CDSE, Deputy Director of Power Generation PCWA, United States
Abstract Submission: Extensive research has examined incipient motion thresholds for different sediment sizes and mixtures in natural channels. However, predicting the threshold of sediment movement for accumulations within concrete channels is less well known. An example is that of the proposed diversion structure design for two dams owned by Placer County Water Agency in California. The structures are designed to provide sediment maintenance flows past the existing dam spillway. To support the design, HEC-RAS 2D was used to predict incipient motion for sediment deposited within the new headpond and optimize the design to reduce the need for manual cleaning of the structure.
This paper describes a flume study conducted in NHC’s Seattle Hydraulics Laboratory to validate the HEC-RAS modeling results. The study used a 1:10 scale physical model of both headpond designs with a range of sediment sizes in a 16-foot-long, 2.5-foot-wide flume. Over 10 scenarios were tested with scaled flows incrementally increased and incipient motion documented for each sediment size in relation to shear stress. This analysis compares the incipient motion threshold predicted by HEC-RAS with physical modeling results. Additionally, the physical modeling results are compared with published data of incipient motion for various sized sediments. The results provide insight into general incipient motion thresholds in concrete channels.
