The Bay is losing sediment, and sand mining is a major factor.
Wetlands and beaches act as natural infrastructure for the Bay Area that make communities more resilient to sea level rise. But rising water threatens these natural features with erosion and drowning, leaving land behind these beaches and wetlands exposed to storms and flooding.
Scientists have known for some time that sand is extracted from the Bay floor faster than it is replaced. Concerned that sediment extraction could undermine conservation and resilience along the Bay’s shorelines, the Bay Conservation and Development Commission (BCDC) and the State Coastal Conservancy commissioned San Francisco Estuary Institute, in partnership with Deltares and the United States Geological Survey (USGS), to more precisely quantify sand sources and sinks.
The new report, “Sand Budget and Sand Transport in San Francisco Bay,” quantifies the Bay's sediment budget.
Key Findings
The Bay has a sediment deficit. Sediment losses of 3.7 million metric tons per year exceeded inflows of two million metric tons per year. The resulting net losses of 1.7 million metric tons per year weigh more than two Golden Gate bridges. Sediment is made of mud and sand, and sand accounts for two-thirds of the total deficit.
Mining and dredging outpace natural replenishment. Sand is removed by mining (1.2 million metric tons per year) and dredging disposal (0.11 million metric tons per year) much faster than it is replaced (0.37 million metric tons per year).
Fine-scale impacts are uncertain. The research was at the scale of the entire Bay. Although we can say that sand mining and dredging affect sediment availability in the Bay, we don’t yet know how changes in mining and dredging practices would affect specific wetlands and beaches. Finer-scale studies would help inform sustainable sand management.
The project was funded by the Bay’s sand industry to meet a 2015 BCDC permit requirement to improve knowledge of sand transport and budgets. We thank the Bay Conservation and Development Commission and the State Coastal Conservancy for their support. Brenda Goeden, Bob Battalio, Craig Jones, Paul Work, and David Schoellhamer provided expert oversight.
Read Sand Budget and Sand Transport in San Francisco Bay.
Read about the project that produced this report and others on sand supply and transport: Research to Understand Impacts of Bay Sand Mining on Sand Supply and Transport in San Francisco Bay and the Outer Coast.
