1. Drainage System Design and Layout
Drainage systems are mainly used to improve crop yield, and to improve the timeliness of field operations. The decision to drain a field is above all else an economic decision.

2. Design Process Flowchart
Select DC,
Spacing
& Depth
Background
Information
(Soils, Topo, Crops)
Determine Drain
Sizes
Installation
Drainage
Needed
Develop System
Layout NO
This flowchart outlines the steps in the drainage system design and installation process. The steps include 1) collecting background information on soils, topography and crops, 2) determining if drainage is needed, 3) determining if there is a suitable outlet of adequate size, 4) selecting a drainage coefficient, and a suitable drain depth and spacing, 5) laying out the laterals and mains for the drainage system, 6) determining suitable depths and grades, 7) sizing the drainage pipes, and 8) installing the system.
Confirm
Outlet
Determine Grades
& Depth NO

3. Design Process Flowchart
Select DC,
Spacing
& Depth
Background
Information
(Soils, Topo, Crops)
Determine Drain
Sizes
Installation
Drainage
Needed
Develop System
Layout NO
One of the most important steps is determining if there is a suitable drainage outlet. As one contractor stated, “all of the easy jobs have already been done.” Suitable outlets might be far away, or may involved crossing one or more fields. The existence and suitability of an outlet should be determined before proceeding with the design process.
Confirm
Outlet
Determine Grades
& Depth NO

4. Drainage Outlets
A suitable outlet may be an open channel or a communal drain. These should be sized so as not to restrict drainage. Often, the limiting factor for the effectiveness of a drainage system is the capacity of the outlet. Many of the outlets in Illinois are undersized. They are based on curves that were developed before patterned tiled drainage systems were commonplace.

5. Design Curves
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.
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.

6. Design Curves
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
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

7. Drainage Outlets
Designs 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.
Designs 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

8. Design Curves http://www.wq.illinois.edu/dg/Equations/prjCapacity.exe
Drainage CFS/ Acre In/Day Curve Acres (Drainage Coefficient) B C D
For 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.
The example above shows design ditch flow rates for draining 100 acres, using each of the three recommended design curves for Illinois. See the video on using the ditch capacity tool to determine these flow rates.

9. Ditch 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
Once the capacity of the outlet channel is known, the next step is 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 outlet is typically set 1 foot above normal depth flow at the design capacity. .

10. Limiting Velocities Velocity
Soil Texture
Maximum 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 cobbles
Graded mixture silt to cobbles
5.5
Coarse gravel,
shales,
or hardpans
6.0
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.
Channels are sized based on a maximum allowable velocity. The velocity of water flow must be high enough to prevent siltation in the channel but low enough to avoid erosion. Here 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.

11. Hydraulic Gradeline
Channels should be designed so that the bottom is lower than or coincident with the hydraulic grade line of the lowest channel or drain entering it. This may require a survey of the entire watershed in which the channel is contained. Special consideration should be given to pipelines or fiber optic cables. These should be identified and their profile should be mapped before designing the channel.

12. Channel Velocity
The most widely used equation for designing outlet channels was developed by Robert Manning in 1890 and is known as Manning's equation:
where
V = average velocity of flow (ft/sec),
n = coefficient of roughness,
R = hydraulic radius (ft),
s = slope of hydraulic gradient (ft/ft).
The flow velocity is linked to the channel geometry and the channel roughness through the Manning Equation. The velocity is inversely proportional to the roughness and directly proportional to the square root of the channel slope.

13. Manning Routine http://www.wq.illinois.edu/dg/Equations/Mannings.exe
The Illinois Drainage Guide contains an app, based on the Manning Equation, that can be used for sizing drainage channels. This routine can be used for channels of many different shapes, including irregular channels, defined by a maximum of 7 points.

14. Design Process Flowchart
Select DC,
Spacing
& Depth
Background
Information
(Soils, Topo, Crops)
Determine Drain
Sizes
Installation
Drainage
Needed
Develop System
Layout NO
The next step in the design process is to select the drainage coefficient, which is the rate at which water is to be removed from a field. It is a value selected to provide adequate drainage for future crops and is expressed in inches per 24 hours. Drainage design may be based on either steady state assumptions or transient assumptions.
For steady state (rainfall-based) drainage design, the depth and spacing of the drains are selected so that the water table will remain in a fixed position, usually a foot below the soil surface, during a steady rain. Since the water table is not fluctuating, the rate at which water leaves the drains is the same as the rainfall rate. This drainage rate is known as the drainage coefficient. For typical row crops (corn, soybeans, etc) a drainage coefficient of 3/8 inch per day is used.
For transient (soil-based) drainage design, the depth and spacing of the drains are selected so that the water table will be drawn down from the soil surface to one foot below the soil surface in one day, or from the surface to two feet below the soil surface in 2 days. The drain flow rate decreases over time as the water table falls and the head decreases. The depth of water removed per foot of water table fall, known as the drainable porosity, is dependent on the soil type. For typical Illinois soils the drainable porosity is approximately 3/8 inch per foot.
Confirm
Outlet
Determine Grades
& Depth NO

15. Drainage Coefficient
The Drainage Guide includes an app for selecting a drainage coefficient. Where field ditches or watercourses provide adequate surface drainage, the drainage area for which a drainage coefficient is selected need only include the area that will be drained by subsurface drains. If the slope of the field is less than 0.2 percent, the higher of the drainage coefficient ranges listed in the table should be selected.
Where surface drainage is not adequate and surface-water or blind inlets must be used to drain depressions, the drainage coefficient must be relatively high so that the drains can remove runoff from the entire watershed of the depressional area. An exception can be made where the depressions are small, as long as surveys are available and the volume of the potholes can be determined accurately. In that case, the drains should be able both to remove water at the appropriate drainage coefficient from the land area that needs drainage, and to remove the water in the potholes within 24 to 48 hours.

