f e r e n t
types of overburden material t h a t w i l l be exposed during t h e mining process.
Overburden e ros ion characteristics should be considered when e s t ima t ing sedi-
ment y i e l d s and runoff concentrations.
2.2.3 Soils
Watershed Soil c h a r a c t e r i s t i c s depend t o a l a rge ex ten t on t h e pa ren t
geologic materials and the predominant weathering processes.
c h a r a c t e r i s t i c s of the watershed determine runoff and eros ion from the
watershed. The magnitude of runoff and eros ion is a function of s o i l
i n f i l t r a t i o n , s o i l permeabili ty, moisture content, s t r u c t u r e , texture, and
con ten t of organic matter.
These soil
The sediment s i z e d i s t r i b u t i o n of t h e runoff is the main design criteria
i n s i z i n g a sedimentation pond. As the s i z e of particle to be removed becomes
smaller the s i z e of a sediment pond requi red becomes la rger . Often t i m e s , the
s ize of pond is impractical and chemical treatment may be necessary to remove
- . ... ..-..- -. .._-... - . - , . . _ ,
2.4
the f i n e particles and a l l ev ia t e the high cos t of constructing a very large pond.
2.2.4 Vegetation
Vegetation plays an important ro l e i n the water balance and s o i l stabi-
l i t y i n a watershed. Through interception, t ranspirat ion, and evaporative
processes, vegetation can subs tan t ia l ly reduce water y ie ld from a watershed.
Vegetation plays an important part i n protecting the soil from erosion.
Vegetation can reduce the amount of erosion through: interception, reduced
r a i n f a l l i n t ens i t i e s by providing an energy-absorbing cover, reduced landsl ide
hazards through binding and lowering soil water content, and reduced overland
and channel erosion by providing added roughness which retards water velocity.
Removal of vegetation exposes underlying soils to great ly increased erosive
forces. Thus, one of the =re cri t ical periods during the mining process is
when reclaimed area s o i l s have not Beveloped s igni f icant vegetation.
, \ .
2.2.5 Topography
Topography, including land and channel slopes, aspect, and surface and
. . channel geometry, is an expression of the morphological, geologic8 and other erosive forces that have acted on the watershed. t u r n 8 these fac to r s are
modified by the topography they have created.
governs water discharge rate and sediment transport rate. Conversely, the
discharge rate and the sediment t ransport can alter the channel form to pro-
duce a new slope.
of w a t e r flow, thus a l t e r i n g sediment transport . Primary topographic con-
s idera t ions i n control l ing erosion are slope and length of slope.
i n slope increases the t ransport capacity of surface runoff, thus increasing
erosion. An increase i n length of slope allows f o r concentration of sheet
flow. This concentration of sheet flow forms rills and gul l ies by erosion
(see Figure 2.1).
\
For example, channel slope
Channel and surface geometry a f f e c t the depth and ve loc i ty
An increase
The watershed shape or geometry can a l so be considered part of watershed
topography. t h e watershed where flow concentrates and the time required.for f l o w
concentration.
The geometry of the watershed determines the amount of area of
. . . . - . ..
2.5
WIND EROSION
RAINDROP EROSION
SHEET EROSION
\ '
R l l l AND GULLY ER,OSION
5 . . \
STREAM AND CHANNEL EROSION
FIGURE 2.1 EXAMPLE OF RILL AND GULLY EROSION (SCS, 1980)
.” . . - . . . .
2 . 6
Many t i m e s topography c o n t r o l s the loca t ion of a sedimentation pond or
Regulations state that sedimentation the type of pond that is constructed.
ponds should be loca ted as close to the source of sediment as possible.
Normally the only f a c t o r that limits loca t ing a pond is s t e e p t e r r a i n ; ponds
are usua l ly cons t ruc ted i n va l l eys or hollows a short d is tance downstream t o
fac i l i t a te cons t ruc t ion .
2.2.6 Hydrology
Runoff that reaches a sedimentation pond is the r e s u l t of complex
i n t e r a c t i o n of a l l t h e prev ious ly discussed watershed c h a r a c t e r i s t i c s ; clima-
tology, geology, s o i l s , vege ta t ion , and topography i n t e r a c t t o make up the
hydrologic process that inc ludes runoff from a p r e c i p i t a t i o n event.
includes the i n t e n s i t y , frequency, duration, d i s t r i b u t i o n of p r e c i p i t a t i o n
events , and how that p r e c i p i t a t i o n uccurs, e i t h e r in the form of snow or r a i n
depending on temperature.
C l i m a t e
Vegetative cover intercepts a f r a c t i o n of this pre-
, c i p i t a t i o n by evapot ranspi ra t ion . The ex is tence and degree of vege ta t ive
cover inc reases w i t h favorable clFmate and so i l conditions.
f r a c t i o n of p r e c i p i t a t i o n , s o i l conditions determine what f r a c t i o n w i l l
. . i n f i l t r a t e and what f r a c t i o n w i l l become su r face runoff. The remaining f r ac -
t i o n that becomes surface runoff w i l l concentrate i n a watershed according t o
t h e geologic and topographic f e a t u r e s of the watershed. I n this manner the
i n t e r a c t i o n of watershed c h a r a c t e r i s t i c s governs t h e magnitude and d i a t r ibu -
O f the remaining
\
t i o n of runoff f r o m p r e c i p i t a t i o n and t h e sediment y ie ld . Consequent
q u a l i t y of runoff f r o m the disturbed area is a f fec t ed by the dynamic
equilibrium of the watershed.
2.3 Location of Major Sources of Sediment
There are four categories for sources of sediment from su r face m
Y, the
ning
a c t i v i t i e s in a watershed. These are the d n e d por t ion of the watershed,
t h e mined or mining po r t ion of the watershed, spoil banks or areas where spoil
is s tockpi led , and haul or access roads necessary f o r mining activit ies. Of
these four categories, the unmined por t ion of the watershed genera l ly y i e l d s
t h e least amount of sediment and is not considered a major source of sediment
y i e ld . The remaining three ca t egor i e s are a direct consequence of mining
opera t ions . Of these three, haul or access roads and s p o i l banks commonly
_. . . - . . . . .
2.7
generate the greatest sediment y ie ld per un i t area. For example, a study done
on a monitored watershed I n Appalachia (EPA, 1976) gave the following com-
parison f o r rates of erosion:
Area Y i e l d
(tons/square mile) ~~ ~~ -~
Unmlned watershed
Mined watershed
Spoi l bank
Eaul road
28
1,930
27 , 000
57,600
However, due t o the much larger percentage of
the mined or mining portion of the watershed,
' most sediment.
2.3.1 Haul and Access mads \ Both haul roads and.access roads to mine
of sediment. In addition, roadways generally
'. . .
the t o t a l watershed covered by
these areas generally y i e ld the
sites cons t i t u t e
remain as a main
a major source
source
throughout the l i f e - o f the mine.
due to an increased r a t e of runoff resu l t ing from a r e l a t lve ly Impermeable
surface, and steep cu t or f i l l slopes associated with roadways.
cept sheet runoff and act as a t ransport mechanism v ia roadside ditches.
Roadways occasionally require c lear ing and steepening of s ide slopes which
increase slope erosion.
y i e ld from roadways include:
Roadways y ie ld la rger amounts of sediment
Roads inter-
Other fac tors which contr ibute to a greater sediment
- Poor locat ion of mads.
- Improper construction/maintenance of roads and s ide slopes.
- Improper planning/construction of road drainage systems.
2.3.2 Areas of Active f in ing
The nature and extent of sediment y ie ld from areas of ac t ive mining are-
dependent on the mining method or procedure u t i l i zed . Four coamon methods of
surface mining are: area mining, contour mining, boxcut mining, and mountain
. .
2 . 8
top removal.
tour mining due to containment of runoff on disturbed areas.
have a long, narrow geometry with more spoil and bench area po ten t i a l i t y ,
draining off the disturbed area d i r ec t ly i n t o an off-s i te drainage system.
addition, t he receiving waters are generally c loser t o the mining operations
for contour mining than for area mining.
In general, area mining is a lesser source of sediment than con-
Contour mines
In
Boxcut mining is a form of contour mining used i n steeper terrain t h a t
reduces spo i l dumped downhill by moving it l a t e r a l l y along the boxcut and
placing it i n areas where coal has been removed.
t h e o r ig ina l grade of the h i l l .
and contour mining as a source of sediment.
This f a c i l i t a t e s maintaining
Boxcut mining f a l l s somewhere between area
Mountain top removal is another form of contour mining used when the eco-
nomics of overburden removal and coal seam thickness f a c i l i t a t e removing the
e n t i r e h i l l t op .
sediment than contour mining. This is due to the regional nature of , and the
Mountain top removal is a l s o considered a lesser source of
\ reduction in , r e l i e f the t o mountain top removal. This generalization is t r u e
only i f the surface drainage is control led internal ly .
The mining methods could be ranked from the la rges t __ - _- con@$bufor -- cf_$edi- . . ment t o the least as contour mining, boxcut mining, mountain top removal, and \
area mining. This is an wet generalization since the sediment y i e ld from a
mined area is si te spec i f ic and is dependent on the amount of on-site erosion
cont ro l measures t h a t are taken.
2.3.3
The areas being cleared and grubbed as pretreatment for surface mining
Areas Being Cleared f o r Mining Act iv i t ies
a c t i v i t i e s a r e one of the major sources of sediment yield.
grubbing expose s o i l on steep slopes and can c rea te a s o i l surface that im-
pedes i n f i l t r a t i o n and/or concentrates runoff. Other fac tors t h a t increase
sediment y i e id are:
overclearing or clear ing too far ahead of the p i t exposing the area f o r a
longer period, and improper placement of the salvaged topsoil .
Clearing and
failure to i n s t a l l perimeter control measures,
2.3.4 Areas i n Process of Reclamation
This last major source occurs from the start of grading operations and
lasts u n t i l s t ab i l i za t ion of the s o i l occurs by vegetative and/or s t r u c t u r a l
. ..I . . . - . -
2.9
.,easures.
y i e l d include cons t ruc t ing excessively steep, long slopes or s t r u c t u r a l l y
uns t ab le drainage channels. Also, improper t i l l a g e practices, p l a n t material
s e l e c t i o n , seedbed preparation, and maintenance can increase sediment y i e l d
Other f a c t o r s i n the reclamation process that can increase sediment
from reclamation areas.
reclamation than a t any o the r t i m e during mining operations; t he re fo re , recla-
mation practices such as t i l l i n g , mulching, and revegetation are very impor-
t a n t i n the con t ro l of erosion.
The p o t e n t i a l f o r e ros ion is greater just after
2.4 5p es and Applications of Sedimentation Ponds
Sedimentation ponds can be an e f f e c t i v e way t o c o n t r o l sediment from
On-site e ros ion p ro tec t ion measures can be l eav ing t h e mine permit area.
taken t o reduce the sediment yield from the mine site, but such measures
r a r e l y c o n t r o l the sediment t o t h e ex ten t that e f f l u e n t requirements can be
- m e t .
dn operator uses before the runoff leaves the mine permit area and e n t e r s
n a t u r a l drainageways downstream. Improper con t ro l of t h e sediment leav ing the
Sedimentation ponds are t y p i c a l l y the last sediment c o n t r o l measure that
t . mine penuit area may cause severe o f f - s i t e damages. \
Sedimentation ponds are generally constructed with embankments, by excaz
va t ion , or a combination of both. The sedimentation ponds presented i n the
following sec t ions ca tegor ize the types of ponds that are cu r ren t ly used by
the mining industry.
exist and may be used i n t h e sediment c o n t r o l plan.
which must be considered at each site to detenaine which basic type of eedi-
mentation pond, or va r i a t ion , w i l l provide the maximum c o n t r o l of sediment. The following sec t ions give a desc r ip t ion of the var ious types of sedimen-
t a t i o n ponds and where their app l i ca t ion is most practical.
However, s eve ra l combinations of t h e var ious types&&' -- There are many f a c t o r s
2.4.1 Excavated Sedimentation Pond
Excavated sedimentation ponds are cons t ruc ted by excavating a p i t or
"hole" i n t h e ground with the use of a bulldozer or backhoe. Generally, t hese
types of sedimentation ponds are l imi ted to conta in sur face runoff from
d i s tu rbed areas a t sur face mines located i n r o l l i n g t o f l a t t e r r a i n and from
s m a l l drainage areas. Sedimentation ponds which are constructed s t r i c t l y by
excavation are not used i n steep sloped t e r r a i n due t o t h e large amount of
. . . , , . .. . . _.. ...- - . . . . . ...... . - - , . . . ._
2.10
excavation t h a t would be required to ,achieve the applicable storage volume
requirements.
drainageway.
These types of ponds are generally located off a natural
The excavated sedimentation pond has been used in conjunction w i t h the
mine p i t which serves as a preliminaG s e t t l i n g basin.
disturbed areas within the mine s i t e is directed i n t o the mine p i t where
s e t t l i n g of the larger s ize particles' occurs.
is pumped i n t o the excavated sedimentation pond where f i n a l a e t t l i n g occurs.
Using pumps to control the inflow in to the excavated sedhenta t ion pond allows
cont ro l of the detention time within $he sedimentation pond. Additionally,
t h e storage volume w i t h i n the p i t can: be u t i l i zed , thereby reducing the sedi-
mentation pond storage requirements as long as the storage volume within the
The runoff from
mom the pi t , the mine atainage
I
I
mine p i t does not i n t e r f e re with mining operations.
There are nmny disadvantages wit+ the excavated sedimentation pond.
is- l imited to applications i n r e l a t ive ly f l a t terrain and control l ing surface
runoff from s m a l l drainage areas. Ins t a l l a t ion of &watering devices i n these
types of ponds is generally very expensive and therefore, they are rarely
ins ta l led .
thus reducing the avai lable storage vblume when a storm event occurs.
r e s u l t of t h i s w i l l be decreased detention times and therefore, decreased
e f f luen t qual i ty . In addition, it i s l d i f f i c u l t t o provide separate pr inc ipa l
and emergency spillways. For these reasons, applications of the excavated
It
1
This leads t o the pond s tor ing water f o r a long period of time
The . . \
sedimentation pond are very W t e d . ~
j
2.4.2 Embankment and Combination PPbankment/Excavated
An embankment sedimentation pond'can be used in any type of te r ra in .
Sedimentation Ponds
Generally, these types of ponds are located on a draincgeway. An embankment
is constructed across the atainageway to form the sedimentat ion pond.
t h e drainageway bed is excavated upstream of the embankment, a combination
When
embankment/excavated sedimentation pond is formed. Excavation upstream of the
embankment provides addi t ional storage volume capacity t o an embankment sedi-
mentation pond.
I
2111
A variety of ou t l e t s may be used,with the embankment and combination
embankment/excavated sedimentation pond. The most common method is t o use a
pipe o u t l e t for the pr inc ipa l spillway and a channel c u t i n t o the top of t h e
embankment as the emergency spillway. Although, several other types of
o u t l e t s are used and are presented in Section 3.9.
As previously mentioned, embankment sedimentation ponds are generally
The topography of ten dictates t h a t t h i s type of constructed on drainageways.
pond be constructed.
There is a p o s s i b i l i t y o f embankment failure due to poor construction or the
use of poor construction materials.
decrease the sedfioent removal e f f ic iency of the pond by adding sediment t o the
Rowever, this type of pond has some disadvantages.
Bank sloughing may occur t h a t can
pond.
increases the maintenance requirements.
t a t i o n pond is generally control led by topography.
Bank sloughing reduces the storage volume capacity.and therefore ,
The Shape of t h e embankment sedfmen-
The combination embankment/excavated sedimentation pond has the advantage
of providing additional storage volume' without increasing the height or s i z e
of t h e embankment. However, exposure ,of t he side slopes due t o upstream exca-
vation may require t h a t the slopes be btabi l ized. I I
I
2.4.3 Sedimentation Pond Spillway Type
Sedimentation ponds have of ten b& c l a s s i f i e d according to the type of I
pr inc ipa l and emergency spillway used.:
binat ion of pr incipal and emergency spillways be provided to safe ly pass the
runoff from a 25-year, 24-hour p rec ip i t a t ion event or larger event spec i f ied
by t h e regulatory authority. I n addition, the elevation of t he crest of the
emergency spillway s h a l l be a t least me foot above the crest of t h e p r inc ipa l
spillway.
emergency spillways.
OSM regulations require t h a t a com-
Therefore, separate o u t l e t s must be provided fo r the p r inc ipa l and
Pr inc ipa l spillways are usual ly constructed by using some type of pipe
ou t l e t . Pnergency spillways are generally constructed by excavating an exit
channel through the embankment or natural ground.
be used for the emergency spillway but due to the costs involved, t h e exca-
vated e x i t channel is the most feas ib le . A complete discussion of spil lways
i s presented i n Section 3.9.
Other types of o u t l e t s may
..
2.12
2.4.4 Multiple Pond'Systems
A multiple pond system is considered to be the use of two or more sedi-
mentation ponds i n a series (one downstream of the other) . The concept of
mult iple ponds is a l so accomplished through compartmentalization of a s ingle
pond.
Solids with higher s e t t l i n g ve loc i t ies w i l l settle i n the f i r s t pond or com-
partment and the f ine r sediments w i l l be s e t t l e d in the f i n a l pond or compart-
ment.
maintenance ( i .eer sediment removal) is l i m i t e d t o the first s e t t l i n g pond or compartment. Also, f i e l d applications have shown t h a t multiple sediment ponds
i n a series are more e f f i c i e n t i n removing f i n e r particles than a s ingle pond
of equal surface area. One disadvantage i n t he use of multiple ponds is that more area is disturbed due t o the construction of additional ponds.
de ta i led discussion on the application and design of multiple pond systems is given in Section 4.3.
The concept of multiple ponds is the occurrence of staged se t t l i ng .
One par t icu lar advantage t o this type of system is t h a t most of the
A
' .
2.4.5 Physical/Chemical Treatment Ponds
Physical/chemical treatment i den t i f i e s the.process of adding chemicals to 5 . . enhance the physical s e t t l i n g charac te r i s t ics of the sediment particles t o be
removed by gravity se t t l ing .
coagulants o r f locculant aids. Chemicals are added to the inf luent of the
sedimentation pond, whereproper mixing must occur, and then the sediment is
f locculated and settled i n the pond. Coal f ines may not be f locculated when
chemicals are added due t o the electrical nature of the coal f ines . Other
treatment measures may be required to remove the coal f ines .
\
The chemicals added are generally re fer red t o as
Physical/chemical treatment is a standard pract ice i n the treatment of
water fo r d o m e s . t i c use. More recently it has been applied i n the mining
industry fo r the removal of sediment t o m e e t e f f luent l imitations. To m e e t
t h e ex i s t ing and proposed ef f luent l imitat ions w i l l s o m e t i m e s require removal
of very f i n e clay and silt sedislents. The volume and surface area of a sedi-
mentation pond required for removal of very f ine sediments are unreasonably
large.
moval of f i n e sediments.
Therefore, physical/chemical treatment measures are required fo r re-
Physical/chemical treatment measures have been used i n the field i n
conjunction w i t h multiple ponds i n a series. In this type of appl icat ionr the
2.13
la rge sediment is settled i n the first pond, reducing the sediment load t o the
second pond where it is only used fo r s e t t l i n g of the f i n e r sediments. The
chemical dose required fo r s e t t l i n g is generally d i r ec t ly related t o the
concentration of solids. Therefore, by reducing the solids concentration i n
t h e f i r s t pond, the amount of chemical required is reduced below t h e amount
t h a t would be required using only one sedimentation pond.
on the ac tua l physical/chemical treatment process and types of coagulants is
given in Section 4.3.
Detailed discussion
Some disadvantages do exist i n the appl icat ion of physical/chemical
treatment.
material s e t t l e d in the sedimentation pond; therefore , consideration m u s t be
given to sediment storage volume and/or frequency of maintenance.
consideration is f i n a l disposal of the s e t t l e d sediment. It should be
rea l ized that the sediment contains the chemicals used f o r coagulation. The
The use of chemicals i n s e t t l i n g solids adds to the volume of
Another
type t i o n
', . . \
such
and/or d i f f i cu l ty of f i n a l disposal w i l l depend on the type of coagula-
chemicals and flocculant aids used.
2.4.6
The dry basin is characterized as a basin which has a dewatering device
Dry Basin versus Permanent Pool
t h a t the Sedimentation pond does not s t o r e the w a t e r runoff from any one
p rec ip i t a t ion event indefini te ly . Examples of dewatering devices include
tr ickle tubes and perforated riser pipe o u t l e t s t h a t &water t o the sediment
s torage l eve l i n the pond.
D r y basins MY be located e i the r on or off drainageways. For dry basin ponds, t he dewatering device is &signed t o dewater the sediment pond a t a
rate which achieves and maintains the required detention time t o achieve
appl icable e f f luent l imitations.
pond is either dewatering o r dry.
