Assessment of Information Products for a Coupled Watershed–Coastal Flood Forecast Modeling System

San Francisco Bay is a highly urbanized estuary and the surrounding communities are susceptible to flooding along the bay shoreline and inland rivers and creeks that drain to the Bay. An integrated forecast model that integrates fluvial and oceanic drivers is being developed for predicting flooding in Bay area tributaries and estuaries. This project involves state-of-the-art coupling of a NWS Distributed Hydrologic Model (DHM) with the USGS Coastal Storm Modeling System (CoSMoS). Results presented here are for a prototype focused on the interaction of the Napa River watershed and the San Pablo Bay. Discharges from the DHM are meteorologically driven and dynamic, allowing for identification of flash flood threats for model grids interior to the tributaries to the Bay. The DHM tributary flows are input to the CoSMoS model which in turn simulates flooding extent in the receiving estuary. We utilize Delft3D-FM, a hydrodynamic model based on a flexible mesh grid, to calculate water levels that account for tidal forcing, seasonal water level anomalies, surge and in-Bay generated wind waves derived from the wind and pressure fields of a NWS forecast model. Flooding extent is determined by overlaying the resulting maximum water levels onto a recently updated 2-m digital elevation model of the study area which resolves the extensive levee and tidal marsh systems in the region, as well as the up-channel migration of storm surge. We use this pilot region to examine storm flooding impacts from a series of storm scenarios that simulate 5-100yr return period events in terms of either tributary or coastal events. These scenarios demonstrate the wide range of possible flooding outcomes considering rainfall recurrence intervals, soil moisture conditions, storm surge, wind speed, and tides. Details on the technical aspects of the Hydro-CoSMoS modeling system are presented in a companion paper.
This paper focuses on assessment of the various flood forecast information products generated by the integrated flood forecast modeling system. The tributary DHM generates forecast information for each grid that are portrayed as discharge, flow hydrographs (peak flow, time-to-peak, duration of high flow), soil moisture, and flood recurrence level. The CoSMoS portrays flood inundation and timing, and duration. Both models can help identify flood impact features such as road-stream crossings, and other critical facilities. A workshop was held with state, federal and local agency staff involved with flood forecasting and warning, and flood mitigation. As part of the workshop, we asked participants to review Hydro-CoSMoS outputs and rate how useful these products would be for theirs jobs. Results of these reviews are presented, and discussion is directed to how users’ assessments could influence design of the to be implemented real-time operational system.