MONROE COUNTY DRAIN COMMISSION

STORM WATER DETENTION METHODOLOGY

MONROE COUNTY DRAIN COMMISSION

1005 SOUTH RAISINVILLE ROAD

MONROE, MICHIGAN 48161

TABLE OF CONTENTS I. INTRODUCTION

A. Purpose..........................................1

B. Objective........................................1

C. Technical Procedures.............................1 II. RUNOFF HYDROGRAPHS

A. General..........................................2

B. Philosophy for Hydrologic Studies................2

C. Selection of a Method............................2

D. Overview of Steps in Hydrograph Development......3

E. Hydrograph Development Procedure.................3

Step 1, Design Storm Frequency..............3

Step 2, Determine Tributary Area............4

Step 3, Estimate Runoff Coefficient.........4

Step 4, Estimate Basin Time of Concentrate..6

Step 5, Determine Rainfall Intensity........7

Step 6, Estimate Peak Flow..................7

Step 7, Development of a Hydrograph........10 III. DETENTION POND

A. Preliminary Design..............................14

B. Detention Pond Layout...........................14 IV. OUTFALL STRUCTURE

A. Preliminary Design..............................16

B. Hydraulics of Outfall Structure

1. Simple Pipe Outfall Structure..............17

2. Drop Inlet Outfall Structure...............17

3. Horizontal Weir Outfall Structure..........21 V. FLOOD ROUTING

A. Background and Procedure........................27

B. Review of Results...............................31 VI. DESIGN EXAMPLES

Example Problem 1....................................32

Example Problem 2....................................42

REFERENCES

LIST OF FIGURES No. Description 1 Overland Flow Curves..................................8 2 Rainfall Intensity - Duration Curves..................9 3 Hydrograph Width at 50% and 75% Peak Flow............13 4 Orifice Pipe Discharge Curves, Outfall Structure.....19 5 Horizontal Weir Discharge Curves.....................26

LIST OF FORMS 1 Peak Flow Computation, Summary Sheet.................11 2 Detention Volume Estimate............................12 3 Storage - Elevation Curve............................15 4 Simple Pipe Hydraulics, Orifice Control..............18 5 Simple Pipe Hydraulics, Pipe Flow Control............20 6 Drop Inlet Hydraulics.............................22-24 7 Weir Hydraulics......................................25 8 Outfall Versus Storage Curve.........................29 9 Detention Basin Routing..............................30

LIST OF TABLES 1 Rational Method Runoff Coefficients...................5 2 Runoff Coefficient, Composite Analysis................6

INTRODUCTION

1. INTRODUCTION

A. Purpose

The purpose of this technical paper is to provide a

uniform method for the computation and submittal of design

plans for review by Monroe County, Michigan. In particular,

this paper deals with the hydrology and hydraulics of

detention facility design.

B. Objective

The major emphasis of the Drain Commissioner’s policy is

to control increases in runoff resulting from development.

Developments that increase runoff rate or volume shall be

required to control the discharge rate of runoff to acceptable

levels similar to existing undeveloped site conditions. All

new and redeveloped projects disturbing over 1 acre of land

(or are less than 1 acre but are part of a larger common plan

of development or sale that would be over 1 acre) must adhere

to the following standards:

a. The minimum treatment volume standard shall be one

inch of runoff from the entire site. The minimum

treatment volume shall have a minimum of 80% removal

of TSS (Total Suspended Solids), as compared with

uncontrolled runoff.

b. Channel protection measures must be met by means of

maintaining a post-development site runoff volume and

peak flow rate at or below existing levels for all

storms up to the 2-year/24-hour event.

c. A long term Operations & Maintenance (O&M) plan for

maximum design performance must be submitted to this

office to ensure the above mentioned measures will

continue to be addressed.

C. Technical Procedures

The procedures presented represent methodologies that are

likely to be familiar to most practitioners. State-of-the-art

approaches, which might place an unwarranted burden on

development submittals, have been avoided. Average values for

hydraulic formulas have been selected and are presented in a

detailed step-by-step format.

The basic procedures for the detention basin design will

be as follows:

1. Develop a hydrograph for existing undeveloped

conditions.

2. Develop a hydrograph for proposed conditions.

3. Limit outfall from the site to existing undeveloped

conditions by storing the difference between existing

and proposed conditions.

4. Determine the type of detention basin to use for the

project. (IE Dry Detention Basin where the basin will

eventually drain dry; Detention Basin with Forebay; Wet

Detention Basin that will retain a permanent water

pond).

