Integrated Flood Forecast Model (Hydro-CoSMoS) for San Francisco Bay

An integrated flood forecast system is being developed for San FranciscoBay which involves coupling a watershed distributed hydrologic model (DHM) with the USGS Coastal Storm Modeling System (CoSMoS) coastal hydrodynamic model. TheHydro-CoSMoS prototype has been developed and is demonstrated for the Napa River watershed and its estuary San Pablo Bay. The DHM used was the NWS Research Distributed Hydrologic Model (RDHM) which represents distributed surface runoff and soil moisture dynamics at a 4-km grid resolution, and routing of the grid flows to the bay. As a hydrologic model, the DHM does not simulate inundation, but does characterize flood flow frequency levels and threats to bridge crossings and other flood impact features. The CoSMoS modelaccounts for tidal forcing, seasonal water level anomalies, surge and in-Bay generated wind waves. CoSMoSis based on the Delft3D-FM, a hydrodynamic model which represents detailed hydraulics using a flexible mesh grid scheme, and generates time series of water levels and inundated areas for the estuary and upstream to the City of Napa where the coastal tide and storm surge influences end.The tributary discharge from the DHM is meteorologically driven and dynamic, as is the CoCMoS; both are linked to NWS forecasts of rainfall, winds, atmospheric pressure and oceanic storm surge. Linkage to the hydrometeorological forecasts allows for operational use of CoSMoS which has previously relied on statistical estimates of river discharge.
This paper focuses on introduction of the technical details in the Hydro-CoSMoS and the results from a case study based on the ArkStorm-like storm scenarios for Napa River watershed and estuary. For demonstration purposes, a scenario was created to illustrate what theHydro-CoSMoS can do and how it works. The scenario is based on actual events that have occurred in the SF Bay area, but we have selected forcings which could cause over 100-yr recurrence interval flooding in the watershed and estuary. The watershed scenario considers the impact of antecedent soil moisture conditions which greatly influences the magnitude of flood runoff throughout the watershed. The coastal scenario involved relatively modest hydrometeorological forcings, but juxtaposedthese to create a tide, storm surge and wind wave event that corresponded to the January 2017 flood events in the region. As a result, we found the effect of each forcing on the flooding and the flooding impacts of the scenario on Napa River watershed and estuary. The detailed hydrodynamic model accurately represented the performance of the recently completed flood bypass channel improvements in the City of Napa. A table-top exercise workshop was held with state, federal and local flood forecast and emergency management officials to simulate how the Hydro-CoSMoS forecast process could occur, and to project the various information products that could be provided. Feedback obtained at the workshop, summarized in a companion presentation, will help refine design of the integrated Hydro-CoSMoS system which is intended for the entire 9-county San Francisco Bay area.