Hydraulic analysis of stream restoration on flood wave propagation

Channel and floodplain restoration can enhance the ability of a channelized or incised reach to temporarily store the flow and dissipate the energy of passing flood waves. Elements of restoration design that can enhance flood wave attenuation include the introduction of meanders, which reduces channel slope and increases channel length, restoring channel-floodplain connectivity, and re-vegetating banks and the floodplain. I examined the efficacy of stream restoration to attenuate floods given the scale at which it occurs and the magnitude of channel change possible by quantifying flood wave attenuation on two restored reaches located in urban and rural catchments and on hypothetical stream reaches representing median values of stream restoration projects in North Carolina using a dynamic flood routing model (UNET in HEC-RAS). Floods routed in impaired and restored reach models of field sites either exhibited very small augmentation to attenuation, largely due to assumed increases in floodplain roughness, or a decrease in attenuation. Modeled sensitivity results indicated that decreases to slope and increases to channel and floodplain roughness demonstrated the highest relative impact to attenuation. Floods of intermediate magnitude (between 2 and 5 yr recurrence interval) were impacted most by restoration, especially those confined to the channel under the impaired morphology but able to access the floodplain under the restored morphology