Designing a subsurface drainage system

To protect crops, a subsurface drainage system must be able to remove excess water from the upper portion of the active root zone 24 to 48 hours after a heavy rain.

More information on excess/drainable soil water

The drainage system capacity selected for most northern Midwest farmlands should provide the desired amount of water removal per day, commonly referred to as the drainage coefficient. This figure is often between 3/8 and 1/2 inch of water removal per day.

Drainage coefficient guidelines

The tables below show drainage coefficient guidelines for crop production on land with adequate surface drainage.

Only refine these drainage coefficient guidelines after you’ve consulted with drainage experts and local drainage contractors. NRCS guidelines suggest you may need to increase the drainage coefficient if one or more of these situations occur:

• The crop has high value (e.g., sugarbeets or other vegetable/truck crops).

• Soils have a coarser texture.

• Crops have a lower tolerance to wetness.

• The topography is flat (implying poorer surface drainage).

• Large amounts of crop residue are left on a field.

• There’s little or poor surface drainage.

• Crop evapotranspiration is low.

• Frequent and low-intensity rain is common.

• Planting and harvest times are critical.

*Figures are from Chapter 14 of the NRCS Engineering Field Handbook.

*Figures are from Chapter 14 of the NRCS Engineering Field Handbook.

If you need to convey surface water to the subsurface drainage system through surface inlets, NRCS guidelines suggest using the drainage coefficients in Table 2, depending on the inlet and soil type. Apply the selected coefficient to the entire watershed that contributes runoff to the surface inlet, unless a portion of the runoff is drained differently.

The goal of drainage system layout and design is to adequately and uniformly drain a field or area. Field topography and outlet location/elevation are typically the major factors considered when planning a drainage system layout, with topography greatly influencing which layout alternatives are possible.

Creating a topographic map

It’s best to create a topographic map of the field that shows the elevations of the potential or existing outlet(s). You can use many methods to create the map, including standard topography surveys, a GPS or a laser system.

The topography map helps the designer assess overall grade and identify a field’s high or low spots that might pose challenges.

Drainage outlet guidelines

• Whether an open channel or a closed pipe, it must be large enough to carry the desired drainage discharge from a field quickly enough to prevent significant crop damage.

• Typically located three to five feet below the soil surface.

• Pumping is sometimes required to create an adequate outlet.

• Locate the bottom of an outlet pipe above the normal water level in a receiving ditch or waterway. Floods or high water levels may briefly submerge the outlet.

• Keep outlets clean of weeds, trash and rodents.

• Protect outlets from erosion, machinery and cattle damage and ice in flowing water.

Selecting the layout

Although there may be many possible layout alternatives for a given field, evaluate specific drainage goals to find the best layout. Goals include removing water from an isolated problem area, improving drainage in an entire field, intercepting a hillside seep, and so on.

Approach system layout and drainage needs in a broad, comprehensive manner, anticipating future needs where possible. Even if you install a drainage system incrementally—some this year, more next year, and so on—system planning shouldn’t be piecemeal.

It’ll be much easier to add to a system if the established mains are already large enough and appropriately located.

Positioning laterals, mains and submains

When selecting a layout pattern for a particular field or topography, orient lateral drains or field laterals with the field’s contours as much as possible. This way, laterals can intercept water as it flows downslope.

On the other hand, you can position mains and submains (also called collectors) on steeper grades or in swales to facilitate the placement of laterals.

A close relationship exists between soil permeability and the recommended drain spacing and depth. When using a system of parallel laterals, drain spacing and depth should be simultaneously considered, based on:

• Soil type.

• Soil permeability and stratification.

• The crops to be grown.

• The desired drainage coefficient.

• The degree of surface drainage.

If there’s an abrupt transition from lighter to heavier soil, it’s better to keep the drains above the heavy layer, when possible. Spacing drains closer together results in a higher drainage coefficient and faster drainage.

Distance between drains

Determining how close is close enough involves balancing costs and benefits. Simply stated, you can only justify the increased cost associated with narrower drain spacings to a point. After that, decreasing profits is the only result.

Aim for uniform depth

An ideal drainage system would have a uniform drain depth. In the real world, topography and system layout determine drains’ actual depths.

A system layout that poorly matches field topography will result in a wide variation of drainage depths and uneven field drainage. Avoid a system layout with many points of minimum cover (2 to 2.5 feet) and excessively deep cuts.

Make decisions about drain spacing and depth after consulting NRCS guidelines and talking to people in the area with drainage experience.

General spacing and depth recommendations

The following table shows general spacing and depth options you might consider during the early planning phase of a new or improved system. It recommends drain spacing in feet between drains. Drainage coefficients (fair, good and excellent) are expressed in inches per day.

The following table is an example of a recommendation from the Minnesota Drainage Guide.

Pipe capacity

The maximum amount of water a drainage pipe can carry (its capacity) depends on the pipe’s inside diameter, the grade or slope at which it’s installed and what the pipe is made of. For example, a smoother pipe has a greater flow capacity, all else being equal.

You can typically get full-flow pipe capacities for specific grades, pipe sizes and pipe materials from a number of sources:

• Drainage calculators.

• Apps and online tools from companies such as Prinsco and  ADS.

• Nomographs (charts) in the Minnesota Drainage Guide.

• Manufacturers’ literature.

• Local drainage contractors and engineers.

Flow capacity

To estimate the required flow capacity (Q) in cubic feet per second (cfs), multiply the area you want to drain by the desired drainage coefficient and divide by the conversion factor (23.8).

