Tackling Drainage Issues for Vegetables and Agronomic Crops

James Adkins, Associate Scientist-Irrigation Engineering; adkins@udel.edu

The recent rainfall events have identified countless deficiencies in drainage across the region. While corrective measures are too late for many crops, there are some in-season options to help drain standing water and reduce disease potential.

Land Leveling
Leveling a field with a tool similar to a Rayne Plane or other land leveler prior to cropping can typically resolve minor puddling issues where the difference in elevation is 6 inches or less. If more than 6 inches of fill is needed this will not be a good option as too much top soil will be removed from the areas surrounding the depression and yield loss will occur. Land levelers are typically run on a diagonal to normal field operations and multiple passes may be required to achieve optimal results.

Surface Drainage
Most farms already utilize some form of surface drainage whether natural or manmade. Field operations including tillage and particularly laying beds for vegetable production often limit the natural flow paths for surface water. Trenches or periodic breaks in a vegetable bed should be put in place just after the beds are formed to allow water to escape as quickly as possible. After a major rain some shovel maintenance of the trenches will be necessary to maximize flow. Likewise, spinner ditches made with a rotary trencher or middle buster plow will likely require a shovel touchup if they have been crossed by a sprayer or planter. Swales or trenches with high flow rates and/or slope should employ a pipe, tile or stone at the outlet to minimize ditch bank erosion. Swales permit a much deeper flow path without the interruption to field operations however it may be necessary to seed the swale to grass to prevent gullying.

Tile Drainage
At this point most of the standing water is the result of a Delmarva Bay, or a deep depression in the middle of the field with no practical way to surface drain. Tile wells are the common solution that can be installed in the cropping season if the affected area is justifiably large. Inlets can be the traditional concrete type, slotted or perforated risers or a blind inlet consisting of stone and filter cloth that can be farmed across. The first step to determine if a tile well is a viable option is to assess whether there is an outlet with adequate elevation difference to successfully drain the depression. This is best determined with a transit, laser or gps survey equipment in coordination with the NRCS for survey and engineering support. For an initial assessment, Google Earth Pro (free download) has a feature where you can a draw a proposed flow path using the path measure tool and check the box labeled “show elevation profile” to view the elevation across the drawn path. https://www.google.com/earth/download/gep/agree.html

Ideally, the tile outlet in the ditch will have at least 1’ of free board above the normal ditch bottom level to prevent the tile line from silting closed and at least 0.1% grade (0.001 ft/ft or 1.2 in/100 ft of tile). Slopes as shallow as 0.02% can be accurately installed with laser and gps controlled tile machines but more slope is desirable to allow sediment to flush out of the tile. If the installation is done with an excavator or backhoe a minimum recommended grade of 0.1% should be followed. The tile should have a minimum of 2’ of cover at all times to prevent damage from equipment as plastic tile is easily crushed by tractors, combines and center pivot irrigation ruts.

While 4 inch flexible drain tile is often adequate in size to drain a small depression in a 12 – 24 hour period it is best suited for a plow installation and should not be trench installed due to its tendency to snake and rise above target grade during backfilling. Ideally 20’ sticks of 6 inch or larger dual wall pipe should be used in open trenches to help achieve target grade and prevent poor installation. When connecting multiple inlets into one tile line, it is preferable to tee into the mainline with a 6-10’ stub for each inlet to prevent a failed inlet from plugging the entire system. Whenever and open trench is dug, be sure to follow the proper safety procedures to prevent injury from trench collapse.

Pattern tiling is becoming more common on Delmarva and is an effective solution to address field that traditionally would be pattern ditched. Pattern tilling typically is used to dry and entire field that suffers from a high or perched water table that leaves the field saturated for extended periods and consists of parallel rows of slotted tile that must be installed with a plow or trencher.

Many folks have accepted pumping as their final solution. While I admit to using a transfer or irrigation pump to remove surface water in a pinch; the high volume of trash and sediment is very hard on seals and plastic impellers. It is preferable to use a trash or mud pump when available. A few Delmarva farmers, including the University of Delaware Warrington farm employ electric lift stations when an adequate surface drain for tile is unavailable. This option can be expensive but is often the only solution to difficult drainage situations on the shore.

For questions regarding the legality of installing any drainage system contact your local NRCS office.

For more information on tile systems and design the University of Minnesota has an extensive fact sheet. http://www.extension.umn.edu/agriculture/water/planning/planning-a-subsurface-drainage-system/