An Experimental Study on Physical Factors Affecting Dispersion and Dilution of the Main Street Stormwater Outfall Pipe in North Myrtle Beach, South Carolina

Understanding how stormwater will disperse from outfall pipes in the coastal environment is a critical aspect of discharge pipe design and evaluation, ensuring that environmental quality standards are met. In order to understand the dispersal, one must consider the processes of dispersion, the characteristics of the discharge from the pipe, the ambient conditions of the discharge area and how these factors are coupled together. Stratification, currents and waves can affect the dispersion and dilution of an outfall’s discharge. Estimates of dilution are based on the relative strength of environmental conditions such as wave orbital velocities, tidal currents and stratification, relative to discharge characteristics, such as density and flow rate. In this study, we investigate the ambient conditions and discharge characteristics of a stormwater outfall pipe in North Myrtle Beach, SC. Previous studies on stormwater outfall dispersion and dilution in the area provide only qualitative descriptions of outfall discharge behavior based on assumed conditions. This study evaluates the percentage of time the dilution of the discharge is tidally dominated, stratification dominated, wave dominated or a combination of these factors over the study period. This objective is accomplished by making in situ measurements of velocity, temperature and salinity of both the discharge and ambient conditions using Acoustic Doppler Velocimeters, an Acoustic Doppler Current Profiler, and temperature and conductivity sensors. The results characterize typical conditions expected close to shore along the South Carolina coastline, and we assess their impact on the dispersion of stormwater from this outfall. This information is valuable for modeling the dispersion of contaminants in this environment and provide accurate data on expected conditions for engineering design purposes.