5. The detention volume must consider and provide for

first flush runoff from the development site. First

flush runoff shall be computed based upon ½” rainfall

over the development site drainage area. Where dry

detention basins are proposed, the first flush volume

must be included in the total detention volume (IE

Total detention volume – required site detention +

first flush volume). Detention basins with Forebay

basin shall have the Forebay designed to meet the first

flush volume and the primary detention basin designed

to accommodate the required site detention volume. Wet

detention basins shall be designed to accommodate the

required first flush volume in the permanent wet pond

volume and the required site detention within the

remainder of the detention basin above the wet pond.

6. Detention systems may be designed as an above ground

or an underground system. Under ground systems may be

as manufactured by Advanced Drainage Systems, Inc.

(ADS), StormTech Chamber Systems, Kennedy Solutions,

Inc or as approved by the Monroe County Drain

Commissioner.

7. Evaluate the operation of the detention facility

through flood routing.

8. All detention system plans shall include a detailed

operating and Maintenance schedule that must be

followed by the detention system owner.

9. All detention systems shall be designed in accordance

with the Monroe County Drain Commissioner Storm Water

Detention methodology and the Low Impact Development

Manual for Michigan. Where conflicts occur between

these standards, the Monroe County Drain Commissioner

Storm Water Detention Methodology shall govern.

RUNOFF HYDROGRAPHS

II. RUNOFF HYDROGRAPHS

A. General

For the detention basin design, a storm hydrograph must be

developed in addition to an estimate of the peak runoff rate.

The hydrograph is required to study the effectiveness of the

proposed storage facility.

The method described herein (Reference 1) is applicable to

small watersheds and studies that are too small to justify

installing a stream gage or undertaking a more sophisticated

computer simulation study. Note, however, that the Design

Engineer should always search for existing streamflow records or

more sophisticated hydrologic analysis that might be available

for the area.

B. Philosophy of Hydrologic Studies

The Engineer has an obligation to provide the best estimate

possible at a cost commensurate with the scope of his project.

He must recognize that error is always present in hydrologic

estimation and that probable error increases if the information

is approximate. The Engineer needs to continually compare and

review his results for consistency and reasonableness.

C. Selection of a Method

The hydrologic method presented is based on the Rational

Method and the assumption that the critical hydrograph is

produced by runoff from the maximum intensity rainfall over a

time duration equal to the basin time of concentration. Most

practicing engineers are familiar with the Rational Method.

The basic equation for the Rational Method is:

Q = CiA (Equation 1)

Q = peak flow (cubic feet per second)

i = design rainfall intensity (inches per hour)

A = drainage area (acres

C = runoff coefficient

D. Overview of Steps in Hydrograph Development

The following outline is a general overview of the steps

required to develop a hydrograph:

1. Select design storm return period

2. Determine tributary area

3. Estimate runoff coefficient

4. Estimate time of concentration

5. Determine rainfall intensity

6. Estimate peak flow

7. Develop the hydrograph

E. Hydrograph Development Procedure

Step 1: Design Storm Frequency

The hydrograph generated for existing conditions will be

based on the ten (10) year frequency rainfall event. The peak

discharge from this hydrograph will be the maximum allowable

outfall from the detention basin. Ultimately, the discharge

from the detention pond should be similar to this hydrograph.

The hydrograph generated for proposed conditions will be

based on the twenty-five (25) year frequency rainfall event.

This hydrograph will be used as the inflow hydrograph for the

storage routing computations.

Step 2: Determine Tributary Area (A)

To apply the Rational Method, a topographic map is used to

define the boundaries of all relevant drainage basins. All

basins tributary to the area of study and sub-basins within the

study area are normally defined. A field check and possibly

field surveys should also be considered. Typically, the

delineated area is determined from the map using a planimeter or

by computation.

Step 3: Estimate Runoff Coefficient (C)

The runoff coefficient is the variable of the Rational

Method that is determined by the Engineer using judgment and

understanding of the basin. Its use in the formula implies a

fixed ratio between runoff and rainfall for any given drainage

area. In reality, this is not totally the case. The

coefficient represents the integrated effects of infiltration,

evaporation, retention, flow routing, and interception which all

affect the time distribution and peak of runoff (Reference 4).

