Drainage System Design and Layout

Design Process Flowchart Background Information (Soils, Topo, Crops) Confirm Outlet Drainage Needed Select DC, Spacing & Depth Determine Grades & Depth Develop System Layout Determine Drain Sizes Installation NO

Design Process Flowchart Background Information (Soils, Topo, Crops) Confirm Outlet Drainage Needed Select DC, Spacing & Depth Determine Grades & Depth Develop System Layout Determine Drain Sizes Installation NO

Drainage Outlets

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 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 Design Curves

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.

Drainage CFS/ Acre In/Day Curve 100 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. Design Curves

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.

Outlet Ditches 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.

Hydraulic Gradeline

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).

Manning Routine

Design Process Flowchart Background Information (Soils, Topo, Crops) Confirm Outlet Drainage Needed Determine Grades & Depth Develop System Layout Determine Drain Sizes Installation NO Select DC, Spacing & Depth

Drainage Coefficient

Impermeable or Restricted-Flow Layer h  d S K K

Design for uniform depth throughout system (depends on layout) Depth will of course vary on flat and rolling topography Drain Spacing & Depth

Depth-Spacing Relationships

Drain Layout Comparison

Drainage Costs

 100 Tile Density Profitability Cost/AcreCrop YieldRate of Return Cost or Yield Ratio (%)‏ Spacing

Design Process Flowchart Background Information (Soils, Topo, Crops) Confirm Outlet Drainage Needed Select DC, Spacing & Depth Determine Grades & Depth Determine Drain Sizes Installation NO Develop System Layout

System Layout

ContourMap

Cost Differential: $50/acre

Design Process Flowchart Background Information (Soils, Topo, Crops) Confirm Outlet Drainage Needed Select DC, Spacing & Depth Develop System Layout Determine Drain Sizes Installation NO Determine Grades & Depth

Drain Sizing Tool

Result Display

Materials

Drainage Materials

Drainage Coeff.

Drainage Coefficient

Design Criteria

10 = 3 x 6 Tile Mathematics 12/2 = 8

Sedimentation Options

Design Chart

Can be Saved

Lateral Specification

Inlets

Questions?