Estimating Boat-Wake-Induced Levee Erosion using Sediment Suspension Measurements

The subaqueous portion of a levee bank in the Sacramento–San Joaquin River Delta of central California was instrumented to quantify the impact of boat-generated waves. Typical erosion rates associated with recreational craft are too small for direct measurement of bank retreat on a per-boat-passage basis; therefore, two independent analytical methods of estimating linear erosion were developed based on colocated suspended sediment concentration and velocity time series. The algorithms were tested using data measured during a field experiment in which a 7.5 m boat was driven past the site over a range of speeds to generate waves of varying size. A cross-shore array of electromagnetic current meters and optical back-scatterance sensors measured the character of boat-generated waves and the resultant sediment suspension. In near-bank, shallow-water (d<0.5m) locations, sediment suspension was closely correlated with the primary boat-wake waves (Hmax<0.21m), indicating that maximum near-bottom orbital velocities were sufficient to erode the fine-grained (mud-silt) bottom materials. Suspension events were short lived (order of 1–5 min), despite very long particle settling times (order of hours), because river currents swept the suspension plumes downstream. This implies negligible sedimentation and resuspension locally. Both algorithms produced strikingly similar erosion estimates, and these values (0.01–0.22 mm/boat passage) compare favorably with direct measurements of cumulative bank erosion in response to multiple, sequential boat passages. Field conditions for which the algorithms are appropriately applied are discussed.