TABLE 1 (REFERENCE 4)

RATIONAL METHOD RUNOFF COEFFICIENTS

Description of Area Runoff Coefficients

Business:

Downtown areas

Neighborhood areas

0.70 to 0.95

0.50 to 0.70

Residential:

Single-family areas

Multi units, detached

Multi units, attached

0.35 to 0.50

0.40 to 0.60

0.60 to 0.75

Residential (1/2 acre lots or more) 0.30 to 0.45

Apartment dwelling areas 0.50 to 0.70

Industrial:

Light areas

Heavy areas

0.50 to 0.80

0.60 to 0.90

Parks, cemeteries 0.10 to 0.25

Playgrounds 0.20 to 0.35

Railroad yard areas 0.20 to 0.40

Unimproved areas 0.10 to 0.30

It is often desirable to develop a composite runoff

coefficient based on the percentage of different types of

surface in the drainage area. This procedure is often applied

to typical “sample” blocks as a guide to selection of reasonable

values of the coefficient for an entire area.

Table 2 is a list of specific runoff coefficients that

shall be used for determining composite runoff coefficients.

TABLE 2

RATIONAL METHOD RUNOFF COEFFICIENTS FOR COMPOSITE ANALYSIS

MONROE COUNTY DRAIN COMMISSIONER

Character of Surface Runoff Coefficients

Streets 0.90

Paved Shoulders 0.90

Gravel Shoulders 0.50

Drives and Walks 0.90

Paved Parking Lots 0.90

Roofs 0.90

Lawns 0.20

Unimproved Areas 0.20

Step 4: Estimate Basin Time of Concentration (Tc)

The basin time of concentration is generally defined as the

time it takes direct runoff to travel from the furthermost point

in the basin to the point at which a hydrograph is desired. In

the application of the Rational Method, the time of

concentration must be estimated so that the average rainfall

rate of corresponding duration can be determined from the

rainfall intensity-duration-frequency curves.

When dealing with pipe systems, the time of concentration

may be readily calculated from the inlet time plus time of flow

in each successive pipe run. The time of flow in each pipe can

be calculated from the velocity of flow as given by the Manning

Formula for the hydraulic conditions prevailing in the pipes.

The inlet time can be estimated by calculating the various

overland distances and flow velocities taken from the most

remote point. A common mistake is to assume velocities that are

too small for the areas near the collections (Reference 4).

Often the remote areas have flow that is very shallow and

velocities cannot be calculated by “channel” equations such as

Manning’s but special overland flow analysis must be considered

(Reference 4). Figure 1 can be used to help estimate time of

surface overland flow.

It is not unusual for the time of concentration at an

initial pickup point to be in the vicinity of ten to fifteen

minutes. Clearly, many factors must be considered in

determining time of concentration including: slope, type of

cover, vegetation, distance, initial wetting period, depression

storage and infiltration characteristics. (Figure 1).

Step 5: Precipitation Intensity-Duration Curve

Figure 2 will be used for rainfall intensity-duration.

Typically, this type of curve is generated from a frequency

analysis of an annual series of peak rainfall events for a given

duration from data for a long-term recording gage (Reference 2

and 3). This figure presents both the ten (10) year and the

twenty-five (25) year storm intensity duration curves. The

graph is entered with the time of concentration and the

associated rainfall intensity is determined. (Figure 2)

Step 6: Estimate Peak Flow

The instantaneous peak flow is calculated using Equation 1:

Q = CiA (cfs.)

Steps 3, 4 and 5 are used to define the parameters on the right

side of the equation. The peak flow is computed directly. Form

1 can be sued to summarize the peak flow computations. (Form 1)

GRAPH

FIGURE 1 – OVERLAND FLOW CURVES

(REFERENCE 4)

GRAPH

Step 7: Development of a Hydrograph

Using the peak flow generated in Step 6, the following

procedure outlines the development of the existing and proposed

hydrographs. (See Form 2).

1. Plot the peak flow calculated in Step 6 at a time to

peak equal to the time of concentration (Tc)

determined in Step 5.

2. Estimate length of hydrograph base (Tb) as three times

the time of concentration (Tc).

Tb = 3 Tc

3. Divide the peak flow (Qp) by the drainage area in

square miles.

4. Enter Figure 3 for the width of the hydrograph at

fifty percent (50%) and at seventy-five percent (75%)

of the peak flow.

5. Divide the widths at fifty percent (50%) and at

seventy-five percent (75%) of the peak flow such that

one third occurs before and two thirds occurs after

the peak.

