Characterization of Suspended Sediment Plumes Resulting from Barge Decanting in San Francisco Bay

Abstract

Abstract : The San Francisco Bay Regional Water Quality Control Board (SFRWQB) and the U.S. Environmental Protection Agency (USEPA) are seeking to prohibit barge decanting during dredging operations. The agencies cite environmental concerns associated with the release of suspended sediments as the primary justification for banning the barge decanting activities. Since approximately 40% to 70% of the volume placed in the scow is water, this will create budgetary cost overruns for future projects. Transporting this volume of water (124 miles round trip) rather than sediment would significantly impact the efficiency and cost of conducting dredging. Projected cost increases without decanting could be more than 40%. To assess potential impacts from barge decanting, acoustic surveys using a 600 kHz acoustic doppler current profiler (ADCP) were conducted to (1) determine the total suspended solids (TSS) concentration above ambient for plumes produced by both barge decanting and buckets associated with mechanical dredging; (2) determine the TSS of the supernatant water during barge loading, after the settling period and during the decanting process as water passes through the standpipe; and (3) determine the percent of dry mass of sediment loss back to the water column as a result of decanting. Results indicated that no distinct, separate, identifiable plume signature associated with barge decanting was detected by ADCP. Based on calculations of TSS from the supernatant discharge, decanting would increase losses by no more than 0.1%. Decanting would, however, increase the effective loading capacity by as much as 50%.