Geometry of inset channels and the sediment composition of fluvian benches in agricultural drainage systems in Ohio

Abstract:

In Ohio, agricultural channels have been constructed or modified to transport surface runoff and subsurface drainage away from agricultural fields. The development of depositional bars, or benches, in these agricultural channel systems has been a focus of maintenance activities. Objectives of the reported study were to: (1) determine if there was a relationship between inset channel geometry, as defined by its benches, and geometry predicted by regional curves that relate bankfull geometry to drainage area; (2) evaluate the effect of channel geometry on bed material particle size distribution; and (3) determine if the particle size distribution of bench materials was related to depth from the surface and position on the bench. The study was conducted on low gradient ditches that are headwaters of the Mississippi River and Lake Erie. An analysis to determine the extent of bench and inset channel formation was undertaken using 0.15 m (0.5 ft) resolution panchromatic digital orthophotos. Geomorphology measurements were made at 13 sites, and a 61 cm (24 in) long split-spoon sampler, with a 7.6 cm (3 in) inner diameter, was used to extract cores from the benches at each site. About 59% of the assessed channels showed inset channel development and bench formation. For 12 of the 13 sites, the bankfull dimensions of the inset channel were consistent with regional curves. All benches showed fining of materials with distance away from the inset channel. It was concluded that benches evolve much like floodplains that build because of overbank accretion. The results will be helpful in determining alternative strategies for improving the drainage function of these systems while enhancing ecological habitat and water quality.