Moving from total risk to community-based risk trajectories increases transparency and equity in flood risk mitigation planning along urban rivers

After several years of drought, 2023 and early 2024 are reminders of the powers of California’s atmospheric rivers and the devastating flooding they can entail. Aged flood-mitigation infrastructure and climate change exacerbate flood risk for some communities more than for others, highlighting the challenge of equitably mitigating flood risk. Identifying inequities associated with infrastructure projects is now legally required by regional water boards in California, but tools are lacking for making this assessment systematically. We propose that risk trajectories, computed by adding a probabilistic wrapper of flood drivers to models already used in flood-risk-mitigation planning, allows planners to quantify the spatial and temporal variability of risk for communities along river and thereby increase procedural equity by making distributional equity more transparent. While our proposed approach is applicable generally, we demonstrate its impact in the case of San Francisquito Creek, California, where risk trajectories combined with a multi-tier engagement model, helped identify and prevent an inequitable risk transfer.