Between prec ip i ta t ion events this type of
The permanent pool sedimentation pond is designed to provide a permanent
s torage volume of water a f t e r the pr incipal spillway has stopped discharging.
The maximum permanent pool elevation is the elevation of the p r i n c i p a l
spillway crest.
and perennial streams when approval of the regulating authori ty is given.
Permanent pool sedimentation ponds can a l so be located off drainageways.
These types of ponds are of ten located on small drainageways
2.14
The main difference between dry basins and permanent pool basins is t h a t
t h e permanent pool provides a constant storage volume fo r water while t he dry
bas in does not.
des a water supply f o r dust control on haul and access roads during dry
periods and the permanent pool helps minfmize resuspension of sediment t h a t
has already settled.
needs t o be designed f o r the addi t ional permanent storage volume of water.
The advantages of the permanent pool basin are t h a t it provi-
The disadvantage of the permanent pool basin is t h a t it
The advantage of the d r y basin pond is t h a t it does not have to be
designed f o r an addi t ional storage volume and therefore, the e n t i r e storage
volume of the basin can be u t i l i z e d during large runoff events.
tage of dry bas ins is t h a t resuspension of the s e t t l e d sediment may occur i f
cont ro l measures a t the i n l e t a r e not taken.
The disadvan-
2.5 Summary of Preliminary Considerations
Preliminary consideration f o r the application and use of a sedimentation -_
I pond i n a surface raining operation is based on the per t inent regulations and
standards t h a t ou t l ine the l eve l of performance required.
knowledge of the watershed charac te r i s t ics , sediment t ransport mechanisms, and
major sediment source locations, the type of sedimentation pond and its appfl-
W i t h a w r u n g
\
ca t ion can be chosen and designed to f a c i l i t a t e meeting regulations and s tan-
dards f o r pond performance.
The criteria for design of sedimentation ponds is established federa l ly
through regulations by the Office of Surface Mining, Environmental Protection
Agency, and the Mine Safety and Health Administration (MSHA).
ter ia a r e intended to prevent, to the extent possible, addi t ional contribu-
t i ons of suspended so l id s outside of the permit area and achieve water qual i ty
standards. Sedimentation ponds are a l so required to meet inspection and large
dam criteria of the ?&HA (30 CFR 77.216) and requirements of state and loca l
agencies with jur i sd ic t ion over the design, construction, or discharge of
sedimentation ponds.
OSM design cri-
The climate, geology, soils, vegetation, topography, and r e su l t an t hydro-
logy of a watershed a f f e c t the magnitude and r a t e of erosion and sediment
t ranspor t t h a t must be treated by a sedimentation pond.
t o r s include t h e seasonal var ia t ion and range of temperature and the magni-
tude, in tens i ty , frequency, duration, and seasonal d i s t r ibu t ion of
Climatological fac-
2.15
precipitation.
face adjustments affect watershed climate and hydrology.
rocks, large-scale erosive forces, and bed structure dictate the general
topography. all of the other watershed characteristics.
Geological processes creating long- and short-term land sur-
Erosion-resistant
Thus, the geology of a watershed is important because it affects
The magnitude of runoff and erosion is a function of soil permeability,
moisture content, structure, texture, and content of organic matter. The
amount of vegetative cover, which is a function of climate and soil charac- teristics, affects the stability of the soil and water yield from a watershed.
Primary factors of topography are the length and steepness of slopes and the
geometry of the watershed. The complex interaction of all these watershed
characteristics determines the magnitude and distribution of runoff from a
precipitation event, which is the element of the hydrologic process of
concern.
There are four categories of sediment sources from surface mining activi-
ties in a watershed. These are the unmined portion of the watershed, the
mined or mining portion of the watershed, spoil banks or areas where spoil is Stockpiled, and haul or access roads necessary for mining activities.
these four categories, the unmined portion of the watershed generally yields
the least amount of sediment and is not considered a major source of sediment yield.
operations.
of
\
The remaining three categories are a direct consequence of mining
The type of pond can be classified into one of three general categories:
excavated ponds; embankement ponds1 and combination excavated/embankment
ponds.
topography of the proposed site.
The major factor in the selection of the type of pond is the
When conditions for treatment of runoff from the disturbed urea warrant,
chemical treatment systems or multiple pond configurations are used.
Normally, chemical treatment systems are utilized when high concentrations of
colloidal size particles are present in runoff from the disturbed area and
gravity-settling methods are not sufficient.
when the cost of constructing a single pond is high because of topographic
constraints, or the gravity-settling method of treatment in a single pond is
not sufficient to meet effluent limitations.
Multiple ponds are normally used
2.16
With sufficient knowledge of the preliminary considerations discussed
previously, the type and application of a sedimentation pond which will be
effective in sediment control can be selected.
...... . ~ _ .
... TABLE OF CONTENTS
CHAPTER 111
Page
I11 . SEDIMENT REMOVAL THROUGH SEDIMENTATION PONDS
3.1 3.1 Site Selection . . . . . . . . . . . . . . . . . . . . . 3.1.1 General Considerations . . . . . . . . . . . . . 3.1.2 Topography Considerations . . . . . . . . . . . . 3.1
3.2
3.1.2.1 Steep Sloped Terrain . . . . . . . . . . 3.1.2.2 Mild Sloped Ter ra in . . . . . . . . . . 3.2
3.3
3.1.3 Sedimentation Ponds m c a t e d on
3.1.4 Sedimentation Ponds b a t e d of f
3.1.5 Source of Sediment . . . . . . . . . . . . . . . 3.1.6 Access ib i l i t y . . . . . . . . . . . . . . . . . . 3.1.7 Mining Considerations . . . . . . . . . . . . . . 3.1.8 F i e l d Inves t iga t ion . . . . . . . . . . . . . . .
Main Drainageways . . . . . . . . . . . . . . . . . Main Drainageways . . . . . . . . . . . . . . . . .
3.4
3.4 3.5 3.5 3.6 3.6
3.2 Data Requirements . . . . . . . . . . . . . . . . . . . 3.8
4 3.2.1 HydrOlOgY b
3.2.2 Sediment Data . . . . . . . . . . . . . . . . . . 3.8 3.8
3.2.2.1 I n f l u e n t Sediment Size Di s t r ibu t ion . . 3.2.2.3 Sediment Yield . . . . . . . . . . . . . 3.9
3.12
3.2.3 Inflow Suspended Solids Concentration . . . . . . 3.12
3.3 E f f l u e n t Limi ta t ions . . . . . . . . . . . . . . . . . . 3.13
3.3.1 Suspended Sol ids Limitation . . . . . . . . . . . 3.3.2 Settleable Solids Limitation . . . . . . . . . . 3.13
3.13
3.4 Trapping Eff ic iency . . . . . . . . . . . . . . . . . . 3.5 S e t t l e a b l e So l ids Concentration . . . . . . . . . . . . 3.6 Storage Volume Requirement . . . . . . . 3.7 Available Storage Volume . . . . . . . . . . . . . . . . 3.8 Sedimentation Pond Configuration . . . . . . . . . . . .
3.16 3.20 3.29 3.30 3.36
3.8.1 Ideal S e t t l i n g . . . . . . . . . . . . . . . . . 3.8.2 Nonideal S e t t l i n g . . . . . . . . . . . . . . . . 3.36
3.39
3.8.2.1 Flow Currents . . . . . . . . . . . . . 3.8.2.2 Delta Formation . . . . . . . . . . . . 3.39
3.40
. . . . . . _-A_.-- . . ...-... . . . .
TABLE OF CONTENTS (continued) CHAPTER 111
\
Page
3.8.2.3 Short C i rcu i t i ng and Turbulence . . . . 3.40 3.8-2.4 Scour and Resuspension . . . . . . . . . 3.40
3.8.3 Control of Nonideal S e t t l i n g . . . . . . . . . . 3.40
3.8.3.1 Short-Circuit ing Factor . . . . . . . . 3.43 3.8.3.2 3.8.3.3 3.8.3.4
Length-to-Width Ratio . . . . . . Permissible I n l e t Velocity . . . . Permissible Flow-Through Velocity . .
3.43 3.44 3.44
3.9 Sedimentation Pond Outlet Control Measures . . . . . . . 3.46
3.9.1 Pr inc ipa l Spillways . . . . . . . . . . . . . . . 3.46
3.9.1.1 Op en Channel Spillways . . . . . . . . . 3.46 3.9.1.2 Drop I n l e t Spillways . . . . . . . . . . 3.47 3.9.1.3 Pipe Culvert Spillways . . . . . . . . . 3.47 3.9.1.4 Eff ic iency of Pr inc ipa l Spillways . . . 3.50
3.9.2 Dewatering Devices . . . . . . . . . . . . . . . . 3.50 3.9.3 P r i n c i p a l Spillway Modifications . . . . . . . . 3.54
3.9.3.1 Weir Troughs . . . . . . . . . . . . . . 3.56 3.9.3.2 Floa t ing Discharge . . . . . . . . . . . 3.56 3.9.3.3 F i l t e r i n p . . . . . . . . . . . . . . . 3.58 3.9.3.4 Gated Spillways . . . . . . . . . . . . 3.58 3.9.3.5 Anti-Vortex Devices . . . . . . . . . . 3.60
3.9.4 Emergency Spillways . . . . . . . . . . . . . . . 3.60 3.9.5 Erosion Control B e l o w Spillways . . . . . . . . . 3.65
3.9.5.1 General . . . . . . . . . . . . . . . . 3.65 3.9.5.2 Riprap . . . . . . . . . . . . . . . . . 3.66
3.10Sunanary . . . . . . . . . . . . . . . . . . . . . . . . ‘ 3 . 6 7
LIST OF TABLES CHAPTER I11
Page
- Table 3.1 Suggested P a r t i c l e S ize Distribution '
for Soil Textural Class . . . . . . . . . . . . . . . . 3.11
Table 3.2 Development of Settleable So l ids S i z e Distribution . . . . . . . . . . . . . . . . . . 3.24
Table 3 . 3 S i ze Distribution for Part i c l e s Smaller than 0.011 orrm . . . . . . . . . . . . . . . . . 3.29
Table 3.4 Stage-Storage Relationship Development . . . . . . . . . 3.36
Table 3.5 Permissible Velocities for Vegetated Spillways . . . . 3.63
Table 3.6 Permissible Spillway Velocities after Aging . . . . . 3.64
.....................
LIST OF FIGURES CHAPTER I11
Page
Figure 3.1 Imhoff cone test apparatus . . . . . . . . . . . . . . . 3.15
Figure 3.2 Requi red trapping eff ic iency t o meet various e f f luen t l imi ta t ions . . . . . . . . . . . . . 3.18
Figure 3.3 Definit ion of trapping eff ic iency f o r various particle s i zes . . . . . . . . . . . . . . . . 3.19
Figure 3.4 Inf luent sediment s ize d is t r ibu t ion . . . . . . . . . 3.21
Figure 3.5 Set t leab le s o l i d s s i z e d is t r ibu t ion . . . . . . . . . 3.24
Figure 3.6 Definit ion of percentage in t e rva l s . . . . . . . . . . 3.27
Figure 3.7 Water rout ing curver S/V versus Tb . . . . . . . . . . 3.31
Figure 3.8 Water routing curve. QI/Qo versus Tb . . . . . . . . . 3.32
Figure 3.9 Definit ion sketch to determine incremental storage volume . . . . . . . . . . . . . . 3.33
Figure 3.10 P a r t i c l e s i z e versus detention . . \ time f o r various depths . . . . . . . . . . . . . . . 3.38
Figure 3.11 Schematic of de l t a formation . . . . . . . . . . . . . 3.41
Figure 3.12 Sedimentation ponds with dead storage spaces . . . 3.42
Figure 3.13 Typical drop i n l e t spillway . . . . . . . . . . . . . 3.48
Figure 3.14 Typical pipe cu lve r t spillway . . . . . . . . . . . . 3.49
Figure 3.15 Subsurface drain . . . . . . . . . . . . . . . . . . . 3.52
Figure 3.16 Single perforat ion of riser barrel . . . . . . . . . . 3.53
Figure 3.17 Siphon dewatering methods . . . . . . . . . . . . . . 3.55
Figure 3.18 Weir trough . . . . . . . . . . . . . . . . . . . . . 3.57
Figure 3.19 Floating w e i r . . . . . . . . . . . . . . . . . . . . 3.59
Figure 3-20 Anti-vortex device . . . . . . . . . . . . . . . . . . 3.61
111. SEDIMENT REMOVAL THROUGH SEDIMENTATION PONDS
The design of sedimentation ponds is based upon sa t i s fy ing e f f luent l i m i -
t a t i ons . These l imitat ions are established fo r specif ied stream water qua l i ty
c r i t e r i a and design precipi ta t ion runoff event. Sedimentation ponds are
usually designed in the preliminary stages of the kine plan and therefore ,
t he re is l i t t l e or no available information on base flow conditions. Because
of t h i s , ponds are designed for the design prec ip i ta t ion runoff event and the
pond ef f luent fo r base flow conditions is tested a f t e r the pond becomes opera-
t iona l . I f base flow ef f luent l imitat ions cannot be satisfied, modifications
can be made to the pond. Generally, the design runoff event w i l l cont ro l the
sedimentation pond design unless sediment inflow fo r base flow conditions is
composed of high concentrations of f ine silts and clays.
The following sections describe the information that is required f o r
designing a sedimentation pond to m e e t e f f luent limitations.
is based upon ideal s e t t l i n g conditions with conservative fac tors incorporated
The pond design
i n t o the design t o account for nonideal s e t t l i n g conditions.
The design begins by select ing a particle s i z e which m u s t be removed i n
Determining the pond t h e pond such t h a t e f f luent l imitat ions are sa t i s f i ed . 5 . . configuration requires an in te rac t ive process which begins by assuming a
depth. The required storage volume and the avai lable storage volume are
determined and compared t o each other. When the required storage volume is
l a rger than the avai lable storage volume, a new depth is assumed and the
design process is repeated. Once the avai lable storage volume is adequate,
t h e pond configuration is checked based upon nonideal s e t t l i n g conditions.
The design procedure and example presented in Chapter vf show how the infor-
mation presented i n the following sections is used in sedimentation pond
design.
\
3.1 Site Selection
3.1.1 General Considerations
Selecting a sedimentation pon location requires consideration of several
fac tors . In a l l cases, sedimentation ponds must be constructed in locat ions
where it w i l l be possible t o direct or d iver t a l l surface runoff from
disturbed areas i n t o sedimentation ponds throughout the l i f e of mining opera-
t ions . Other fac tors which are of primary importance and should be considered
3.2
in se lec t ing a sedimentation pond location include the mpography of t he mine
site, locat ing major sources of sediment, access ib i l i ty of the sedimentation
pond, a v a i l a b i l i t y of construction materials, and the direct ion of mining. I n
many instances these fac tors will l i m i t the number of viable locations t h a t
are avai lable f o r sedimentation pond construction. In. par t icu lar , availabi-
l i t y of su i t ab le sites for a sedimentation pond location w i l l be control led,
t o a large extent , by the topography of the mine site. In addition, ponds
must be constructed pr ior to any disturbance of the mine area. Through care-
f u l planning practices and f i e l d investigation, the sedimentation pond loca-
t i o n s which w i l l meet t h i s objective can be ident i f ied.
3.1.2 Topography Considerations
3.1.2.1 Steep Sloped Terrain
Throughout the United States surface mining operaions are of ten located
i n s teep sloped te r ra in . This is t rue for the Appalachian mining region, the
Rocky Mountain, and parts of northern California and Washington.
regions which are characterized by steep sloped t e r r a in , the topography beco-
mes the most important control l ing factor i n the site se le t ion f o r a sedimen-
t a t i o n pond location.
pond locat ion is t o determine where an adequate storage volume can be
provided.
I n the
!
The main problem i n finding a su i tab le sedimentation
Where surface mining operations are located in the upper part of a
watershed, the topography is characterized by steep slopes and v-shaped
drainageways.
close t o the mining operation as possible; therefare , the only site that is often avai lable for a sedimentation pond location is the v-shaped drainageway
d i r e c t l y downstream of the surface mine operation.
incorporates the use of an embankment w i l l have t o be used.
of the sedimentation pond can be increased by excavating upstream of the
embankment. Often t i m e s the storage capacity provided by the embankment
including any upstream excavation does not provide the storage volume required
to achieve e f f luent l imitations. To Overcome the problem of sedimentation
pond locat ion i n steep sloping t e r r a in , there are two a l te rna t ives avai lable
t o the operator.
It is usually desirable to locate the sedimentation pond as
A sedimentation pond which
The storge volume
. - . . . . . - . . . .
3 . 3
. -
The f i r s t a l t e r n a t i v e i s to cons t ruc t a sedimentation pond a t a loca t ion
f a r t h e r downstream within t h e watershed where the slope of t h e drainageway
becomes milder and t h e shape of t h e drainageway becomes u-shaped.
vantage of this a l t e r n a t i v e is t h a t runoff from a much l a r g e r area w i l l need
/
The bisad-
to be contained and treated, thereby r equ i r ing t h e cons t ruc t ion of a much
larger s t ruc tu re .
The second a l t e r n a t i v e is to cons t ruc t a series of sedimentation ponds
located i n the steep, narrow drainageways where t h e runoff from t h e disturbed
mining areas passes through each sedimentation pond (mul t ip le sedimentation
ponds).
It should be noted here t h a t t h e r e are o the r a l t e r n a t i v e s t o seaimen-
t a t i o n ponds f o r sediment and eros ion con t ro l .
area, s e v e r a l devices and techniques have been used.
Sediment Control Measures f o r Small Areas in Surface C o a l Mining" (OSH, 1982)
'for f u r t h e r consideration of a l t e r n a t i v e s to sedimentation ponds.
Depending on t h e size of the
Refer t o "Design of
3.1.2.2 Mild Sloped Terrain
There is much more f l e x i b i l i t y in s e l e c t i n g a sedimentation pond loca t ion 5 i n mild sloped t e r r a i n . The phys ica l c o n s t r a i n t s imposed by the topography
are less than for s teep ' s loped t e r r a i n and the re fo re , more a t t e n t i o n may be
directed toward t h e o the r primary f a c t o r s considered in t h e s e l e c t i o n of a
sedimentation pond site.
\
Sedlmentation ponds may be located on or of f drainageways. Small draina-
geways are o f t en s e l e c t e d for a sedimentation pond location where an embank-
ment Is used with or without excavation t o provide t h e s to rage volume
required. Due to t h e milder drainageway p r o f i l e and milder slopes of t h e
v a l l e y , t h e sedimentation pond located in t h e mild sloped t e r r a i n w i l l nor-
mally have a greater length and width f o r any he ight of dam spec i f i ed , thereby
provid ing more s torage capacity.
O f f drainage loca t ions are genera l ly p re fe r r ed when t h e r e is a suitable
l o c a t i o n available f o r sedimentation pond cons t ruc t ion .
areas are good loca t ions f o r sedimentation ponds. An embankment can be
cons t ruc t ed across t h e downstream end of t h e depression area and t h e s to rage
volume may be increased by excavation.
Natural depression
3.4
3.1.3
Sedimentation ponds located on main drainageways are usua l ly found i n
Sedimentation Ponds irocated on Hain Drainageways
su r face mining opera t ions located i n steep t e r r a i n . Due to topographic
c o n s t r a i n t s , this may be the arost cos t -e f fec t ive way t o control sediment.
main drainageway may be either an ephemeral or perennial stream. The sedimen-
t a t i o n pond located on a perennia l stream will have a permanent pool, whereas
sedimentation ponds loca ted on main drainageways which are ephemeral may be
e i t h e r a dry bas in or permanent pool.
The
The disadvantage of l oca t ing a sedimentation pond on 01 main drainageway
is that t h e su r face runoff from both d is turbed an6 undisturbed areas w i l l have
t o be detained long enough t o achieve e f f l u e n t l imi t a t ions . This requires
much l a r g e r s to rage volume be provided and the re fo re , the cons t ruc t ion of a
much l a r g e r sedimentation pond structure. I n addi t ion , chances of a sedimen-
t a t i o n pond being washed away during a major flood event are increased due t o
c o n t r o l of runoff from a l a rge r drainage area. Sedimentation ponds located on
drainageways which are perennial streams must be desgined t o m e e t base f l o w
w a t e r q u a l i t y l imi t a t ions . When t h e sedimentation pond is removed, reclama-
t i o n of t h e drainage channel w i l l be required. The channel w i l l have t o be . . \ restored t o i ts o r i g i n a l shape, slope, and channel pro tec t ion .
It is p r e f e r r a b l e t o select a sedimentation pond loca t ion which w i l l no t
be located h a main drainageway. However, t h e topographic c o n s t r a i n t s of t h e
mine site area may be such that the main drainageway is t h e only possible site
for a sedimentation pond location.
3.1.4 Sedimentation Ponds irocated of f Main Drainageways
Off nutin drainageways sedimentation ponds are genera l ly used in r o l l i n g
and mild t e q r a i n where t h e topography does not restrict t h e loca t ion t o t h e
e x t e n t a t do s i n steep sloped t e r r a i n . The types of sedimentation ponds '-7-
used'
embankment and excavation. The sedimentation ponds may be cons t ruc ted as
e i t h e r a permanent pool or dry basin.
