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Drainage

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Introduction

• Water is component of all landscape designs that cannot be ignored.

• Water issues include:1. Too much2. Not enough3. Water being at an undesirable point4. Water flowing across an undesirable point5. 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 frost

heave.

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Site Analysis

• Before starting to survey a site for drainage purposes it is important 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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Site Analysis--cont.

• Also check the map for drainage easements. – They should be labeled "d.e." on the plat and are usually located

along property lines.

– A drainage easement indicates that water will be probably flow along the easement after rainfall.

– Erosion can be a problem along drainage easements.

– Structures, fences, roads, etc. should not be constructed within drainage easements.

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Drainage

• Drainage is the natural or artificial removal of surface and sub-surface water from a given area.

• Drains can be either

surfaceor subsurface.

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Need For Drainage

• A landscape design that does not properly control runoff may cause 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 and shaped surfaces.

• Shaped surfaces– Swales– Ditches– Berms

• Surface drainage works best with small sites or for sites with a small amount of runoff.

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Subsurface Drain

• Subsurface drain is a system of collecting 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, what

are 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 it’s 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 structure or 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 of runoff 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 rates from small <20 acre areas that are relatively uniform in topography and vegetation.

• Peak runoff rates are important when sizing drainage structures, especially pipes.

• Rational method uses a simple equation:

€

Q = CIA

Q = ( )Runoff rate cfs

C = Dimensionless coefficient

I = ( / )Rain fall intensity in hr

A = ( ) Area ac

The difficulty is getting accurate values for each variable.

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Runoff Coefficient (C)

• The runoff coefficient (C) is defined as the ratio of the peak

runoff 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 the

watershed.

• Values for “C” are usually determined from tables.

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Coefficient Table

Area Description Runoff Coefficient C

Business

Downtown 0.70-0.95

Neighborhood 0.50-0.70

Residential

Single-Family 0.30-0.50

Multiunits, detached 0.40-0.60

Multiunits, attached 0.60-0.75

Residential (suburban) 0.25-0.40

Apartment 0.50-0.70

Industrial

Light 0.50-0.80

Heavy 0.60-0.90

Railroad yard 0.20-0.35

Unimproved 0.10-0.30

Parks, cemeteries 0.10-0.25

Playgrounds 0.20-0.35

Character of surface Runoff Coefficient C

Pavement

Asphaltic and concrete 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

Steep, 7 percent 0.15-0.20

Lawns, heavy soil

Flat, 2 percent 0.13-0.17

Average, 2-7 percent 0.18-0.22

Steep, 7 percent 0.25-0.35

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Rainfall Intensity (I)

• The rainfall intensity used in the rational method is based on a specific rainfall duration and recurrence interval.

• The recurrence used depends on the importance of the project.– Terraces and waterways are designed for a 10-year recurrence.

– Spillways for dams may require a design based on a recurrence interval of 100 years or more.

• The rainfall intensity can be determined from an intensity-duration-recurrence interval chart.

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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 time required 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 length and slope.

• Tables are available for TOC.

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TOC Table

Time of concentration for small watersheds (min).

Drainage way gradient (slope), %

Maximum lengt h of 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

4,000 86 66 33 27 21 15

6,000 119 91 46 37 2 9 20

8,000 149 114 57 47 36 25

10,000 175 134 67 55 42 30

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Area

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

• Watershed area can be difficult 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 says a 50 year reoccurrence interval is appropriate. The drainageway is 850 feet long and has a slope of 1.25 %.

€

Q = CIA The first step is determining the C value.

Character of surface Runoff Coefficient C

Pavement

Asphaltic and concrete 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

Steep, 7 percent 0.15-0.20

Lawns, heavy soil

Flat, 2 percent 0.13-0.17

Average, 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 to determine an appropriate value for the rainfall intensity.

• The time of concentration is used to determine the intensity.

Time of concentration for small watersheds (min).

Drainage way gradient (slope), %

Maximum lengt h of 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

4,000 86 66 33 27 21 15

6,000 119 91 46 37 2 9 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 will

cause 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 of 7 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 = CIA

= 22 x 10 x 1.25

= 275 cfs

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Mixed Watershed

• The previous example assumed that the entire watershed had the 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 =C1 x A 1( ) + C2 x A 2( ) + ... 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 % slope and 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 0.75x( ) + 0.06 3.4x( ) + 0.15 2.2x( )

0.75 + 3.4 + 2.2

€

Cw =5.25 + 0.204 + 0.330.75 + 3.4 + 2.2

=1.0596.35

= 0.167

Character of surface Runoff Coefficient C

Pavement Asphaltic and concrete 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 Steep, 7 percent 0.15-0.20 Lawns, heavy soil Flat, 2 percent 0.13-0.17 Average, 2-7 percent 0.18-0.22 Steep, 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).

Drainage way gradient (slope), % Maximum length of 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 rainfall intensity.

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Mixed Watershed Example--cont.

• With a TOC of 6 minutes and a reoccurrence interval of 100 years, the rainfall intensity can be determined from the chart.

• Rainfall intensity = 12 in/hour

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Mixed Watershed Example--cont.

• The peak runoff rate from the mixed watershed is:

€

Q = CIA

= 0.167 12 6.35x x

= 12.72.. 12.7 or cfs

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Questions?