Pumped outlets for subsurface drainage
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Generally speaking, surface drainage isn’t as effective as subsurface drainage for satisfying the drainage needs of many soils.
Enhancing surface drainage with subsurface, or tile, drainage can potentially improve crop productivity and farm efficiency where wet soils persist. Compared to surface drainage alone, improving tile drainage reduces runoff and peak outflow rates—in some cases, dramatically.
Tile drainage may decrease sediment, phosphorus and organic nitrogen. However, it may lead to increased losses of mobile constituents, such as nitrate nitrogen and some salts.
What pumped outlets are
Pumped outlets (Figure 1) can improve drainage at a reasonable cost in areas that lack a natural gravity outlet. A pump drainage system consists of a:
Collection system (tile or surface drainage system).
Figure 2 shows a typical pumping plant.
When to use pumped drainage
For pumped drainage to be feasible, weigh the economic benefits of improved drainage against the startup and operating costs associated with the pumping plant. Locally assess potential economic benefits, as those depend on the:
Adequacy of the current drainage system.
Crop to be grown.
Operating costs depend on the pumping plant design—in particular, size of the pump and lift—and rainfall characteristics over the drainage season.
Planning pumped outlets
Plan the drainage system of the area served by pumps to both:
Meet the area’s drainage needs.
Efficiently operate the pump.
Divert runoff from areas outside the drainage area to a suitable outlet. Protect the drained area against backwater or overflow from the outlet with perimeter dikes designed to prevent overtopping.
Pumping lift is one of the most important factors in the pumping plant’s initial and operating costs.
The pump intake depends on the tile main’s placement. The highest water level in the sump shouldn’t exceed the bottom of the tile main, as shown in Figure 1. Although higher water levels may be possible in some cases, this will usually compromise the tile system’s effectiveness.
The natural outlet’s condition determines the pump’s discharge level. If water levels in the outlet are relatively stable, you can set the discharge pipe just above the maximum anticipated water level, so water can freely discharge into the outlet.
If outlet water elevation considerably fluctuates, you can lower the outlet elevation, reducing operating costs. For this condition, install a valve to prevent backflow when the discharge pipe is submerged.
The required pump capacity must match the drainage system’s capacity so the pump can continuously operate during peak flow periods, while satisfying drainage discharge requirements. For reference:
1 acre-inch per day drainage rate = 19 gallons per minute (gpm)
1 cubic foot per second (cfs) = 448 gpm
As the growing season progresses, drainage requirements will likely decrease and the pump will run intermittently.
Centrifugal pumps are the most common type of pump type used in agricultural drainage. There are three types: Radial flow, axial flow and mixed flow.
Required discharge flow rate and head may dictate which type of pump is best suited for the pumping station. You can also use submersible pumps, as illustrated on Figure 2.
Use electric drive power units whenever you can bring electric power to the site at reasonable cost. A direct-connected, vertical-shaft motor is best for low-maintenance operation.
The sump storage volume, shape and position are important because together with pump capacity, they determine the intermittent operating characteristics of the pump.
The sump must be large enough so the pump doesn’t excessively start and stop. Storage below the minimum water level serves as sediment storage and minimum clearance for the suction pipe. There should be a bottom clearance of one-third of the suction pipe’s diameter.
The sump can be a pit, tank, section of ditch or a low area that serves as a collection point for the drainage system. Concrete silo staves or corrugated metal make inexpensive sump walls as long as a stable foundation is possible. When using a tank, securely anchor it so it isn’t pushed upward when the water table is high.
Limit pumps to 10 or fewer cycles of operation per hour for automatic operation. A cycle of operation includes both running and standing time. Running time shouldn’t be less than three minutes.
Calculate the minimum storage (S) in cubic feet for automatic operation using the below formula, where n equals the desired number of cycles per hour and Q is the pumping capacity in gallons per minute (gpm).
S in ft3 = (2 x Q in gpm) / n in cycles per hour
Then, choose the storage area and depth so their product equals or is greater than S. For an economical operation, the sump should be large and shallow, not small and deep. Storage depths of 2 feet are recommended for closed sumps and 1 foot for open sumps.
You’re designing sump storage for a pumping plant designed to serve a 100-acre tile drainage system with a drainage coefficient (water removal rate) of 3/8 inch per day. The pump will run at eight cycles per hour.
The flow rate for the drainage system (Q) is 3/8 inch per day × 18.9 gpm per acre × 100 acres = 709 gpm.
Using the formula (2 feet × Q in gpm) / n in cycles per hour, the required sump storage volume = (2 × 709) / 10 = 142 cubic feet.
With a storage depth of 2 feet and a circular sump, the sump diameter must be √(142 × 4)/(2 × 3.14) = 9.5 feet
Corey, A.T. (1981). Pumped outlets for drainage systems. Transactions of the ASAE, 24(6): 1504-1507.
American Society of Agricultural Engineers (ASAE). (1998). Design of agricultural drainage pumping plants (EP369.1 DEC94., pp. 830-836).
Larson, C.L., & Allred, E.R. (1956). Planning pump drainage outlets. Agricultural Engineering, 37(1), 38-40.
U.S. Department of Agriculture Natural Resource Conservation Service (NRCS). Minnesota Drainage Guide (1984).
Reviewed in 2018