14 drainage

8/4/2019 14 Drainage

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Drainage


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Introduction

Water is component of all landscape designs that cannotbe ignored.

Water issues include:

1. Too much

2. Not enough

3. Water being at an undesirable point

4. Water flowing across an undesirable point

5. Frost heave

Too much water can be handled by drainage.

Not enough water can be resolved by using irrigation.

Drainage can also be used to move water from unwanted

areas. Drainage structures can be used to reroute water.

Drainage can also be used to reduce the effects of frostheave.


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Site Analysis

Before starting to survey a site for drainage purposes it isimportant to evaluate the site.

If the site adjoins a waterway, do not remove the vegetation

adjacent to and along the stream bank.

This vegetation is an essential buffer zone that will help maintain

the water quality and curb erosion problems.

Check your survey or plat for the location of nearby flood plains.

If the land is in a flood plane, it is reasonable to expect the area will

be inundated with water at some point.

It is important that no structures, especially homes, are built within

a designated flood plane.


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Drainage

Drainage is the natural orartificial removal of surface

and sub-surface water from

a given area.

Drains can be either surface

or subsurface.


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Need For Drainage

A landscape design that does not properly control runoff maycause damage to and devaluation of the property.

To prevent damage or devaluation of property, three questions

must be answered.

What is the elevation of the design property in relation to adjacent

properties. Will water run onto the property, if so, were does it enter and were

does it exit?

How will the landscape plan change the drainage at the site.

Drainage is needed to handle rooftop, driveway, and overland

run off.

Four main issues to consider when caring for soil and grass roots

are fertilization, drainage, aeration, and thatch control.


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Eight Drainage Principles

1. Water flows downhill

2. Whenever it rains you have the potential for runoff.

3. The greater the intensity of the rain--the greater the potential for

runoff.

4. Reducing the permeability of the soil increases runoff.

5. Increasing the non-permeabile area will increase runoff.6. Water or silt on walkways during, or after a rain, is an indication

of poor design.

7. A good landscape plan includes drainage in the plan.

8. Drainage plans rely upon slope, pipes, berms or other

structures to control the direction the water flows.


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Slope

Any area that is exposed to rainfall should always have some

slope to direct the flow of water.

Water will puddle on flat, horizontal surfaces.

The amount of slope varies with the surface and the conditions

of the site.

Turf areas = 2 - 3%

Paved areas = 2%

Foundations = special requirements

One recommendation is

a six inch drop within

the first 10 feet.


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Surface Drains

Surface drainage is controlling the

flow of water using slope andshaped surfaces.

Shaped surfaces

Swales

Ditches

Berms

Surface drainage works best withsmall sites or for sites with a smallamount of runoff.


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Subsurface Drain

Subsurface drain is a system ofcollecting and disposing of rain water.

Common means of collection are a drain

grate or perforated pipe.


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Drain Outlet

Both surface and sub surface drains must have an outlet. Modification of existing outlets is usually not very problematic,

changing the location of an outlet may cause problems.

One alternative is to direct the water towards the street.

May require a permit.

Greater problem if the drain is a redirect and not the natural path.

Part of drainage plan that most municipalities require for

development.


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Drain Outlet--cont.

If codes do not allowed the redirection of water to the street, whatare the options?

Unless you already have a landscape drainage system in place

(allowing you to route the runoff into that system), you have two

(2) options.1. Channel the water to a location on

the site (but make sure its not a

neighbor's!) where it's less

troublesome and where it can

percolate into the ground.


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Drain Outlet-cont.

2. Build a pond and direct the water into it.

A pond may be constructed of stone or

concrete

A storm detention cell may be a

code requirement.

or natural.


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Estimating Runoff

Before a decision is made on the

type and size of drainage structureor storage structure that is needed,

the peak runoff rate and total

volume of runoff must be

determined.

The peak rate of runoff is

required when sizing

drainage channels and

pipes.

The total amount ofrunoff is needed to size a

pond.


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Estimating Peak Runoff Rates

Several different methods are available. Rational

Useful for estimating peak runoff rates from small areas.

Does not estimate volume of runoff.

USDA-NRS Technical Release 55 (TR-55)

Most popular method

Two methods

Tabular method

Graphical discharge method

US Army Corps of Engineers HEC-1 Model


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Rational Method

The rational method is useful for estimating peak runoff ratesfrom small


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Runoff Coefficient (C)

The runoff coefficient (C) is defined as the ratio of the peakrunoff rate to the rainfall intensity.

The runoff coefficient mathematically indicates whether the

runoff is likely to be high or low for the watershed.

The value of C depends on the type and characteristics of thewatershed.

Values for C are usually determined from tables.


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Coefficient

Table

Area Description Runoff Coe ffic ient CBusiness

Downtown 0.70 -0 .95 Neighbor hood 0.50 -0.70

Residenti alSingle -Family 0.30 -0.50 Multiunits, detached 0.40 -0.60 Multiunits, attached 0.60 -0 .75 Residenti al (suburban) 0.25 -0.40

A partment 0.50 -0.70 Indust rial

Light 0.50 -0.80 Heavy 0.60 -0 .90 Railroad yard 0.20 -0.35 Unimproved

0.10 -0.30

Parks, ceme t eries 0.10 -0.25 Playgrounds 0.20 -0.35

Character o f surface Runoff Coe ffic ient CPavement

Asphaltic and concret e 0.70 -0 .95 Brick 0.70 -0.85

Roofs 0.75 -0 .95 Lawns, sandy soil

Flat, 2 percent 0.05 -0.10 Average, 2 -7 percent 0.10 -0.15 St eep, 7 percent 0.15 -0.20

Lawns, heav y soil Flat, 2 percent 0.13 -0.17

Ave rage, 2 -7 percent 0.18 -0.22 St eep, 7 percent 0.25 -0.35


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Rainfall Intensity, Duration & Recurrence Interval

To find the correct

value for rainfall

intensity from the

chart, the time of

concentration must

be known.


