Assessing Compound Flood Hazard Risk for the San Francisco Bay Area

Abstract:

As climate change causes sea levels to rise, increases precipitation variability, and leads to larger, more frequent storms, communities are faced with ever increasing risks from flooding. Communities require reliable assessment of the compound risk they face to decide how to mitigate their flood risk. For some, the primary driver of flooding is readily identified but for others, face the challenge that several factors may contribute to this risk since flood hazards rarely occur in isolation. During extreme precipitation events, coastal, fluvial (riverine), or pluvial flooding (excess runoff and surface flooding) can happen concurrently, but their compounding effect is not always taken into account when mapping flood hazard.

The goal of this study is to integrate the effects of coastal, fluvial, and pluvial hazards by using preexisting hazard maps and models to develop a map of the compound flood hazard for the San Francisco Bay Area, CA USA. We build on different models that quantify coastal, fluvial, and pluvial flooding hazards to back out the flooding location and the compounding flood depth. For all three flooding hazards, we use existing flood maps to create baseline parameters such as time frame, sea level rise, land cover, and storm severity and validate against historical data. To get a sense of flood risk, instead of only hazard, we overlay characteristics from both census data (for demographic and socioeconomic information) and tax assessor records (for housing stock) onto our map to identify communities at risk.

Our results suggest that a one size fits all approach to compound flood risk mitigation is problematic since communities face different challenges and have different needs. Some communities like Foster City are primarily affected by sea level rise, which has been an important focus in recent planning efforts in the San Francisco Bay Area. Others are less exposed to coastal flooding, but are affected by fluvial and pluvial flooding, leading to a high compound hazard. Yet other communities are exposed mainly to pluvial flooding, but are more vulnerable, due to low household income and substandard housing, and thus more at risk than current maps indicate. Our analysis highlights the importance of considering other drivers of flooding besides sea level rise to better understand who is at risk.