Two-Stage Agricultural Ditch - Hydraulic and Sediment Impacts (Sebewaing River Basin, Michigan)

Agricultural ditches are normally constructed and maintained as trapezoidal channels with flat bottoms. Over time, a two-stage channel often develops, consisting of a narrow low-flow channel flanked by low, grassed berms. These berms act as a new floodplain within the original channel. The benefits and impacts of the two- stage channel are evaluated from an ecological, hydraulic, hydrologic, and sediment transport perspective. Examples are taken from the Sebewaing River watershed, Michigan. Four different reaches of the watershed were selected as being representative of the system as a whole and having a two-stage channel present. Cross-sections and slopes were measured at each of these reaches and bed material samples were taken for grain size analysis. This data was then input into HEC-RAS and SIAM (Sediment Impact Analysis Model) in order to examine hydraulic and sediment transport capacities, as well as channel stability. A sensitivity analysis was performed using these same tools to determine the effects of varying slopes and channel roughness. The impacts of widening agricultural ditches (to accommodate a floodplain bench) on downstream peak flows were also evaluated.