6. Sketch the total hydrograph from the above points.

7. Determine area between curves.

8. Estimate required volume.

9. Allowable peak discharge equals existing peak.

FORM 1 Project Name: ____________________

DETENTION POND PEAK FLOW COMPUTATION SUMMARY SHEET

Rational Formula Q=CiA

PARAMETER EXISTING

CONDITIONS PROPOSED CONDITIONS

1. Design Storm Frequency 10 Year Design 25 Year Design 2. Tributary Area A = Acres A = Acres 3. Runoff Coefficient C = C = 4. Time of Concentration Tc = Min. Tc = Min. 5. Rainfall Intensity i = In./Hr. i = In./Hr. 6. Peak Flow Q10 = CFS Q25=

= CFS

NOTE: Allowable Discharge = Acres x 0.20 CFS / Acre = CFS

PROCEDURE:

1. Design Storm (Frequency Given).

2. Delineate and Planimeter Tributary Area.

3. Determine Runoff Coefficient, See Tables 1 & 2.

4. Determine Time Of Concentration For Site, See Figure 1,

May Require Sewer Velocity Computations

5. Enter Figure 2 With Time Of Concentration To Determine Rainfall

Intensity, 10 Year Design - Existing, 25 Year Design - Proposed.

6. Calculate Peak Flow Using Rational Formula, Q = CiA

7. Calculate the Allowable Discharge from the site: (.20 CFS / Acre)

DETENTION POND

III. DETENTION POND

A. Preliminary Design

Based on the hydrographs developed in Chapter II, Runoff

Hydrographs, a preliminary estimate of detention pond volume can

be made. This estimate is made by plotting both the existing

condition hydrograph and the proposed condition hydrograph on a

common scale and determining the area between the two

hydrographs. This area represents the volume of storage that is

required assuming that the final detention pond discharge

hydrograph resembles the existing condition hydrograph (See Step

8, Form 2).

B. Detention Pond Layout

Following the preliminary estimate of volume, the designer

must prepare a detention pond layout plan. Typically, this

would be done on a topographic map that would show both the plan

and the grading of the proposed pond. From this plan and

grading layout, a detention pond storage-elevation curve can be

developed. See Form 3 for the procedure. This curve is

required for the detailed flood routing computation that will be

conducted to verify the operation of the proposed system. (Form

3)

OUTFALL STRUCTURE

IV. OUTFALL STRUCTURE

A. Preliminary Design

Following the generation of inflow hydrographs and a

preliminary layout of a detention pond, an outfall structure

must be selected. The hydraulic characteristics of the outfall

structure are summarized on a discharge-rating curve for use in

the flood routing analysis of the detention pond.

The hydraulics of the discharge structure must be such that

under the maximum available head condition, the outflow does not

exceed the peak flow for existing conditions. The maximum

available head condition typically occurs when the detention

pond is full and the outlet is not subject to backwater

restrictions. It is also desirable for the outfall hydrograph

to be similar to the existing condition inflow hydrograph.

Several types of outfall structures can be designed or

utilized including pipes, weirs, drop inlets, or combinations of

pipes and weirs. The Design Engineer must select the type of

outfall structure best suited for his particular installation

and the outflow characteristics he is attempting to achieve.

B. Hydraulics of Outfall Structure

Once the type of structure is selected, the hydraulic

characteristics need to be established. The maximum discharge

must not exceed the maximum of the existing inflow hydrograph.

Ideally, the detention pond outflow hydrograph should be similar

to the existing inflow hydrograph.

1. Simple Pipe Outfall Structure

The hydraulics of a typical simple pipe outlet

are a function of two controls: Case 1, the size of

the pipe inlet (orifice control); Case 2, the length

of the pipe (pipe flow control). For pipe inlet

control, the depth of water above the pipe in the pond

is controlled by the restrictive characteristics of

the inlet. This is typically analyzed using the

orifice equation. The phenomenon is similar to inlet

control in culvert hydraulics.

Pipe flow control (similar to outlet control in

culvert hydraulics) is based on a headloss analysis of

the outlet pipe, including losses for friction and

minor losses such as entrance and exit conditions.

Both orifice and pipe flow conditions must be

plotted on a common graph with the final outfall curve

being a combination of both orifice and pipe flow.

The controlling condition is that which gives the

higher pond level relative to discharge.

Form 4 presents a detailed procedure for

evaluating orifice control characteristics for a

simple pipe. Figure 4 presents a series of orifice

pipe discharge curves that can be used to simplify the

analysis. Form 5 presents a detailed procedure for

evaluating pipe flow control characteristics for a

simple pipe.

2. Drop Inlet Outfall Structure (Circular Stand

Pipe)

The hydraulics of the drop-inlet type outfall

structure are more complex than the simple pipe

outfall. In addition to orifice control (inlet) and

pipe flow control (outlet), weir control is also a

consideration. During the initial outflow, prior to

development of an orifice flow condition, the crest of

the drop-inlet will act as a weir. In this range,

weir hydraulics will control the pond elevation and

the discharge rating curve. Whichever condition gives

the higher pond level relative to discharge is the

controlling condition.

