Development of a decadal-scale estuarine geomorphic model for Suisun Bay, California: calibration, validation, and idealized time-stepping
Geomorphic evolution of estuarine habitats and landscapes over decadal timescales (>10 years) is sensitive to sediment supply from the watershed as well as estuarine hydrodynamics. Sediment supply to Suisun Bay, California is subject to natural as well as anthropogenic influence, beginning with the drastic input of sediment during the hydraulic mining period of the late 19th century. Today sediment supply is declining due to reduction of the hydraulic mining sediment pulse, reservoir storage, and land use practices. Future climate change, land use change, and sea level rise are some of the many factors that may alter sediment supply and threaten ecologically beneficial estuarine habitats. We have developed an estuarine geomorphic model based on a traditional tidal timescale hydrodynamic/sediment transport model, by idealizing boundary conditions, applying novel calibration procedures, and implementing a simplified time-stepping method. The Regional Oceanic Modeling System (ROMS) was developed for Suisun Bay, for the purpose of hindcasting historical geomorphic change and modeling future scenarios of geomorphic change. Seaward boundary conditions were idealized using tidal harmonic prediction for tidal stage and velocity, and a synthetic time-series for sediment concentrations was constructed by applying typical seasonal wind patterns and the spring-neap tidal signal. Calibration of these idealized boundary conditions and bed sediment parameters was accomplished using sediment flux data for the boundaries of Suisun Bay from water years 1997 and 2004. Calibrating to sediment fluxes guarantees that modeled net geomorphic change will not exceed the total supply of sediment from landward and seaward sources. The successful simulations of 1997 and 2004 allow for the development of a time-stepping method that reduces computational expense. The method involves simulating the two distinct sediment-transport seasons of Suisun Bay as month-long periods, then extrapolating the results of each compressed period for the entire season. Computational time for hindcasting and future scenario simulations is now reduced by 85% using this simplified method.