water Article Assessment of Flood Forecast Products for a Coupled Tributary-Coastal Model Robert Cifelli 1,*, Lynn E. Johnson 1,2, Jungho Kim 1,2 , Tim Coleman 3, Greg Pratt 4, Liv Herdman 5 , Rosanne Martyr-Koller 5, Juliette A. FinziHart 5, Li Erikson 5 , Patrick Barnard 5 and Michael Anderson 6 1 NOAA Physical Sciences Laboratory, 325 Broadway, Boulder, CO 80305, USA; [email protected] (L.E.J.); [email protected] (J.K.) 2 Cooperative Institute for Research in the Atmosphere at the NOAA Physical Sciences Laboratory, Boulder, CO 80305, USA 3 Cooperative Institute for Research in Environmental Sciences at the NOAA Physical Sciences Laboratory, Boulder, CO 80305, USA; [email protected] 4 NOAA Global Systems Laboratory, 325 Broadway, Boulder, CO 80305, USA; [email protected] 5 USGS Pacific Coastal and Marine Science Center, 2885 Mission St., Santa Cruz, CA 95060, USA; [email protected] (L.H.); [email protected] (R.M.-K.); jfi[email protected] (J.A.F.); [email protected] (L.E.); [email protected] (P.B.) 6 California Department of Water Resources, 3310 El Camino Avenue, Sacramento, CA 95821, USA; [email protected] * Correspondence: [email protected] Abstract: Compound flooding, resulting from a combination of riverine and coastal processes, is a complex but important hazard to resolve along urbanized shorelines in the vicinity of river mouths. However, inland flooding models rarely consider oceanographic conditions, and vice versa Citation: Cifelli, R.; Johnson, L.E.; for coastal flood models. Here, we describe the development of an operational, integrated coastal- Kim, J.; Coleman, T.; Pratt, G.; watershed flooding model to address this issue of compound flooding in a highly urbanized estuarine Herdman, L.; Martyr-Koller, R.; environment, San Francisco Bay (CA, USA), where the surrounding communities are susceptible to FinziHart, J.A.; Erikson, L.; Barnard, flooding along the bay shoreline and inland rivers and creeks that drain to the bay. The integrated P.; et al. Assessment of Flood Forecast Products for a Coupled tributary-coastal forecast model (Hydro-Coastal Storm Modeling System, or Hydro-CoSMoS) was Tributary-Coastal Model. Water 2021, developed to provide water managers and other users with flood forecast information beyond what 13, 312. https://doi.org/10.3390/ is currently available. Results presented here are focused on the interaction of the Napa River w13030312 watershed and the San Pablo Bay at the northern end of San Francisco Bay. This paper describes the modeling setup, the scenario used in a tabletop exercise (TTE), and the assessment of the various Academic Editor: flood forecast information products. Hydro-CoSMoS successfully demonstrated the capability to Venkatesh Merwade provide watershed and coastal flood information at scales and locations where no such information is Received: 1 December 2020 currently available and was also successful in showing how tributary flows could be used to inform Accepted: 12 January 2021 the coastal storm model during a flooding scenario. The TTE provided valuable feedback on how to Published: 27 January 2021 guide continued model development and to inform what model outputs and formats are most useful to end-users. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: coastal flooding; flood forecast; tributary-coastal; San Francisco Bay; tabletop exercise published maps and institutional affil- iations. 1. Introduction The San Francisco Bay (the Bay) area is home to over 7 million people and supports Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. one of the most prosperous economies in the U.S. [1]. The area is highly urbanized, This article is an open access article encompassing 9 counties, 3 major cities (San Francisco, Oakland, and San Jose), and distributed under the terms and US Highway 101 adjacent communities. The region is susceptible to coastal flooding conditions of the Creative Commons along the bay shoreline as well as storm water and flash flooding along inland rivers and Attribution (CC BY) license (https:// creeks that drain 483 watersheds [2] to the Bay (Figure1). According to a 2013 California creativecommons.org/licenses/by/ Department of Water Resources (CA-DWR) report, over 355,000 people in the Bay area 4.0/). are exposed in the 100-year flood plain and this number increases to over 1 million people Water 2021, 13, 312. https://doi.org/10.3390/w13030312 https://www.mdpi.com/journal/water Water 2021, 13, x FOR PEER REVIEW 2 of 25 Water 2021, 13, 312 2 of 21 ment of Water Resources (CA-DWR) report, over 355,000 people in the Bay area are ex- posed in the 100-year flood plain and this number increases to over 1 million people in in the 500-year flood plain [3]. Estimated value of exposed structures in the 100- and the 500-year flood plain [3]. Estimated value of exposed structures in the 100- and 500- 500-year floodplains are $46.2 billion and $133.8 billion, respectively. Water, transportation, year floodplains are $46.2 billion and $133.8 