Presenter/s: Erich Hester
Symposium Session: 2024 - 10 Big Scale Hydrology - Redefining Stream Power
Topics covered: climate change, flow augmentation, and hydraulics
ABSTRACT
Increased peak flows and associated flood risk, as well as decreased baseflow and associated impacts to floodplains and aquatic ecosystems, are common effects of human land use (e.g., urbanization, agricultural development, and certain forestry practices) that can be exacerbated by climate change. Enhancing floodplain capacity through river restoration to increase surface and groundwater storage along river networks has potential to mitigate such impacts. Yet this potential has been poorly quantified at the watershed scale. We simulated the effect of varying the amount and location of floodplain and Stage 0 restoration on surface water storage in a series of watersheds using the U. S. Army Corps of Engineers Hydrologic Engineering Center’s River Analysis System (HEC-RAS). First, we modeled a synthetic 4th-order watershed using average stream geometry and hydrology for the Virginia Piedmont with storms ranging in size from the 2-year down to monthly discharges. Model results indicate that Stage 0 techniques (simulated as low banks/shallow channel) were more effective at inducing floodplain exchange and flood wave attenuation than restoring bankfull floodplains (simulated as higher banks/deeper channel). The incremental effect of an individual restoration project varied depending on where it was in the 4th-order channel network, and on the amount of previous restoration that had already occurred in the watershed, with tradeoffs between enhancing flood attenuation and enhancing floodplain exchange. As expected, flood attenuation and floodplain exchange both increased with percent of channel network restored, yet Stage 0 approaches exhibited substantial flood attenuation potential even when implemented in relatively small portions of channel networks. Second, we simulated the cumulative effect of Stage 0 restoration (simulated as multiple/shallow channels, fully connected floodplain) along Meadow Creek, a tributary to the Grande Ronde River in the Columbia River Basin, similarly focusing on storm hydrographs. We again found Stage 0 restoration reduced peak flows at multiple scales, with potential to mitigate effects of land use change and climate change. Important future directions include extending this analysis to a) additional processes such as groundwater exchange to evaluate effects of restoration on baseflow between storms, and b) annual timeframes to assess effects on seasonal low flows and potential to mitigate climate impacts on such flows. Overall, our results indicate that floodplain and Stage 0 restoration approaches have substantial potential to reduce peak flows, increase floodplain storage, and increase system resilience to climate change. We emphasize the importance of viewing watersheds as a whole to understand the potential impacts of restoration projects, and watershed level planning to prioritize which stream reaches have the greatest benefit in supporting improved hydrologic response.