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Designing Two-Stage Agricultural Drainage Ditches

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  701P0904,  . (doi: 10.13031/2013.17443)
Authors:   Andy D Ward, Dan Mecklenburg, Anand Jayakaran, Larry Brown

The Ohio Department of Natural Resources and the Ohio State University have developed a twostage ditch design procedure in collaboration with county, state and federal agencies, faculty at other institutions in Ohio, Illinois and Minnesota, and watershed groups. The goal of this work is to develop a practical procedure that can be used throughout the Midwest to correctly size the main channel, to provide a minimum bench width to ensure stability, and to size the crosssectional capacity of the second stage to carry a design discharge to prevent over-bank flow based on a recurrence interval that satisfies local, county, watershed, or state requirements. Several demonstration designs have been installed and will be monitored to evaluate how they enhance stability, water quality, and ecological function. Highly modified channels drain extensive portions of productive agricultural land in the U.S.A. In many of these areas, most natural channels have been deepened and straightened to facilitate the flow of water from agricultural subsurface drainage outlets and to maximize conveyance. Work done periodically to maintain the drainage function typically includes removal of woody vegetation and deposited sediment, and stabilizing bank slope failures and toe scour. Drainage ditch form (pattern, profile, and dimension) was measured on ditches in Northwest Ohio. Additional measurements have been made in the Wabash River and Great Miami River Watersheds. Apparent benefits exist for incorporating fluvial process derived form into ditch construction and maintenance. To facilitate drainage, and reduce the frequency of over bank flows ditches are typically constructed such that flows as large as perhaps 5-50 year recurrence interval are contained within the ditch. The constructed ditch channel is often oversized for small flows and provides no floodplain for large flows. In response to this imbalance fluvial processes work to create a small main channel by building a floodplain or bench within the confines of the ditches. If conditions allow, these benches can reach a stable size, thickly vegetated with mostly grasses. The small main channel will often meander slightly within the ditch and is sized by nature to carry the effective discharge. Evidence and theory both suggest ditches prone to filling with accumulated sediment may require less frequent dipping out if constructed in a two-stage form. Second, channel stability may be improved by a reduction in the erosive potential of larger flows as they are shallower and spread out across the bench. Stability of the ditch bank may also be improved where the toe of the ditch bank meets the bench rather than the ditch bottom. Here the bank height is effectively reduced and the shear stresses on the toe of the bank are less. The probable dimensions of the low-flow channel can be empirically determined based on regional studies similar to those that are conducted for natural streams. A two-stage ditch has the potential to create and maintain better habitat. Two-stage ditches might also be useful in improving water quality particularly for nutrient assimilation. The primary costs of two-stage ditches are increased width and more initial earthwork.

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