Simulation of Nature-Based Management Scenarios for Restoring Thermal Pollution and Flood Risk in Urban Rivers

Urbanization causes flooding and thermal pollution as part of urban stream syndrome, which is characterized by rivers receiving warm runoff from impervious areas, more solar radiation due to absent riparian forests, and less cool inflows from subsurface waters. In this study, an updated version of the i-Tree Cool River model coupled with the US Army Corps of Engineers flood hazard model, HEC-RAS, is presented to address the effectiveness of the nature-based solutions on real-world river restoration challenges. The coupled model used the HEC-RAS model water surface profiles needed for flood hazard analysis in restoration planning, remotely sensed imagery of channel, solar radiation, and temperature data, and Streamstats estimated flows. The updated model was validated in steady state in a river within the New York City drinking water supply area and then used on a 17.5 km reach of the Los Angeles (LA) River in southern California where a multi-million dollar riverine restoration project is planned, to estimate temperatures for the base case and restoration scenarios. The model simulated the LA River average temperature in the base case at 29.5°C, which decreased by 0.3℃ when warm surface inflows were converted to cooler groundwater inflows by green infrastructure; by 0.7°C when subsurface hyporheic exchange was increased by removal of armoring and installation of riffle-pool bedforms; by 3.6℃ when riparian forests shaded the river; and by 6.4°C when floodplain forests were added to riparian forests to cool surface reservoirs and local air temperatures. The simulated decreases in river temperature lead to increased saturated dissolved oxygen levels, reaching 8.7 mg/L, up from the 7.6 mg/L in the base case scenario. The simulated restoration is capable of providing improved fish habitat and reducing eutrophication and hypoxic zones to assist real-world nature-based management solutions.