By Jon Witter, Jessica D’Ambrosio and Justin McBride
A grant program through H2Ohio was recently announced by the Ohio Department of Agriculture to support the installation of two-stage ditches in counties draining to the western basin of Lake Erie. The program will be administered by the Ohio Department of Agriculture through the County Soil and Water Conservation Districts and Northwest Ohio County Engineers Offices. Contact your local Soil and Water Conservation District or County Engineer for more information about the program.
This program represents a significant investment in infrastructure that integrates the benefits of conservation and protection of water quality with the need for reliable drainage. We briefly outline ditch management approaches in the following article along with very basic information on potential trade-offs when considering a conservation channel design versus a traditional (trapezoidal) ditch design.
Ditch design options
The traditional trapezoidal ditch design is a good solution for surface drainage and works well in most applications if it stays well vegetated, provides adequate drainage capacity, and does not undergo frequent maintenance.
Drainage ditches that exhibit chronic bank erosion or failure, overflow too frequently, or require excessive maintenance may be good candidates for the design of a conservation channel, either a two-story ditch or a ditch. two-stage self-forming. Conservation channel designs have been developed to mimic the benefits of wetlands and floodplains in ditch banks while improving the drainage capacity needed for agricultural production and flood protection.
In ditches with adequate width, slope, and drainage area, a two-story ditch has sediment transport dynamics that mimic natural stream systems allowing for a better balance between sediment movement and deposition along the length of the ditch, which reduces siltation of the ditch bed and blockage of tile outlets.
Floodplains and water quality
One of the benefits of small floodplains in a two-story ditch is their ability to process and retain some of the nutrients that might otherwise be transported downstream to rivers and lakes. Think of banks or floodplains as small buffer strips or long linear wetlands at the bottom of the ditch that flood frequently as the water level rises. Additionally, tile spillage that can occur for several days or weeks after a rainfall event spills onto vegetated floodplains in a two-stage ditch providing additional nutrient processing that normally bypasses field edge buffers through rows of buried tiles. Additionally, two-stage floodplains treat runoff from the entire watershed above, not just a small portion of the adjacent field during and shortly after a rainfall event. In unstable ditches, floodplains also help stabilize and protect against bank failures and erosion of ditch slopes.
Trade-offs to consider
Selecting a conservation channel design approach involves trade-offs that must be considered during the planning and design phases of a project. To make room for small floodplains, a two-stage design requires more space to implement. Depending on the existing topography and drainage area of the watershed, this may be only a few extra feet in width or as much as tens of feet. The best and most cost-effective projects typically minimize earthworks volumes and avoid disturbing existing high-quality natural aquatic habitats.
A two-story ditch will have greater capacity and better handle frequent but intense rain and runoff events; however, these drainage improvements are likely to have little impact on the more extreme flood events that occur infrequently. The increased capacity is expected to improve field traffic conditions and crop growth in wet years.
Although a larger ditch may provide better drainage, some drainage maintenance programs may have concerns about the ability to maintain these drainage improvements (i.e. finally, the construction costs of the two-stage design are generally higher than a traditional maintenance activity due to increased earthworks; however, the grant program will cover up to 100% of the costs State and local conservation professionals can provide information and recommendations to help landowners and ditch maintenance personnel make informed decisions about the drainage design options best suited to their operations.
This column is provided by the OSU Department of Food, Agricultural and Biological Engineering, OSU Extension, Ohio Agricultural Research & Development Center and the College of Food, Agricultural and Environmental Sciences.