Procedure

A. Select Outfall Structure Type

B. Evaluate Hydraulics of Structure

- Maximum discharge not to exceed peak of

existing inflow hydrograph

- Orifice Control, Pipe Control, Weir

Control

C. Plot Discharge Rating Curve

- Combination of Highest Control Elevations

D. See Form 6, Drop Inlet Hydraulics

3. Weir Outfall Hydraulics

The hydraulics of a horizontal rectangular weir are

the least complex of the alternatives illustrated. Form 7

presents the procedure for generating a discharge rating

curve for this type of structure. Figure 5 can be used to

simplify the computations.

FORM 4 PROJECT NAME: _____________________

DETENTION POND SIMPLE PIPE HYDRAULICS

CASE 1, ORIFICE CONTROL

PIPE DATA D = DIAMETER (INCHES) A = PIPE AREA (SQ. FT.) I.E. = INVERT ELEVATION

PIPE DATA D= __________ " A= __________ SQ.FT. I.E.= __________ FT. G= 32.2

(1)

H (FT)

(2)

Q (CFS)

(3)

WSU

DISCHARGE FORMULA FOR H GREATER THAN D Q = 0.6 A√(2Gh) = (C.F.S.) h= H - (D/24) = FT. G = 32.2 FT/SEC2 Q = C.F.S. DISCHARGE FORMULA FOR H LESS THAN D Q = 3L (H3/2) = (C.F.S.) (Weir Flow) H = FT. Q = C.F.S. L = D/12 = FT. PROCEDURE BY COLUMN (1) ASSUME VALUES FOR H. (2) CALCULATE Q BY FORMULA

OR USE DISCHARGE CURVES (3) DETERMINE UPSTREAM ELEVATION

WSU = I.E. + H (4) PLOT DISCHARGE RATING CURVE

FORM 5

PROJECT NAME: DETENTION POND

SIMPLE PIPE HYDRAULICS CASE 2 – PIPE FLOW CONTROL

PIPE DATA D = DIAMETER (INCHES) A = PIPE AREA (SQ. FT.) I.E. = INVERT ELEVATION WP = WETTED PERIMETER N = ROUGHNESS L = LENGTH (FEET)

WP = L = N =

(3)

ENT (FT)

(5)

WSD (FT)

PIPE DATA D = IN A = SQ.FT. I.E. = FT.

(1) Q(CFS)

(2)

V (FPS)

(4) HF

(FT)

(6)

WSU

G= 32.2

PROCEDURE BY COLUMN (1) ASSUME DISCHARGE VALUES (2) COMPUTE PIPE VELOCITY; V = Q/A (ASSUME PIPE FULL) (3) CALCULATE ENTRANCE LOSS; ENT = 0.5 V2/2G; G = 32.2 FT./SEC.2 (4) CALCULATE FRICTION LOSS; HF = 0.45 V2N2L/R4/3 R=A/W P= R4/3= HF=V2 (5) ASSUME DOWNSTREAM ELEVATION = ELEVATION OF CROWN OF PIPE = WSD (6) DETERMINE UPSTREAM INLET ELEVATION; WSU = ENT + HF + WSD (7) PLOT DISCHARGE RATING CURVE

FORM 6: (Page 2 of 3)

OUTFALL HYDRAULICS – CIRCULAR DROP INLET

C. PIPE FLOW CONTROL (NO BACKWATER)

(1) (2) (3) (4) (5) (6)

Q (CFS) V (FPS) ML (FT) HF (FT WSD WSU

PROCEDURE BY COLUMN:

(1) Assume discharge values (ie, 0.00, 0.10, 0.20, ETC)

(2) Compute pipe velocity (V=QiA)

(3) Calculate minor losses

(Entrance = 0.5 V2 / 2G) + (90° Bend = 0.75 V2 / 2G) = 1.25 V2 / 2G

(4) Calculate friction loss, HF = 0.45 V2 N2 LP / R1/3

WP = Wetted pipe Perimeter _______, R = A / WP = _________

R 1/3 = ___________, HF = V2 ________

(5) Assume downstream elevation (WSD) = Elevation of crown of pipe

(6) Determine upstream elevation WSU = ML + HF + WSD

(7) Plot the discharge rating curve (Page 3 of 3)

FLOOD ROUTING

V. FLOOD ROUTING

A. Background and Procedure

Following the generation of hydrographs, the preliminary

layout of a detention pond, and selection of an outfall

structure, the design must be checked to assure that it

functions as intended. Flood routing calculations are performed

to provide this check.

