Modeling climate change impacts on flooding and community vulnerability Novato, Ca

This study investigates the effects of climate change on compound fluvial and coastal flooding using coupled hydrologic and hydraulic models to examine the changing physical characteristics and socio-economic impacts of flooding in a small basin typical of many flood-prone areas on the United States west coast. Novato Creek is located in Marin County, California along the northeastern edge of San Francisco Bay. With steep, densely populated communities upstream and major regional transportation and utility infrastructure in low-lying areas near the bay, the watershed is highly vulnerable to climate change. Downscaled daily precipitation projections from an ensemble of 10 Global Climate Models indicate that the magnitude of a two-day, 50-year (2% annual chance of occurrence) storm will increase by an average of 16% ± 6% by mid-century (2040-69) and 31% ± 8% by late-century (2070-99) under a high emissions (RCP 8.5) trajectory. These increases in precipitation, combined with sea level rise (SLR) projections of 0.6 to 1 m (1.9 to 3.4 ft) by mid-century and 1.0 to 1.9 m (3.4 to 5.2 ft) by late-century, will lead to median increases in flood extent of 42% to 77% by mid-century and 82% to 83% by late-century. Socio-economic impacts from this compound flooding are significantly greater than the impacts of SLR or changing precipitation alone. The total number of people impacted by flooding will increase from 6,500 under historic storm conditions to at least 9,400 by mid-century and 11,400 by late-century. The length of roads impacted by flooding will increase 60% to 170%, and as many as 2,400 to 3,250 homes and other buildings will be exposed to flooding. The scale of these projected impacts makes clear the importance of considering compound flood effects when planning for climate change adaptation.