Managing mud: Predicting how estuary-scale geomorphic changes affect intertidal habitats

Tidal wetland restoration in the breached ponds of the South Bay Salt Pond Restoration Project relies on capturing recirculated estuarine sediments to build subsided former marshes up to mature marsh plain elevations.  This large new sediment demand, combined with significant increases in tidal prism caused by tidal inundation of the ponds, will affect South Bay sediment dynamics, sediment budget, extent of offshore mudflats and shoreline erosion rates at the estuary scale. Using analysis of historic bathymetric change and hydrodynamic circulation studies carried out by USGS, we have developed a sediment budget model for the South Bay. This model, combined with a set of empirical geomorphic tools, allows us to predict how intertidal mudflats would respond over the next 50 years under no action and maximum tidal restoration scenarios for the ponds.  This analysis shows that most of the restoration sediment demand is located in the upper reaches of the Bay (Far South Bay) where sediment naturally accretes.  Here, under a no action scenario we predict gain in mudflat area whereas under full tidal restoration we anticipate significant loss of mudflats due to increased sediment demand in the restored ponds.  In the rest of South Bay, we expect that restoration actions will not affect the rate of mudflat loss. The sensitivity of predicted mudflat change to variability in the sediment input parameters was also tested. The adaptive management program incorporated in the Project allows opportunities for anticipating, minimizing, or mitigating these landscape-scale changes.  This could be done by phasing restoration or partially filling subsided ponds to reduce the rate of sediment demand, restoring mudflats as transitional or permanent intertidal habitats within restored ponds, and by removing Bayfront levees to allow offshore mudflats to migrate shoreward as well as replenishing sediment supply through shoreline erosion.