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Abstract 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. 

Abstract Notwithstanding the large number of studies on bedforms such as dunes and antidunes, predicting equilibrium bedform type and geometry for a given flow regime, sediment supply and caliber remains an open problem. Here, we present results from laboratory experiments specifically designed to study how upper regime bedform type and geometry vary with sediment supply and caliber. Experiments were performed in a sediment feed flume with flow rates varying between 5 and 30 l/s and sand supply rates varying between 0.6 and 20 kg/min. We used both uniform and non‐uniform sands with geometric mean diameters varying between 0.22 and 0.87 mm. Analysis of our data and data available in the literature reveals that the ratio of total (bedload plus suspension) volume transport rate of sediment to water dischargeQ_s/Q_wplays a prime control on upper regime equilibrium beds. Equilibrium bedforms transition from washed out dunes (lower regime) to downstream migrating antidunes (upper regime) forQ_s/Q_wbetween 0.0003 and 0.0007. For values ofQ_s/Q_wgreater than 0.0015, the bedform length increases withQ_s/Q_w. At these high values ofQ_s/Q_w, equilibrium in fine sand is characterized by upstream migrating antidunes, cyclic steps, and significant suspended load. In experiments with coarse sand, equilibrium is characterized by plane bed with bedload transport in sheet flow mode. Standing waves form at the transition between downstream migrating antidunes and upstream migrating bedforms. 

Intermittent floodplain channels are low‐relief conduits etched into the floodplain surface and remain dry much of the year. These channels comprise expansive systems and are important because during low‐level inundation they facilitate lateral hydraulic connectivity throughout the floodplain. Nevertheless, few studies have focused on these floodplain channels due to uncertainty in how to identify and characterize these systems in digital elevation models (DEMs). In particular, their automatic extraction from widely available DEMs is challenging due to the characteristically low‐relief and low‐gradient topography of floodplains. We applied three channel extraction approaches to the Congaree River floodplain DEM and compared the results to a channel reference map created through numerous field excursions over the past 30 years. The methods that we tested are based on flow accumulation area, topographic curvature, and mathematical morphology, or the D8, Laplacian, and bottom‐hat transform (BHT), respectively. Of the 198 km of reference channels the BHT, Laplacian, and D8 extracted 83%, 71%, and 23%, respectively, and the BHT consistently had the highest agreement with the reference network at the local (5 m) and regional (10 km) scales. The extraction results also include commission “error”, augmenting the reference map with about 100 km of channel length. Overall, the BHT method provided the best results for channel extraction, giving over 298 km in 69 km2 with a detrended regional relief of 1.9 m. Further, these analyses allow us to shed light on the meaning and use of the term “low‐relief landscapes”. 

Flow regime, sediment supply and base level control geometry and evolution of alluvial channels and floodplains. Single thread rivers subject to constant forcing can reach equi-librium conditions in which the amount of sediment deposited on the floodplain through point bar deposition and overbank sedimentation is balanced by erosion of floodplain sed-iment through channel migration. At equilibrium, floodplain slope and sediment size dis-tribution, reach-averaged channel geometry (width and depth) and channel migration rates do not change in time. In response to changes in sediment supply and floodplain width, channel geometry and migration rate, floodplain slope and size distribution are expected to evolve in space and time. Predicting this response remains an open problem for geoscien-tists and engineers. Here we use an equilibrium solution of a 1D morphodynamic frame-work of channel-floodplain evolution to investigate how equilibrium conditions change as a function of sediment supply and floodplain width. Sediment is modeled here as a mix-ture of two grain sizes, sand and mud. Channel migration rate and width are functions of near-bank flow properties and floodplain characteristics. We zero the model using input parameters based on the pre-1930 ~ reach of the Minnesota River from Mankato to Jordan, USA, where data is available for proper field scale model verification. We then use the validated model to quantify the long-term (equilibrium) response of the schematic reach to changes in sediment supply magnitude and size distribution, as well as to changes in floodplain width. 

Flow regime, sediment supply and base level control geometry and evolution of alluvial channels and floodplains. Single thread rivers subject to constant forcing can reach equi-librium conditions in which the amount of sediment deposited on the floodplain through point bar deposition and overbank sedimentation is balanced by erosion of floodplain sed-iment through channel migration. At equilibrium, floodplain slope and sediment size dis-tribution, reach-averaged channel geometry (width and depth) and channel migration rates do not change in time. In response to changes in sediment supply and floodplain width, channel geometry and migration rate, floodplain slope and size distribution are expected to evolve in space and time. Predicting this response remains an open problem for geoscien-tists and engineers. Here we use an equilibrium solution of a 1D morphodynamic frame-work of channel-floodplain evolution to investigate how equilibrium conditions change as a function of sediment supply and floodplain width. Sediment is modeled here as a mix-ture of two grain sizes, sand and mud. Channel migration rate and width are functions of near-bank flow properties and floodplain characteristics. We zero the model using input parameters based on the pre-1930 ~ reach of the Minnesota River from Mankato to Jordan, USA, where data is available for proper field scale model verification. We then use the validated model to quantify the long-term (equilibrium) response of the schematic reach to changes in sediment supply magnitude and size distribution, as well as to changes in floodplain width.