South Bay restoration

Delft3D Flexible Mesh models of Alviso Slough and Pond A8 were used to study how breaching salt pond levees and reconnecting creeks change erosion, deposition and sediment export in South San Francisco Bay.

Partners: U.S. Geological Survey, Deltares Netherlands, Santa Clara Valley Water District (now Valley Water), South Bay Salt Pond Restoration Project

All case studies

Morphodynamic modeling of salt pond breaching and creek reconnection

The South Bay Salt Pond Restoration Project aims to restore about 60 km² of former industrial salt ponds to tidal habitat. In the Alviso complex, which lies downstream of the former New Almaden mercury mines, breaching levees raises the tidal prism and scours the sloughs, and the scour can remobilize legacy mercury. The 2018 USGS report built and tested a two-dimensional Delft3D Flexible Mesh model of Alviso Slough. The mesh had 2,316 cells, from about 20 m in the sloughs to 150 m in the ponds. It was calibrated against bathymetric surveys from 2010 to 2012 and run to March 2017.

The model reproduced the observed erosion and deposition patterns with skill, although suspended sediment concentrations were harder to match. Two scenarios were added: a 0.5 m higher mean water level and five additional breaches of about 30 m. Both led to infill of the A5, A7 and A8 ponds. A 2023 conference paper reports a seven-year hindcast and again finds that the model captures the morphodynamic trend.

For the Santa Clara Valley Water District, Deltares used a model derived from the Alviso Slough model to simulate a reconnection of Calabazas and San Tomas Aquino Creeks to Pond A8, with one or two levee breaches, over 5.5 years. During high river discharge, flow velocities in both creeks increased, and so did sediment export, most strongly with two breaches. The report calls its results qualitative and recommends further simulations.

Downloads

Report: van der Wegen, M., Reyns, J., Jaffe, B., and Foxgrover, A. (2018). Modeling morphodynamic development in the Alviso Slough system, South San Francisco Bay, California. U.S. Geological Survey.
Report: Röbke, B. R., and van der Wegen, M. (2018). Calabazas-San Tomas Aquino Pond A8 reconnection: Preliminary scenario simulations. Deltares report 11200020-002-ZKS-0003, prepared for the Santa Clara Valley Water District.
Paper: van der Wegen, M., Reyns, J., Jaffe, B., et al. (2023). Successful hindcast of 7 years of mud morphodynamics influenced by salt pond restoration in south San Francisco Bay. Coastal Sediments 2023, 1129-1134.