Grade control for urban flood channel design

Grade control by drop structure is a common practice to design a steep channel. For a given design discharge, the drop height is determined by the pre-selected permissible flow Froude number or flow velocity. Design parameters for the energy dissipation over a drop include channel lining roughness, approach flow Froude number, drop number, and downstream jump in the stilling basin. All these design parameters shall be collectively optimized to provide a safe and functional structure. This paper reviews the current design procedure and then presents a methodology to determine the drop height by maximizing the energy dissipation. Design parameters include the limiting Froude number and channel roughness. The sensitivity test conducted in this study suggests that the limiting Froude number for drop structure designs be between 0.6 and 0.8. The optimal dissipation rate can be 80-85% of the specific energy associated with the approach flow.