Promoting Naturalized Drainage Channel Design
In September of 2003, a 2,400 feet stretch of the Crommer Ditch in the Hillsdale County, MI portion of
the St. Joseph Watershed was modified to a ditch design created by Ohio State University and Ohio
Department of Natural Resources and funded by The Nature Conservancy and the Great Lakes Commission.
Two stage ditches function differently from conventional trapezoid designs by increasing ditch stability,
improving drainage and ecological function, decreasing sediment and nutrient transport, and reducing
maintenance.
The modified two-stage ditch has a primary channel sized to handle bank-full flows, and a new low-level
vegetated bench that will accommodate higher flows (Figure 1). The resulting design might be visualized as
“a stream within a ditch,” where the primary channel is allowed to meander slightly within the constructed
floodplain corridor.
(a)
(b)
Figure 1.
The sizing and design criteria for the two-stage ditch will be based on a regional curves assessment
procedure that uses local conditions (i.e., land use, topography and climate) to define the cross-section shape
of the channel as a function of the drainage area. Once the design dimensions of the ditch have been
determined, the construction phase of the project site will begin, which will include site engineering,
excavation of ditch banks, broadening any existing second stage bench, leveling spoil and flattening the bank
side slopes. Tile lines will be extended where necessary, and splash aprons and animal guards will be
installed at all existing tile outlet extensions. The newly formed berms, banks, and benches will then be
seeded and fertilized. Sampling equipment will be installed after thaw in the spring of 2004 in order to
obtain data on the performance of the ditch.
Benefits of a two-stage ditch over a conventional ditch are potentially both improved drainage function
and ecological function. Drainage benefits may include increased ditch stability and reduced maintenance.
Additionally, the two-stage ditch has the potential to create and maintain better habitat. The narrow deep
main channel provides better water depth during periods of low flow. Grass on the benches can provide
quality cover and shade. The substrate in the main channel is improved as the two-stage form increases not
only sediment conveyance but also sorting, with fines deposited on the benches and courser material forming
the bed. Two-stage ditches might also be useful in improving water quality particularly for nutrient
assimilation. Work has been initiated on the ecology of these ditches and the role of the channel and benches
in improving water quality and habitat.
This type of restoration/modification will be looked at more closely in the future as a means to find a
win/win situation when dealing with both drainage issues and the environment in agricultural watersheds.