In situ measurement of sediment resuspension caused by propeller wash with an underwater particle image velocimetry and an acoustic doppler velocimeter

Flow induced by propellers of waterborne vessels can cause sediment resuspension in estuaries, bays and harbors, where sediments are usually contaminated. Bottom shear stress due to propeller wash is the key parameter that determines the initiation of sediment resuspension and the subsequent erosion. A novel self-contained underwater miniature particle image velocimetry (UWMPIV) system has been developed and deployed to study sediment resuspension under propeller wash in a US Navy harbor in San Diego, CA. Near bed profiles of mean velocity and Reynolds stresses were measured to evaluate the bottom shear stress, and to validate the shear stress measured with an acoustic Doppler velocimeter (ADV) that is simultaneously deployed with the PIV system. The critical shear stress was estimated by directly observing PIV images and identifying the moment when sediment resuspension started. PIV measurement became unfeasible as the propeller speed increased and the optical access was blocked by high level of suspended solids. However, the development of the bed erosion was able to be recorded in PIV images at several intervals when sediment concentration was relatively low and the sediment bed was visible. The observed time series of cumulative erosion depth agreed well with an erosion rate model that depends linearly on the bottom shear stress excess.