FIELD DRAINAGE
The most important part of agricultural drainage is the field drainage system, which consists of a set of underground networks of drainage tiles, and in some instances surface drains. Drainage tile networks terminate at an outlet pipe within drainage channels.
The effectiveness of land drainage, and its impact on agricultural productivity and economic return, will only be as good as the condition of the tile lines and tile outlets, regardless of the condition of the drainage channel into which the tiles empty.
For best performance, tile lines should be undamaged and tile outlets should be above normal water levels in the drainage channel to assure free drainage. Any maintenance needs assessment should start with an evaluation of the function of the field drainage system.
The first point to consider is the condition of the field drainage system, including the location of existing tile lines and outlets, elevations of outlets above the bottom of channel, the number of tile outlets buried under sediment within the drainage channel, the number of tile lines that are blocked or broken before reaching the outlet.
The second consideration is the effect of fluctuations in water level in the drainage channel on the overall efficiency of the field drainage system. To determine the effect of the water elevation in the drainage channel on tile-outflow efficiency information is required on outlet flow rates for different water depths in the drainage channel.
The final thought relates to the amount of time flow of a given depth is maintained in the drainage channel relative to the elevation of the tile outlets. The depth/time relationship is defined by a stage-duration curve, which represents the percent of time flow in the drainage channel is at a given stage or depth. This information, with information on field drainage outlet elevations, is used to determine the percent of time the outlet is submerged and how much benefit will be achieved by deepening the channel.