- *'U main drainageway loca t ions are embankment or some combination of
The of f main drainageway loca t ion has seve ra l advantages over t h e on main
drainageway loca t ion .
t i o n can genera l ly be constructed closer t o t h e sediment source and the re fo re ,
designed f o r a smaller i n f l u e n t volume. This location avoids unnecessary
A sedimentation pond i n an of€ main drainageway lOCa-
.. . - .. . _ _ - - . .
3.5
treatment of runoff from undisturbed areas. E s s e n t i a l l y , the base flow is
zero, therefore the pond is generally designed based on precipi ta t ion events.
Gated o u t l e t s can be used on sedimentation ponds located i n an off main
drainageway s i te to control the detention t i m e between runoff events. ~ l s o ,
the chances of an apbanhent sedimentation pond f a i l i n g during a major storm
event are reduced since only the runoff from a much smaller area w i l l have to
be controlled.
whether an off main drainageway location w i l l be feasible fo r the operator t o
construct.
However, again topography w i l l play an important r o l e as t o
3.1.5 Source of Sediment
During the development of a mine plan, the locations that w i l l be major
sources of sediment i n
Major sediment sources
spoil p i l e s , and areas
major sediment sources
throughout t he l i f e of
stages.
the surface mining operation should be ident i f ied.
include haul and access roads, areas being cleared,
being reclaimed. As mining progresses the locations of
change. Thus, the location of sediment sources
the mine should be considered during the planning
Sedimentation ponds should be located as close to m j o r seaFment sources
as possible.
t ages from both a sediment control and construction viewpoint.
t h e sediment as close t o the source as possible may require the construction
Locating sedimentation ponds i n t h i s manner has several advan-
Controlling
of several smaller sedimentation ponds as opposed to one or two la rger ponds.
The smaller seaFmentation ponds may be constructed d i r ec t ly downstream of the
major sediment sources thus requiring sediment control of only the disturbed
areas. The net e f f e c t of this is the inf luent volume is reduced by avoiding
co l lec t ion of runoff from undisturbed areas, thereby reducing the required
storage volume to achieve eff luent l imitations.
3.1.6 Accessibil i ty
Improper, o r lack of , maintenance for sedimentation ponds is one of the
major reasons fo r poor sediment removal eff ic iencies . Often the l a c k of main-
tenance is due t o inaccess ib i l i ty to the location of the sedimentation pond.
Sedimentation ponds are often constructed i n locations that are remote from
t he surface mining operation and therefore, access roads t o the sedimentation
3.6
pond are given l i t t l e regular a t t e n t i o n and maintenance.
a sedimentation pond should be a primary consideration i n t h e planning and
design of sedimentation ponds.
The access road should be designed considering t h e type of equipment
The a c c e s s i b i l i t y t o
which w i l l use the road.
such t h a t t h e road drainage w i l l be directed i n t o t h e sedimentation pond.
Other sediment c o n t r o l measures or t h e construction of another sedimentation
pond w i l l be requi red downstream of t h e access road t o prevent o f f - s i t e damage
i f road drainage i n t o the sedimentation pond is not possible.
Where poss ib le , t h e access road should be designed
3.1.7 Mining Considerations
Throughout t h e l i f e of t h e sur face mining opera t ions , t h e loca t ions of
t h e major sources of sediment w i l l cons tan t ly change due to the progression of
mining. Sedimentation pond loca t ions should be selected cons ider ing t h e
d i r e c t i o n of mining so it w i l l be poss ib le t o direct or d i v e r t a l l s u r f c e
&off from d i s tu rbed areas i n t o t h e pond throughout t h e l i f e of t h e mining
opera t ions .
a minimum t o avoid f i l l i n g of t h e sedimentation pond prematurely.
I n a l l cases, t i m e of exposure of cleared land should be kept t o \
. . \
3.1.8 Field Inves t iga t ion
F i e l d i n v e s t i g a t i o n is e s s e n t i a l in the development of an e f f e c t i v e sedi-
ment c o n t r o l plan.
t h a t must be considered in s e l e c t i n g a sedimentation pond loca t ion , several
pre l iminary sedimentation poncl l oca t ions can be selected using the most r ecen t
topographic maps of t h e mine area.
ducted t o v e r i f y information from topographic maps, survey t h e phys ica l
f e a t u r e s of each s i te , and i d e n t i f y any problems which may be encountered a t
each si te.
Af te r the operator has an understanding of the factors
A f i e l d inves t iga t ion should then be con-
There are several su r face f e a t u r e s t h a t should be noted a t each p o t e n t i a l
sedimentation pond loca t ion . These f ea tu res inc lude s o i l type, vege ta t ive
cover, p r e f i l e and s ide slopes of t h e drainageway, channel shape, channel pro-
t e c t i o n , and t h e c a p a b i l i t y of each s i t e to provide the design s to rage volume.
The previous ly mentioned f ea tu res are a l l interrelated in inf luenc ing t h e
e r o s i o n a l p o t e n t i a l of t h e s i te and t h e s u i t a b i l i t y of t h e s i te as a sedimen-
t a t i o n pond location. An overview of each si te should be conducted not ing any
3.7
problems r e l a t ing t o unstable s o i l s or erosion.
denced by small landslides, bank sloughing, and gully or till erosion.
These features w i l l be evi-
The s o i l type and vegetative cover a t the s i t e should be investigated and
noted.
s o i l . I f a large area w i l l be cleared fo r mining operations and the construc-
t i o n of the sedimentation pond, protective measures may be required to protec t
t h e b a r r e n so i l .
desirable t o d is turb s o i l s which would become unstable. This w i l l happen f o r
s o i l s which have l i t t l e cohesion and the problem is increased i f the sedimen-
t a t i o n pond is constructed in steep sloped te r ra in .
disturbed, a continous sloughing of the banks w i l l occur which w i l l reduce the
sediment removal eff ic iency of the pond as w e l l as threaten the s t a b i l i t y of
t he embankment. However, sedimentation ponds located in s o i l s which have a
high clay content may pose a problem in achieving ef f luent l imitations.
t o turbulence within the pond and wave action on the banks, high con-
centrat ions of col lo ida l particles could r e s u l t and w i l l require the addition
of coagulants or f locculants under base flow conditions.
Vegetation reduces the erosion poten t ia l and tends t o s t a b i l i z e the
Where s o m e type of excavation might be required, it is not
d
Once unstable s o i l s are
Due
i
I n s teep sloped areas where sedimentation ponds are o f t e n constructed on
drainageways, the drainageway channel shape and protection should be invest i -
gated.
w i l l have to be r e s to r id after the removal of t he embankment.
have developed an armoring Layer of a ce r t a in s i z e particle o r it may be pro-
tected only by vegetation.
downstream of the sedimentation pond and the magnitude of scouring W i l l be
greater fo r the vegetation-lined drainageway than fo r the drainageway which
has already developed an armoring layer.
\
The drainageway channel protection should be noted since the channel
The channel may
It can be expected t h a t scouring w i l l occur
Once i n the f ie ld , the designer should ver i fy the information on the
topographic maps and survey the physical features a t each site.
f i e l d investigation, a review and comparison of the advantages and disadvan-
tages of each s i te can be evaluated to select the best sedimentation pond
location.
After the
3.8
3.2 Data Requirements
Information t h a t is required t o design a Sedimentation pond can be broken
i n t o three categroies: hydrology, sediment data, and inflow suspended so l id s
concentration. The following sect ions describe what spec i f ic information is
required fo r sedimentation pond design.
3 2 1 Hydrology
Hydrologic information required t o design a sedimentation pond includes
the peak inflow rate and the runoff volume fo r the design storm event. In
addition, where ponds receive the inflow from the mining p i t , pumping, or are
located on a perennial stream, the inflow rate f o r base flow conditions must
be determined.
For the design storm event, an inflow hydrograph must be developed from
which the peak inflow r a t e and runoff volume can be determined. There are
severa l references available which describe inflow hydrograph development
(OSM, 1982; Bureau of Reclamation, 1977; Barfield, 1981; Soi l Conservation
Service, 1975 1. I .
For sedimentation ponds which receive inflow by pumping, the designer
! w i l l have to determine the inflow r a t e f o r base flow conditions. For ponds \
which a re located on perennial streams, the designer can use h i s t o r i c a l data
i f available.
Therefore, the inflow rate for base flow conditions or from pumping w i l l
generally have to be measured a f t e r the pond has been constructed.
However, t h i s type of information may be very limited.
3.2.2 Sediment Data
The sediment data required f o r pond design are the p a r t i c l e s i z e distri-
bution and total sediment yield during a runoff event. The design of a pond
occurs during the planning stages before ac tua l mining starts. Therefore,
Information on the particle s i z e d is t r ibu t ion of the sediment runoff from the
disturbed area is not generally avai lable f o r the spec i f ic s i t e .
following sections discuss the methods of obtaining sediment s i z e d is t r ibu t ion
and sediment yield.
The
3.9
3.2.2.1 Influent Sediment Size Distribution
The most important sediment data required to design a sedimentation pond
to meet effluent limitations is the particle size distribution of the sediment
influent. The particle size distribution should represent the worst condition
during the life of the mine. From past and present experience, the worst con-
dition occurs during the reclamation phase. Two conditions during the recla-
mation phase must be considered.
The first condition to be considered is before the topsoil or "A" horizon
has been replaced.
size distributi
replaced. L l G - Pondition, the particle size distribution of the eroded soil will be represented by - the txpsd-1.
particle size distribution with the highest percentage of particle sizes in
the silt range (0.001 to 0.074 m) will be selected for the design influent particle size distribution.
distribution is to obtain size distribution Information from previous anh nearby mining operations. When mining operations within the same area or
areas with the same soil texture exist, determination of particle size distri-
butions of sediment runoff from existing analysis can be used.
ticle size distribution from a nearby site is used, several considerations and
comparisons must be made so the information does represent the site under
consideration.
The soil which is eroded, and hence the influent particle 1 be represented by the graded overburden. The second -
condition to be - - 1 7 onsider d is after the topsoil or "A" horizon has been 8'
Whichever condition results in a c- -
\ The best way to estimate the particle size
'
t \
Before a par-
1. Soil characteristics at both sites should be very similar including soil types below the surface which are disturbed during mining.
2. Slopes, drainage, and sediment transport characteristics of both sites should be evaluated and compared.
3. The type of mining and amount of area disturbed at both sites should be evaluated.
4. Data from as many samples and sites should be collected and eva- luated to provide a good estimate.
5. The magnitude of the runoff event during which the sample was collected should be considered.
When these data do not ex i s t , but nearby sites do exist, sampling and labora-
to ry analysis should be conducted whenever possible.
Another method f o r developing particle s ize d is t r ibu t ion information is based on the site spec i f ic soil textural class and physical properties.
Generally, s o i l physical propert ies occurring at a spec i f ic si te can be iden-
t i f i e d using information given in s tandard Soil Conservation Service (SCS)
s o i l surveys. These invest igat ions c o n s i s t of c lass i fying physical, chemical,
and biological cha rac t e r i s t i c s of s o i l s extending to depths of up t o s i x feet.
The U.S. Department of Agriculture (USDA) publishes reports and maps of their
soil surveys, usually on a county basis.
A procedure f o r determining particle s i ze d is t r ibu t ion based on s o i l
textural class is presented f o r use w i t h t h i s manual.
simply a name given each s o i l which designates the ranges of sand, silt, and
c lay s i zes it contains.
descriptions, other soil survey data i n t he vicini ty , s o i l data from the mine
p lan , f i e l d estimation by a soil s c i e n t i s t , or laboratory analysis. After
determining the textural c lass i f ica t ion , the corresponding particle s i z e
groups are then determined from Table 3.1.
A textural class is
This class can be obtained from SCS s o i l series
I
r Where the mining area has several s o i l textural c l a s s i f i ca t ions within
For \
the drainage bowdary, a composite s i z e dis t r ibut ion can be developed.
each pa r t i cu la r soil t ex tu ra l c lass i f ica t ion , the sediment s i z e d is t r ibu t ion
given in Table 3.1 W i l l be multiplied times the f rac t ion of the disturbed area
that each s o i l textural class covers. The values fo r each soil textural class
are then aUded together t o form a representative composite s i z e d is t r ibu t ion . An example of developing a representative composite s i z e d is t r ibu t ion is given
i n the design example i n Chapter VI.
The sediment s i z e d is t r ibu t ion based on tex tura l class is not recommended
f o r use i f amre detailed soil data are available a t the mine site. Also, it
is impor tan t t h a t the s o i l data describing the material b e l o w the surface
(exposed during mining) be considered during development of t he particle s i z e
d is t r ibu t ion .
than the information on which it is based.
changes and modifications t o the pond after construction, the particle s i z e
d i s t r ibu t ion u t i l i z e d should be a conservative estimate.
The designer should r ea l i ze that the design can be no better
To help eliminate s ign i f i can t
3.11
n a Y)
F A
V Y
n n
0 a - - a L
E a I-
s1
m n
m -
0 c
N
U C
In a
0 N
0 w
0 N
m c
m
u C
v) a
it 0 a
0 *)
0 hl
0 hl
m N
m
E
-I
6 C
v) a
m (Y
m N
0 c
0 n
0 -
I
m c
m
0 W
0 N
I
m
m
0 c
5:
0 c
m N
m N
E 0
3 * a 0 % 0 c .I v)
-
0 -
0 c
0 c
0 w
0 R
I
m
0 c
0 (Y
m W
. . .. . . . .
3.12
The s i z e d is t r ibu t ion for base flow conditions can be s igni f icant ly d i f -
f e r e n t from the s i ze d is t r ibu t ion based upon the surface and overburden so i l s .
Generally, the s i z e d is t r ibu t ion fo r base flow conditions w i l l be composed of
smaller particle s izes . Sampling of the base flow s i z e d is t r ibu t ion is recom-
mended to accurately design for base flow ef f luent Limitations.
For sedimentation ponds which receive inflow by pumping, the sediment
s i z e d is t r ibu t ion is very d i f f i c u l t to predict.
s i z e d is t r ibu t ion can be developed from the overburden so i l . Once the pond is
operational, the ef f luent w i l l have to be t es ted and pond modifications may be
required.
An i n i t i a l estimate of the
3.2.2.2 Sediment Yield
Sediment y ie ld of the mining area is required to determine the sediment
s torage volume of the pond and ca lcu la te the average e f f luent concentration
f o r the design storm event.
upon the annual sediment yield and the frequency of sediment removal.
lef t to the designer to decide how often the sediment w i l l be removed from the
pond.
Loss Equation (USLE). There are several references which are avai lable which
describe the use of the'USLE (OSM, 1982; Barfield, 1981).
The required sediment storage volume is dependent
It is i
The annual sediment y ie ld can be determined using the Universal Soil t \
Sediment y ie ld fo r the design stonn event must be determined 80 the
average e f f luent concentration can be calculated. The Modified Universal Soil
Loss Equation (MUSLE) can be used to ca lcu la te the sediment y ie ld from the
design storm event. of MUSLE.
The previously mentioned references also describe the use
3.2.3 Inflow Suspended .Solids Concentration
The inflow suspended so l ids concentration is required f o r both base flow
conditions and the design runoff event. For base flow conditions, the
in f luen t suspended solids concentration w i l l have to be measured since it is very d i f f i c u l t to predict.
For the design runoff event, the average inf luent suspended so l ids con-
c e n t r a t i o n can be computed knowing the storm runoff volume and sediment yield.
The average inf luent suspended so l id s concentration is computed as:
6
3.13
where, CI = average i n f l u e n t suspended solids concentration (mg/l),
Y = storm sediment y i e l d (lbs),
y = u n i t weight of water (62.4 1b / f t3 ) , and
It should he recognized t h a t t h i s concentration is t h e average suspended
So l ids concent ra t ion during t h e storm and higher suspended s o l i d s con-
c e n t r a t i o n s would be expected when t h e peak inflow rate occurs.
/ 3.3 E f f l u e n t Limitations ,-> I .
Design procedures f o r sedFmentation ponds developed in this manual are- L r based on meeting So l ids e f f l u e n t l imi t a t ions .
t h a t t h e r e are o the r e f f l u e n t q u a l i t y limitations on i ron , manganese, and pH.
It assumed that t h e manual w i l l be used for design of sedimentation ponds
i n t h e planning s t ages of mining and that t h e mining operation is con t ro l l ed
by New Source Performance Standards (NSPS).
l i m i t a t i o n s are based on flow condition and t h e state of mining operation.
The opera tor should be aware
'1 The NSPS solids e f f l u e n t q u a l i t y
! \ 3.3.1 Suspended Solids Limitation
Solids e f f l u e n t q u a l i t y l i m i t a t i o n during base f l o w for a c t i v e su r face
mining, underground mining, and coal prepara t ion areas is 35 mg/l total
suspended solids (TSS) for the average of d a i l y values for 30 consecutive days
and a maximum of 70 mg/l TSS f o r any one day. Por post-mining cona i t ions , ' t he
discharge f r o m underground mine drainage is a l s o subject t o these suspended
solids l i m i t a t i o n s . 3.3.2 S e t t l e a b l e Sol ids Lfmitation
During any discharge or w e r f l o w r e s u l t i n g from a p r e c i p i t a t i o n event
less than or equal to t h e 10-year, 24-hour p r e c i p i t a t i o n event, t h e discharge
i s subject to s o l i d s e f f l u e n t q u a l i t y l i m i t a t i o n s of 0.5 m l / l settleable
solids (SS).
event greater than t h e 10-year, 24-hour p r e c i p i t a t i o n event the discharge is
During any discharge or werflow r e s u l t i n g from a p r e c i p i t a t i o n
I
3.14
not s u b j e c t t o a s o l i d s e f f l u e n t q u a l i t y l imi t a t ion . These a l t e r n a t e limita-
t i o n s during p r e c i p i t a t i o n events only apply i f :
1. The treatment f a c i l i t y is designed, constructed, operated, and main- t a i n e d to contain a t a minimum t h e volume of water which would d ra in i n t o t h e treatment f a c i l i t y from a c t i v e ' n l n i n g areas and reclamation areas during t h e 10-year, 24-hour p r e c i p i t a t i o n event (or snowmelt of equivalent v0lume)t
2. The treatment f a c i l l t y is designed, constructed, operated, and main- t a i n e d t o cons i s t en t ly achieve t h e e f f l u e n t l i m i t a t i o n s set by the regula tory agencies f o r a l l e f f l u e n t q u a l i t y l imi t a t ions ;
The volume of settleable solids i n the e f f l u e n t from a sedimentation pond
is determined by a simple procedure known as t h e Imhoff cone test (see Figure
3.1).
mixed sample.
cone are gent ly s t i r r e d with a rod to f r e e any particles which may be c l ing ing
t o t h e s i d e s of t h e cone, and s e t t l i n g is allowed to occur f o r an a d d i t i o n a l
15 minutes. The volume of settleable s o l i d s i n t h e cone is then recorded as
The Imhoff cones are f i l l e d to t h e one - l i t e r mark with a thoroughly
S e t t l i n g is allowed to occur for 45 minutes, t h e s i d e s of the
' milliliters per l i t e r (from %tandard Methods f o r Examination of Water and
Wastewater,' 15th e d i t i o n ) .
perature 25OC, 77.F) or t h e r e s u l t s are ad jus ted to room temperature. It
should be poin ted ou t that s o m e d i f f i c u l t y exists in reading the Imhoff cones.
When dea l ing with f i n e particles such as silt, it requires practice in
def in ing t h e volume of settleable solids. It is recommended t h a t t hese
readings be taken in t h e presence of persons who have experience in performing
the Imhoff cone test.
The test is normally performed a t room texn-
: \
P a r t i c l e s i z e s smaller than one micron (0.001 mu) are assumed non-
Therefore, particle sizes settleable under gravitational fo rces alone.
smaller than one micron are not considered settleable s o l i d s i n this manual. - A well-designed sedimentation pond w i l l remove p r a c t i c a l l y a l l of the
sand-sized particles. Therefore, t h e settled volume in t h e b o t t o m of t h e
Imhoff cone w i l l be composed pr imar i ly of silt.
The sma l l e s t particle which w i l l set t le through the e n t i r e he ight of t h e
Imhoff cone during t h e test can be computed. Based upon Stoke's Law8 test
condi t ions , and assuming a s p e c i f i c g rav i ty of t h e particle to be 2.65, this particle size is computed as 0.011 rmn (GI. Stokes's f a w is based upon i d e a l
s e t t l i n g and t h e r e are aeveral re fe rences ava i l ab le which d iscuss Stoke's L a w
... _ r . . . - . .. .