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Time of Concentration (TOC)

The time of concentration for a watershed is defined as the timerequired for water to flow from the most remote point of the

watershed to the outlet.

The peak rate will occur when the entire watershed contributes

to the runoff.

The time of concentration is a function of drainageway lengthand slope.

Tables are available for TOC.


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Area

The area used is the number of acres in the watershed above the

outlet.

Watershed area can bedifficult to determine.

When a map is

available a planimeter

can be is used for this

purpose.

Another method is placing a grid over the map and counting

squares.

If the map is digital, mapping software can calculate area.


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Rational Method Example

Determine the peak runoff for a1- 1/2 acre lot that has grass

planted on heavy soil with an average slope of 3%. The client saysa 50 year reoccurrence interval is appropriate. The drainageway is

850 feet long and has a slope of 1.25 %.

Q = CIAThe first step is

determining the C

value.

Character o f surface Runoff Coe ffic ient CPavement Asphaltic and concre te 0.70 -0 .95 Brick 0.70 -0.85

Roofs 0.75 -0 .95 Lawns, sandy soil

Flat, 2 percent 0.05 -0.10 Average, 2 -7 percent 0.10 -0.15 St eep, 7 percent 0.15 -0.20

Lawns, heav y soilFlat, 2 percent 0.13 -0.17

Ave rage, 2 -7 percent 0.18 -0.22 Steep, 7 percent 0.25 -0.35

C = 0.18 to 0.22

Use 0.22


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Example--cont.

The next step is todetermine anappropriate value forthe rainfall intensity.

The time of concentration is used

to determine theintensity.

Time of concentration for small wat ersheds (min).

Drainage way gradient (slope), %

Maximum lengt hofflow (ft) 0.05 0.10 0.50 1.00 2.00 5.00

500 18 13 7 6 4 3

1,000 30 23 11 9 7 5

2,000 51 39 20 16 12 9

4,000 86 66 33 27 21 15

6,000 119 91 46 37 29 20

8,000 149 114 57 47 36 25

10,000 175 134 67 55 42 30

7 min

This example shows one of the problems of using tabular data.

What do you do when the data falls in between columns and/or

rows?

In this case the lower number was used knowing that this willcause the calculated peak flow to be slightly higher.

A drainageway of 850 feet at 1.25% slope =


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Example--cont.

With a TOC of7 minutes

and a 50 year interval, the

IDR graph can be used to

estimate rainfall intensity.

I = 10 in/hr


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Example--cont.

Solving for peak runoff:

Q = CI

= 22 x 10 x 1.25

= 275 cfs


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Mixed Watershed

The previous example assumed that the entire watershed hadthe same surface and slope.

This seldom happens, therefore the equation must be modified

to accommodate mixed watersheds.

This is accomplished by calculating a Weighted C.

Cw=

C1x A

1 C2 x A2 ...Cn An A


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Mixed Watershed Example

Determine the peak runoff for a watershed that consists of .75

acres of impervious surface, 3.4 acres of lawn at 1.8 % slopeand sandy soil and 2.2 acres of lawn at 0.75% slope and heavy

soil. The drainageway is 400 feet long with a slope of 1.2%.

The first step is to determine the weighted C.

Cw =0.70 x 0.75

0.06 x 3.4

0.15 x 2.2 0.75

3.4

2.2

Cw =5.25 + 0.204 + 0.33

0.75 + 3.4 + 2.2=

1.059

6.35= 0.167

r ct r f s rf c ff ffici tt

s ltic c cr t 0.70-0.95rick 0.70-0.85

Roofs 0.75-0.95Lawns, sandy soil

lat, 2 rc nt 0.05-0.10rage, 2-7 ercent 0.10-0.15

teep, 7 percent 0.15-0.20Lawns, eavy soil

lat, 2 percent 0.13-0.17verage, 2-7 percent 0.18-0.22teep, 7 percent 0.25-0.35


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Mixed Watershed Example--cont.

With a drainageway length of 400 feet and a slope of 1.2% the best

number for TOC is 6 minutes.

Time of concentration for small watersheds (min).

rainage way gradient (slope), %Maximum lengthof flow (ft)

0.05 0.10 0.50 1.00 2.00 5.00

500 18 13 7 6 4 3

1,000 30 23 11 9 7 5

2,000 51 39 20 16 12 9

The next step is to determine the time of concentration and rainfallintensity.


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With a TOC of6 minutesand a reoccurrence

interval of 100 years, the

rainfall intensity can be

determined from the

chart.

Rainfall intensity = 12

in/hour


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The peak runoff rate from the mixed watershed is:

Q = CIA

= 0.167 x 12 x 6.35

= 12.72.. or 12.7 cfs


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Questions?

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