Research on urban flood hydrology—especially in small watersheds—is urgently needed in response to a dearth of small catchment studies, rapid urban growth, increased flash flooding, and precipitation variability. This study compares stormflow between three small catchments in the South Carolina Sandhills of the USA: two heavily urbanized, and one forested catchment that serves as a proxy for pre-urban conditions. Comparisons of runoff volume, runoff coefficient, peak discharge, and shape of dimensionless unit hydrographs (DUH) show significantly larger stormflow volume and peak instantaneous discharge in the urban catchments that exceed those in the forested catchment by more than an order of magnitude. Changes in hydrograph shapes are visualized by DUHs and measured by skewness and kurtosis. Urban kurtosis values of DUHs are significantly higher than forested kurtosis values (p-values < 0.01) implying a difference in timing of stormflow volumes. In addition to changes in hydrograph shapes, urban runoff coefficients are significantly higher (p < 0.0001) than forested values. Urban times-to-peak were correlated with rainfall duration. Urbanization of hilly, forested areas with sandy soils, such as the Sandhills environment investigated in this study, can cause pronounced increases in runoff and peak discharge that result in substantial non-stationarity and increased flood risks. These hydrologic changes reveal a substantial shift in stormwater responses to anthropogenic alterations that represent a pervasive and on-going aspect of the “Anthropocene”.
