STORMWATER RUNOFF AND TIDAL DISCHARGE: INVESTIGATIONS OF FIVE TIDAL CREEKS FROM THE CENTRAL COAST OF SOUTH CAROLINA

Changes in the natural environmental related to urbanization lead to anthropogenic stress in coastal regions, such as increases in stormwater volume and pollutants entering aquatic ecosystems. These stresses can be observed as alterations in the hydrology and ecology in tidal creeks and salt marsh wetlands. In this study, we collected and modeled hydrology data for five different ebb-dominant tidal creeks along the central coast of South Carolina. Water velocity and cross-sectional area of flow were observed under a variety of tidal conditions over multiple years using an acoustic Doppler current profiler (ADCP). Hydraulic geometry relationships were developed using measurements of width, depth and velocity to estimate the discharge of tidal creeks. Total discharge volume (or tidal prism) and nutrient loading was calculated from modeled time-discharge regression equations. Estimates of runoff volume entering the creek system were modeled for four different unit hydrograph design storm events (corresponding to local standards for volume and peak flow reduction requirements). These tidal flow and stormwater runoff models were also used to evaluate the thresholds at which stormwater runoff volume surpasses the volume of the tidal prism, which could result in depression of salinity levels and/or increase in pollutants that would be detrimental to the biota of these tidal creek ecosystems. A presumptive total maximum daily load (TMDL) for total nitrogen (TN) and total phosphorus (TP) was developed for four of the creeks using the results. The goal was to inform regulators and managers about potential numeric criteria for these nutrients in estuarine waters in this region. Understanding these relationships between hydrology, water quality, and development in tidal creek watersheds will help managers protect the resiliency and productivity of these valuable ecosystems.