Agricultural drainage channels serve as outlets for subsurface drainage systems (i.e., tile lines) that are common in fields with poorly drained soils. Drainage ditches traditionally have been constructed with trapezoidal cross sections (Figure 2.1A) and uniform slopes. Unfortunately, in some cases, this practice is not self-sustaining and requires regular “dipping” or “clean-out” to maintain drainage capacity or costly reconstruction of channel banks to eliminate bank failures and erosion.
In some cases, trapezoidal ditches evolve to a two-stage form developing an inset channel (i.e., the first stage) within the larger trapezoidal channel (Figure 2.1B). Research has shown the inset channel that forms is selfflushing as drainage water is concentrated within the narrow channel, keeping soil particles from depositing within the inset channel. The small floodplains, which form from deposited sediments that eventually vegetate, anchor the ditch sideslope and reduce the likelihood of failing ditch banks. Research has shown that these floodplains provide important water quality benefits.
In channels where a two-stage geometry has formed and there is a need to increase drainage capacity, the landowner may elect to widen the channel at the interface between the first and second stages (Figure 2.1C). Implementing the two-stage ditch approach eliminates or reduces the need for regular maintenance; however, it is more costly to construct initially and typically requires additional land to construct. In some states, including Ohio, cost-share programs may be available to offset construction costs. In some cases, the additional costs are offset partially or entirely by a reduction in long-term maintenance costs.