An enhanced low-complexity hydraulic model for assessment of floodplain rehabilitation alternatives

Alluvial floodplains provide many beneficial functions, including the storage of floodwaters that reduce peak downstream flows during storm events. Transport of sediment/nutrients to downstream lakes is slowed when floodplain flow residence time is increased allowing nutrient-rich particulates to settle out. Anthropogenic activity (e.g., construction of roads, culverts, berms, railroads, and landcover alterations) may disconnect floodplains or otherwise impair this function contributing to downstream flood damage and harmful algal blooms. This has resulted in increased interest in floodplain projects designed to restore function. Hydraulic modeling is integral to the assessment of potential floodplain reconnection, however, developing, calibrating, and applying these models is resource and data intensive. Low-complexity models (i.e., those that simplify process) are useful for rapid identification of hydrologic connectivity, especially over large scales but may not accurately represent local inundation extents. Previous work used Height Above Nearest Drainage (HAND; a low complexity model based on the elevation gain from the stream network to each point in the watershed along each point’s drainage flow-path) to evaluate flooding on the National Hydrography Dataset (NHD) reach scale. However, NHD reaches may span many geomorphic regimes; and applying HAND to NHD reaches results in averaging of these regimes. Stream geomorphic assessment (SGA) reaches are finer in resolution and more geomorphically-consistent than reaches at the NHD scale. In this work, we evaluate whether integration of the SGA-level data provide more local detail for the Mad River watershed in Vermont. The NHD and SGA reach results are compared with a calibrated HEC-RAS model to evaluate if the SGA-enhanced analysis improves the accuracy as compared to the NHD analysis. Future work will involve using this enhanced model to simulate and optimize suites of river and floodplain restoration projects.