Q in cfs = [area in acres x drainage coefficient in inches per day] / 23.8

To use the equation in this form, area and dc must be in units of acres and inches per day, respectively.

Pipe grade, material and diameter

Once you determine Q, you can select the pipe grade, material, and ultimately the diameter to meet the required flow capacity. Topographical constraints typically determine the pipe grade, so the pipe size is selected after the material is selected (e.g., corrugated polyethylene pipe, smooth-interior pipe, etc.).

Flow velocity

Besides flow capacity, design drainage systems to provide a certain minimum velocity of flow so it self-cleans or self-scours.

If fine sands and silt are present, the minimum recommended velocity is 1.4 feet per second to prevent sediment accumulation in the system. Drainage systems in more stable soils can tolerate slower flow velocities, as low as 0.5 feet per second.

Grades are supported by the American Society of Agricultural Engineers (ASAE) EP260 standards. Flatter grades result in slower flow and run the risk of failure. Always avoid reverse grades.

Drains not subjected to fine sand or silt have a minimum velocity of 0.5 feet per second, while drains where fine sand or silt may enter have a minimum velocity of 1.4 feet per second. CPE drains refer to corrugated polyethylene plastic pipe, and smooth refers to smooth-wall plastic pipe or concrete or clay tile. 

*Recommendations for drain sizes are from the Minnesota Drainage Guide from the Natural Resource Conservation Service (NRCS).

Flow velocity example

Example: Find the flow capacity needed to drain 80 acres with a 1/2 inch per day drainage coefficient:

Q(cfs) = 80 acres x 0.5 inches per day / 23.8 = 1.7 cfs

Because excess water velocities could cause some pressure problems at drain joints or tube openings that might result in unwanted soil erosion around the drain, there are also suggested maximum grades for drain sizes and soil types. Chapter 4 of the Minnesota Drainage Guide outlines these suggestions.

Potential land area that can be drained

The following tables show the potential land area that can be drained with various grades, drain sizes, and pipe materials using 1/4-, 3/8-, and 1/2-inch drainage coefficients.

When computing drain size with any tool or chart, always round an intermediate size to the nearest larger commercially available size. For example, if a calculation calls for a 6.8-inch diameter pipe and a 7-inch pipe isn’t available, select an 8-inch pipe.

In tables 6, 7, and 8, CPE denotes corrugated polyethylene pipe (3 to 8 inches, n=0.015; 10 to 12 inches, n=0.017; more than 12 inches, n=0.02). Smooth denotes smooth-wall CPE, concrete or clay tile (n=0.01).

What are drain envelopes?

A drain envelope, or “sock,” is a material placed around a drain pipe to provide either a hydraulic function, which facilitates flow into the drain, or a barrier function, which prevents certain-sized soil particles from entering the drain.

Drain envelopes aren’t filters. Filters become clogged over time; drain envelopes do not. Many types of envelope material exist, from thick gravel and organic fiber to thin geotextiles.

Synthetic drain envelopes have a rather long useful life, provided they’re not left in the sun for extended periods or exposed to excessive ultraviolet radiation.

Do you need one?

Fine-textured soils with a clay content of 25 to 30 percent are generally considered stable and therefore don't need drain envelopes. A geotextile sock is recommended for coarse-textured soils free of silt and clay. These soils are considered unstable even if undisturbed, so particles may wash into pipes.

For intermediate soils (clay contents of 25 to 30 percent or less), it’s best to let a professional contractor or soil and water engineer determine whether an envelope is needed, because soil movement is more difficult to predict.

Subsurface tile drainage systems can convey soluble nitrate-nitrogen (N) from the crop root zone.

Implementing nitrogen fertilizer best management practices can reduce the potential loss of nitrate-N. Adding perennial crops to the rotation may also reduce N losses to surface waters and decrease drainage.

Farmers installing new or improved field drainage systems should consider using crop management practices and landscape structures that reduce nitrogen, sedimentation and water discharge rates.

Purpose of surface inlets

Surface inlets remove ponded water that forms in closed basins or potholes in a field. These inlets, however, can provide a direct pathway for surface waters that may carry sediment and other pollutants to drainage ditches and other downstream surface water.

The general public, resource managers and others are concerned about surface inlets’ potential impacts to both the quality and quantity of downstream waters.

Configurations

From a water quality perspective, almost any inlet configuration is preferable to using an open pipe that’s flush with the ground surface. Of the traditional intakes available, the slotted or perforated riser is a good option because it promotes some settling of sediments in the basin during flow events.

Farmers in some areas have begun replacing traditional inlets with blind or rock inlets. These have the advantage of being farmable, and anecdotal evidence suggests they can effectively remove water.

There are still questions, however, about the effective life of rock inlets. University of Minnesota researchers have been investigating the performance characteristics of these and other drainage practices, such as bioreactors, controlled drainage and constructed wetlands.

Why installation is important

A great deal of careful consideration goes into installing a drainage system. Drain depth, grade, pipe size and field layout are all extremely important design factors that’ll determine how well a system performs.

In addition, the installation method is key to a successful system. It’s why you should take special care to ensure every installation is on grade and of high quality.

Because quality installation is important, an experienced installer is usually an asset. It’s also important to know the equipment’s limitations.

Equipment limitations

Although pull-type and tractor-mounted drainage plows or trenchers often can adequately perform, they face limitations in the field. If these are improperly accounted for, it can result in installation and performance problems.

Field irregularities such as dead furrows, lines, swales and rocks can pose installation problems for these machines. In addition, operators have found it difficult to make cuts deeper than five feet.