San Francisco Bay Long-Term Management Strategy (LTMS) for Dredging and Disposal. Report 2. Baywide Suspended Sediment Transport Modeling

Field data analysis was used to examine suspended sediment transport in Central San Francisco Bay, and to develop and verify a two-dimensional numerical fine-grained sediment transport model. This study concerned the dispersion and fate of disposed dredged material in Central San Francisco Bay, California. The 1992 monitoring survey spanned a 2-week period in June, and used three boat-mounted acoustic Doppler current profiling (ADCP) systems to obtain repeated cross-sectional transects near the Golden Gate, the entrance to South Bay, and Richmond Point. Between acoustic transects, water samples were obtained over depth for salinity, total suspended material (TSM) concentration, and particle size determinations. Acoustic backscatter data were used to produce correlated suspended material concentration and flux fields. Discrete measurements were fit to empirical discharge and suspended flux models and integrated over a neap-spring-neap tidal sequence to estimate net transport. There was an observed net transport of suspended sediment seaward at the Golden Gate over the neap to spring sampling period. The total net transport was 188,000 metric tonnes seaward over 14 lunar days. Fluxes during the ebb flows averaged 44% greater than the flood flows, and instantaneous strong-ebb TSM fluxes were often more than twice those on flood tidal phases. Suspended sediment concentrations increased during the strongest part of the ebb at all Central Bay sampling ranges. Wind forcing did not appear imponant to this process. Suspended concentrations quickly returned to normal levels after the passage of peak ebb flows. Spatial variability in the TSM fields was relatively large. The ADCP backscatter intensity measurements correlated well with TSM especially on the Golden Gate range where a broadband unit was employed.