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10. Subsurface drainage outlet considerations
The location of the outlet of a drainage system is important to the proper water-removal function of the system. Place the outlet to allow free flow (unsubmerged condition) most of the time. Typically, water flows into the lateral drain pipes, which connect to submains and a main that conveys the water toward the system outlet. The invert of the outlet pipe should be at least 1 ft below the ditch bottom (Figure 10).
At times, the field is relatively flat with not enough grade to allow gravity flow of the water toward the outlet, or the drainage ditch is not deep enough to allow the outlet to flow into it. In these cases, a pump station is needed where the main moves drainage water into a sump from which it is pumped out to remove the excess water (Figure 10). For information about design and sizing of pumps stations, see Scherer (2015).
Figure 10. Left: Subsurface drainage outlet flowing into a drainage ditch that is at least 1 ft above the ditch bottom. Right: A relatively flat field with not enough grade to allow gravity flow of the drainage water into a drainage ditch, so the drainage discharge flows into a sump from which it is pumped out into the ditch.
Scherer, T. F. (2015). Tile drainage pump stations for farm fields (AE1747). North Dakota State University Extension. Link