3.15
FIGURE 3.1 IMHOFF CONE TEST APPARATUS (SAWYER, 1978)
3.16
(Barfield, 1981; Shames, 1962). A l l particles larger than 0.011 nm~ would
sett le during the test.
mm would be expected to settle depending upon the concentration of each par-
t icle s i z e within the sample.
select a particle s i z e of a par t icu lar s i z e d is t r ibu t ion t h a t must he removed
so that the se t t l eab le so l id s concentration meets e f f luent l imi ta t ions when
t h e sample is placed i n the Imhoff cone.
only a percentage of the pa r t i c l e s smaller than 0.011
The objective of *is design manual is t o
3.4 Trapping Efficiency '
To m e e t e f f luent l imitat ions, sedimentation pond design must be based on
sediment s ize d is t r ibu t ion and TSS concentration of t he base flow or design
storm runoff entering the pond.
common method f o r developing the pond design c r i t e r i a to meet a specif ied
e f f luent l imitat ion is by determining the percent of sediment removal
required.
(E) and is equal to the weight of sediment flowing i n t o the pond minus the
weight of sediment leaving the pond divided by the weight of sediment flowing
i n t o the pond and then multiplied by 100 to obtain eff ic iency In percent.
Thus, the trapping eff ic iency is given by:
Based on present state of the art , the most
The percent of sediment removal is ca l led the trapping eff ic iency
L
: \
wo x 100 wI - E = wI '
where, wI - weight of sediment flowing i n t o the pond,
Wo = weight of sediment flowing out of the pond,
During base flow, the sedimentation pond w i l l be i n a steady-state
condition where the water inflow volume equals the water outflow volume. The
w a t e r volume can be changed to a weight of w a t e r .
sediment by t h e weight of water w i l l y ie ld a concentration of TSS.
t h e trapping eff ic iency becomes
Dividing the weight of
Therefore,
x 100 cI - co E = ( 3 . 3 )
.. . ... . . .. .. . .
3.17
where, CI = average sediment concentration
Co = average sediment concentration
i n t o the pond,
o u t of t h e pond.
For base f l o w , e f f l u e n t l imi t a t ions are stated as a concentration of TSS.
Therefore, a r e l a t i o n s h i p between the i n f l u e n t TSS Concentration and t h e
t r app ing e f f i c i e n c y can be developed i f t h e e f f l u e n t TSS concentration is
known. Once t h e i n f l u e n t TSS concentration has been measured, t h e required
t r app ing e f f i c i e n c y can be determined i f the e f f l u e n t TSS concentration is
known. Figure 3.2 presen t s t h i s r e l a t i o n s h i p f o r a range of e f f l u e n t con-
c e n t r a t i o n l imi t a t ions . Knowing t h e i n f l u e n t TSS concentration, the required
t r a p p i n g e f f i c i e n c y to limft the e f f l u e n t concentration to a s tandard can be
determined from Figure 3.2.
During t h e design p r e c i p i t a t i o n runoff event, t h e development of pond
The condition during a storm runoff is design criteria is more d i f f i c u l t .
.dynamic i n that the inflow to t h e pond is represented by a runoff hydrograph;
t h e outflow is based on t h e water surface e l eva t ion in t h e pond and t h e
discharge capac i ty of t h e outflow device. I n addi t ion , e f f l u e n t l i m i t a t i o n s
for the design p r e c i p i t a t i o n runoff event are stated as a concent ra t ion of
b settleable sediment. !
To design f o r t h e design runoff event requires that a practical approach
be taken. The method used to route the inflow hydrograph through the sedimen-
t a t i o n pond is based upon the inflow volume being equal to the outflow volume.
(Water rou t ing is discussed in Section 3.6.) Therefore, the t rapping e f f i -
c iency for the design runoff went can also be computed using Equation 3.3.
However, for t h e design runoff event, t h e suspended solids concent ra t ion and
t h e t r app ing e f f i c i e n c y are both unknown. - By d e f i n i t i o n , t h e t rapping e f f i c i ency is t h e weight of sediment removed
i n the pond.
c e n t r a t i o n and s i z e d i s t r i b u t i o n . I n addi t ion , it is assumed t h a t the
i n f l u e n t sediment is evenly d i s t r i b u t e d in t h e water inflow. Therefore, when
a
pe rcen t of sediment removal or t rapping e f f i c i e n c y is equal to t h e percent of
I t h e s i z e d i s t r i b u t i o n t h a t is l a r g e r than di . Figure 3.3 p re sen t s t h e def i -
The i n f l u e n t sediment is represented by the sediment con-
sedimentation pond is designed t o remove a c e r t a i n particle size ( a i l , t h e
n i t i o n of t h e t rapping e f f i c i ency f o r various particle s i zes . This estimate
of t r app ing e f f i c i e n c y is conservative s ince it assumes none of the particles
100
90
80
n
a? Y * 0
w' 70 3 :
w
- LL
>LL
a z 60 a a
0: t-
50
40
. I 10
- . ... . . . . . . . ....
3.113
I 100 lob0
INFLUENT SUSPENDED SOLIDS CONCENTRATION (mQ./L.)
FIGURE 3.2 REQUIRED TRAPPING EFFICIENCY TO MEET VARIOUS EFFLUENT LIMITATIONS
. . . . . . .
. . . - . .. . . .. , . .. , . . .~-,. ._. . _ _ ...._ , . . _.
3.20
smaller than the selected particle s i ze ( d i ) w i l l set t le i n the pond.
Actually, a percentage of the particles smaller than d i w i l l settle.
Therefore, f o r each particle s ize , a trapping eff ic iency can be determined
from the inf luent s i z e d is t r ibu t ion , and the suspended so l id s concentration
can be calculated by rearranging Equation 3.3.
E co = ( 1 - - 100) cI (3.4)
To determine whether the e f f luent requirements a re s a t i s f i e d , a rela-
t ionship between the suspended so l ids concentration and the settleable solids
concentration is required. This re la t ionship is presented in the following
section.
3.5 Set t leab le Solids Concentration
Effluent l imi ta t ions during runoff events and post-mining reclamation are
stated in terms of a volume of s e t t l eab le so l ids per one l i ter of sample. To - relate the settleable solids J i d t a t i o n -+ -the- *sign - . - . of - sedimentation ponds, c-- .__c_
a relat ionship between settleable solids and total suspended solids m u s t be
considered.
settle in the bottom of an Imhoff cone i n one hour of quiescent se t t l i ng .
-- Set t leab le solids are defined as the volume of particles t h a t . _
Knowing the inf luent sediment s i z e dis t r ibut ion, a particle s i z e t o be
settled in t h e pond is selected and the se t t l eab le so l id s concentration is determined. If the settleable so l id s concentration is l a rger than e f f luen t
l imitat ions, a smaller particle s i z e is selected and a new settleable so l id s
concentration is computed. Likewise, if the settleable so l id s concentration
is Qnaller than the e f f luent l imitat ions, a larger particle s i z e is se lec ted
and the ettleable so l id s concentration is computed. Therefore, an I
interat i i process is required to determine the particle s i z e that t h e s ea - / e entat ion pond must remove so the pond ef f luent satisfies the settleable
s o l i d s l imitat ion.
The first s t ep in computing the se t t l eab le sol ids concentration is t o
ad jus t the inf luent sediment s i z e d is t r ibu t ion by subtract ing out the non-
s e t t l e a b l e s i z e s ( < 0.001 man). Given the s i z e d is t r ibu t ion in Figure 3.4, it
can be seen that ten percent of the sediment is smaller #an 0.001 CPILI.
Therefore, the 90 p e r c e n t of the s i z e d is t r ibu t ion which is settleable m u s t be
. . . _ _ - - , . - . _ _ .
3.21
6 0 0 6
0 0 0 0 0 QD (0 0 cu
tl3NId l N 3 3 U 3 d
8 0
0 9
E E Y
W
w J 2 I- a a a
w .tn
3.22
red is t r ibu ted 60 that it makes up 100 percent of the s i z e dis t r ibut ion.
3.2 shows how t o develop a s i z e d is t r ibu t ion i n which a l l particle s i zes are
se t t l eab le .
Table
The settleable s i z e d is t r ibu t ion is presented i n Figure 3.5.
A re la t ionship between the ef f luent suspended solids concentration, t he
s e t t l e a b l e particle s i z e dis t r ibut ion, and the settleable so l ids concentration
is required.
suspended so l id s concentration to se t t l eab le so l id s based on d iscre te particle
s e t t l i n g and the geometry of the Imhoff cone. The volume of settleable so l id s
is given by
Barf ie ld (1981) developed an equation f o r the conversion of
A
d i 6 [ ( l - X0’ + c (TI Axil C* ss = - W
i = 1 0
where, SS = settleable so l id s concentration (mg/l),
C* - average e f f luent suspended so l id s concentration fo r the settleable s i zes (mg/l),
W = dry bulk density of the settled so l ids (mg/ml),
(3.5)
Xo = f rac t ion of particles in the e f f luent d i s t r ibu t ion smaller than = 0.011 nun,
d, = smallest particle which w i l l settle through the e n t i r e height of an Imhoff cone (0.011
d i = mean particle s i ze of the in t e rva l (m)8 and
A X i = f r ac t ion of e f f luent sediment s i z e d is t r ibu t ion which has a mean particle s ize of d i .
The average e f f luent suspended so l id s concentration fo r the settleable s i zes
is given as
where, E, y v# Y are 88 defined previously, and
k = f rac t ion of the pa r t i c l e s in the inf luent s i z e d is t r ibu t ion which are se t t leab le .
I n the previous example, k would equal 0.90 since 90 percent of the inf luent
s i z e d i s t r ibu t ion is se t t leab le .
3.23
Table 3.2. Development of Settleable Solids Size Distribution.
Settleable Solids Influent Size Distribution
Particle Size Distribution column 3 x (100/90) (mm) ( 0 finer) column 2 - 10 ( 0 finer) (1 ) (2) ( 3 ) ( 4 )
0.001
0 0042
0.01
10
16
26
0
6
16
0.0
6.7
17.8
. . 0.04 50 40 44.4
0.10
0.20
0.66
72
90
100
62
80
90
68.9
88.9
100.0
3 - 2 4
PARTICLE SIZE (mm)
FIGURE 3.5 SETTLEABLE SOLIDS SIZE DISTRIBUTION
3.25
The dry bulk dens i ty of the settled solids (W) should be r ep resen ta t ive
of settled silt s ince t h i s is the s i z e range tha t w i l l settle during t h e
Imhoff cone test. A r ep resen ta t ive value of W for settled silt is 7 0
l b / f t 3 or approximately 1120 mg/l.
The f r a c t i o n of the particles in t h e e f f l u e n t size d i s t r i b u t i o n which are
smal le r t h a n a, (0.011 mm) is denoted as X,,. When a particle s i z e t o be
removed i n a pond is equal to or smaller than 0.011 nun, w i l l always be
1.0 and a l l of the particle s i z e s i n t h e e f f l u e n t are equal to or smal le r than
0.Ollmm. A l l of the particle s i z e s which have a diameter of 0.011 x n or
l a r g e r w i l l settle i n an Imhoff cone test.
determines what percent of the particle sizes smaller than 0.011 w i l l settle
during t h e test.
The second term i n Equation 3.5
When a particle s i z e to be removed i n t h e pond is l a r g e r than 0.011 nun,
x, than 0.011 mm. For this condition, the e f f l u e n t w i l l conta in particle sizes
greater than 0.011 mu. A l l particle s i z e s greater than 0.011 rmm w i l l settle
in the Imhoff cone during the test. The first term in Equation 3.5 describes
the percent of the e f f l u e n t s i z e d i s t r i b u t i o n which is l a r g e r than 0.011 rmn
and the re fo re , w i l l sett le during the Imhoff cone test. For t h i s condi t ion ,
is equal t o t h e percent of the e f f l u e n t size d i s t r i b u t i o n which is smaller
2b can be completed as
% of settleable s i z e d i s t r i b u t i o n smal le r than 0.011 nrm x = o % of settleable size d i s t r i b u t i o n smaller than s i z e to
be removed i n sedimentation pond
The design of a sedimentation pond to meet e f f l u e n t l i m i t a t i o n s requires
t h a t a particle s i z e to be removed be se lec ted .
select a particle size of 0.011 mn. This makes x, i n Equation 3.5 equal t o
1.0.
smaller than 0.011 ran. To eva lua te the second term in Equation 3.5, the par-
t ic le sizes smaller than 0.011 w must be r e d i s t r i b u t e d i n t o a s i z e d i s t r i b u -
t i o n i n which the particle s i z e s smaller than 0.011 rn comprise the e n t i r e
s i z e d i s t r i b u t i o n .
3.5, it c a n be seen that 19.5 percent of the settleable size d i s t r i b u t i o n is
smaller t h a n 0.011 nm. This percentage of the settleable s i z e d i s t r i b u t i o n is
t hen r e d i s t r i b u t e d t o be 100 percent.
A good s t a r t i n g p i n t is t o
Therefore, the e f f l u e n t size d i s t r i b u t i o n is made up of particles
Using the settleable size d i s t r i b u t i o n presented i n Figure
_ _ _ . . . - . . . . .
3.26
This procedure starts by breaking up the settleable s i z e d i s t r i b u t i o n
smaller than 0.011 mm i n t o s e v e r a l percentage i n t e r v a l s . This is shown i n
Figure 3.6. The s i z e range f o r each increment is then t abu la t ed and the mean
size ( d i ) is determined. This procedure is shown i n Table 3.3. I n this
example, percentage increments of 0.04 were chosen. There is no set value for
the percent increments.
r e s u l t . The particle s i z e range f o r each increment is then t abu la t ed (column
1, Table 3.3). The particle s i z e ( d i ) i n the middle of each increment is then
t a b u l a t e d in column 2 of Table 3.3 as mean s i z e .
r e d i s t r i b u t e t h e s i z e d i s t r i b u t i o n smaller than 0.011 m. This is
accomplished by div id ing each percent increment (column 3) by the sum of
column 3. For this example, the first four e n t r i e s in column 4 are found by
d iv id ing 0.04 by 0.195. Column 4 is the A X i va lue used in Equation 3.5
corresponding t o the di value (column 2) . Knowing this information, the
'settleable s o l i d s concentration in the e f f l u e n t can be determined from
Equation 3.5.
However, smaller s i z e d increments w i l l y i e l d a better
The f i n a l s t e p is to
I f the settleable s o l i d s e f f l u e n t limitations are not s a t i s f i e d , a par-
t i c le s i z e smaller than 0.011 nm is chosen to be removed. The va lue of &, i n Equation 3.5 w i l l still be equal to 1.0. However, the particle size range
i n column 1, Table 3.3 w i l l change. The particle s i z e range w i l l now have the
upper l i m i t of the selected particle size instead of 0.011 mu. Therefore, the
t r app ing e f f i c i ency , e f f l e u n t concent ra t ion , particle size range, increment
size, and A X i
cen t ra t ion can be computed.
w i l l have new values and the new settleable solids con-
When the computed settleable s o l i d s concentration is less than the
e f f l u e n t l i m i t a t i o n s , larger s i z e d particles w i l l be allowed in the e f f l u e n t .
Therefore, a particle s i z e l a r g e r than 0.011 nun is selected to be removed i n
the pond. I n Equation 3.5, the second term w i l l r e sa in the same as that which
w a s computed f o r a particle s i z e of 0.011 m bu t w i l l be reduced by a factor
of %. poin t . The va lue of w i l l no longer be equal to 1.0. For this condition,
rr, can be computed as defined previously. W i t h the new t rapping e f f i c i ency , e f f l u e n t concentration, and value of G, the settleable solids concent ra t ion
can be computed using Equation 3.5.
i nc rease r ap id ly as the particle s i z e t o be removed i n the pond is increased
This is one of the main reasons f o r s e l e c t i n g 0.011 nun as a s t a r t i n g
The settleable solids concentration w i l l
. ... ~ .,.. .
3.28
Table 3.3. Size Distribution for Particles Smaller than 0.011 nun.
( 1 ) Particle Size Range
( 3 ) Percent in Size
Range of ( 2 ) Settleable Size
Mean Size Distribution
I
0.001 - 0.0023 0.0015
, 0.0023 0.0046 0.0035
0.0046 - 0.0064 0 . 0054
0.0064 - 0.0088 0 0075
0.0088 - 0.011 0.0100
c
0.04
0.04
0.04
0.04
0.035 - 0.195
0.205
0.205
0 205
0 - 205
0.180
1.0
-
3.29
since a l l particles larger than 0.01 1 mn w i l l s e t t l e i n an m o f f cone.
Therefore, when a new pa r t i c l e s i z e is selected, a particle s i z e i n t he range
of 0.015 mm t o 0.02 xmn should be t r i e d so the designer can understand how f a s t
t h e se t t l eab le so l ids concentration increases. . When the designer has calculated the particle s i z e which must be removed
in the sedimentation pond to meet ef f luent Limitations, criteria €or the sedi-
mentation pond design can be determined. The determination of the design par-
t i c l e s i z e to meet e f f luent .limitations may seem confusing. Following through
the example given i n Chapter V I w i l l help the operator understand how t o
design a sedimentation pond to s a t i s f y se t t l eab le so l ids e f f luent lLmitations.
3.6 Storage Volume Requirement
I n the design process, there is an i t e r a t i o n procedure t h a t is required
between t he information presented in Sections 3.6, 3.7 and 3.8. Knowing the
particle s i z e t o be removed (Section 3.5)# a depth is assumed and the
corresponding required detention time is determined (Section 3.8). The
avai lable storage volume for the selected depth is determined (Section 3.7). The required storage volume is then determined (Section 3.6) and compared t o
\ t h e avai lable storage volume. I f the avai lable storage volume is less than :
t he required storage voiume, the depth is Increased and the In te ra t ion is
repeated. When the avai lable storage volume is greater than the required
storage volume, the depth, detention time, storage volume, and outflow rate
are established. The pond surface area, length, and Width are then checked to
ensure t h a t the selected particle size is s e t t l e d in the pond.
Flow routing through a sedimentation pond is determined by the rate of
inflow, storage capacity of the pond, and outflow capacity €or given reservoi r
levels . Numerous methods of reservoir routing have been developed which
include the b d l f i e d Puls Method, Rippl Mass Curve, and several others.
Descriptions of these methods can be found in hydrology texts and manuals.
A simplified method is used in this manual. The simplified rout ing
method is used to determine the required storage volume and s i z e the p r inc ipa l
spillway t o produce the required detention time so t h a t e f f luent requirements
are m e t . The simplified routing procedure requires that the peak inflow rate
and runoff volume are known.
determined from the inflow hydrograph. This method implies two assumptions,
The peak Inflow r a t e and runoff volume can be
\
3.30
t he shape of the inflow and outflow hydrographs are t r iangular and the i n i t i a l
water surface elevation is a t the elevation of the pr incipal spillway c re s t .
Therefore, the areas under the inflow and outflow hydrographs are equal.
Water routing through sedimentation ponds can be solved using Figures 3.7
and 3.8. Figure 3.7 is a graph showing the relat ionship between the time base
of the inflow hydrograph (a) and the r a t i o of the required storage volume (S)
t o the runoff volume (V) f o r a range of detention t i m e s . Figure 3.8 presents
t h e
the
the
relat ionship between Tb and the r a t i o of t he peak outflow rate (Qo) t o
peak inflow rate (QI)
inflow hydrograph is determined as:
f o r a range of detention times. The time base of
V 1800 Qx Tb =. (3.7)
where, = time base of. inflow hydrograph (hours),
V
Q~ = peak inflow rate ( c f s ) .
= water runoff volume ( f t 3 ),
I
The time base can be
t hydrograph. Knowing
detention time f o r a
t he required storage
Figures 3.7 and 3.8.
b
computed based on the information from the inflow
the t i m e base of the inflow hydrograph and the required
selected particle size to be s e t t l e d (Section 3.8.11,
volume and peak outflow rate can be determined using
3.7 Available Storage Volume
The sedimentation pond storage volume should provide an adequate sediment
s torage volume and an adequate detention storage volume so ef f luent limita-
t i o n s a re sa t i s f i ed . A t each sedimentation pond site, a relat ionship between
the depth and the storage volume is required s ince the trapping eff ic iency
depends on depth and storage volume of the pond.
The method u t i l i z e d to develop the depth and storage volume relat ionship
requires a topographic map of the location of the pool area and embankment of
t he sedimentation pond.
pool elevations can be determined using a planimeter and the scaled topography
map. For example, i n Figure 3.9 the incremental storage volume between a pool
a t elevation E2 and a pool a t elevation E3 is d e t e d n e d by measuring (with a
An incremental value of storage volume between two
3 . 3 3
FIGURE 3.9 DEFINITION SKETCH TO DETERMINE INCREMENTAL STORAGE VOLUME
3.34
planimeter) the pool surface area a t elevations E2 and E3-
storage volume is then calculated as the increase i n elevation (E3 - E2) t i m e s
t he average surface area of the pool [(AE Thus, a t ab le r e l a t ing
storage t o stage can be developed (see Table 3.4). A graph of t he s tage ver-
The incremental
+ AE~)/~]. 2
sus storage volume is then plot ted.
