Stormwater management is a multi-billion dollar enterprise in the United States that depends on human decisions at local to regional scales to achieve watershed scale environmental goals. This research project investigated how decision-making processes influence actions taken to manage stormwater and the subsequent environmental outcomes at the watershed scale. The project compared the Cleveland and Denver metropolitan regions, which are characterized by distinct institutional and environmental conditions, in order to distill general lessons about stormwater management dynamics in urbanizing regions.

The investigators determined how individual and institutional decision-making processes influence stormwater management actions through analysis of policy documents and a survey of stormwater managers (Objective 1). From a survey of stormwater managers across Cleveland and Denver metropolitan areas, we learned that stormwater managers’ primary priorities align with regional emphasis on quality versus quantity goals and stormwater co-benefit priorities align with individual stormwater managers’ environmental orientation (e.g., values, attitudes). The research team then quantified how stormwater management actions influence flow regime at watershed scales through a combination of field monitoring and modeling (Objective 2).  In an analysis of streamflow responses to storm events in Denver, we found that streamflow responses had shorter duration and higher peak flows in watersheds with more impervious surface cover, whereas there was little relationship between streamflow and impervious surface cover in the Cleveland area. With a meta-analysis of 52 modeling studies and new modeling efforts in Cleveland and Denver, we found that as a larger fraction of impervious area is treated by green stormwater infrastructure, the same size rain events produce lower peak streamflows and produce less street flooding. Using computer models, we found the magnitude of peak flow reduction is dependent on where green infrastructure is located in the watershed, the city’s climate, and the design choices made for each type of green infrastructure. As typically designed, permeable pavement is more effective at reducing peak flows and stormwater volumes than bioretention cells or swales. For green infrastructure to have a significant effect on urban streamflow, more than 14-20% of the impervious area needs to be treated by green infrastructure. However, across 20 US urban areas, more than half of the cities treated stormwater from less than 12% of their impervious areas. Further, the team evaluated how flow regime modifies the ecosystem health status of urban watersheds (Objective 3). Rates of photosynthesis were slowed by storms in Cleveland and Denver and took many days to recover to pre-storm levels. However, respiration was only minimally disturbed by storm events. Cumulatively, the greater sensitivity of photosynthesis to storm events and the long recovery time relative to the storm event return interval resulted in streams that were rarely autotrophic and might suffer from low oxygen conditions. The supply and movement of fine sediment varied dynamically between the three Cleveland watersheds and was influenced by stream geomorphic characteristics. Specifically, suspended fine sediments during large storms were found to be relatively low in the most impervious watershed in Cleveland. Findings from these objectives were integrated to predict environmental outcomes from stormwater management strategies through a Bayesian network model linking decision-making to environmental outcomes (Objective 4). This unified model demonstrated that decision makers wanting to reduce peak flows and improve stream health should focus their efforts on increasing the amount of green infrastructure rather than emphasizing particular types of green infrastructure.

New scientific insight into stormwater management for urban aquatic ecosystem health is targeted to inform future directions of stormwater programs in U.S. urban areas. This research provided education and training opportunities around engineered stormwater systems and urban watersheds for K-12, undergraduate, and graduate students through hands-on research and data-based course activities. Data generated from this project were used to generate K-12 lessons on urban ecosystems, disturbance, and ecosystem ecology, which can further be used to develop skills in data literacy (https://datanuggets.org/2023/04/surviving-the-flood/).

Last Modified: 10/04/2023
Modified by: David M Costello