Computational Assessments of Scenarios of Change for the Delta Ecosystem (CASCaDE)

California's delicate balance between water supply and ecosystem preservation is under increasing pressure from a growing population and habitat loss. The locus of many of these issues is San Francisco Bay, where freshwater from the Sacramento/San Joaquin Delta meets saline water from the Pacific Ocean. Suisun Bay is the furthest landward subembayment of San Francisco Bay, and is therefore most responsive to freshwater flow. Water withdrawals from the Delta adversely impact the estuarine ecosystem and habitats. Increasing the quality of habitat in Suisun Bay, however, would decrease the ecosystem stress caused by freshwater flow diversions. Current goals of ecosystem restoration include the creation and maintenance of beneficial wetlands and shallow-water habitat (Goals Project 1999).

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 the Bay is an ongoing issue, beginning with the drastic input of sediment during the hydraulic mining period of the late 19th century (Gilbert 1917). Today sediment supply is declining due to reduction of the hydraulic mining sediment pulse, reservoir storage, and land use practices (Wright and Schoellhamer 2004). 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 (Scavia et al. 2002, Pont et al. 2002). Hydrodynamics are directly modulated by the varying morphology of the Bay (and vice-versa), so there is a feedback between hydrodynamics and geomorphology.

Objectives

The specific objectives of the research are as follows:

1. Develop a tidal timescale hydrodynamic/sediment transport model of Suisun Bay based on existing public-domain software.
   
2. Implement idealized boundary conditions for the seaward boundary of the domain, due to the lack historical data for hindcasting simulations.
   
3. Calibrate and validate the complete model with reference to sediment flux data at the landward and seaward boundaries of Suisun Bay from 1997 and 2004 (McKee et al. 2006; Ganju and Schoellhamer 2006).
   
4. Evaluate the accuracy of a time-stepping procedure which utilizes a reduced set of hydrographs to represent average conditions over several decades.
   
5. Develop historically based landward boundary conditions for hindcasting simulations based on the historical geomorphic data from Cappiella et al. (1999).
   
6. Hindcast the bathymetric change from 1867-1990, to further validate the model.
   
7. With the calibrated, validated model, evaluate the geomorphic response to climate change scenarios, including sea-level rise, altered streamflow patterns, and modified wind forcing.

A detailed description of the entire project, along with the proposal and supplemental materials can be found at http://sfbay.wr.usgs.gov/cascade.

Publications

Ganju, N.K, Knowles, N., and Schoellhamer, D.H., in press, Temporal downscaling of decadal sediment load estimates to a daily interval for use in hindcast simulations. Journal of Hydrology.

Ganju, N.K., and Schoellhamer, D.H., in press, Calibration of an estuarine sediment transport model to sediment fluxes as an intermediate step for robust simulation of geomorphic evolution. Continental Shelf Research.

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