It is l e f t ' to the designer to decide how often sediment w i l l be removed
from the pond.
removal can be computed using procedures described in Section 3.2.2.2.
sediment y i e ld is converted to a storage volume by dividing the y ie ld by the
The sediment y ie ld during the time period between sediment
The
u n i t weight of the deposited sediment. Lara and Pemberton (1963) developed an
equation to ca lcu la te the unl t weight of the settled sediment based upon sedi-
ment s i z e d is t r ibu t ion and type of reservoir operation. This equation appears
i n several references (Barfield, 1981; Bureau of Reclamation, 1977) and the
designer may consult these to compute the u n i t weight of the settled sediment.
A un i t weight of 70 lbs/ft3 is suggested t o simplify the design.
value of u n i t weight, the required sediment storage volume can be computed.
Using this
The corresponding depth of the sediment i n the pond can be found from the
stage-storage curve.
: The characteristics of sediment deposition are such that the large sized \
particles wil l settle near the i n l e t of the pond resu l t ing i n the formation of
a del ta . Delta formation is described in Section 3.8.2.2. Because the la rger
s ized particles settle near the i n l e t , the sediment storage volume should be
provided near t he i n l e t of the pond.
provided a t this location, accumulated sediment a t the inlet w i l l require f re-
quent remova 1 e
If t h e sediment storage volume is not
The detention storage volume is the storage volume required to produce
the required detention t i m e . This is the volume that is used in the w a t e r
routing procedure presented i n Section 3.6.
determined from the stage-storage curve and is measured as the avai lable
s torage volume above the elevation of the pr inc ipa l spillway crest. The ele-
vation of the pr inc ipa l spillway crest is usually chosen as the maximum depth
of the sediment storage volume unless a permanent pool is provided.
permanent pool is provided, the permanent pool elevation w i l l be a t the eleva-
t i o n of the pr inc ipa l spillway crest.
The detention storage volume is
When a
. ..
3.35
Table 3.4. Stage-Storage Relatlonshlp Developent.
Stage (feet)
Storage (feet31
0
E2
E3
E4
3.36
3.8 Sedimentation Pond Configuration
The design of the sedimentation pond conf igura t ion is based upon ideal
s e t t l i n g condi t ions . I n actual f i e l d s i t u a t i o n s , ideal s e t t l i n g conditions
are o f t e n d i f f i c u l t to reproduce.
f a c t o r s h t o the design which account f o r nonideal s e t t l i n g conditions.
following s e c t i o n s d iscuss the design of the pond conf igura t ion based upon
ideal s e t t l i n g , f a c t o r s which produce nonideal s e t t l i n g , and what f a c t o r s are
used to compensate for nonideal s e t t l i n g conditions.
This n e c e s s i t a t e s the need to incorpora te
The
3.8.1 Ideal S e t t l i n g
Based upon ideal s e t t l i n g conditions, t h e r e is a direct r e l a t i o n s h i p
between t h e de ten t ion s to rage depth of the pond and t h e detention t i m e . This r e l a t i o n s h i p
I where, Vs =
D =
TD =
can be expressed as
D 3600 TD
=
particle s e t t l i n g ve loc i ty ( f p s ) , deten t ion storage depth ( f t ) , and
de ten t ion time (hours).
The particle s e t t l i n g ve loc i ty is defined by Stoke's L a w and is dependent
upon temperature of the water, particle size, and specific g rav i ty of t h e par-
ticle. To determine t h e design particle size as presented in Section 3-58 the
temperature of the water was assumed 7 7 0 F, s i n c e this is part of the Imhoff
cone test and sets t h e criteria which must be s a t i s f i e d . I n t h e f ield, t h e
temperature of the water runoff w i l l be closer t o 500 F. For the same par-
t i c l e size, s e t t l i n g w i l l t ake longer in the water which is 50° F t h a n i n t h e
water which is 77O F.
upon the water being 50. F. A s s d n g a water temperature of SO0 F and the
s p e c i f i c g r a v i t y of t h e particle to be 2.65, Stoke's Law may be wri t t en as
'
Therefore, design of the sedimentation pond is based
Vs = 2.254 d2 (3.9)
w h e r e , Vs 5 particle s e t t l i n g ve loc i ty (fps) and
d = particle diameter (m).
3.37
The r e s u l t of combining Equations 3.0 and 3.9 is
D 3600 TD 2.254 d2 = (3.10)
Figure 3.10 presents the relat ionship between the particle diameter and deten-
t i o n time f o r various depths using Equation 3.10. To sett le any size par-
t icle, the required detention time f o r various depths can be found from Figure
3.10 or computed by Equation 3.10.
There is a l so a direct re la t ionship between the flow length of the pond
and the detention time. This re la t ionship is represented as
L u '€I 3600 TD
where, V, = horizontal flow veloci ty through the pond ( fps ) ,
L = f l o w length of the pond (f t) , and
T~ = detention time (hours).
The horizontal flow veloci ty through the pond can be computed as
QO VH = - wD1
where, Qo = peak outflow rate ( c f s ) ,
W = average width of t he pond (f t) , and
D1 = total depth of the pond (ft).
Combining Equations 3.11 and .3.12 results i n
3600 T Q Dl O L =
wDl
(3.11)
(3.12) ,
(3.13)
where, TI), G8 W are as defined i n Equation 3.11 and
D1 = sediment storage depth plus detention storage depth, and
TD, = detention time f o r depth D1 from Equation 3.10.
Equation 3.13 gives the required flow length of the pond t o settle the design
p a r t i c l e s ize . This equation is used as a check a f t e r the pond storage volume
3.39
and outflow rate have been established. The total depth is used i n Equation
3.13 since the p a r t i c l e w i l l be required to sett le this depth j u s t after the
pond construction is completed.
achieved, .measures described in Section 4.2 can be taken to increase the flow
If the required flow length cannot be
length of the pond.
3.8.2 Nonideal Se t t l i ng
As presented so f a r , t he design of sedimentation ponds have been based on
ideal s e t t l i n g conditions.
Impossible t o provide i d e a l s e t t l i n g conditions.
s e t t l i n g are caused by severa l factors .
conditions; they are a l l i n t e r r e l a t ed and cause deviations from ideal s e t t l i n g
reducing the eff ic iency of the sedimentation pond.
va r i a t ion form ideal s e t t l i n g are:
However, i n the f i e l d it is d i f f i c u l t and of ten
Variations from ideal
These factors are not independent
The conditions causing
- - Reservoir deposition
Flow c u r r e n t s within the pond
- Short c i r cu i t i ng and turbulence
- Scour and resuspension
3.8.2.1 Flow Currents
V a r i o u s types of flow currents can exist within sedimentation ponds. The
most common being those caused by wind blowing Over the surface of the pond.
Convection currents can a l s o exist due t o s ign i f icant differences of
temperature within the pond.
Often during storm runoff events, the inflow to the pond is t yp ica l ly
more dense due t o high suspended so l id s concentrations. This r e s u l t s i n a
densi ty current t h a t flows along the bottom of the pond. This local ized
increase i n flow can cause scour and resuspension of settled solids and signi-
f i c a n t l y reduce the t r a p eff ic iency i f the o u t l e t t o the pond is located near
the bottom.
A l l types of currents t ransport suspended material throughout the pond
both v e r t i c a l l y and horizontal ly and d i s t o r t the flow pa t te rn from that
assumed under ideal se t t l i ng .
the pond.
The r e s u l t is a reduction in the performance of
3.40
3.8.2.2 Delta Formation
As the sediment-laden inflow passes from the i n l e t channel t o the sedi-
mentation pond, the forward velcotiy of the flow is reduced due t o increase i n
f l o w width and flow depth. This re su l t s in the larger sized particles being
deposited almost immediately as flow enters the sedimentation pond.
Deposition of the larger sized particles near the inlet of the pond w i l l
r e s u l t i n t he formation of a delta. Figure 3.11 shows a del ta formation near
t h e i n l e t of the pond. The <a will continue t o grow larger and w i l l gra-
dually migrate downstream within the pond.
mation is reduced detention time.
given enough t h e t o settle to the bottom of the pond.
The consequence of a delta for-
Therefore, small p a r t i c l e s i zes are not
3.8.2.3 Short Circuit ing and Turbulence
Short c i r cu i t i ng is the flow of water through a Sedimentation pond
d i r e c t l y from the i n l e t to the o u t l e t resu l t ing in dead storage areas and
reduced detention t i m e s .
shapes which have shor t c i rcui t ing.
caused by flow currents (as previously (Uscussed), high i n l e t w l O C i t i e 6 , high
o u t l e t flow rates, sedimentation pond geometry, and improper location of
i n l e t s and outlets. When short c i r cu i t i ng 0ccum8 the ef fec t ive width of the
f l o w area through the pond is reduced and t h e l l o w veloci ty through the pond
is greater. This e f f e c t reduces the s e t t l i n g charac te r i s t ics and increases
po ten t i a l f o r scour and resuspension of s e t t l e d sediments.
Figure 3.12 presents some typical sedimentation pond
Short c i r cu i t i ng and turbulence are !
3.8.2.4 Scour and Resuspension
Scour and resuspension is caused.by density c u r r e n t s and high flow rates
through the sedimentation pond. The scour veloci ty is defined as t h a t velo-
c i t y of flow required t o i n i t i a t e motion of a discrete par t ic le .
of the design p a r t i c l e s i z e u i l l r e s u l t i n e f f luent l imitat ions not being
s a t i s f i e d .
Resuspension
r
3.8.3 Control of Nonideal Se t t l ing
The s ign i f i can t fac tors which a f f e c t ideal s e t t l i n g are shor t c i r cu i t i ng ,
turbulence, and scouring of the s e t t l i n g sediment.
c r i t e r i a areset t o minimize these e f fec ts . These criteria are length-to-width
In the following sect ions
3.4:.
POUD .
FIGURE 3. . I 1 SCHEMATIC OF DELTA FORMATION (AFTER GRAF, 1974)
RISER (OUTLET) Dt FLOW PATHS
NORMAL POOL
. DEAD STORAGE
. .
;
. DEAD STORAG
FLOW PATHS . .
ISER (OUTLET) DEAD STORAGE
DEAD STORAGE
NORMAL POOL FLOW PATHS
INFLOW
FIGURE 3.12 SEDIMENTATION PONDS WITH DEAD STORAGE SPACES (AFTER HAAN & BARFIELD, 1978 )
3.43
r a t i o , sho r t - c i r cu i t i ng f a c t o r , permissible inlet ve loc i ty , and permissible
flow-through ve loc i ty . The sedimentation pond must meet these criteria t o
ensure that the desired sediment removal is a t t a ined .
3.8.3.1 Short-Circuit ing Fac tor
Research by Camp (1946) on various types of s e t t l i n g basins has r e s u l t e d
in the development of a shor t - c i r cu i t i ng compensation f a c t o r based on the
shape of the bas in geometry. It has been recommended tha t the su r face area of
a s e t t l i n g bas in be increased t o account for nonideal s e t t l i n g condi t ions
according t o
(3.14)
where, A = su r face area of the pond a t the e l eva t ion of t h e p r i n c i p a l sp i l lway crest ( f t 2 ) ,
FSC = shor t - c i r cu i t i ng f a c t o r ,
QO = outflow rate ' ( c f s ) , and
vs = s e t t l i n g ve loc i ty of the design particle size (fps). : \
The value of FSC is genera l ly 1.2. Equation 3.14 w i l l y i e l d the required
surface area that is needed in the pond a t the e l eva t ion of the p r i n c i p a l
sp i l lway crest.
face area does no t meet the requirement of Equation 3.14.
There are three measures that can be taken if the pond sur-
The pond side
slopes can he excavated, the e l eva t ion of the p r i n c i p a l sp i l lway crest can be
raised, or app l i ca t ion of multiple ponds.
3.8.3.2 Lenqth-to-Width Ratio
The ratio between the f l o w length and the e f f e c t i v e width of the sedimen-
t a t i o n pond is used as a design a i d to minimize short c i r c u i t i n g .
a length-to-width ratio allows f o r u t i l i z a t i o n of the f u l l su r f ace area of the
sedimentation pond and helps maintain a cons tan t ho r i zon ta l v e l o c i t y through
the sedimentation pond.
t h e water m u s t flow from the inlet to the o u t l e t of the pond. The width used
i n the computation is the e f f e c t i v e width of the sedimentation pond. This is
determined by div id ing the sur face area by the length from the i n l e t t o the
Specifying
The length that is used is the shortest d i s t ance that
i
3.44
ou t l e t . The surface area of the pond is measured a t the elevation of the
pr inc ipa l spillway crest.
It is general practice to specify a minimum length-to-width r a t i o of
2: l . Larger length-to-width r a t io s w i l l promote improved performance and
values of up t o 5: l have been recommended. The length-to-width r a t i o is
determined after the pond storage volume and outflow rate have been
established. I f t h i s r a t i o cannot be sa t i s f i ed , the flow length can be
increased. Section 4.2 describes the measures that can be taken to increae
t h e flow length.
Both the short-circuit ing factor and the length-to-width r a t i o compensate
f o r nonideal s e t t l i n g conditions. The short-circui t ing factor determines only
t h e required surface area, whereas the l e n g t h - W i d t h r a t i o defines the shape
of the surface area.
3.8.3.3 Permissible I n l e t Velocity
For a sedimentation pond to be ef fec t ive in sediment removal, the velo- . -- I \ c i t y of the flow i n t o the pond mus t be small enough t o prevent short cir-
cuiting. The criteria is used t o l i m i t the Roude number in the i n l e t channel
t t o 1.0. The Froude number is defined as
V
\
(3.15) Fr = - 4z
where, Fr = Froude number,
v P veloci ty in the inlet channel (fps) 8
g = gravi ta t iona l acceleration (32.2 ft/sec2) 0 and
D = depth of flow in the inlet channel ( f t ) . I f the Froude number in the i n l e t channel is greater than 1.0, inlet cont ro l
measures w i l l be required. These measures are discussed in Section 4.2.
3.8- 3.4 Permissible Plow-Through Velocity
The horizontal flow velocity through the pond must be less than the scour
veloci ty of the design particle s i ze to avoid resuspension of the s e t t l e d
sediment. The scour velocity fo r a specific particle s ize is determined by
3.45
(3.16)
where, Vsc - scour velocity,
B = Shields' crt icial shear stress parametr (0.047 for uniform sand),
g = gravitational acceleration ( 32.2 f t/sec 2) ,
6, = specific gravity of particle (usually 2.6 t o 2-81,
d = diameter of spherical particle ( f t ) 8
F = Darcy-Weisbach friction factor (usually 0.02 t o 0.03).
Assuming that Shields' cri t ical shear stress parameter is equal t o 0.05, the
. specific gravity of the particle is 2.65, and the Darcy-Weisbach f r i c t i o n fac-
tor is 0.025, Equation 3.16 can be reduced to
1/2 V '= 1.67 d SC
I where, Vsc - scour velocity (fps) and
d = particle diameter (mm). t \ The horizontal velocity through the pond is
QO va = - WD
where, vx = horizontal flow velocity (fps),
QO = outflow rate (cfs),
w - average width of sediment pond ( f t ) , and
D = detention storage depth ( f t ) .
(3.17)
(3.18)
I f the horizontal velocity through the pond is greater than the scour velocity
for the particle that must be settled, the depth can be increased to reduce
the horizontal velocity which will also increase the width and decrease the
outflow rate.
3.46
3.9 Sedimentation Pond Outlet Control Measures
Sedimentation ponds must provide a pr inc ipa l spillway and an emergency
spillway. Pr incipal spillways are designed to provide su f f i c i en t detention
time during the design prec ip i ta t ion event t o meet the ef f luent l imi ta t ions
and dewater the pond. Rnergency spillways are designed to work in conjunction
with the pr inc ipa l spillway and pond storage to safe ly discharge the peak
runoff r e su l t i ng from the design s t o m . The design procedure presented i n
Chapter V I develops the design discharge f o r s iz ing the pr inc ipa l spillway and
the elevat ion above the b o t t o m of the pond.
no t covered in this section. Several references provide design procedures f o r
s i z i n g standpipe and culvert-type spillways (U.S. Department of
Transportation, 1965; Barf ie ld 19811 Bureau of Reclamation, 1974; Soi l
Conservation Semice, 1969). The following discussion presents important
design considerations f o r pr inc ipa l and emergency spillways, various types and
Actual design of the o u t l e t is
configurations, and effectiveness.
\
! 3.9.1 Pr inc ipa l Spillways
The pr inc ipa l spillway i s sized to provide a discharge rate as determined
: through design of t h e sedimentation pond.
uat ion of local drainage .conBitions, water r igh ts , economics, land-use
cons t ra in ts , and requirements of local, state and federa l regulations. Design
of the p r inc ipa l spillway should not be independent of the design of the ear th
embankment and emergency spillway.
A c t u a l design should include eval- \
The types of pr inc ipa l spillways commonly used can be c l a s s i f i e d into
three categories: open channel, drop inlet and pipe culverts. The type of
spillway used is based on local site-specific conditions.
3.9.1.1 Op en Channel Spillways
open channel spillways should only be used when a l l other a l t e rna t ives
have been shown to be infeasible .
dewatering the pond.
drainage basins. Design of open channel spillways to meet e f f luent standards
during base f l o w and design storm conditions is very d i f f i c u l t . When an open
channel is used €or the pr inc ipa l spillway it often is designed f o r the
emergency capacity, or the emergency spillway is a l so an open channel.
This type of spillway provides no means of
Typically open channel spillways are located on small
Open
3.47
channel spillways do not provide as much control of discharge or f l e x i b i l i t y
t o modification as drop i n l e t s or pipe culverts. Further discussion of open
channel spillways is covered in Section 3.9.4 on emergency spillways.
3.9.1.2 Drop I n l e t Spillways
Drop i n l e t spillways are one of the most common types of pr inc ipa l
spillways used for sedimentation ponds. A drop inlet spillway is quite
f l ex ib l e i n design, offers good control of dbcharge, and is w e l l adapted t o
sedimentation ponds.
i n diameter.
cleaning. When the design discharge f o r meeting eff luent requirements r e su l t s
I n a spillway s i z e smaller than 12 inches in diameter, a 12-inch pipe is used
with an o r i f i c e of the required s i z e opening affixed to the inflow end of the
drop inlet.
A recommended minimum s i z e fo r drop inlets is 12 inches
This m i m i m u size provides access ib i l i ty for maintenance and
Configuration of a typical drop inlet is shown In Figure 3.13. A drop
i n l e t has two main features , the barrel and riser. The riser and barrel can
be of concrete, reinforced concrete polyvinyl chloride (PVC) 8 corrugated or smooth m e t a l pipe.
\ The select ion of the type of material used should consiUer
! s i te conditions and economics. \
I n designing drop inlets an impor tan t consideration is anchoring of the
riser and barrel on the bottom of the pond, and seepage along the barrel.
Fai lure of the riser to s t ay anchored is a k o n problem.
rit3erS should consider the s i z e of the riser, local soil type, type of pond,
and weather condltions. I f the pond is a permanent pool it is susceptible to
freezing, and the forces created by the forming Ice should be considered.
Anchoring of
Seepage along the barrel is o f t e n the cause of dam embankment failure.
The problem generally occurs due t o the lack of compaction around the barrel . during construction of the embankment.
3.9.1.3 Pipe Culvert Spillways
Another type of pr inc ipa l spillway commonly used is the pipe cu lver t ,
also re fer red t o as a “ t r i c k l e tube.” It’ consis ts of a pipe l a i d in the earth
i n such a manner that the entrance elevation of the pipe ( a t the upstream end)
establishes the normal pool elevation in the pond. Figure 3.14 shows a typ-
i c a l pipe culver t arrangement. Pipe cu lver t spillways require the same con-
3.48
/
FOQT IN G AHTI -SEEP 2d
COLLARS RIP RAP PROTECT8QN
FIGURE 3.13 TYPICAL DROP INLET SPILLWAY
3.49.
PROTECTION COLLARS
FIGURE 3.14 TYPICAL PIPE CULVERT SPILLWAY
3.50
s i d e r a t i o n as drop i n l e t spil lways do for seepage, minimum s i z e and type of
material.
3.9.1.4 Eff ic iency of Pr inc ipa l Spillways
I n t h i s s e c t i o n the e f f i c i ency of p r inc ipa l spil lways 3s q s c u s s e d . l n
general . I n Section 3.9.3 various modifications to p r i n c i p a l spil lways and
t h e i r e f f e c t on discharge q u a l i t y are presented. The s ize of the p r i n c i p a l
spil lway is designed to convey the discharge required to achieve removal of
sediment. The design discharge is determined during design of the sediment
pond.
c i p a l spil lways on discharge q u a l i t y is based on loca t ion , i n r e l a t i o n t o the
geometry of t h e pond, and flow c h a r a c t e r i s t i c s a t the i n l e t end of t h e
Once t h e design discharge is properly determined, the e f f e c t of prin-
Spi 11-y
The primary concerns i n loca t ion of t h e p r i n c i p a l spil lway are an ef fec-
?ive sur face area and s h o r t c i r c u i t i n g . As discussed in Sections 3.8.2 and
!S. 3, t h e length-to-width ra t io should be a minimum of 2: I , and a ratio of
5: l is recommended. As the d i s t ance between t h e spil lway and t h e pond i n l e t ! \decreases, t h e e f f e c t i v e sur face area decreases. As the e f f e c t i v e sur face
area decreases, the occurrence of short c i r c u i t i n g and turbulence is more
l i k e l y , and the overall e f f i c i ency of t h e pond is reduced.
pond e f f i c i e n c y is variable and based on site-specific conditions.
a n es t imate of the reduced e f f i c i ency can be based on the reduction i n effec-
t i ve -face area of t h e pond.
