The role of dams on tides and storm surge, Charleston Harbor

Tides and storm surge events are extending far inland due to human development (e.g., sea level rise, decreased river flows, dredged channels), increasing interactions with inland structures, such as dams. Structures commonly restrict flow, which reflects energy, locally amplifying and accelerating tidal waves. How dams affect natural (convergent) estuaries/rivers remains poorly understood. To investigate the longitudinal variability of tides and storm surge, we study the Greater Charleston Harbor, where tides extend to a dam (Cooper River) and swamps (Ashley and Wando Rivers), using long-term observations from 32 stations (water levels, velocity) and a 1D analytical box model. Preliminary observational results captured increasing tidal amplitudes, speeds, and velocity-water level phase lags in seaward reaches of strong landward convergence, revealing partial reflection, and near the Cooper River dam, revealing full reflection. In all other reaches, tides attenuated, creating mid estuary regions of peak amplitude, and on the Cooper River, an additional region of minimum amplitude. Observations and modeling show peak amplitude locations were variable with longer periods and smaller incident amplitudes, which oscillated over fortnights and 18.6 years, peaking further landward. They also extended the reflected wave further from the dam, shifting quasi-nodal points of low amplitude further seaward. This created a complicated interaction between partial reflection from convergence and full reflection from the dam. The tidal observations and modeling were extended to storm surge events, which were similarly captured as propagating waves. Surge waves in all three rivers amplified with geomorphic convergence, causing amplitudes and water levels to peak further inland than diurnal tides. On the Cooper River, peak amplitudes and water levels occurred at the dam, where long reflected waves elevated most of the system. Preliminary results suggest dam effects on local systems depend on wave period and amplitude, other sources of reflection (e.g., convergence), and the longitudinal location of the dam. While dams may reduce river flooding, this study shows dams can elevate water levels from tides and storm surge, potentially increasing marine flooding risks for inland communities as sea levels rise.