Journal of Waterway, Port, Coastal, and Ocean Engineering

Process-based models are suitable tools for reproducing storm-driven erosion. However, their performance has been mainly examined on mild-slope sandy beaches and their use on steep beaches still represents a challenge. Here, open-source process-based model XBeach experiments were combined with topographical measurements collected for two storms (16- and 5-year return period) to obtain a reliable model. The model parameters “facua” (parameterized wave asymmetry and skewness sediment transport component), “bermslope” (upslope transport term for semireflective beaches), and “wetslope” (critical avalanching submerged slope) were utilized for calibration and validation. The 16-year storm simulations on an exposed beach revealed that whether bermslope increased and “facua” must be reduced, and vice versa, to properly simulate erosion. Adding bermslope provided excellent results for these storms when using facua and wetslope values close to the recommended values. In a groin-protected site, XBeach was successfully calibrated and validated for the tested storms using these parameters, although with different values. These experiments demonstrated that the appropriate use of these parameters can satisfactorily simulate morphological changes on steep beaches for different hydrodynamic conditions and coastal settings (exposed and groin protected).

Swash flows are commonly modeled using the nonlinear shallow water equations (NSWEs). In the derivation of the NSWEs, directly from depth-averaging the Navier–Stokes equations, a so-called momentum correction factor, β, emerges. In this study we present a numerical model of the NSWEs that includes β, which is allowed to vary in space and time, and feedback onto the flow. We apply this model to a swash flow, by making use of the vertical flow structure calculated by use of the log-law boundary layer and free flow region. We thereby examine its influence on the swash-flow predictions of a dam-break swash event described in the literature. The numerical results show that the momentum correction factor has a significant effect on the shoreline motion, and flow adjacent to the shoreline, which results in an overprediction of the shoreline with respect to the standard (β = 1, NSWE) approach. Given that consideration of β should yield a more complete description of the swash dynamics, the implication is that the log-law boundary layer model does not describe the flow structure in the swash tip region well. The implication of this is that to achieve accurate modeling at the flow uprush tip, at which point the largest bed shear stresses are typically exerted, a different submodel is required in that vicinity. Equally, it suggests that classical NSWEs also cannot describe the flow at the tip well, and that accurate prediction is achieved despite this inherent deficiency.

The rehabilitation timing of a damaged rubble mound structure depends on the remaining capacity of the deteriorated structure. A laboratory experiment was conducted in a wave flume to measure the remaining capacity of a low-crested rubble mound inside the surf zone on a sand beach. The formation of a bar and trough feature modified wave conditions at the toe of the structure. Wave transmission over and through the structure increased with lowering of the mound crest. The mound with a double armor layer on smaller core stones was exposed to irregular wave action lasting 22.2 h. Some of the core stone was visible through holes of the thinned armor layer but remained in place. Crest lowering reduced the wave action on the damaged mound. On the other hand, the mound with a single armor layer did not stabilize itself because of core stone removal after 7.8 h. The rubble mound structure was resilient as long as the core stone was protected.

The scattering of gravity waves by vertically staggered multiple porous barriers having variable porosity is analyzed within the framework of linearized water wave theory in two dimensions. The barriers are assumed to follow quadratic pressure boundary conditions to account for energy dissipation with the changes in the wave height, which is often neglected in the case of Darcy’s law. A generalized code based on the dual boundary element method (DBEM) is developed for solving the boundary value problem. Three different wave barrier configurations are considered with the same volume of materials required for its construction to identify the best-performing barriers. The hydrodynamic performance of the barriers with progressively decreasing porosity is found to be better than the barriers with gradually increasing porosity or barriers with constant porosity. The scattering coefficients attain optimal at integer multiples of half the wavelength. It is appropriate to select relative spacing in the range of 0.2–0.3 for better hydrodynamic performance for any field design condition. For deeper water depths (k₀h > π), increasing the relative submergence depth beyond 0.2 is insignificant to the change in hydrodynamic performance. For the practical field range of k₀h and for a threshold wave transmission coefficient of 0.2, the appropriate relative submergence depth is 0.6. The results of this study would help in the hydrodynamic design of a progressive wave absorber.

The bed level change in rivers and estuaries due to several dynamic parameters such as tide, waves, river discharge, and wind is a common problem. The current practice is to predict the changes through numerical modeling and compare it through field measurements. The empirical relations to be adopted for the sediment transport estimation form an integral part of morphodynamic models. In this study, five different total sediment transport formulae have been incorporated in the morphodynamic routine of a circulation model. The hydrodynamic model accounts for tide-induced flows, tide being the dominant forcing in the estuaries considered for the study. The preceding sediment transport formulae for predicting bed level changes (erosion and accretion) are subjected to an assessment, through statistical approaches, to find the most suitable formula for different hydrodynamic settings of a macrotidal estuary (Hooghly) and a microtidal estuary (Cochin). The results are validated with the field measurements and finally the appropriate empirical formula is designated based on a detailed analysis. The AW model outscores in the Brier skill score (BSS) analysis and thus qualifies to be most suited sediment transport model for the macrotidal estuarine dynamics with a score of 0.55, and the AW model is followed by the VR model with a score of 0.52. As per BSS, the EH model falls under the Good model category with a score of 0.48, for the microtidal estuary and thus qualifies to be the better suited sediment transport model among the models considered.

Abstract

Colocated sediment pore pressures at depths of approximately 0.02 and 0.22 m below the sand surface and near-bed water velocities were measured for approximately 2 weeks in approximately 1 m mean water depth on an ocean beach near Duck, North Carolina. These measurements suggest that storm wave-driven liquefaction processes may enhance local shoreward sediment transport. During the passage of tropical storm Melissa, wave heights in 26-m water depth (NDBC 44100) were 1–4 m, and storm surge (approximately 1 m) and wave-induced setup increased the water depth on the beach. Upward vertical gradients in pressure heads between the sensors increased with the storm approach, with the largest values observed before the maxima in local wave heights, wave periods, and water depths. The large gradients in pore pressure exceeded several liquefaction criteria and usually occurred when near-bed velocities were upward- and shoreward-directed.

Practical Applications

Observations on an ocean beach show that during storms the water pressure below the sand surface can be greater than the pressure near the sand surface. When that difference in pressure (called a pressure gradient) becomes large, the sediments fluidize, that is, they act like a fluid in a process called liquefaction. Unlike dry or partially wet sediments, the fluidized sand is moved easily by currents. During tropical storm Melissa, large upward-directed pressure gradients fluidized the sediment, usually as the crest of an ocean wave passed over the surface. Therefore, the sediments possibly became free to move as a liquid when the currents from waves were directed both upward and toward the shore. The liquified sediments could be transported shoreward, leading to changes to the sand surface (accretion and erosion). This phenomenon is important to understand because it contributes to how, where, and when sediment might be transported on an ocean beach.