Hydrological Assessments of Tidal Creeks to Inform Nutrient Management Recommendations
Ellis, Kathryn Keller
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The purpose of this study was to collect and model hydrology data in four ebb-dominant tidal creeks in the central coast of South Carolina to inform regulators and managers about numeric criteria for nitrogen and phosphorus. Discharge, velocity, and creek geometry (width, depth, and cross-sectional area) were measured with an acoustic Doppler current profiler during multiple campaigns of differing tidal conditions over a two year period (2015-2016). The total discharge volume (or tidal prism) was calculated from modeled time-discharge regression equations, and was combined with Total Nitrogen and Total Phosphorus concentrations to calculate loads. A presumptive Total Maximum Daily Load was created for each creek based on the modeled nutrient loads, as well as modeled stormwater runoff inputs. Results showed that smaller, urbanized creeks may be less resilient to stormwater inputs than larger creeks in forested watersheds. Hydraulic geometry curves were developed to define how stable creeks behave, and help predict the behavior of ungauged creeks. Additionally, the porewater of adjacent tidal marshes was monitored and the results indicated that the smaller, more urbanized creeks had the highest porewater nutrient concentrations, indicating that porewater may be driving nutrient loading. Finally, this study found that the unique hydrology of bidirectional flow in tidal creeks may require the expansion of the traditional watershed delineation practices focusing on the upstream (headwaters) areas to also include some downstream contributing areas.