THE EFFECTS OF SEASONAL VARIABILITY, IMPERVIOUS SURFACES, AND SOIL PERMEABILITY ON THE FLASHINESS OF HEADWATER STREAMS IN THE PIEDMONT OF NORTH CAROLINA

Impervious surfaces within headwater drainage basins can result in profoundly flashy discharge in first order streams and can significantly alter their morphology. Three first order streams drain the Davidson College campus including a wooded basin in the Davidson College Ecological Preserve, a basin consisting of manicured landscape and buildings from Patterson Court, and a mostly urbanized basin from Baker Sports Complex. The variability of the landscape provides a natural laboratory to examine how basin variability impacts how streams respond to rain events. Using ArcGIS, we mapped basin area and impervious surfaces. This data was then used to normalize discharge based on drainage basin area, enabling a more accurate comparison storm events.

Discharge from each stream has been recorded at 15-minute intervals using a pressure transducer and data logger since the fall of 2013. While the extreme flashiness has often damaged individual gauges, our compiled results show important behavioral differences between the basins under a variety of seasonal conditions. Our preliminary results highlight a number of factors that impact the drainage basins in different ways. For instance, the most natural basin varies in its flashiness depending on the season and presumably the amount evapotranspiration occurring in the basin. These seasonal differences led us to additionally hypothesize that sediment size within the stream may vary seasonally as well, as the stream energy balance fluctuates and we present the results of grain size analysis from summer and fall of 2015. More broadly, seasonal variability between basins and a non-linear relationship between impervious surface and flashiness suggest that the system is more complicated than a simple relationship between impervious surfaces and flashiness. Other likely factors include soil permeability and storm sewer transmission rates. Since soil permeability and storm sewer design and quality are the legacy of historical land use, stream flashiness may be the result of previous anthropogenic landscape alterations as much as modern ones.