HYDROLOGICAL AND WATER QUALITY ASSESSMENT OF A RAPIDLY URBANIZING SOUTHEASTERN PIEDMONT WATERSHED
The purpose of this dissertation research was to assess the change in hydrological and watershed processes influencing water quality in a rapidly urbanizing SE Piedmont watershed. Specifically, this dissertation research assessed the effectiveness of engineered stormwater control measures (SCMs) and stream restoration projects in a rapidly urbanizing watershed to maintain the pre development hydrologic and water quality regime in compliance with local stormwater and water quality regulations. The hydrologic and water quality benefits of a network of the existing engineered SCMs and alternative engineered SCMs that included distributed backyard rain-gardens and additional offline bio-retention basins were simulated in the most developed sub-watershed of the study watershed using the Model of Urban Stormwater Improvement Conceptualization (MUSIC). Model simulation results indicated that the post-development simulation with existing engineered SCMs network in comparison to without-engineered SCMs network lowered the annual load of total suspended sediment (TSS), total phosphorus (TP), and total nitrogen (TN) by 56.7%, 50.7%, and 9.5%, respectively. Model simulations indicated that mandatory 85% and 70% TSS and TP annual load reductions, respectively could be obtained by diverting runoff from 70% and higher of the contributing drainage area of the existing engineered SCMs into additional offline bio-retention basins. The effectiveness of the existing engineered SCMs network in maintaining the predevelopment runoff hydrology of five developing sub-watersheds (10% to 54% suburban development) was evaluated with the unit hydrograph, unit impulse response, and Mann-Kendall trend test approaches. The measured reduction in peakflow discharge and increase in direct runoff coefficient and runoff duration is attributed to the engineered SCMs in the most developed sub-watersheds, whereas little difference in runoff response could be attributed to the stream restoration projects. The three approaches applied to assess the change in hydrologic responses from different BDC sub-watershed provided similar results. Finally, a residual mass balance approach was applied to assess the in stream transport and retention dynamics of sediment, nutrients, and organic carbon (OC) in two restored and two unaltered or "natural" stream reaches of the study watershed during different flow regimes. The restored stream reaches indicated a net retention of TSS, N (PN, TN, TDN, and DON), P (TP and PP), and OC during baseflow monitoring periods. Whereas, the restored stream reaches exhibited a net export of TSS, NO3-N, TP, PP, and POC during storm events. The predominately forested and unaltered stream reach exhibited a net retention of ortho-P and a decline in per unit flux of most of the other water quality constituents during baseflow and storm runoff events. The suburban unaltered stream reach with significant engineered SCMs indicated the downstream mobilization of most of the water quality constituents during baseflow and storm events. Overall, this dissertation provided a comprehensive assessment of the alterations of the hydrological and biogeochemical processes in an urbanizing SE Piedmont watershed and an assessment of the effectiveness of current Stormwater Control and Stream Restoration practices through stormwater modeling, analytical, and field based monitoring approaches.