On comparison of peak flow reductions, flood inundation maps, and velocity maps in evaluating effects of restored wetlands on channel flooding

With growing interest in using restored and/or new constructed wetlands as additional flood mitigation systems, it has become essential to investigate the effectiveness of wetlands in reducing the risk of flood hazards. This study examines and compares evaluation techniques based on peak flow estimation, flood inundation area estimation, and velocity maps to assess the performance of wetlands in reducing flood hazards in a Midwestern watershed affected by changing precipitation patterns. School Branch, one of the main branches of Eagle Creek watershed, located in central Indiana, was selected as a study area. A coupled hydrologic-hydraulic model approach was proposed to simulate the outflow generated by various design storms. Then the water profile and water velocity in the river were calculated and finally the inundation and velocity maps were generated. Also, the effect timing of storm on runoff modulated by wetlands was investigated by applying design storms to different months of year. The results of this study show that wetlands can reduce the peak flow up to 42%, flood areas up to 55% and maximum velocity up to 15%. Additionally, a sub-basin that had maximum peak flow reduction for a specific design storm did not necessarily have a simultaneous maximum reduction in flood inundation area, indicating the variability in how wetlands mitigate flooding in different sub-basins of a watershed. Also, deeper wetlands are more effective in reducing the impacts of storms with higher return periods. For example, at the study site, deeper wetlands (D=1.8m) were able to reduce peak flows up to 20% for a 500 year storm whereas shallow wetlands (D=0.5m) reduced the peak flow by 11% for the same storm.