Convergent coastal-plain estuaries have been shortened by dam-like structures worldwide. We used 31 long-term water level stations and a semi-analytical tide model to investigate the influence of a dam and landward-funneling on tides and storm surge propagation in the greater Charleston Harbor region, South Carolina, where three rivers meet: the Ashley, Cooper, and Wando. Our analysis shows that the principle tidal harmonic (M2), storm surge, and long-period setup-setdown (~4–10 days) propagate as long waves with the greatest amplification and celerity observed in the M2 wave. All waves attenuate in landward regions, but, as they approach the dam on the Cooper River, a frequency dependent response in amplitude and phase progression occurs. Dam-induced amplification scales with wave frequency, causing the greatest amplification in M2 overtides. Model results show that funneling and the presence of a dam amplify tidal waves through partial and full reflection, respectively. The different phase progression of these reflected waves, however, can ultimately reduce the total wave amplification. We use a friction-convergence parameter space to demonstrate how amplification is largest for partial reflection, when funneling and wave periods are not extreme (often the case of dominant tides), and for full reflection, when funneling and/or wave periods are small. The analysis also shows that in the case of long period events (>day), such as storm surges, dams may attenuate the wave in funneling estuaries. However, dams may amplify the most intense storm surges (short, high) more than funneling with unexpected consequence that can greatly increase flood exposure.

We studied the temporal and spatial changes in the flood water elevation and variation in the surface extent due to the flooding resulting from Hurricane Florence (September 2018) using the observation from an Unmanned Aerial Vehicle Synthetic Aperture Radar (UAVSAR) and Sentinel-1. The novelty of this study lies in the estimation of changes in the flood depth during the hurricane, and investigating the best method. Overall, flood depths from SAR were observed to be well-correlated with the spatially distributed ground-based observations (R2 = 0.79 to 0.96). The corresponding change in water level (∂h/∂t) compared well between the remote sensing approach and the ground observations (R2 = 0.90). This study highlights the potential use of SAR remote sensing for inundated landscapes (and locations with scarce ground observations), and it emphasizes the need for more frequent SAR observations during flood to provide spatially distributed and high temporal repeat observations of inundation.

As tides propagate inland, they become distorted by channel geometry and river discharge. Tidal dynamics in fluvial-marine transitions are commonly observed in high-energy tidal environments with relatively steady river conditions, leaving the effects of variable river discharge on tides and longitudinal changes poorly understood. To study the effects of variable river discharge on tide-river interactions, we studied a low-energy tidal environment where river discharge ranges several orders of magnitude, the diurnal microtidal Tombigbee River-Mobile Bay fluvial-marine transition, using water level and velocity observations from 21 stations. Results showed that tidal attenuation was reduced by the width convergence in seaward reaches and height convergence of the landward backwater reaches, with the channel convergence change location ~40-50km inland of the bayhead and seaward of the largest bifurcation (~rkm 90-100). River events amplified tides in seaward regions and attenuated tides in landward regions. This created a region of river-induced peak amplitude seaward of the flood limit (i.e., bidirectional-unidirectional current transition) and passed more tidal energy inland. Tidal currents were attenuated and lagged more with river discharge than water levels, making the phase lag dynamic. The river impacts on the tides were delineated longitudinally and shifted seaward as river discharge increased, ranging up to ~180 km. Results indicated the location and longitudinal shifts of river impacts on tides in alluvial systems can be estimated analytically using the ratio of river discharge to tidal discharge and the geometry convergence. Our simple analytical theory provides a pathway for understanding the tide-river-geomorphic equilibrium along increasingly dynamic coasts.