drainage system design and layout. design process flowchart background information (soils, topo,...

Drainage Drainage System Design System Design

and Layout and Layout

Design Process FlowchartDesign Process Flowchart

Background Information

(Soils, Topo, Crops)

Confirm Outlet

Drainage Needed

Select DC, Spacing & Depth

Determine Grades & Depth

Develop System Layout

Determine DrainSizes

Installation

NO

NO

Drainage Drainage OutletsOutlets

Outlet channels designed according to Curve B will provide excellent agricultural drainage in Illinois. Use this curve for drainage of truck crops, nursery crops, and other specialty crops. Designs based on curve B will provide the best drainage that can normally be justified in agricultural areas.

Design Design CurvesCurves

Channels that are designed according to curve C will provide good agricultural drainage in Illinois. This curve is the one most often recommended for drainage of Illinois cropland


Drainage OutletsDrainage OutletsDesigns based on curve D provide satisfactory agricultural drainage as long as frequent overflow does not cause excessive damage. This curve is generally recommended for pasture or woodland. It may also be adequate for drainage of general cropland in northern Illinois, provided that the landowner carries out an excellent maintenance program. Designs based on curve D provide the minimum amount of drainage recommended in Illinois.

Drainage CFS/ Acre In/Day Curve 100 Acres (Drainage Coefficient)

B 20 4.8

C 8 2.0

D 5 1.2For comparison: For a 100 acre watershed, RCN = 75, Avg. Slope = 1% in Central Illinois

A 2 Yr., 24 Hr. Rainfall yields 1” of Runoff and would result in a Peak

Flow of 30 CFS.

A 10 Yr., 24 Hr. Rainfall yields 2” of Runoff and would result in a Peak

Flow of 70 CFS.


Ditch ConfigurationDitch Configuration

Once you know what the capacity of the outlet channel must be, you need to determine the size that will enable it to convey the desired amount of flow without letting the water surface rise above a predetermined elevation. The following sections describe some basic hydraulic concepts that will help you design a channel of the proper size

.

Outlet Outlet DitchesDitches

Velocity The velocity of water flow must be high enough to prevent siltation in the channel but low enough to avoid erosion. Listed on the next page are the maximum velocities for drainage areas of 640 acres or less. The velocity should be no lower than 1.5 feet per second. A lower velocity will cause siltation, which encourages moss and weed growth and reduces the cross section of the channel.

Soil TextureMaximum Velocity

(ft/sec)

Sand or sandy loam 2.5

Silt loam 3.0

Sandy clay loam 3.5

Clay loam 4.0

Clay or silty clay 5.0

Fine gravel, cobbles, or

graded loam to cobbles5.0

Graded mixture silt to cobbles 5.5

Coarse gravel, shales,

or hardpans6.0

Hydraulic Hydraulic GradelineGradeline

Channel Channel VelocityVelocity

The most widely used equation for designing outlet channels was developed by Robert Manning in 1890 and is known as Manning's equation:

whereV = average velocity of flow (ft/sec),

n = coefficient of roughness,R = hydraulic radius (ft),

s = slope of hydraulic gradient (ft/ft).

2

1

3

249.1sR

nv

Manning Manning RoutineRoutine




Confirm Outlet

Drainage Needed




Installation

NO

NO


Drainage Drainage Coefficient Coefficient

Impermeable or Restricted-Flow Layer

DK

Sd h hc

42

22( )

h

dS

K K

•Design for uniform depth throughout system(depends on layout)

•Depth will of course vary on flat and rolling topography

Drain Spacing & DepthDrain Spacing & Depth

Depth-Spacing Relationships

0

20

40

60

80

100

120

2.5 3 3.5 4 4.5 5

Drain Depth (ft)

Dra

in S

pa

cin

g (

ft)

3/8" Drainage Coeff icient 1/2" Drainage Coeff icient

3/4" Drainage Coeff icient

Drain Layout ComparisonDrain Layout Comparison

Drain Layout Comparison

10 358820 182530 125640 97250 80260 68870 57480 51790 460

100 452

Spacing (ft) Costs ($/acre)

Drainage Costs

Corn Soybean10 24.2 52.720 24.2 52.730 24.2 52.740 24.2 52.750 24.2 43.3560 22 33.1570 22 23.880 17.6 14.4590 15.4 5.95100 13.2 3.4

Spacing (ft)Increase Yield (bu/acre)

10 4.8720 9.5530 13.3740 16.8150 17.1660 15.8270 14.6380 10.8590 6.66100 4.42

Spacing (ft) Rate of Return (%)

100

Tile Density

Pro

fita

bili

ty

Cost/Acre Crop Yield Rate of Return

Co

st o

r Y

ield

Rat

io (

%)

Spacing




Confirm Outlet

Drainage Needed




Installation

NO

NO


System System Layout Layout

ContourContourMapMap

90.0 ft

91.5 ft

93.0 ft

94.5 ft

96.0 ft.

90.0 ft

91.5 ft

93.0 ft

94.5 ft

96.0 ft.

Optimized for Drainage Water ManagementOptimized for Cost of Installation

90.0 ft

91.5 ft

93.0 ft

94.5 ft

96.0 ft.

90.0 ft

91.5 ft

93.0 ft

94.5 ft

96.0 ft.

Optimized for Drainage Water ManagementOptimized for Cost of Installation

System LayoutSystem Layout

Cost Differential: $50/acreCost Differential: $50/acre




Confirm Outlet

Drainage Needed




Installation

NO

NO


Drain Sizing Tool Drain Sizing Tool

Result Display

Materials

Drainage Materials Drainage Materials

DrainageCoeff.

Drainage Coefficient Drainage Coefficient

DesignCriteria

10 = 3 x 6 10 = 3 x 6 Tile Mathematics Tile Mathematics

12/2 = 8 12/2 = 8

SedimentationOptions

DesignChart

Can be Saved

Lateral Specification Lateral Specification

Inlets Inlets

Questions?

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