Written by Chris Hay, former SDSU Extension Water Management Engineer.
Subsurface (tile) drainage has become increasingly popular in eastern South Dakota in recent years. Increasing trends in precipitation, high agricultural commodity prices, rising land prices and the advent of computerized and GPS guided drain installation equipment have all contributed to the increased interest in subsurface drainage. Subsurface drainage is needed on some South Dakota soils to provide adequate root zone aeration and allow for timely field operations. Good design, along with quality materials and installation, helps ensure the drainage system will function effectively with minimal maintenance.
When beginning a drainage design, it is useful to gather background information on soils, topography, climate, and crops. A good topographic survey, in particular, is important in designing a drainage system. A topographic map will help identify elevations of potential outlets, the general grade of the field, and identify high and low spots that could present some challenges. A drainage system will only perform as well as the outlet, so it is essential that there is a suitable outlet with adequate capacity to carry the water from the drainage system and that meets all legal and regulatory requirements for drainage outlets. Neighbor relations may be impacted by drainage, so it is also important to discuss drainage plans with neighbors. Drainage problems are often not limited to a single property, so working with neighbors to address drainage problems can result in more effective solutions and less potential for disputes.
The drainage system should be designed to remove excess water from the active root zone within 24 to 48 hours of a large rain in order to protect crops. The rate at which the drainage system can remove water from the soil is commonly called the drainage coefficient, which is usually expressed as the depth of water removed in a day. For pattern tiled systems, the drainage coefficient is determined by the drain depth, drain spacing, and soil properties. For single, targeted drain lines, the drain depth and soil properties will determine the effective distance from the drain that will be adequately drained. The layout of the drainage system will determine the uniformity of drainage for the field or area. Drainage system layout is chosen to best match field topography and outlet location. Topography will dictate what layout options are practical.
Drainage systems should be designed such that both minimum and maximum grade recommendations are followed. This is to ensure that flow velocities are within an acceptable range. The grade should be sufficient to prevent sediments from accumulating in the drains and shallow enough to prevent excessive pressure that could result in erosion of soil around the drain. In addition to minimum grades, the use of drain envelopes or "socks" should be considered for soils high in fine sands and silts, particularly if shallower grades must be used. The grade on which the pipe is laid, the area to be drained, the drainage coefficient, and pipe material will determine the size of drainage pipe needed. There are a number of resources for determining pipe sizes including manufacturer's literature, online and slide calculators, and charts and tables.
This is only a brief introduction to some of the considerations that go into a good drainage design. More information on drainage design can be found in the Illinois Drainage Guide and Planning An Agricultural Subsurface Drainage System.
Additionally, SDSU Extension, in cooperation with University of Minnesota Extension and NDSU Extension, hosts a series of drainage design workshops in the winter that provide in-depth training on drainage design.