Flood risk mitigation services of wetlands vary with seasonal changes to surface and soil water storage

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Jason Sauer and Nancy Grimm, School of Life Sciences, Arizona State University, Tempe, AZ, Olga Barbosa, Instituto de Ciencias Ambientales y Evolutivas, Universidad Austral de Chile, Valdivia, Chile, Elizabeth Cook, Julie Ann Wrigley Global Institute of Sustainability, Arizona State University, Tempe, AZ, Daniel L. Childers, School of Sustainability, Arizona State University, Tempe, AZ

Background/Question/Methods
Wetlands are gaining increased attention and even traction as forms of urban ecological infrastructure to reduce flood risk in cities. And yet, there is a notable absence of studies exploring the broad effects of incorporating wetlands into urban stormwater management systems on flood risk, or discussions on important characteristics of wetlands that might impact their abilities to reduce flood risk. We asked how wetland seasonal differences in surface and soil water storage affect flood risk from extreme rainfall events in an urban stormwater management system in Valdivia, Chile that incorporates wetlands. We monitored surface and soil water storage in six wetlands across Valdivia, Chile, between 2017-2018 and again from 2019-2020, and altered wetland storage properties in the city’s official stormwater management model in EPA SWMM to reflect wetland average wetland storage conditions during the early dry season, late dry season, early wet season, and late wet season. We modeled the stormwater management system’s reaction to a 6-hour, 100-year return period storm event to estimate changes in stormwater management services of these wetlands across the seasons.

Results/Conclusions
We found that flood mitigation services varied substantially across seasons, with the worst flooding occurring during the peak of the wet season, when wetland soils were saturated and there was abundant surface water, and the least amount of flooding occurring during the peak of the dry season, when surface water was absent from many of our wetlands and soil moisture had also been reduced. We found that changes in flood risk in the system also extended beyond our wetlands of study, such that when wetland were full, non-wetland nodes that were downdrain of the wetlands tended to experience greater flooding. We find our results to indicate that wetlands do not always reduce flood risk in an urban stormwater management system, and under certain conditions may even increase this flood risk. Cities planning the creation, restoration, conservation, or inclusion of wetlands should be certain to model changes to flood risk from extreme weather events under a variety of antecedent wetland storage conditions.