The level a t which the inlet of t+e p r i n c i p a l spil lway e.xists wi th in the
Because the sediment settles to the
The reduction i n
Bowever,
pond a f f e c t s the e f f i c i ency of the pond.
bottom of the pond, it is clear t h a t there w i l l be less sediment a t the sur- face than near t h e bottom of the pond. Thus discharging from near t h e sur-
face of the pond can improve t h e e f f ic iency . This characteristic has been
shown through use of f l o a t i n g weir devices and is fXschssed f u r t h e r in Section
3.9.3.2.
Due to the tu rbu len t nature a t the p r i n c i p a l spil lway inlet, scour and ' resuspension of s e t t l e d sediment is l ike ly .
s i o n around a spil lway is related tn the elevation of the s e t t l e d sediment.
As the l e v e l of settled sediment approaches the e leva t ion of the i n l e t of the
sp i l lway, scour and resuspension increase. Scour and resuspension are o f t en
The amount of scour and resuspen-
3.51
associated w i t h dewatering.
avoid some resuspension and scour a t a l l t i m e s .
t e r i n g methods is presented in the following section.
When dewatering is required it is impossible to
Comparison of various dewa-
3.9.2 Dewatering Devices
Dewatering is usually required to drain the sediment pond between runoff
storms so adequate storage volrnae within the pond is maintained.
devices a re not necessary when draining below the pr incipal spillway is not
required. Several methods of dewatering are used, including perforated
risers, subsurface drainage, a s ingle perforation with associated use of a
skimmer baf f le or a type of gate valve, siphon arrangement attached t o the
riser and/or pumping.
Dewatering
The use of perforated s tandpipe or riser for dewatering i s required by
some states. However, sediment is carried out of the pond through the per- dorat ions because of resuspension of settled so l id s due to turbulence near the
perforat ions or because sediment is allowed to accumulate too high along the
riser barrel. \ U s e of a perforated riser is not recommended.
In the subsurface drain arrangement, a (four-inch) perforated plastic
t pipe network is l a i d in a trench in the bottom of the pond and covered with a
f a b r i c f i l t e r and sand as shown in Figure 3.15. The pipe is connected to the
riser and the pond is dewatered through the sand f i l t e r /per fora ted pipe
arrangement by gravity.
\
There are two advantages of a subsurface drain arrangement: ( 1 ) complete
dewatering of the s e t t l e d sediment I s possible to a id in removal and disposal,
and (2 ) no turbulence or resuspension of settled sediment is associated w i t h
t h i s method.
f i l t e r fabric due to the nature of the settled sediment, the permeability of
t h e settled sediment could r e su l t in exceedingly long dewatering t i m e , and the
added expense of i n s t a l l i ng t h i s type of pipe arrangement.
However, major disadvantages are clogging of the sand f i l t e r and
A s ingle perforation at the sediment cleanout leve l w i t h a skimmer-baffle
is shown in Figure 3.16.
and is capable of completely draining the pool to the sediment clean-out
level . With a skimmer, the perforation is non-clogging, f a i r l y easy t o
construct , and an e f f i c i e n t skimmer of surface debris. S o m e type of valve can
a l s o be used to gate the perforation which allows control over the desired
The s ingle perforation method is easy to construct
3.52
PERFORATED PIPE IN TRENCH EDGE OF POOL
A L
NT
P L A N NOTE : S = 15' TO 25' -
* EMBANKMENT
1 I ,
1
PERFORATED PIPE IN TRENCH
0.5% MINIMUM GRADE J-
S E C T I O N A - A
FIGURE 3.15 SUBSURFACE DRAIN (EPA, 1977)
3.53
L
i RISER
4" MAX. DIA. HOLE I, v
MAXIMUM SEDIMENT .
1 STORAGE 1"'" .L v LL
I ' I I SEDIMENT CLEANOUT LEVEL
(60% OF MAXIMUM SEDIMENT I , STORAGE LEVEL)
SINGLE PERFORATION CROSS SECTION
MAXIMUM SEDIMENT STORAGE LEVEL
CUT IN HALF LENGTHWISE
S I N G L E PERFORATION WITH SKIMMER E L EVATI 0 N
FIGURE 3.16 SINGLE PERFORATION OF RISER BARREL ( E P A , 1977)
3.54
detention t i m e before dewatering the pond. With the perforation, gated
dewatering can be done a f t e r the runoff event is Over and the required removal
of sediment has Occurred, thus reducing the amount of sediment discharged
during dewatering.
With the siphon methods of dewatering, a ( f o u r h c h ) pipe siphon can be
subs t i tu ted fo r the s ing le perforation as described previously (Figure 3.17).
The length of siphon depends on the dewatering t i m e desired. In each case,
t h e i n l e t t o the siphon is placed a t the elevation of the sediment clean-out
l e v e l t o f a c i l i t a t e drainage without removing sediment. The siphon is a l s o an
e f f i c i e n t skimmer of surface debris, w i l l always drain the pond to the sedi-
ment clean-out level , and has a higher discharge capacity than the s ing le per-
fo ra t ion method with the same s i z e of opening.
For excavated ponds without a permanent pool, risers may not be prac-
tical.
pond.
Therefore, a self-priming or portable pump can be used to dewater t h e
The e f f e c t of dewatering devices on the discharge qua l i ty depends grea t ly
on the leve l of sediment in the pond. When sediment is allowed to accumulate
up t o the dewatering o u t l e t , the amount of scour and resuspension of settled
! sediment increases, decreasing the discharge quali ty. Therefore proper main- \
tenance and s e d h e n t removal can decrease the e f f ec t s of dewatering. It is
recommended t h a t the sediment be cleaned out when it reaches 60 percent of t he
design sediment storage.
devices, the a b i l i t y t o maintain the discharge qua l i ty can be r e l a t ed to the
level of cont ro l a t the dewatering device.
forations provide less control than a s ingle perforation with a b a f f l e skimmer or a siphon type arrangement. A gate on the dewatering opening provides t h e
most cont ro l by enabling the operator to vary the detention t i m e and physi-
c a l l y ver i fy t h a t the sediment has settled before dewatering the pond.
For properly designed and constructed dewatering
Perforated risers and s ing le per-
3.9.3 Principal Spillway Modifications
The purpose of modifications to atop i n l e t or pipe culver t spil lways is
t o reduce shor t c i rcu i t ing , eliminate turbulence, and thus increase t rapping
e f f i c i ency of the pond. After proper s iz ing, the effect iveness of a p r inc ipa l
spil lway is related to location within the pond, discharge po in t from within
t h e pond, and turbulent flow conditions a t the out le t . As discussed i n
- . . . . . . . . . . . . - - . - . . . . . .
I/i1 HOLE AT SEDIMENT _ _ CLEANOUT LEVEL AND I " FROM 'END OF PIPE
3.55
- - . . .
RISER
FLOW 4
4" PIPE
SEDIMENT CLEANOUT LEVEL
!/: HOLE .AT SEDIMENT CLEANOUT LEVEL AND I" FROM END OF PIPE
I- I 2"
I I
. . A. SHORT SIPHON CROSS - SECTION
RISER -
FLOW .+--
u 41' PIPE
B. LONG SIPHON CROSS-SECTION
FIGURE 3.17 SIPHON DEWATERING METHODS (€PA, 19801 '
. . . . . . . . . .. . . . . .. . - . __ . . .
3.56
Section 3.8.2.3, shor t c i r cu i t i ng and turbulence can reduce the trapping
ef f ic iency of the sedimentation pond. Studies of ex is t ing ponds have shown
poor pond performance as a r e su l t of short c i r cu i t i ng and turbulence a t the
o u t l e t of the pond (reference EPA, 1980; EPA, 1979; EPA, 1976). Several stu-
dies have made recommendations as t o what spillway modifications can help
improve the sediment removal eff ic iency of a par t icu lar pond.
discussion presents some of the more commonly used modifications and t h e i r
effects on discharge q u a l i t y .
The following
3.9.3.1 Weir Troughs
Drop inlets and cu lver t spillways are s ingle point ou t l e t s t h a t usually
c r e a t e shor t c i r cu i t i ng and resuspension.
t h e point discharge is a w e i r trough connected t o the out le t .
One modification t h a t elfminates
A w e i r trough
.discharges along the length of the w e i r and creates less turbulence than a
s ing le discharge point.
flow-through area and ef fec t ive surface area are a l so increased. Thus, by
By discharging from more than a s ing le point t he
I reducing turbulence and increasing the e f f ec t ive surface area, a weir trough
o u t l e t can provide improved discharge qua l i ty over that of a s ingle poin t
: discharge ou t l e t . Figure 3.18 shows a typical w e i r trough arrangement. I n . \
appl icat ion of a weir trough, s t r u c t u r a l i n t eg r i ty and maintenance are
required f o r e f f ec t ive operation and performance. Weir troughs are suscep-
t ib le to the same s t ruc tu ra l problems as baffles (see Section 4.2.2.2).
3.9.3.2 Float ing Discharge
Typical pr inc ipa l spillways are generally fixed and discharge from the
same point elevation. As the water surface elevation in the pond rises above
t h e spillway elevation, the concentration of sediment increases due t o the
s e t t l i n g of particles from the surface down. om this it is easy t o see that
the ndnFmum concentration within the pond w i l l be near or a t the water surface
elevation. Therefore, the discharge qua l i ty can be improved by discharging
from the surface of the pond.
f i e l d tested by OSM.
The r e s u l t s of the tes t showed settleable so l id s concentration in the
discharge to be consis tent ly less f o r the p ivota l e l b o w type o u t l e t as com-
pared to a typical perforated riser ou t l e t .
A variable elevation discharge o r i f i c e has been
Three tests were conducted using d i f fe ren t o u t l e t s izes .
This device w a s adapted from
I
3 .57
P L A N
rTROUGH NORM
EMERGENCY SPILLWAY
PROTECTION RAP
E L E V A T I O N
FIGURE 3.18 WEIR TROUGH
3.58
f loa t ing w e i r spillways or ig ina l ly suggested fo r management of f i s h ponds.
Figure 3.19 shows the device tes ted by OSM. The f loa t ing weir (or incl ined
am) allows removal of surface water regardless of surface elevation. The
f loa t ing w e i r consis ts of a riser pipe connected to the d ra in via a p ivota l
90° e l b o w designed and constructed to enable quarter rotat ion about the axis
of the drain.
weights and f lo t a t ion jugs t o maintain the o r i f i c e t o two to four inches below
t h e water surface (OSM, dra f t report) . Discharge is controlled by varying the
o r i f i c e size.
dewatering of the pond.
Buoyancy and submerged depth of the o r i f i c e are adjusted by
Another advantage t o this device i s t h a t it can a l so provide
3.9.3.3 F i l t e r i n g
As i n municipal water treatment, f i l t e r i n g of the discharge can grea t ly
improve the qua l i ty by removing f ine r sediments t h a t do not sett le out in the
pond.
riiser pipe f i l t e r s include cloth or f iberg lass wraps and gravel cones placed
: around the f i l t e r s . The f i l t e r wraps have been found to be f a i r l y e f f ec t ive
Riser pipe f i l t e r s have been used t o improve trapping efficiency.
i n trapping f i n e particles. €lowever, the f i l t e r wraps become clogged very
! rapidly (Oscaryan, 1975). This clogging may cause the water leve l in the pond \
to rise above the riser crest, thus negating the f i l t e r e f fec t .
Another inexpensive f i l t e r i n g mechanism is the use of s t r a w bales around
an out le t . Straw bales,
l i k e f i l t e r wraps, are effect ive in trapping f ine particlest however, they
require frequent maintenance.
This method is nrost applicable to pipe culverts.
3.9.3.4 Gated Spillways
A gated spillway gives the operator complete control of the discharge
from the pond.
p l e t e lys to re the runoff from a r a i n f a l l event.
f o r s e t t l i n g of the sediment, the gate valve is opened and the pond allowed to
dra in .
With the gate closed the pond is allowed to f i l l and com-
After an adequate time period
Gated spillways a re applicable only t o ponds t h a t do not have a constant
base flow o r ponds on ephemeral drainages.
s t o r e the e n t i r e runoff volume from the design ra i .n fa l l event. Often times
ponds with gated spillways are designed to s to re t w i c e the design runoff
The pond should be designed to
,
3.59
I
, . - \
OR1 F ICE
/-
PIVOTAL ELBOW
(FIGURE 3.19 FLOATING WEIR
3.60
volume due t o the nature of one or more r a i n f a l l events to occur within a
shor t t i m e period.
Sl ide gates or bu t t e r f ly valves a r e usually used for control at the
downstream end of the o u t l e t conduit.
end (within the pond) of the spillway, however, access must be provided when
t h e pond is full. Again, proper maintenance is necessary to keep the valves
Gates can also be used a t the upstream
i n good working condition.
Gated spillways have an ind i r ec t e f f e c t on discharge water qual i ty .
s t a t e d previously, a gated spillway enables the operator to increase the
detention time within the pond. Thus, t he longer the operator is able to
store the runoff i n the pond, the amre s e t t l i n g can take place, and thus
improve the discharge vater quality.
As
3.9.3.5 Anti-Vortex Devices
An anti-vortex device is used t o reduce turbulence a t the o u t l e t and to
I reduce the range of headwater depth where slug-flow act ion preva i l s and t o
a l l o w f u l l pipe flow to occur a t a lower headwater depth. Slug-flow ac t ion
f r e s u l t s from the induction of air into the conduit by entrance drawdown and b
vor t i ces immediately upsteam of the i n l e t .
discharge eff ic iency values may be reduced *by up t o 50 percent (scs8 1975).
If no anti-vortex device is used,
Anti-vortex devices include grills, racks, vertical plates, of f ixed
s o l i d hoods placed t o break up the vor t ices or t o prevent their formation
where they could feed air i n t o the conduit (Figure 3.201. In order t o be
ef fec t ive , M e hood or grill must be placed immediately above the entrance and
the area between the ins ide of the anti-vortex device and the outside of the
riser must be equal t o or greater than the mea ins ide the riser.
Another anti-vortex device is a th in , vertical plate normal to the cen-
t e r l i n e of t h e dam and firmly attached to the top of the riser.
p l a t e must equal the diameter of the riser plus 12 inches and height must equal t h e diameter of the barrel.
Length of the
3.9.4 Emergency Spillways
Emergency spillways are used to convey large flood events sa fe ly out of
the pond without overtopping or breaching the dam. For dams less than 20 f e e t
i n height or 20 acre-feet i n ac t ive storage, OSM requirements cal l for design-
3.G!.
! -
! \
L L - W - a a
m . t
z - I K
f, u w
F I I
a I U
2 .o
I- 0 w v)
-
w 0 - > W 0 X w I-
0 > I
z
a
F: a
3.62
ing the combination of the pr incipal and the emergency sp i l lway t o safe ly con-
vey the runoff resu l t ing from the 25-year, 24-hour precipi ta t ion event. For
l a rger dams, the spillways must safely dicharge the runoff resul t ing from the
100-year, 24-hour precipi ta t ion event or a larger event as required by the
regulatory agency.
account the design Uscharge of the pr inc ipa l spillways. In general,
emergency spillways consis t of a crest section, a conveyance section, and a
discharge section. mere are two types of emergency spillways, overflow
spillway and channel spillway.
The design of the emergency spillway should take i n t o
Selection of the type of emergency spillway is dependent on the s o i l s and
Vegetated emergency spillways have higher protection climate of the site.
from damaging erosion than earth spillways.
where a vigorous grass growth can be sustained by normal maintenance without
I r r iga t ion .
They are applicable t o sites
Earth spillways are used in those areas where vegetative growth cannot be
I maintained. They are similar t o vegetated spillways but are designed fo r
later permissible ve loc i t ies and less frequent use. Normally, they w i l l
require more maintenance a f t e r a flow event. ! h Rock emergency spillways are applicable on undisturbed land where parent
bedrock material is present.
locities must be ascertained for the spec i f ic site based on a knowledge of
hardness, condition, durabi l i ty , weathering charac te r i s t ics , and s t ruc ture of
t h e rock formation.
Allowable frequency of use and permissible ve-
Excavated open channel spillways are t o have cut-and-fill slopes in earth
and rock which are stable against s l iding. If the dam is to be permanent, cu t
slope s t a b i l i t y is t o be evaluated for the long-term natural moisture con-
dit ions. Side slopes s h a l l be s t ab le f o r the material i n which the spillway
is constructed and s h a l l not be steeper than 3 horizontal t o 1 v e r t i c a l i n
ea r th and 1 horizontal t o 1 vertical in rock.
The exit channel should be s t r a igh t whenever possible. Slope of the
constructed exit channel should f a l l within the range established by discharge
requirements and permissible ve loc i t ies based on spillway material (Tables
3.5 and 3.6).
veloc i t ies .
Riprap may be used t o stabilize the spillway f o r higher design
Spillway discharge should be a t a point hwnstream from any part
_..... .. ... .
3.63
Table 3.5. Permissilbe Velocities for Vegetated Spillways (So i l Conservation Service, 1981).
Permissible Velocity in fps
Erosion Ftesistant Soils2 Easily Erodible Soils2
Slope of Exit Channel i n percent
Slope of E x i t Channel i n percent
Vegetation 0 to 5 5 t o 10 o t o s 5 t o 1 0
Bermudagrass \ Bahiagrass
a
Buffalograss 5 Kentucky bluegrass
Smooth bromegrass 7 . Tal l fescue Reed Canarygrass
Sod-f orming grass legume mixtures
5
7
6
4'
Lespedeza sericea Weeping lovegrass Y e l l o w bluestem 3 . 5 N/A3 Native grass mixtures Annuals
6
5
4
2.5
5
4
3
N/A
SCS-TP-61
As defined i n TR-52
Use on slopes steeper than 5 percent is not recommended.
.- - . . . .. .
...
3.64
Table 3.6. Permissible Spillway Velocities a f t e r Aging l .
Original Material Excavated Feet per second
Fine sand, non-colloidal
Sandy loam, non-colloidal
S i l t loam, non-colloidal
Alluvial silts, non-colloidal
Ordinary firm loam
Volcanic ash
3.00
3.50
3.50
3.50
_ . . , Fine gravel 5.00
' St i f f clay, very colloidal
Graded, loam to cobbles 8 non-colloidal
! Alluvial s i l t68 C O l l O i d a l \
Graded, S i l t to cobbles, C O U O i & l
Cobbles and shingles
Coarse gravel, non-colloidal
Shales and hardpans
5.00
5.00
5.00
5.50
5.50
6.00
6.00
Recommended i n 1926 by Special Committee on Irrigation Research, American Society of Civil mglneers.
Values shown apply to water transporting colloidal silts.
3.65
of the ear th embankment. If this is not prac t ica l , a wing d ike should be
constructed t o prevent flows from encroaching on the downstream toe of the
dam.
Elevation of the crest of the emergency spillway is dependent upon the
type of sp i l lway t o be used. In a l l cases, the design depth of water over the
spillway must be a minimum of one foot below the elevation of the settled
height of the &am.
emergency spillway for events less than the lO-year, 24-hour prec ip i ta t ion
event as long as ef f luent l imitations are achieved. Therefore, t h e elevation
difference between the principal spillway and the emergency spillway is site
dependent.
design storm.
The current OSM regulations allow discharge through the
Lucal state requirements may specify f u l l containment of the
3.9.5 Erosion Control Below Spillways
3.9.5.1 General
During operation, the ou t l e t discharge from the pr incipal spillway of a
sedimentation pond is a highly concentrated, fast-moving jet (with its asso- c i a t ed turbulence) t h a t has considerable poten t ia l f o r causing damage
downstream.
erosion downstream of the s t ructure; undenuine the ou t l e t ; form a wide, deep
acour hole i n the o u t l e t area; and possibly endanger the aafety of the dam
embankment. Protection is necessary to prevent the jet and i ts associated
turbulence from causing erosion u n t i l the jet flow has dissipated to a d l d e r ,
non-scouring flow. The m6t conanon method of protect ing the channel from ero-
sive forces caused by high ve loc i t ies and turbulent flow is to l i n e the chan-
nel w i t h riprap. I n this manner, the channel is protected from erosion u n t i l
the outflow jet has dissipated to a milder flow condition of decreased ve-
l o c i t y and turbulence.
