Ditch design options

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 Lake Erie Basin. The program will be administered by the Ohio Department of Agriculture through county Soil and Water Conservation Districts and County Engineer offices in Northwest Ohio. Contact your local Soil and Water Conservation District or County Engineer for more information on the program.

This program represents a significant investment in infrastructure that integrates conservation benefits and water quality protection with the need for reliable drainage.  We briefly describe ditch management approaches in the following article along with some very basic information on potential tradeoffs when considering a conservation channel design over 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 remains well-vegetated, provides adequate tile drainage capacity, and doesn’t undergo frequent maintenance. 

Drainage ditches that have chronic bank erosion or failure, flood out too frequently, or require excessive maintenance may be good candidates for a conservation channel design, either a two-stage ditch or a self-forming two-stage ditch. Conservation channel designs were developed to mimic the benefits of wetlands and floodplains within the ditch banks while improving drainage capacity needed for agricultural production and flood protection. 

In ditches with adequate width, slope, and drainage area, a two-stage ditch has sediment transport dynamics that mimic natural stream systems allowing for a better balance between the movement and deposition of sediments along the length of the ditch, which reduces siltation of the ditch bed and blockage of tile outlets. 

Floodplains and water quality

One benefit of the small floodplains in a two-stage ditch is their ability to treat and retain some of the nutrients that might otherwise be transported downstream to rivers and lakes. Think of the benches or floodplains as small buffer strips or long, linear wetlands in the bottom of the ditch that inundate frequently as the water level rises.  In addition, tile discharge that may occur for many days or weeks after a precipitation event spills out on to vegetated floodplains in a two-stage ditch providing additional treatment of nutrients that normally bypass edge-of-field buffers through buried tile lines. Furthermore, the two-stage floodplains treat runoff from the entire watershed above it, not just a small portion of the adjacent field during and shortly after a rainfall event. In unstable ditches, the floodplains also help stabilize and protect against bank failures and erosion of the ditch sideslopes.

Tradeoffs to consider

The selection of a conservation channel design approach involves tradeoffs that should be considered during the planning and design phases of a project. To make room for the small floodplains, a two-stage design requires more space to implement. Depending on the existing topography and watershed drainage area this may only be a few additional feet of width or as large as tens of feet. The best and most cost-effective projects typically minimize earthwork volumes and avoid disrupting existing high quality natural aquatic habitats. 

A two-stage ditch will have greater capacity and better handle the frequent, but intense rainfall and runoff events; however, these drainage improvements will likely have little impact on the most extreme flood events that rarely occur. Increased capacity should lead to improved conditions for trafficability in fields and crop growth in wet years. 

While a bigger ditch may provide better drainage, some drainage maintenance programs may have concerns over the ability to maintain these drainage improvements (i.e., weed control, mowing, etc.), especially if they only have access to smaller equipment. Finally, construction costs of the two-stage design are generally higher than a traditional maintenance activity due to increased earthwork; however, the grant program will cover up to 100% of the costs. Conservation professionals at the local and state level can provide information and recommendations to help landowners and ditch maintenance staff make informed decisions on which drainage design options might best suit 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.