Low-head or Run-of-River (RoR) dams exist on all types of rivers throughout the United States, yet the exact mechanisms of how sediment moves around the structures have not been well researched. Due to the increasing use of RoR dams in small hydroelectric projects, there is a need to better understand the controlling factors of how sediment passes over these dams. A one-dimensional morphodynamic model was developed to investigate the effects of RoR dams
on channel morphology over long time scales. The model solves the gradually varied flow equations to compute the flow field in the vicinity of the dam, computes grain-size-specific sediment transport rates, and uses sediment mass conservation and vertical storage bookkeeping to calculate the evolution of bed elevation, the bed surface grain-size distribution, and the vertical pattern of stratigraphy. The model’s hydraulic calculations were calibrated using data collected from a series of flume experiments performed with a model RoR dam to better capture the non-hydrostatic flow
over a dam. Numerical experiments designed to investigate how the grain-size distribution of the sediment supply rate, flow rate (steady and unsteady), and dam height act as controls on sediment passage over RoR dams were conducted using parameters reported in the literature for a RoR dam in northern Delaware. These one-dimensional simulations were complemented with a few simulations using, a two-dimensional morphodynamic model, Nays2DH. The 1D simulation results show that the stored sediment upstream of RoR dams does depend on the sediment supply, dam height, grain-size and flow discharge. Once sedimentation in the reservoir has reached equilibrium, high flow events will reduce or scour the sediment while lower flows will typically increase the amount of sediment behind the dam. Finally, a dam that stores more sediment will have greater downstream effects in terms of changes to grain-sizes and bed elevation due to the increased time it takes to pass sediment over the dam and reach an equilibrium condition on the upstream side of the dam.