! \
If protect ive measures are not taken, pond discharge can cause
Where the pond w i l l discharge onto an area which had not previously been
exposed to flow, there is the likelihood of severe erosion from the flow over
loose so i l s . On the other hand, if the pond discharges i n t o a well-armored
na tura l channel, the downstream erosion a f f ec t s w i l l be minimal.
the o u t l e t and the channel at the o u t l e t should be straight so Uischarge does
no t impinge on any of the channel banks a shor t distance Bownstream. By pro-
per ly choosing an ou t l e t location and geometry, the amount of downstream ero-
Alignment of
3.66
sion is minimized.
below principal spillways are discussed in the following sections.
The application and design of r iprap f o r erosion control
3.9.5.2 Riprap
Riprap consis ts of a layer of d i scre te fragments of durable rock
possessing su f f i c i en t s i z e t o withstand the dynamic, erosive forces generated
by the flow of water. Riprap should be hard, dense, and durable t o withstand
long exposure to weathering. In surface mining operations, r ip rap is t he mos t
common and economical means of preventing erosion of channel bed and banks
upstream and par t icu lar ly downstream of sedimentation ponds where there is a
high erosive poten t ia l due to contraction of flow, flow alignment, changes i n
slope, and etc. When the material is of su f f i c i en t s i ze , shape, gradation,
and hardness, r iprap is excellent erosion protection.
The important fac tors to be considered i n designing rock r iprap protec-
t i o n are: rock durabi l i ty , density, s ize , weight, shape, and angularity;
direction and maganitude of the velocity of flow near the rockt bed or bank
slope; and angle of repose of the rock. In addition, the desired l eve l of
.p ro tec t ion may not be provided by the r iprap i f design criteria concerning
! rock gradation, placement, r ip rap thickness, and f i l t e r design are not \
considered . There a re many means and methods by which r iprap protect ion can be
constructed and placed.
methods of placement:
Following is a categorization of r iprap materials and
- Dumped r iprap
- Hand-placed r iprap
- Wire-enclosed r iprap (gabion)
- Grouted r iprap
When available i n su f f i c i en t s ize , dumped rock riprap is usually the most
economical material f o r bank protection. Dumped rock r iprap has many advan-
tages over other types of protection, including i ts f l e x i b i l i t y and the ease
of local damage repair .
manner but is not complicated.
embankment, rocks can be dumped d i rec t ly from trucks from the top of the
embankment.
dropping down the slope in a chute or pushed downhill with a bulldozer.
Construction must be accomplished in a prescribed
I f r iprap is placed during construction of the
To prevent segregation of s izes , rock should never be placed by
3.67
Dumped r iprap can be placed with a minimum of expensive hand work.
appearance of dumped r iprap is natural , and after a time, vegetation w i l l grow
between the rocks: Final ly , i n temporary channels when usefulness of the pro-
t ec t ion is finished, the rock is salvageable.
The
m p e d r iprap is extensively used on surface mine sites due to the
ava i l ab i l i t y of rock and the ease of placement.
f o r the proper s t a b i l i t y and erosion control.
r iprap are avai lable (Simons, Li ti Associates, fnc., 1982; ~ a r f i e l d , 19818
Bureau of Reclamation, 1977).
Sizing of r iprap is Important
Several references f o r s iz ing
3.10 Summary
Sedimentation pond design is based upon meeting e f f luent l imitat ions f o r
t h e design storm runoff event.
pond is determined such t h a t e f f luent l imitat ions are sa t i s f i ed .
configuration i s then determined t o provide the required s e t t l i n g conditions.
.'This requires an i n t e ra t ive process.
lished, the principal spillway is sized to produce the required detention time
and the emergency spillway is then shed so that the combination of pr inc ipa l
and emergency spillways are adequate. The f i n a l step in the design process is
A particle s i z e t h a t must be removed in the
The pond
\ Once the pond configuration is estab-
! \
to check the e f f luent f o r base flow conditions a f t e r the pond is operational.
The design example i n Chapter VI presents h o w the previous sect ions are
in t e r r e l a t ed in the design process.
4.7
81 ~ORIGINAL STREAM BANK
A
L A
-1
s..J
PLAN VIEW ,
SECTiON B-B
"""~-~~~ FIGURE 4. '3 LOOSE ROCK
(II ~ ~(,\~~~T ~~~VI~~ CHECK DAM
'Q7~'
4.20
POINTS A SHOULD BE HIGHER THAN POINT B
PROPER PLACEMENT OF STRAW IN DRAINAGEWAY (OSM, 1981
BALE BARRIER
\. 10
FIGURE 4.10
. . . . . . . . . .
TABLE OF CONTENTS CHAPTER V
Page
V. MAINTENANCE FOR WATER QUALITY CONTROL
5.1 Pond Maintenance . . . . . . . . . . . . . . . . . . . . 5.1
5.1.1 Accessibility . . . . . . . . . . . . . . . . . . 5.2 5.1.2 Monitoring/Maintaining
Sediment Storage Volume . . . . . . . . . . . . . 5.2 5.1.3 Bank Stability and Maintenance . . . . . . . . . . 5.3
5.1.3.1 Vegetation Stabilization . . . . . . . . 5.3 5.1.3.2 Riprap Stabilization . . . . . . . . . . 5.4 5.1.3.3 Rill and Gully Control . . . . . . . . . 5.4
5.1.4 Maintenance of Inlet and Outlet Structures . . . . 5.4
V. MAINTENANCE FOR WATER QUALITY CONTROL
5. I Pond Maintenance
The useful life of a pond is partially a function of the maintenance of
the pond and the embankment.
serve the structural integrity of the dam to ensure that essential design
features of the outlets are maintained, and to ensure adequate storage and
capacity for the pond.
major problems.
program than it is to make repairs after an extended period of negligence.
Tie primary purposes of maintenance are to pre-
Hinor problems should be repaired before they become
It is usually less costly to implement aregular maintenance
Written instructions for maintenance and operation of the structure (and
any r e w r e d monitoring equipment) should be prepared as part of the design. These instructions should establish the frequency, and describe the nature of,
inspections.
inlet and outlet structures.
gates, specific instructions should be given regarding the operation of the
gates
Instructions should also be provided for routine maintenance of
If a spillway is controlled by manually operated
’ ‘1 A record of all inspections and any maintenance performed. on the pond should be kept. The date, last major rainfall, sediment storage level, and
any problems should be noted. If maintenance has been performed, the date and
type of repair should be noted. These two records w i l l aid the operator in
determining if chronic problem areas exist in the pond design.
t \
After an area has been stabilized and successfully revegetated, sedimen-
tation ponds may be removed.
Sedimentation pond is usually addressed in the d n h g and reclamation plan
submittal.
this chapter.
The decision for removal or retention of a
The options available to the operator will be discussed later in
Repairs of embankments and emergency spillways are extremely important
for the proper functioning of the sedimentation pond system.
regions, embankments are usually stabilized by mulching and then by
establishing a good vegetation cover.
establishment of a vegetative cover, riprap or mulching may be used. The use
of either a vegetative cover or riprap is not specifically required by OSM.
However, both of these measures will aid in stabilizing the embankment or
spillway and will usually reduce the required maintenance. The design
In humid
Where conditions do not allow the
- ._ . .- . . _-. . . .. ... ._
5.2
engineer should check w i t h the regulatory authority t o determine regionally
accepted methods t o s t a b i l i z e the embankment and spillway.
Studies invest igat ing the performance of exis t ing sediment ponds f o r the
control of erosion from surface mining operations .have shown- t h a t once the
pond has been constructed and operated maintenance of sediment level , i n l e t s
and ou t l e t s is one of the main causes of poor pond performance (EPA 1980, EPA
1979).
5 -1 .1 Accessibi l i ty
Location of t he pond is of prime importance. The pond should be
accessible f o r construction, k n i t o r i n g , and maintenance.
maintenance should be considered during the planning of the pond.
engineer should consider the type of equipment used f o r construction and main-
tenance and the r o o m required fo r this equipment to function e f f i c i en t ly . I n
a ' w e l l designed pond, the heavier sediment w i l l *posit near the i n l e t of the
Accessibi l i ty for
The design
- . pond. Therefore, access t o the i n l e t end of the pond is essent ia l . A w e l l i ' constructed and regular ly maintained road is very helpful for providing proper
maintenance, including sediment removal, r iprap repair, or embankment repair.
Adequate access ib i l i t y is v i t a l if chemical f locculation is being used. : \
5.1.2 Monitoring/Maintaining Sediment storage Volume
Most sedimentation ponds are designed with su f f i c i en t annual sediment
storage volume for a number of years. ow ever, designing for excess s torage
volume aOes not guarantee that this storage volume w i l l not be exceeded by a
l a rge storm event.
I n order to ensure adequate storage volume, the avai lable sediment
storage volume i n a pond must be monitored.
is helpful f o r monitoring.
clean-out l eve l is t o i n s t a l l a s t a f f gage i n the pond and to determine the
sediment accumulation l eve l t h a t requires clean out. Most design manuals
(Virginia Soil and Water Conservation COmmission, 1980) recommend clean out
when the accumulated sediment reaches 60 percent of the design sediment
s torage volume. An acceptable schedule should be established by the design
engineer, operator, and regulatory agency. It is the respons ib i l i ty of the
operator t o ensure t h a t the schedule is followed.
Pre-defining the clean-out leve l
One of the simplest means of pre-defining the
5.3
Clean out of sediment is usually handled by a small dragline, clamshell
bucket, or a backhoe f o r w e t ponds and by a front-end loader fo r dry ponds.
If a front-end loader is used, the ponds should be su f f i c i en t ly dr ied i n order
t o support t he weight of equipment.
by dragl ines operating from the banks, cleaning is =re d i f f i c u l t .
cases dredging may be necessary.
professionals experienced i n this procedure.
For large ponds which cannot be cleaned
I n such
Dredging w i l l of ten require the service of
Sediment removed from a pond is usually incorporated i n t o the s p o i l
material. I f the removed sediment I s found to contain acid- or toxic-forming
materials, the sediment w i l l have to be disposed of in a more control led
manner.
U s e of t h i s material as a topsoi l subs t i t u t e may be very useful f o r
underground coal mining a c t i v i t i e s where the amount of avai lable topso i l is
limited. I f chemical f locculat ion is used to improve the eff ic iency of the
pond, use of the accumulated sediment w i l l probably not be su i tab le as a top-
soil subs t i t u t e because of the possible toxic e f f e c t s of the chemicals.
Sediment removed from a pond may be used as a subs t i t u t e for topsoi l .
1 Chemical and physical analyses are needed before any material can be used as a
s u b s t i t u t e f o r topsoi l .
5.1- 3
Depending on the pond surface area, location, and local climate, main-
Bank S t a b i l i t y and Maintenance
tenance of s ide slopes is important. Wave action, excavation, and removal of
vegetation w i l l promote the erosion of side elopesand subsequent increase i n
solids concentration of the pond.
are dbcussed.
Several methods and maintenance procedures
5.1 . 3.1 Vegetation S tab i l iza t ion
Maintenance of vegetative measures should occur on a regular basis, con-
s i s t e n t with favorable p lan t growth, soil , and climatic conditions. This
involves regular seasonal work for f e r t i l i z i n g , liming ( i f appl icable) ,
pruning, f i re controls , reseeding, and weed and pest control. open channel
spil lways are subject to rapid infes ta t ion of w e e d s and woody plants .
, should be eradicated or cu t back since they of ten reduce drainageway effi-
These
ciency. Well-maintained vegetation w i l l provide a comfortable margin of ero-
s i o n cont ro l .
5.4
5.1.3.2 Riprap Stab i l iza t ion
Large storms may displace the r iprap and allow erosion of the underlying
material. Displacement and damage to the r iprap w i l l usually occur where flow
v e l o c i t i e s are highest. Typically, discharge s t ruc tures and spillway areas
experience the mos t damage. I f displacement of the r iprap has occurred, t h e
r ip rap f i l ter blanket should be checked fo r damages. Repairs should be made
as soon as prac t ica l . These areas should be checked fo r erosion or s i l t i n g of
t h e channel i n order to assess the impact of the carrying capacity of the
channel.
Riprap is commonly used to control the upstream embankment of the dam
from damages due to wave action. I f r ip rap is damaged, the s i z e of the r ip rap
may have to be increased.
damages wherever the r iprap is displaced.
The r iprap f i l ter blanket should be checked f o r
5.1.3.3 Rill and Gully Control
Concentrated water flow w i l l cause rills and gu l l i e s on the embankment
slope. Vegetation and r iprap will often s top their development; however, a
c e r t a i n amount of erosion is expected on any earth embankment.
densi ty a t which rills and gu l l i e s become uncontrollable is d i f f i c u l t to
define and is dependent on the s o i l s , climate, and land use of the local area.
OSM requires rills or gu l l i e s deeper than nine inches in reclaimed areas to be
f i l l ed , graded, or otherwise s t ab i l i zed (i-e.., s t r a w mulch) (30 CFR 816.106).
U s e of this r u l e as a guideline for embankment s t ab i l i za t ion is suggested and
w i l l preclude the formation of large gul l ies .
more s t r ingen t maintenance program.
: The s i z e and \
Sta t e agencies may require a
5.1.4 Maintenance of I n l e t and Oultet Structures
Maintenance of i n l e t and o u t l e t s t ruc tures is M extremely important
requirement i n achieving e f f ec t ive sediment control. A l l water-handling
s t ruc tu res should be inspected after every major storm. Erosion damages
requi re prompt repair to prevent fur ther damage and to help prevent similar
damage i n t h e future.
Sediment buildup i n the i n l e t section and behind check dams and f i l ter
barriers should be checked. Sediment and other debris removed from these
5.5
areas should be disposed of in a manner t h a t w i l l prevent sediment from being
carried back in to the waterways a t the mine. Possible use of t h i s material as
a subs t i t u t e s o i l medium should be considered.
barriers should be replaced before they become clogged or overtopped.
Straw bales and sandbag
When vegetation is used to s t a b i l i z e the area or as a vegetative f i l t e r ,
t o p dressing with f e r t i l i z e r is usually required. F e r t i l i z e r w i l l help keep a
dense s tand and provide growth of desirable plants. Areas where f a i l u r e s have
been experienced i n the establishment of vegetative protection m u s t be
promptly treated. Timely maintenance w i l l reduce cos ts in t he long run. I f
t h e area continues t o exhibit vegetation f a i l u r e due to e i t h e r high or pro-
longed w a t e r flow, mote extensive s t ab i l i z ing measures such as r iprap may be needed.
Pipe culver t spillways should be examined fo r s t ructural s t a b i l i t y both
a t the i n l e t and a t the discharge point. Trash racks should be cleaned of
debris. I f gates or valves are wed, they should be t e s t ed to see t h a t they
- \ work freely. I
TABLE OF CONTENTS CHAPTER VI
Page
. . Vf. DESIGN PROCEDURES AND MAMPLE
6.1 Design Procedure . . . . . . . . . . . . . . . . . . . . 6.1 6 . 2 DesignBcample . . . . . . . . . . . . . . . . . . . . . 6.4
. . . . . . . . . -
LIST OF TABLES CHAPTER VI
Page
Table 6.1 Development of Influent Size Distribution . . . . . . . 6.6
Table 6.2 Summary of Sedimentation Pond Design . . . . . . . . . . 6.20
LIST OF FIGURES CXAPTER V I
Page
F i g u r e 6.1 Sedimentation pond site . . . . . . . . . . . . . . . . 6.5
F i g u r e 6.2 Sediment s i ze distribution . . . . . . . . . . . . . . 6.7
Figure 6.3 Stage-storage area and stage-storage curves of Pond Si te No. 1 . . . . . . . . . . . . . . . 6.11
Figure 6.4 Sedimentation Pond S i t e No. 2 . . . . . . . . . . . . . 6.15 : \ Figure 6.5 Stage-storage area and stage-storage
curves of Pond Si te No. 2 . . . . . . . . . . . . . . . 6.17
V I . DESIGN PROCEDURES AND MAMPLE
6.1 Design Procedure
Step 1. Site selection (Section 3.1)
The sedimentation pond loc t ion is selected considering the f a c t o r s presented i n Section 3.1.
The peak inflow r a t e and runoff volume f o r the design storm event are determined.
Step 3. I n f l u e n t sediment size d i s t r i b u t i o n (Section 3.2.2.1)
The s i ze d i s t r i b u t i o n of the inflowing sediment is required. Where e x i s t i n g information from t h e mine site or nearby mine sites is a v a i l a b l e , it should be used. When there is no e x i s t i n g data, a s i z e d i s t r i b u t i o n can be developed using information from so i l
. surveys.
! Step 4. Sediment y i e l d (Section 3.2.2.2)
Determine the annual sediment y i e l d and the storm sediment y ie ld . \
Step 5. Inflow suspended s o l i d s concentration (Section 3.2.3)
Using t h e storm sediment y i e l d and the storm runoff volume, deter- mine the average in f luen t suspended solids concentration.
Step 6. Settleable solids concentration (Section 3.5)
Develop t h e settleable sediment s i z e d i s t t i b u t i o n (particles > 0.001 mm) f r o m the in f luen t sediment size d i s t r i b u t i o n . Select a particle size t o be removed i n t h e pond. Determine t h e t rapping e f f i c i e n c y from the settleable sediment size d i s t r ibu t ion . Determine the average e f f l u e n t suspended s o l i d s concentration using t h e t rapping e f f i c i ency , sediment y i e ld , and runoff volume (Equation 3.6). Calcula te the settleable solids concentration (SS) from Equation 3.5. I f SS > 0.5 m l / l , select a smaller s i z e particle and repeat procedure. I f SS E 0.5 d/l, go t o Step 7 and design pond to remove selected particle s i ze . If SS < 0.5 m l / l , select a l a r g e r s i z e particle and repeat procedure.
6 . 2
Step 7 . Available storage volume (Section 3.7)
Develop stage-storage curve for sedimentation pond location. Determine the required sediment storage volume and the corresponding depth. Determine the available detention storage depth by
I) = embankment height - (embankment settlement + required
freeboard + emergency spillway depth..+ sediment
storage depth)
Determine the available detention storage volume above the sediment storage depth from the stage-storage curve.
Step 8 . Required storage volume (Section 3.6) . A s s u m e a detention storage depth and determine the required deten- t i o n t i m e fo r the design particle s i z e from Figure 3.10. t he t i m e base of the inflow hydrograph. storage volume from Figure 3.7. from Figure 3.8. avai lab le storage volume. If t he avai lable storage volume is less than the required storage volume, e i t h e r
Calculate Determine the required
Determine the required outflow rate Compare the required storage volume t o the
(a) Increase the embankment height and determine the new avai lable storage volume. Repeat Step 8.
(b) Excavate ehe pond side slopes and a develop new stage- storage curve. R e p e a t Step 8.
(c ) Construct a pond downstream and re turn t o Step 1.
I f the available storage volume is larger than the required storage volume, check the required surface area (Section 3.8.3.1). If t he measured surface area is less than the required surface area, (1) excavate pond side slopes or ( 2 ) r a i s e pr incipal spillway crest. I f the measured surface area is greater than the required surface area, check length-width r a t i o (Section 3.8.2.2) and ca lcu la te required length t o settle design particle size (Section 3.8.1). I f the length is not large enough, increase the flow length (Section 4.2). If the length criteria is met , check scouring (Section 3.8.3.4). I f t h e scouring velocity is smaller than the horizontal velocity, increase the depth and return to Step 7 . is greater than the horizontal veloci ty , go t o Step 9.
I f the scouring veloci ty
Step 9. Principal spillways (Section 3.9.1)
Select pr incipal spillway type and design for the peak outflow rate and the corresponding head.
6.3
Step 10. Emergency spillway (Section 3.9.4)
Select emergency spillway type and design the spillway system to pass the peak discharge from the 25-year, 24-hour runoff event.
Step 11. Erosion control b e l o w the spillways (Section 3.9.5)
Size the riprap b e l o w the principal spillway.
Step 12. Check base f l o w conditions after pond is operational.
6 . 4
6.2 Design Example
The design example is to i l lus t ra te the procedures of s i z ing a detention
pond t o meet the e f f luen t Standard. Design of pr inc ipa l spillway (Step 9 ) ,
emergency spillway (Step 101, erosion control below the spillway (Step 111,
and base f l o w condition (Step 12) are not Included because methodologies can
be found in various texts and references.
Step 1. Considerations f o r sedimentation pond select ion have been presented i n Section 3.1. I n this design example, a si te is preselected and presented i n Figure 6.1.
Step 2. Hydrology
Design Event
10-year, 24-hour
25-year, 24-hour
' Step 3. Sediment s i z e d i s t r ibu t ion
Q I (cfs)
77
91.5
V (acre-feet)
2.31
3.35
For t he purpose of i l l u s t r a t i n g the meghodology, develop in f luen t s i z e d i s t r ibu t ion from Soil 'Surveys (Table 6.1).