Flood routing is an accounting technique that inventories

flow at the site. The basic principal of the technique is as

follows:

INFLOW - OUTFLOW = STORAGE (Equation 2)

Flood routing is typically performed on a time interval

basis for the duration of the storm. A convenient time

interval, DT is selected. The time interval must be short

enough to adequately account for hydrograph peaks. The

detention pond storage for time interval DT is:

(Equation 3)

[INFLOW (FT3) - OUTFLOW (FT3)] DT (SEC) = STORAGE (FT.3) SEC SEC

If the beginning of the interval is designed by subscript

“1" and the end of the interval is designated by subscript “2",

the following relations can be used to express storage, inflow,

and outflow for an interval DT

DS (change in storage for DT) = S2 - S1

Inflow (average rate of inflow x DT) = I1 + I2 DT 2

Outflow (average rate of outflow x DT) = 01 + 02 DT 2

Substitution of these relations into Equation 3 gives:

I2+I1 ∆T - 01+02 DT = S2 - S1 (Equation 4) 2 2

Typically, quantities, I1, 01 and S1 are known at the

beginning of a storm event. For example; inflow, outflow and

storage would normally be zero at the beginning of an event.

Assuming an inflow hydrograph has been developed, I2 would also

be known.

Grouping the known values together, Equation 4 can be

rewritten as:

I1 + I1 + 2S1 - 01 = 2S2 + 02 (Equation 5) DT DT

Although S2 and 02 are unknown, they are related by the storage-

elevation relationships. Knowing the storage-elevation

relationships of the proposed pond (See Storage-Elevation Curve)

and the discharge-elevation characteristics of the outflow

structure (See Discharge-Rating Curve), a graph can be generated

relating:

2S + 0 versus 0

DT

Form 8 presents the procedure for generating this Outfall

versus Storage Curve.

The flood routing computations can now be undertaken by

solving Equation 5 for successive time intervals DT. The result

will be the ordinates of the outflow hydrograph routed through

the proposed detention pond and proposed outfall structure.

The use of this procedure is further discussed by an

example in the following section using Form 9, Detention Basin

Routing.

FORM 8

FLOOD ROUTING COMPUTATIONS PROJECT NAME ____________________

FLOOD ROUTING COMPUTATIONS OUTFLOW VS. STORAGE

PROCEDURTE BY COLUMN: (1) Pond elevation, increments from bottom (2) Outflow at the pond elevation (See discharge – rating curve) (3) Outflow at pond elevation, (See storage – elevation curve) (4) Calculate 2S / T, T = 300 seconds (5) Calculate (2S / T) + O (6) Plot outfall VS storage relation, Column (2) VS Column (5)

(1) (2) (3) (4) (5)

POND

ELEVATION

O

OUTFLOW

(CFS)

S

STORAGE (FT3)

2S / DT

CFS

(2S / DT) + O

(2S

/DT)

+O

O = OUTFLOW (CFS)

DETENTION BASIN ROUTING (FORM 9)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

Routing Interval

Inflow Time Min.

T1 Min

T2 Min

I1 Inflow CFS

I2 Inflow CFS

2S1 DT CFS

01 CFS

2S2 + 02 DT CSF

02 CFS

2S2 DT CFS

S2 C.F.

Procedure by Column (1) Number of Interval, for Accounting Only (2) Inflow time from Inflow Hydrograph (3) Time at Beginning of Interval (4) Time at End of Interval (5) Inflow at Beginning of Interval (6) Inflow at end of Interval Note: (2) to (6), From Inflow Hydrograph

(7) Calculate 2S1 (8) Outflow at T1, T1 (initial Interval) = 0.0 (9) (5) + (6) + (7) - (8) = (9), Equation 5 (10) Outflow at T2 (from Outflow-Storage Curve)(11) (9) - (10) = (11) (12) Calculate Storage at T2, S2 = (11) x DT 2

DESIGN EXAMPLES

B. Review of Results

Following the flood routing computations, the Engineer

should review the results to assure that the detention pond

maximum outflow and the outflow hydrograph is consistent to the

allowable limits as determined by the existing condition inflow

hydrograph.

The results should also be reviewed for reasonableness and

overall performance in relation to the site development.

Example Problem 1

Problem:

A commercial development of 10.00 acres is being

proposed. The drainage plan for the site must meet

the Monroe County, Michigan criteria for design of

detention facilities for developing areas. The

designer must submit for review a detention pond

design and appropriate flood routing calculations.

