Rainfall-Runoff Mechanisms and Flood Mitigation in a Coastal Watershed with Numerous Wetlands and Ponds

This study analyzed mechanisms of flooding in Blackwater River Watershed, located in coastal Virginia and hydraulically connected with mid-Atlantic Ocean. The analysis was based on the examination and simulation of the rainfall-runoff relationship, and such an analysis is very important for conventional water resource management and dealing with hydrologic extremes (e.g., floods and droughts, as well as ecological and pollution discharges). The rainfall-runoff relationship is a quantitative description of the hydrologic cycle, a dynamic process that can be interactively influenced by various factors, namely climate, topography, soils, land use and land cover, and land management practice. In the past 60 years, there is no significant changes in precipitation patterns, so climate change can be downplayed. The rainfall-runoff relationship has not been changed by human activities and was found to be independent of drainage areas within the watershed. The overall storage capacity tended to be smaller in an upstream than a downstream drainage area. The drainage area above Dendron, Virginia, had a runoff coefficient of zero to 0.6, while the drainage area above Franklin had a runoff coefficient of 0.05 to 0.32. The observed data at Dendron and Franklin, Virginia, indicated that baseflow accounted for more than 50% of the total streamflow at the annual scale and in spring and winter. Such a percentage was smaller in summer and fall because a higher evapotranspiration lowered the water table. Regardless of the seasons, the shallow aquifer beneath the watershed was discharging groundwater into the Blackwater River all the time. Although the current Soil and Water Assessment Tool (SWAT) model had limitations in mimicking the baseflow variations and representing the storages across the study watershed, it was judged to be good enough for the model to be used for screening possible flood-mitigation scenarios. Moreover, the historical floods incurred by the study watershed were primarily caused by storms with an above-normal intensity and/or duration. Using gated outlet structures to regulate the water levels in the storages can be a cost-effective flood-mitigation measure for the Blackwater River Watershed