billion, respectively. Water, transportation, and emergency management agencies across the 9 county San Francisco Bay region rely and emergency management agencies across the 9 county San Francisco Bay region rely on flood forecasts to assess risk and inform their flood mitigation processes. Of special on flood forecasts to assess risk and inform their flood mitigation processes. Of special concern is combined coastal and tributary flooding events that compound the effects of concern is combined coastal and tributary flooding events that compound the effects of either one alone (i.e., compound flooding events; [4]). This has been assessed by the either one alone (i.e., compound flooding events; [4]). This has been assessed by the U.S. U.S. Geological Survey (USGS) Coastal Storm Modeling System (CoSMoS) for the top Geological Survey (USGS) Coastal Storm Modeling System (CoSMoS) for the top 12 trib- 12 tributaries in a simplified manner (i.e., one-way coupling, daily flows; [5]), but not with utaries in a simplified manner (i.e., one-way coupling, daily flows; [5]), but not with real- real-time atmospheric forcing and coupling, or short time steps. time atmospheric forcing and coupling, or short time steps. FigureFigure 1. 1.Map Map showingshowing importantimportant geographicgeographic features in the region region around around San San Pablo Pablo Bay Bay at at the the northern end of San Francisco Bay where the case study described in the text was conducted. In- northern end of San Francisco Bay where the case study described in the text was conducted. Insert sert shows the location relative to the greater San Francisco Bay area. The legend indicates color shows the location relative to the greater San Francisco Bay area. The legend indicates color shading shading for elevation (m), Federal Emergency Mangement Agency (FEMA) flood zones A (Areas forwith elevation a 1% annual (m), Federal chance Emergencyof flooding), Mangement V and VE (Coastal Agency areas (FEMA) with flood a 1% zones or greater A (Areas chance with of a 1% annualflooding chance and an of additional flooding), Vhazard andVE associated (Coastal with areas storm with waves), a 1% or urban greater regions, chance and offlooding water bodies. and anThe additional outline of hazard the Napa associated river wate withrshed, storm stream waves), channels, urban regions,and county and boundaries water bodies. are Theindicated outline by ofline the colors Napa corresponding river watershed, to the stream legend. channels, CNRFC and fore countycast points boundaries are identified are indicated by inverted by line red colors tri- correspondingangles. to the legend. CNRFC forecast points are identified by inverted red triangles. Currently,Currently, the the National National Oceanic Oceanic and and Atmospheric Atmospheric Administration Administration (NOAA) (NOAA) National National WeatherWeather Service Service (NWS) (NWS) and and the the National National Ocean Ocean Service Service (NOS) (NOS) provide provide flood flood and and high- high- waterwater forecasts forecasts atat selected points points along along major major rivers rivers surrounding surrounding the theBay Bay and andin and in along and along the Bay. The California-Nevada River Forecast Center (CNRFC, https://www.cnrfc. noaa.gov/) has 22 river forecasts points on major inflows in the San Francisco Bay Area, Water 2021, 13, 312 3 of 21 which are all upstream of the head of tide locations such that there is no accounting of the coastal influence on forecast water levels (see Figure1). Site-specific forecasts for small tributaries surrounding the Bay are not provided except in a categorical manner (e.g., Flash Flood Guidance in [6]). In contrast, a distributed hydrologic model (DHM) can provide flow forecasts for every grid or stream reach in a watershed. For example, the National Water Model (NWM — see [7] for a description of the NWM) has more than 11,000 stream reaches in the nine county region surrounding San Francisco Bay. The NOS San Francisco Bay Operational Forecast System (SFBOFS, https://tidesandcurrents.noaa.gov/ofs/sfbofs/ sfbofs_info.html) also provides water level forecasts in the Bay and the intertidal zone of the major tributaries. However, these coastal water level forecasts are mainly for navigation purposes and have relatively coarse resolution for estuary flood inundation. In the Bay area, the intertidal and small stream interface zone represents a significant fraction of the total area susceptible to flooding. San Francisco Bay agencies responsible for flood mitigation, water supply, water qual- ity impacts, and coordinating emergency response efforts have expressed a need for more accurate and timely information on precipitation, tributary flows, and coastal flooding in order to carry out their respective missions. To address the needs of water agencies for more information in the tributary and intertidal zone for both planning and real time operation purposes, the CA-DWR supported a joint NOAA-U.S.