Given:

Existing

Proposed

Area (acres

10.00

10.00

Runoff Coefficient

0.25

0.80

Time of Concentration

(min.)

40.00

25.00

Solution:

1. Generate Peak Flows, Form 1.

2. Estimate Detention Volume, Form 2.

3. Layout Detention Pond and Generate Storage-Elevation Curve,

Form 3.

4. Selected Outflow Structure and Generate Discharge Rating

Curve(Use Simple Pipe, Form 4, Form 5, and Figure 4.)

5. Generate Outflow-Storage, Flood Routing Curve, Form 8.

6. Conduct Flood Routing Using Form 9.

7. Review Results.

EXAMPLE PROBLEM #1

FORM 1

PEAK FLOW COMPUTATION

SUMMARY SHEET

RATIONAL FORMULA

Q = CiA

PARAMETER

EXISTING CONDITIONS

PROPOSED CONDITIONS

1. Design Storm

Frequency

2. Tributary Area

3. Runoff

Coefficient

4. Time of

Concentration

5. Rainfall

Intensity

6. Peak Flow

10 - Year

A = 10.00 Acres

C = 0.25

Tc = 40 Min

i = 2.5 In/Hr.

Q = 6.2 CFS

25 - Year

A = 10.00 Acres

C = 0.80

Tc = 25 Min

i = 3.9 In/Hr.

Q = 31.2 CFS

PROCEDURE

1. Design storm (frequencies given)

2. Delineate and planimeter tributary area

3. Determine runoff coefficient, See Tables 1 & 2

4. Determine time of concentration for site, see Figure 1, may

require sewer velocity computations

5. Enter Figure 2 with time of concentration to determine

rainfall intensity 10-year existing, 25-year proposed

6. Calculate peak flow using rational formula: Q = CiA

I1 + I2 + 251 - 01 = 2S2 + 02 DETENTION BASIN ROUTING (FORM 9) DT DT EXAMPLE

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)

ROUTING INTERVAL

INFLOW TIME MIN

T1 MIN

T2 MIN

I1 INFLOW CFS

I2 INFLOW CFS

2S1 DT CFS

01 CFS

2S2 + 02* DT CFS

02 CFS

2S2 DT CFS

S2 C.F.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

0

1.9

4.2

8.2

13.8

31.2

24.2

15.6

12.0

9.1

7.0

5.3

3.8

2.4

1.2

0.0

0.0

0.0

1.9

4.2

8.2

13.8

31.2

24.2

15.6

12.0

9.1

7.0

5.3

3.8

2.4

1.2

0.0

0.0

0.0

0.0

0

1.9

7.3

17.5

38.0

76.0

121.8

150.6

166.7

176.0

179.9

179.8

176.6

170.8

162.7

152.4

141.1

130.1

0

0.0

0.7

1.5

2.7

4.3

5.3

5.7

5.8

6.0

6.2

6.2

6.1

5.9

5.8

5.7

5.6

5.4

1.9

8.0

19.0

38.0

80.3

127.1

156.3

172.5

182.0

186.1

186.0

182.7

176.7

168.5

158.1

146.7

135.5

124.7

0.0

0.7

1.5

2.7

4.3

5.3

5.7

5.8

6.0

6.2

6.2

6.1

5.9

5.8

5.7

5.6

5.4

5.3

1.9

7.3

17.5

35.5

76.0

121.8

150.6

166.7

176.0

179.9

179.8

176.6

170.8

162.7

152.4

141.1

130.1

119.4

285

1095

2625

5295

11400

18270

22590

25005

26400

26985

26970

26490

25620

24405

22860

21165

19515

17910

PROCEDURE BY COLUMN (1) Number of Interval for Accounting Only (2) Inflow time from inflow hydrograph (3) Time at beginning of interval (4) Time at end of interval (5) Inflow at beginning of interval (6) Inflow at end of interval. Note: (2) to (6) from inflow hydrograph

(7) Calculate 2S1, _T = Routing interval (Sec) or from (11) T S, (initial interval) = 0.0 (8) Outflow at T1, T1 (initial interval) = 0.0 (9) (5) + (6) + (7) - (8) = (9), Equation 5 (10) Outflow at T2 (From outflow-storage curve) (11) (9) - (10) = (11) (12) Calculate storage at T2, S2 = [(11)xDT] / 2

Results, Example Problem 1

The results of the flood routing analysis indicates that a

maximum discharge of 6.2 cfs is generated and a maximum storage

capacity of 26,935 c.f. is required. The elevation in the

detention pond at maximum storage is 87.8 which allows 2.2 feet

of free board on the pond. The discharge velocity from the 10-

inch pipe at maximum flow is 114 fps, therefore some erosion

protection should be provided downstream. Not that 6.2 cfs

discharge from the retention pond is equal to the discharge from

a ten year rainfall on the undeveloped site.

