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.

Research on bedrock rivers primarily focused on bedrock incision and, to the best of our knowledge, morphodynamic models have not yet considered the variability of sediment grain size and the presence of small scale bedforms in low-slope (slope < 0.005) bedrock reaches. Further, very few models can quantify spatial and temporal changes in the fraction of channel bed covered with alluvium (alluvial cover) within these reaches. Here we present a novel formulation of alluvial morphodynamics of low-slope bedrock reaches transporting non-uniform bed material. The formulation is implemented in a one-dimensional model and validated against laboratory experiments on bedrock reaches downstream of stable alluvial-bedrock transitions, where the flow accelerates in space. The validated model is used to study the alluvial morphodynamics of bedrock reaches upstream of stable bedrock-alluvial transitions. Equilibrium results show that the interactions between flow, sediment transport and non-erodible bedrock surface result in a flow decelerating in the streamwise direction. The effects of this spatial flow deceleration are 1) a streamwise increase in alluvial cover, and 2) the formation of a pattern of downstream coarsening of bed surface sediment. We then investigated the effects of sea level rise/fall on the location of alluvial-bedrock and bedrock-alluvial transitions. In the case of sea level rise, alluvial-bedrock transitions migrate downstream and bedrock-alluvial transitions migrate upstream. Opposite migration directions are expected in the case of sea level fall.

Key Points: 8 • Bed heights of bedload-dominated rivers modeled by Distinct Element Method (DEM) 9 simulations follow a Gaussian distribution. 10 • The standard deviation of bed height, s η , increases as the shear stress increases. 11 • Peak entrainment of bed particles occurs at a distance 2s η above the average bed 12 height. Abstract 14 We investigate the statistics of bed height variability and particle entrainment height un-15 der steady state bedload transport conditions using distinct element method (DEM) sim-16 ulations. We do so in the context of a theoretical probabilistic formulation derived to 17 better capture spatial variation in sediment exchange between bed material load and al-18 luvial deposits (Parker et al., 2000). Using DEM simulations, we set the foundation for 19 a physics-based closure of this probabilistic framework toward its practical implemen-20 tation. Towards this, we perform DEM simulations for bedload transport under simi-21 lar boundary conditions to those of Wong et al. (2007) laboratory experiments: a bed 22 of gravel particles of median grain size 7.1mm with lognormal grain size distribution trans-23 ported under bed shear stresses ranging from τ 0 = 8.70 to 13.7 Pa. We first validate 24 these simulations by demonstrating that they capture measurable transport and height 25 variations from experimental measurements. We then compute the statistics of both the 26 bed height and entrainment height as they vary with bed shear stress. We find that vari-27 abilites in both bed height and entrainment height variabilities follow Gaussian distri-28 butions, for which: (1) the standard deviation of bed height variability s η increases with 29 shear stress, and (2) the peak entrainment height occurs a distance of twice the stan-30 dard deviation of bed height variability (2s η) above the mean bed height. We discuss 31 implications of these results and next steps for understanding these transport statistics 32 under a broader range of conditions. 33

Notwithstanding the large number of studies on bedforms such as dunes and antidunes, performing quantitative predictions of bedform type and geometry remains an open problem. Here we present the results of laboratory experiments specifically designed to study how sediment supply and caliber may impact equilibrium bedform type and geometry in the upper regime. Experiments were performed in a sediment feed flume with flow rates varying between 5 l/s and 30 l/s, sand supply rates varying between 0.6 kg/min and 20 kg/min, uniform and non-uniform sediment grain sizes with geometric mean diameter varying between 0.22 mm and 0.87 mm. The experimental data and the comparison with datasets available in the literature revealed that the ratio of the volume transport of sediment to the volume transport of water Qs/Qw plays a prime control on the equilibrium bed configuration. The equilibrium bed configuration transitions from washed out dunes (lower regime), to downstream migrating antidunes (upper regime) for Qs/Qw between 0.0003 and 0.0007. For values of Qs/Qw greater than those typical of downstream migrating antidunes, the bedform wavelength increases with Qs/Qw. At these high values of Qs/Qw equilibrium bed configurations with fine sand are characterized by upstream migrating antidunes or 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 bed configurations with upstream migrating bedforms.