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 measure- ment 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,0.5 m) locations, sediment suspension was closely correlated with the primary boat-wake waves (Hmax,0.21 m), 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. DOI: 10.1061/~ASCE!0733-950X~2002!128:4~152! CE Database keywords: Sediment suspension; Levees; Erosion; Ship motion; California; Measurement.