Climate change and management decisions could transfer flood risk to socioeconomically disadvantaged communities along the San Francisquito Creek, California

Abstract:

Risk-mitigation infrastructure such as levees and seawalls are built to prevent flooding up to a specific event, referred to as the design event. Estimates of the magnitude and probability of the design event, like the 100-yr flood, rely on historic conditions which are becoming increasingly less representative of current conditions, as climate change increases the probability of extreme events including heavy precipitation and elevated coastal sea levels. Combined with changes in land use, river bathymetry, and infrastructure modifications, the current paradigm of flood risk-mitigation by preparing for a historically determined static level is no longer adequate in accounting for the increasing uncertainty of flooding events.

Our research quantifies how climate change and infrastructure modifications alter the probability of flooding for different communities along the San Francisquito Creek, California. The San Francisquito Creek flows from the Santa Cruz mountains into South San Francisco Bay, and runs through five municipalities which have a large variation in wealth. To evaluate the potential for flooding, we use a hybrid modeling approach which combines thousands of synthetic simulations of upstream river discharge and downstream coastal water levels with the HEC-RAS flow model to produce along-river water levels. We model synthetic joint climate conditions for current river discharge and sea level, as well as combinations of increased river discharge and sea level. The synthetic joint climate conditions are used as boundary conditions for multiple river configurations to evaluate how joint changes to the climate and infrastructure alter the likelihood of flooding.

Infrastructure modifications that reduce the risk of flooding during the 100-yr event upstream near affluent communities, increase the risk of flooding downstream to socioeconomically disadvantaged communities. Increases to river discharge exacerbate this risk. Thus, evaluation of a single event fails to acknowledge the broader risks experienced at different along-river locations. Risk-mitigation infrastructure, while intended to minimize the total impact of flooding, may increase socioeconomic disparities, emphasizing the need to integrate social and environmental sustainability into flood-risk management.