16. Design Depth and Spacing
Spacing.exe
The spacing and depth required to keep the water table at the desired level are influenced by the permeability of the soil, depth to the barrier, the amount and frequency of rainfall, capillary movement, and topography. Spacing and depth also influence each other. In general lateral spacing increases with drain depth for a specified drainage coefficient. Spacing and depth recommendations are given in the Drain Spacing app for specific soils in Illinois . For soils not listed in the guidelines, there are general drain spacing and depth principles.
Drains in rapidly permeable soils should be spaced 200 to 300 feet apart, while those in moderately rapidly permeable soils should be spaced 100 to 200 feet apart. Where soil permeability is moderate, spacing should be 80 to 100 feet apart. In slowly permeable or moderately slowly permeable soils, drains should be spaced 30 to 70 feet apart or 60 to 80 feet apart, respectively.
With respect to general principles for depth, drains in moderate to moderately permeable mineral soils in humid areas should be installed at a depth of 3 to 5 feet. At this depth the drains will lower the water table to not less than 2 to 4 feet. Because the upward capillary action is limited in very sandy soils, the drains should be no deeper than 4 feet. In slowly permeable clay soils, the rate of lateral water movement does not increase with depth. Therefore, the drain is usually placed approximately 1 foot below the desired water table

17. Drain Spacing & Depth Design for uniform depth throughout system
Design for uniform depth throughout system
(depends on layout)
Depth will of course vary on flat and rolling topography
When possible, subsurface drains should be placed at uniform depths. The range of grades on which they can be placed depends to some degree upon the topography of the land. The grade should be great enough to prevent silting but flat enough to prevent flow from exceeding the allowable velocity and subjecting the drain to excessive pressure. Too much flow would cause erosion around the drain. The grade should be as great as possible on flatlands. However, adequate drain depth should not be sacrificed to increase grade. The minimum grades of small drains can be evaluated with the Minimum Grade program listed above.

18. System Layout
Next to specifying the outlet, the most important step is laying out the system. The objective is to have the highest possible depth uniformity. The design should start with a contour map of the entire field. The map should be very detailed, and any surface or subsurface objects in the field should be mapped.

19. System Layout X
Normally, mains and submains are placed on the steepest slopes, and the laterals should be more on the contours. Placing the laterals parallel to the contours give greater flexibility in operating the system.

20. System Layout
Drainage systems can be installed in different patterns. Common patterns are shown here. These include parallel, herringbone, double main, and targeted systems. When planning or installing a drainage system, it’s important to properly document the system layout.

21. Cost Differential: $50/acre
System Layout
Here are two possible designs for the same field. One is optimized for the cost of installation, and the other for drainage water management. Based on comparisons between fields, there is about a $50 per acre cost differential on fields in Illinois.
Cost Differential: $50/acre

22. Design Process Flowchart
Select DC,
Spacing
& Depth
Background
Information
(Soils, Topo, Crops)
Determine Drain
Sizes
Installation
Drainage
Needed
Develop System
Layout NO
The next step is to determine depths and grades. Details of this step are outlined in one of the labs for this module. In general, slopes should not be less than a tenth of a percent, and the depth range should be kept as narrow as possible. Care should be taken to not have the drains too shallow. They should be placed below the mean annual depth of frost penetration, to prevent frost heaving.
Confirm
Outlet
Determine Grades
& Depth NO

23. Drain Sizing Tool
The online version of the Illinois Drainage Guide contains an application for sizing drains. The use of this application will be covered in a lab and in homework exercises.

24. Lateral Specification
There are other applications in the Drainage Guide, including an application to determine the allowable longest run for a pipe of a given size, and the quantities of material needed for a field of a given size.

25. Main Sizing Worksheet
During this module, you will develop a worksheet for sizing drainage pipes. This worksheet can be used for real-life applications.

26. Inlets
Surface-water inlets allow surface water to enter subsurface drains directly. Because of the high cost of carrying surface water in buried drains, inlets are recommended only for draining low areas where it is not feasible to install a surface drainage system. If it is necessary to use a surface-water inlet, non-perforated tubing or conduit should be placed on each side of the riser. Since surface-water inlets may be a source of weakness in a drainage system, they should be set off to one side of the line to reduce the hazard to the main line.
Blind inlets remove both surface and subsurface water. They are most useful in open fields because they do not hinder farming operations. Since blind inlets remove impounded water at a much lower rate than surface-water inlets, they should not be used where there is a large volume of impounded water.