Sediment Pathways in San Francisco South Bay

Abstract

San Francisco Bay is the largest estuary on the west coast, and it supports a surrounding population of over 7 million people. Human developments have reduced the amount of fine sediments delivered to the Bay-Delta system in recent years, which threatens habitat restoration initiatives and suitable water quality in the southern reach of the Bay (South Bay), where an ample supply of fine sediments is needed to support tidal flat development and to maintain conditions that limit the excessive growth of algae populations. A better understanding of how sediments enter and are redistributed within South Bay is needed to anticipate how the dynamics and supply of fine sediments will change under future conditions.  

In this study, a pair of process-based numerical models was applied to investigate fine sediment pathways in San Francisco Bay. An offline coupling between a DELWAQ water quality model and a Delft3D FM hydrodynamics model provides an efficient method to study suspended sediment transport in large, complex systems.   The DELWAQ model was applied to various hydrodynamic scenarios to better understand the effects of wind, density-driven flows, and freshwater discharge on sediment dynamics in San Francisco Bay. The results suggest that sediment import and export at the mouth of South Bay is linked to density-driven circulation induced by horizontal salinity gradients between South Bay and Central Bay. During years with particularly high volumes of freshwater discharge from the Sacramento and San Joaquin Rivers, South Bay is more likely to import sediments, and extreme pulses of freshwater were shown to enhance sediment connectivity between North Bay and South Bay. Sediment exchange between the channel and the flats in South Bay is facilitated by the tide, and wind wave resuspension influences the magnitude of sediment fluxes into and within the southern reach.  

This study advances our understanding of sediment dynamics in the complex San Francisco Bay estuary. With further validation, the DELWAQ model developed in this research can be used as a tool to predict system response to future conditions brought on by climate change and human-induced alterations to the watershed. Large-scale management decisions regarding the Bay Area and the Central Valley could alter the long-term trajectory of the estuary and determine whether it will maintain ecological viability while adapting to changing hydrodynamic and environmental conditions. Insights learned from sediment transport modelling can adequately capacitate the Bay's leadership as well as countless population-dense estuaries around the world confronting analogous challenges.