Example Problem 2

Problem:

As an alternate design to Example Problem 1, the designer is

considering a wet design detention pond to improve the

landscaping characteristics of the site. To maintain a level in

the detention pond, he will use a drop inlet structure. Also,

he has agreed to improve the downstream receiving ditch so that

an allowable outflow of 10 cfs can be discharged from his site.

Design a circular pipe drop inlet with a crest elevation of 85.0

to maintain 3.0 feet of water in the pond at all times. Perform

flood routing computations to verify the operation of the

system.

Given:

- Proposed Inflow Hydrograph, See Problem 1

- Allowable Discharge Equals 10.0 cfs

- Pipe Length Equals 50 feet

- Storage-Elevation Curve

DETENTION BASIN ROUTING (FORM 9)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

Routi

ng

Inter

val

Inflo

w

Time

Min

T1

Min

T2

Min

I1

Inflo

w

CFS

I2

Inflo

w CFS

2S1

DT

CFS

01

CFS

2S2 + 02

DT

CFS

02

CFS

2S2

DT

CFS

S2

C.F.

1

2

3

4

5

6

7

8

9

10

11

0

5

10

15

20

25

30

35

40

45

50

0

5

10

15

20

25

30

35

40

45

50

5

10

15

20

25

30

35

40

45

50

55

0

1.9

4.2

8.2

13.8

31.2

24.2

15.6

12.0

9.1

7.0

1.9

4.2

8.2

13.8

31.2

24.2

15.6

12.0

9.1

7.0

5.3

0

1.6

5.7

13.7

29.2

65.3

109.3

136.0

149.9

157.1

159.1

0

.3

1.7

2.7

3.8

5.1

6.3

6.8

6.9

7.0

7.1

1.9

7.4

16.4

33.0

70.4

115.6

142.8

156.8

164.1

166.2

164.3

.3

1.7

2.7

3.8

5.1

6.3

6.8

6.9

7.0

7.1

7.0

1.6

5.7

13.7

29.2

65.3

109.3

136.0

149.9

157.7

159.1

157.3

240

855

2055

4380

9795

16395

20400

22485

23565

23865

23595

12

13

14

15

16

17

18

19

20

55

60

65

70

75

80

85

90

95

55

60

65

70

75

80

85

90

95

60

65

70

75

80

85

90

95

100

5.3

3.8

2.4

1.2

0.0

0.0

0.0

0.0

0.0

3.8

2.4

1.2

0.0

0.0

0.0

0.0

0.0

0.0

157.3

157.5

144.9

134.8

122.6

109.7

97.4

85.6

74.3

7.0

6.9

6.9

6.8

6.6

6.3

6.0

5.8

5.5

159.4

151.8

141.6

129.2

116.0

103.4

91.4

79.8

68.8

6.9

6.9

6.8

6.6

6.3

6.0

5.8

5.5

5.1

152.5

144.9

134.8

122.6

109.7

97.6

85.6

74.3

63.7

22875

21735

20220

18390

16455

14610

12840

11145

9555

Procedure by Column

(1) Number of Interval, for Accounting

Only

(2) Inflow time from Inflow Hydrograph

(3) time at beginning of Interval

(4) Time at end of Interval

(5) Inflow at beginning of Interval

(6) Inflow at end of Interval

Note: (2) to (6), from Inflow

Hydrograph

(7) Calculate

(8) Outflow at T1, T1 (Initial Interval) =

0.0

(9) (5) + (6) + (7) - (8) = (9), Equation 5

(10) Outflow at T2 (from Outflow-Storage

Curve)

(11) (9) - (10) = (11)

(12) Calculate Storage at T2, S2 = (11) x

DT

2

Review Results, Example Problem 2

With a drop inlet type outfall structure, the discharge rating

curve is controlled by weir and orifice hydraulic properties.

The maximum required storage is 23,865 c.f. This corresponds to

an elevation of 88.3 which provides 1.7 feet of freeboard. The

maximum discharge from detention is 7.1 cfs which is below the

allowable of 10.0 cfs from this problem. The associated maximum

discharge

velocity is 9.0 fps. Some type of erosion protection should be

provided.