Reflection of Storm Surge and Tides in Convergent Estuaries With Dams, the Case of Charleston, USA.

Convergent estuaries have been shortened by dam‐like structures worldwide. Here, we evaluate 31 long‐term water level stations and use a semi‐analytical tide model to investigate how landward‐funneling and a dam influence tidal and storm surge propagation in the greater Charleston Harbor region, South Carolina, where three rivers meet: the Ashley, Cooper, and Wando. Results show that the phase speed and amplification of the principal tidal harmonic (M2) is larger than other long waves such as storm surge (∼1–4 days) and setup‐setdown (∼4–10 days). Further landward, all waves attenuate, but, as they approach the dam on the Cooper River, a frequency dependent response in amplitude and phase progression occurs. A semi‐analytical tidal model shows that funneling and the presence of a dam amplify tidal waves through wave interference from partial and full reflection, respectively. The different phase progressions of the reflected waves interact with the incident wave to increase or decrease the summed overall wave amplitude. Using a friction‐convergence parameter space, we demonstrate that dominant tides in 23 estuaries and the tidal, storm surge, and setup‐setdown waves in the Cooper River can be delineated into three regimes that describe landward amplification or attenuation associated with funneling, a dam, or both. The regime of each tidal constituent is consistent but can change with the duration and height of each storm surge event; dam associated wave interference can attenuate long‐duration events, while the most intense events (short duration, high water) are amplified by dams more than funneling and greatly increase flood exposure. Plain Language Summary: Most ports and cities are located along estuaries and deltas where flood hazards are increasing partially due to human modifications of channel geometry and land use. Dams, salt barrages, and surge barriers are common in estuaries. They modify estuarine geometry, regulate river flow, protect against flooding, and prevent salt intrusion. Many estuaries are naturally convergent, wide near the sea and narrower landward. Dams are barriers which shorten an estuary. Like ocean swell at a seawall, tides reflect off dams and often increase tidal range. Here we investigate how dams and landward changes in estuarine geometry influence tides and storm surges. Using measurements from the greater Charleston Harbor, SC, we find that constructing a dam either increases or decreases water levels, depending primarily on estuary convergence and surge event duration, as well as flow resistance and river flow. The presence of dams results in higher water levels, and thus flood exposure, when convergence is weak and storm surge at the sea has a short duration and high water levels. Further, channelization that extends inland, resulting from dredging of shipping channels, increases the magnitude and seaward extent of dam effects with increasing flood risk. Key Points: Estuarine geomorphic funneling and dams produce partial and full reflections, respectively, with magnitudes that are frequency dependentThree convergence regimes emerge: dominant tides have near peak amplification, overtides attenuate, and long duration surges mildly amplifyDams reflect and amplify long waves—increasing flood exposure—the most in weakly convergent estuaries with low friction [ABSTRACT FROM AUTHOR]