55 percent of area is sand 4 0 percent of area is sandy clay
5 percent of area is s i l t y c lay
Figure 6.2 presents the inf luent s ize dis t r ibut ion. t i o n is extended with a s t r a i g h t l i n e to a particle size of 0.001 mm.
The dis t r ibu-
Step 4. Sediment y i e ld (use USLE for annual sediment y i e ld and MUSLE f o r storm sediment y i e ld )
Annual sediment y ie ld = 115 tons Storm sediment y ie ld = SO tons (looyear, 24-hour stom)
... .
6 . 5
FIGURE 6. I SEDIMENTATION ' POND SITE
6.6
!
A % I 0 , L
w m 0 . 0 - . - -
A
c;-
A
c 1
cc.
I--
8
1 m N
1 m N
? (Y
m 0
0
N c:
m m
c . c
. ( Y
hl 0 x N
8 0' V
x 0 c
m
I
I
I
0 . 2
m N
2 (Y
Q) *\
* 9 c . c
K e4 . -
...... .
6.7
n
E E Y
w
W -I 52 c
.6 0
6 J cd w K - J c3 -
1J3NId lN33C13d
6.8
Step 5. Average inflow suspended so l ids concentration
15,926 mg/l 50 tons x 2000 lb/ton x l o 6 = 2 3 2.31 ac-ft x 43,560 f t /ac x 62.4 Ib / f t cI
Step 6. Settleable so l id s concentration
From Figure 6-28. 21 percent of the inf luent s i ze d is t r ibu t ion is non-settleable (< 0.001 mm). The redis t r ibuted se t t l eab le s i z e d is t r ibu t ion is a l so presented i n Figure 6.2.
Star t with d = 0.011 m.
From Figure 6.2, trapping efficiency E = 0.82 and f rac t ion of settleable solids K = 0.798 using Equation 3.6.
(1.0 - 0.82) x 0.79 x 50 tons x 2000 lb/ton
2.31 ac-ft x 43,560 f t /ac x 62.4 l b / f t 2 3 c* = 0
x l o 6 = 2265 mg/l
The s e t t l eab le so l ids concentration is determined as follows:
t b
P a r t i c l e Influent Effluent (di/O- 01 1 l6 Size Range Mean Size 4 4. x Axi
( a i l
0.001 -0.002 0.0014 0.05 0.28 1.19 x 10'6 ,
0.002 -0.0038 0.0028 0.05 0.28 7.62 x 10-5
0.0038-0.0072 0.0052 0.05 0.28 3.12 10-3
0.0072-0.011 0 . 0089 0.16 4.49sx 10-2 - 0.03 - 1.0 0.048 0.18
From Equation 3-58 settleble solids concentration can be calculated as
2265 m /1 ss = l120 mi,d [ (1.0 - l = O ) + 0.0481 0.10 ml/l < 0.5 m l / l
6.9
Try with a larger size of settleable s o l i d s , d = 0.020 nnn.
% of settleable s i z e d i s t r i b u t i o n smaller than 0.011 m o % of settleable size d i s t r i b u t i o n smaller than 0.020 rmn x =
0.18 0.22 =- = 0.818
From Figure 6.2, E = 0.78 and K = 0.79
(1.0 - 0.78) x 0.79 x 50 tons x 2000 -/ton
2.31 ac- f t x 43,560 f t /ac x 62.4 l b / f t 2 3 c =
x lo6 = 2768 mg/l
2768 mg/l 1120 m g / m l [(loo - 0.818) +‘(0.818 x 0.048)l ss =
= 0.55 ml/l > 0.5 m l f l
Try with a smal le r s i z e of settleable solids, d = 0.018 mn.
% of settleable s i z e d i s t r i b u t i o n smaller than 0.011 nun % of s e t t l e a b l e size d i s t r i b u t i o n smal le r than 0.018 nnu
E
‘0
0.18 0.215 = - = 0.837
From Figixre 6.2, E - 0.785 and K - 0.79 (1.0 - 0.785) x 0.79 x 50 t ons x 2000 &/ton
2.31 ac- f t x 43,560 f t /ac x 62.4 lb/ f t 2 3 C =
x lo6 = 2705 mg/l
ss = 2705 1 120 mg/l mg/ml [(loo - 0.837) + (0.837 x 0.048)J
= 0.49 ml/l E 0.5 ml/l
I n order t o m e e t the 0.5 m l / l standard, the pond is designed to remove a l l particles equal to and larger than 0.018 ma.
Step 7. Available s to rage volume
Based on the selected s i te (Figure 6.l), the stage-storage curve is obtained using the rnethodology described in Section 3.7 and is pre- s e n t e d i n Figure 6.3. f o r three t i m e s the annual sediment y i e l d
The sediment s torage volume should provide
6.10
I
3 yr x 115 ton/yr x 2000 %/ton p: o.23 ac-ft P
3 2 70 l b / f t x 43,560 f t /ac vs
From Figure 6.3, depth of sediment = 4.2 f t
Total maximum embankment height (assumed) = 16 f t Allow 5 percent settlement (0.5 x 16) = 0.80 f t
Required freeboard = 1.0 f t Allow 2.5 f t for emergency spillway 2.5 f t
Haximum depth of sediment storage = 4.2 f t
Available detention storage depth (Di) 16 - ( 0 . 8 0 + 1.0 + 2.5 + 4.2) = 7.5 f t
Stage a t maximum detention storage depth 7.5 + 4.2 = 11.7 f t
The avai lable storage volume is described by taking the difference between the storage volume at the stage of the maximurn detention s torage depth and the storage volume a t the elevation of t he prin- cipal spillway.
Pram Figure 6.3, avai lable detention storage volume 1.71 - 0.23 = 1.48 ac-ft
t \
Step 8. Required storage volume
The t o t a l depth (+I of the pond is used i n the following com- putat ions and is equal t o the sum of the sediment storage depth, detention storage depth, and the permanent pool depth ( i f used).
T h e base of inflow hydrograph
2 2V 2 x 2.31 ac-ft x 43,560 f t /ac
*I o.73 hours
T b a - = 3 77 f t /sec x 3600 secfhx
For d = 0.018 mm and D = 11.70 f t , T
TD = 4.50 hours (From Figure 3.10)
6.11
15
10 '
0
SURFACE AREA ( F T ~ )
1.0 2.0
STORAGE VOLUME (ACRE - FEET)
FIGURE 6.3 STAGE - SURFACE AREA AND STAGE - STORAGE CURVES OF POND SITE N0. I
6.12
i
For Tb = 0.73 hours and TD = 4.50 hours,
- r 0.950 ( P r o m Figure 3.7) V
QO
QI - = 0.05 (From Figure 3.8)
S = 0.950 x 2.31 ac-ft = 2.19 ac-ft > 1.48 ac-ft
Since the pond location w i t h an assumed 16 ft embankment height can- not provide the required storage volume, t he designer has three a l te rna t ives .
(a) Increase the detention storage depth and r e t u r n t o Step 7 and determine the new available storage.
(b) Excavate the pond side slopes no steeper than 2 horizontal t o 1 vertical and re turn to Step 7 and develop new stage- storage curve.
( c ) Construct another pond downstream.
For purposes of i l l u s t r a t i n g multiple pond design, alternative ( c ) is chosen.
t Step 8c. Determine the sediment removal i n the upstream pond \
S 1-48 U-ft o.64 - I c V 2.31 ac-ft
S V
TD = 0.4 hours
For - = 0.64 and Tb = 0.73 hours,
(Figure 3.7)
Qo
QI - L: 0.360 (Figure 3-81
L
6.13
. . 1
For TD = 0.4 hours and DT = 11.70 f t ,
(Equation 3-10) DT 2.254 d2 = 3600 TD
11.7 f t 2.254 x 0.4 hrs x 3600 aecfhr
d=j7- 2.254 X 3600 TD =
e 0.060
Therefore, only particles larger than 0.060 m can be removed i n t h e first pond based upon idea l s e t t l i n g .
Check surface area
A (measured a t depth - 4.2 f t ) = 3490 f t 2
Qo = 0.360 x 77 CfS = 27.7 CfS
From Equation 3.14, required surface area
A 1.2 X 27'7 ft3/sec = 4096 ft2 > 3490 f t 2 rep 2.254 x (0.06012
Due t o nonideal s e t t l i n g conditions, the smallest particle which w i l l be removed using the ava i lab le surface area is /--
I 0.065 ZUU 2.254 x A 2.254 X 3490 8 . .
Check length-to-width ratio
L (measured a t depth = 4.2 f t ) * 130 f t
130 = 4.9 > 2.0 L W 26.8 f t - =
.. .
6.14
Check Scouring
V t i = - = *O 27*7 ft3/sec * 0.14 fps (Equation 3.18) WD 26.8 f t x 7.5 f t
V = 1.67 dli2 = 1.67 x (0.065)'/2'= 0.43 fps (Equation 3.17) sc
The smallest particle which w i l l be removed i n the f i r s t pond is 0.065 mn which corresponds t o a removal e f f ic iency of 69.5 percent (Figure 6.2).
Design of Downstream Pond (repeat Steps 1 through 8 )
Step 1.
step 2.
S i t e selection (Figure 6.4). Se lec t a second pond j u s t downstream of t h e f i r s t pond.
Eydrology (10-year, 24-hour design event)
10-year, 24-hour design event
F i r s t pond (%I
Q ( c f s )
27 i7
V (acre-feet)
2.31
Additional contr ibut ing area (Qr) 22.0 0.39
Design parameter (QI Second pond) 49.7 2.70
25=year, 24-hour design event
First pond (h)
V (acre-f eet 1
3.35
Additional contr ibut ing area (Qs) 26.1 0.56
Design parameter 117.6 3.91
Notes: 1. The design inflow peak flow for second pond should be deter- mined by reservoir rout ing and added to t he hydrograph of the addi t iona l contr ibut ing area. For i l l u s t r a t i o n of eedimen- t a t i o n pond design, the peak outflow from the f i r s t pond is added d i r ec t ly t o the peak f l o w of t h e addi t iona l contr ibut ing area.
I
. * _ . . . . . ..-.
G .15
FIGURE 6.4 SEDIMENTATION POND SITE N0.2
6.16
2. I f t h e r e is any permanent pool s torage i n the first pond, the stored volume should be subt rac ted from the t o t a l volume for designing t h e second pond.
Step 3. Sediment size d i s t r i b u t i o n
U s e the same sediment size &.istribution.as the f i r s t pond.
Step 40 Sediment y i e l d (use USLE f o r annual sediment y i e l d and MUSLE for storm sediment y i e l d )
From add i t iona l con t r ibu t ing drainage area (10 acres)
Annual sediment y i e l d = 19 t ons S t o m sediment y i e l d - 8.3 t o n s
Step 5. Inflow suspended s o l i d s concent ra t ion
Since the design is to remove a l l particles l a r g e r t h a n 0.018 m, c a l c u l a t i o n of t h e inflow suspended solids concent ra t ion is not required.
.A
I Step 6. Settleable solids concent ra t ion
Must remove a l l particles Larger than 0.018 mn. t \
Step 7. Available storage volume
The stage-storage curve is presented i n Figure 6.5. Since t h e first pond has a t rapping e f f i c i e n c y of 69.5 percent , the sediment storage volume required & t h e second pond is equal t o 30.5 percent of the upstream sediment y i e l d plus the sediment y i e l d from the a d d i t i o n a l con t r ibu t ing drainage area.
(115 ton/yr x 0.305) + 19 ton/yr] x 3 yr x 2000 lb/ton v = I 8 70 lb/ft3 X 43,560 f t 2 / a C
= 0.11 ac - f t
From Piguke 6.5, depth of sediment = 1.3 f t I
Using the same embankment se t t lement , required freeboard, and emergency spil lway depth as the first pond, t h e a v a i l a b l e de ten t ion s to rage depth is
D1 = 16 (0.80 + 1.0 + 2.5 + 1.3) = 10.4 f t
.. . . . - - . , . - .. ..
6.17
\ \ I I
10
0 I .o 2.0 . 3.0 4.0 5.0
STORAGE VOLUME (ACRE - FEET)
FIGURE 6.5 STAGE - SURFACE AREA AND STAGE -STORAGE CURVES OF S ITE NO. 2
. _ _ - _ . . . . .
6.18
Stage a t maximum detention stage depth
10.4 + 1.3 11.7
From Figure 6.5. available storage volume is
3.75 - 0.11 = 3.64 ac-ft
Step 8. Required storage volume
Time base of inflow hydrograph
2 2V 2 x 2.70 ac-ft x 43.560 f t /ac
'1 49.7 f t /sec x 3600 sec/hr l.31 hours
T b = - = 3
For d = 0.018 x n and DT = 11.7 f t8
For Tb = 1.31 hours and TD = 4.5 hours8
S - = 0.930. S = 0.930 x 2.70 ac-ft = 2.51 ac-ft < 3.64 ac-ft V
QO
QI - = 0.0708 Qo e 0.070 x 49.7 P 3-48 C f S
Since the required storage volume is less than the available storage volume, decrease the depth and repeat Step 8 .
Assume ~1 - 8.5 ft
From Figure 6 . S 8 Stage a t maximum detention depth
For
For
8.5 + 1-3 a= 9.8 f t
Available storage = 2.80 - 0.11 = 2.69 ac-ft
d = 0.018 and DT P 9.8 ft
TD = 3.9 hours
Tb = 1.31 hours and TD = 3.9 hours
s - = 0.910. S = 0.910 x 2.70 = 2.46 ac-it < 2.69 ac-ft V
6.19
= 0.09 x 49.7 = 4.47 c f s QO
*I Qo - = 0.09,
Check surface area
A (measured at depth = 1.3 f t ) = 5300 f t 2
- = 6120 f t 2 > 5300 f t 2 4.47
2.254 x (0.018) 2 A P 1.2 X rep
The pond must be excavated t o meet the surface area requirement or raise the p r i n c i p a l spillway crest t o elevation 2.0 f t . - This w i l l provide more sediment storage than is required. w i l l exist i f a dewatering device is not provided.
A permanent pool
A (measured a t depth - 2.0 f t ) = 6500 f t 2
Check ava i lab le storage volume
Available storage = 3.20 - 0.25 = 2.95 ac-ft > 2.46 ac- f t
Check length-to-width ratio
L (measured a t depth = 2.0) - 250 f t
L 250 W 26
. - - = 9.6 > 2.0
Check scourinq
Qo 4.47 ft3/eec e: o.02 fps v,=-- WD1 26 f t x 8.5 ft
= 1.67 d1l2 = 1.67 x (0.018)’/2 = 0.22 fps vS
vH < ’s
A summary of t h e &sign is presented in Table 6.2. I
6.20
Table 6.2. Summary of Sedimentation Pond Design.
Description Pond N o . 1 PondNo. 2
1 .
2 .
3.
4 .
5.
6 .
'7.
8.
1 9.
10 .
1 1 . ! \
12.
13.
14
15.
16.
17.
18.
19.
=Sign inflow (PI), CfS
Design runoff volume (v), ac-ft
Annual sediment yield, tons
Storm sediment yield, tons (10-year, 24-hour storm)
Inflow suspended solids concentration, mg/l
Minimum s ize of particle se t t l ed , m
Detention t i m e , hours
Pr incipal s p i l l a y elevation, f t
Sediment storage required, ac-f t
Sediment storage provided, ac-ft
-off detention depth provided, f t
Runoff detention volume provided, ac-f t
Surface area required, f t
Surface area provided, f t
Length-to-width ratio
Scour veloci ty (Vsc), f p s
2
2
Flow-through Velocity (v,) 8 f p S
10-year, 24-hour design outflow, cfs
25-year8 24-hour design outflow, c f s
77
2.31
115
50
15,926
0 065
0.4
4.2
0.23
0.23
7 . 5
1-48
3490
3490
4 .9
0.43
0.14
27.7
63.8
49.7
2.7
54
--
--- 0.018
4.5
2.0
0.11
0.25
8.5
2.95
6120
6500
9.6
0.22
0.02
4.47
113.13
VII. REFERENCES
American Arblic Health Association, American Water Works Association, Water Po l lu t ion Control Federation, 1981, "Standard Methods f o r the Examination of Water and Wastewater," 15th Edition, American public Health Association, Washington, D.C.
Anderson, C. E. and J. M. miggs8 1979, "Planning Erosion Control f o r Coal Mining and Restoration," Iowa State University, Dept. Agr icu l tura l Engineering, Jour. Soil Water Conservation 34: 234-236.
Bar f i e ld , B. J., R. C. Warner and C. T. Haan, 1981, Applied Hydrology and Sedimentology f o r Disturbed Areas, Oklahoma Technical Press; Sti l lwater, Oklahoma.
Bondurant, J. A*, C. E. Brockway and M. J. Brown, 19758 "Some Aspects of Sediment Control, University of Kentucky; ~ e x i n g t o n , =I pp- 117-121.
Bureau of Reclamation, 1974, Design of Small mutts, A Water Resources Technical Publ ica t ion .
camp, T. R., 1946, "Sedimentation and the Design of S e t t l i n g Tanks," Transactions of t h e X C E r V o l . 111, pp. 895-958.
) hen, m a r n g - ~ i n g , 1975, "Design of Sediment Retention asi ins," National Symposium on Urban Hydrology and Sediment Control, University of Kentucky, k f i n g t o n , pp* 285-2930
t \
Curtis8 D. C. and R. H. McCuen, 19778 "Design Eff ic iency of Stormwater Detention Basins," Jour. Water Resources Planning and Management Division, ASCEt VOl. 103, ppo 125-1390
Environmental P ro tec t ion Agency, 1981, "Development of Methods to Improve Performance of Surface Mine Sediment Basins, Phase If and 111," EPA Contract NO. 68-03-2677.
Environmental Protection Agency, 1980, "Development of Methods to Improve Performance of Surface Mine Sediment Basins, Phase I-," EPA 660/7-80-072.
Environmental Pro tec t ion Agency, 1980, "Demonstration of Debris Basin Effec t iveness in Sediment ~ o n t r o l , " EPA 600/7-80-154.
Environmental P ro tec t ion Agency, 1979, "Evaluation of the Perfonnance Capab i l i t y of Surface Mine Sedimentation Basins," EPA 440/1-79-200.
Environmental Protection Agency, 1979, "Removal of Trace Elements from A c i d Mine Drainage," E ~ A 660/7-79-101.
Environmental Protection Agency, 1978, "Erosion and Sediment Control HandhOkt" E P A 440/3-78-003.
- . _ _ .. .- _ . .. . . . .
7.2
Environmental Pro tec t ion Agency, 1976, "Erosion and Sediment Control: Surface Mining in t h e Eastern United States, Vol. 1 Planning and Vol. 2 Design," EPA 625/3-76-006.
Environmental Pro tec t ion Agency, 1976, "Effectiveness of Surface Mine Sedimentation Basins," EPA 660/2-76-117.
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Environmental Pro tec t ion Agency, 1976, "Methods f o r Control of Fine Grained Sediments Resulting from Construction Activity, ' EPA 440/9-76-026.
Environmental P ro tec t ion Agency, 1972, "Guidelines f o r Erosion and Sediment Control Planning and Implementation," EPA R2-72-72-015.
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Guy8 H. Fe8 1979, "Performance and Environmental E f fec t s of Sedimentation .Ponds Required for Surface Coal Mining Operations,' IES 25th Annual Technical Meeting, Seattle, Washington.
'; Haan, C. T. and B. J. Bar f i e ld , 1978, Hydrology and Sedimentology of Surface Mined Lands, I n s t i t u t e f o r Mining and Minerals Research, Kentucky Center f o r Energy Research Iabora tory , University of Kentucky, Lexington, Kentucky.
: \ Hazen, A * , 1904, 'On Sedimentation" Transactions ASCE Vol. 53:45-71.
H i l l , Re D - , 1975, "Sediment Control and Surface Mining," Polish-United States Symposium Environmental P ro tec t ion in open P i t Coal M.ning, Univers i ty of Denver Research I n s t i t u t e , Denver, Colorado (Septr 89-95).
Lara, J. M. and E. L. Pemberton, 1963, " In i t i a l Unit Weight of Deposited Sediments," USDA Miscellaneous Publication.
Mallory, C. W. and M.A. Nawrocki, 19748 "Containment Area F a c i l i t y Concepts f o r Dredged Ua te r i a l Separation, Drying and &hauling," U.S. Army Engineering Waterways Experiment S t a t i o n , DACW 39-73-(2-0136.
McCarthy, R. E., 1973, 'Preventing the Sedimentation of Streams i n a P a c i f i c N o r t h w e s t Coal Surface Mine," Research and Applied Technology Symposium on Mined Land Reclamation, P i t t sburgh , Pennsylvania (133-143).
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.. . . ..-. ._- - -. -. _._., .. . , , . - .. . . .. .. ..
7.3
i
Office of Surface Mining, 1981, %esign Manual for Water Diversions i n surface Mine Operations," prepared by Shcm68 W. & Associates, Inc., OSM Contract No. J5 10 1050.
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_i . - . , . . . . - _. . .. . , . .
7.4
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