Can Restoring Tidal Wetlands Reduce Estuarine Nuisance Flooding of Coasts Under Future Sea‐Level Rise?

Wetland restoration is an increasingly popular nature‐based method for flood risk mitigation in coastal communities. In this study, we present a novel method using hydrodynamic modeling and harmonic analysis to quantify wetlands' ability to reduce future nuisance flooding. The method leverages a hydrodynamic model calibrated to present day data and was run for a range of future sea‐level rise (SLR) and wetland restoration scenarios to quantify changes to tidal harmonic amplitudes and phases. The harmonic constituents are used to generate water surface elevations over a time period of interest (e.g., one year) and compared to critical exceedance thresholds such as levee elevations. Then, changes to nuisance flooding are calculated by counting the number of hours critical thresholds are exceeded under different SLR and wetland restoration scenarios. We applied the method to Coos Bay, Oregon, USA as a test case. We found restoration reduces the number of hours nuisance flooding occurs in downtown Coos Bay from 15 hr (present day conditions) to 0 hr (fully restored condition) under median SLR (82 cm by 2100). Restoration had spatially variable impacts on reducing peak flood elevations with minimal impacts near the estuary mouth and greatest impact 32 km inland. The effectiveness of restoration was heavily dependent on future SLR. Restoration was maximally effective in 2050 under all SLR scenarios, less effective in 2100 under median SLR, and not effective under high SLR. Modeling results suggest increased tidal prism and accommodation space are driving restoration‐associated reductions in tidal amplitudes. We developed a new way to estimate the flood reduction co‐benefits resulting from wetland restoration in coastal bays. This is accomplished through the use of harmonic analysis trained on computational hydrodynamic models, which helps reduce computational runtimes to allow long‐term analysis and run a greater number of scenarios than with physics‐based modeling alone. We then applied this methodology to a study site in Coos Bay, Oregon, USA. We found that restoration can significantly reduce expected flooding in both downtown regions and along major transportation corridors in 2100 under low to moderate sea‐level rise (SLR) scenarios. The expected magnitude of the flood impact reduction depended on location within the estuary and the SLR scenario. Results indicate restoration is more effective at reducing tidal flooding at inland sites, with less impacts near the coast. We also found that restoration effectiveness was driven by increased flood accommodation space, allowing floodwaters to spread. We developed a model which estimates reductions in nuisance flooding resulting from wetland restoration under sea‐level rise (SLR) We apply this model to an estuary in the Pacific Northwest and find that attenuation is maximized at low and medium SLR We find that flood damage reduction is both spatially and temporally variable, with more benefits realized farther inland We developed a model which estimates reductions in nuisance flooding resulting from wetland restoration under sea‐level rise (SLR) We apply this model to an estuary in the Pacific Northwest and find that attenuation is maximized at low and medium SLR We find that flood damage reduction is both spatially and temporally variable, with more benefits realized farther inland