Journal of Waterway, Port, Coastal, and Ocean Engineering

A 2D barotropic numerical model is developed for the Gulf of Kutch with a view to synthesize available information on tides and currents in the Gulf. A comparison of model results with moored current meter observations shows that the model reproduces the amplitudes and phases of surface elevations and currents satisfactorily relative to the observations. We describe the important tidal constituents in the Gulf by using model results. Computed M₂ residual currents show the presence of topographically generated eddies. An analysis of momentum balance shows that the dynamics of tidal propagation in the Gulf is characterized by a balance between the pressure gradient and friction near the coast, whereas in the central region, local acceleration balances the pressure gradient. The natural period of oscillations of the Gulf was determined to be about 10 h based on the model results.

Topographic data from three different beach nourishment projects were analyzed using empirical orthogonal functions (EOFs) to determine the response of the fills at different time and space scales. The studied nourishments were undertaken at Ocean City, Md., Silver Strand, Calif., and Sylt, Germany, encompassing different nourishment schemes, forcing conditions, and general beach properties. Time series of measured beach topographies were used as input data in the EOF analysis, which produced maps describing the dominant spatial patterns in the data. These patterns displayed the effect of the fills, severe storms, and natural morphological features such as shore-attached shoals and rhythmic features. The evolution in time of the eigenvectors showed that the perturbation of the natural conditions typically caused by a fill disappears after about 1 year, provided that the material is available for cross-shore adjustment by the normal wave climate. If the fill material is placed high up on the beach, a series of severe storms may be needed to adjust the topography back to its natural state.

A linear, 2D numerical model is developed to investigate the hydrodynamic characteristics of a bottom-mounted fluid-filled membrane in normally incident waves. The static shape of the membrane, which is noncircular when the densities of the internal and external fluids are different, is computed by considering the equilibrium of the external loads with the membrane hoop stress. A second-order differential equation governing the membrane deformation due to dynamic loads is derived from the membrane theory of cylindrical shells and it is solved using the finite-element method. The motions of the internal and external fluids are computed, respectively, by two boundary-element models, which in turn are coupled with the finite-element model of the membrane. Both the kinematic and dynamic boundary conditions are satisfied at the fluid-membrane interface, and the steady-state hydrodynamic problem is solved in the frequency domain. The present model and its results are compared with those in a previous study and the differences discussed. Through a parametric study, the density ratio of the internal and external fluids is shown to be an important parameter influencing the dynamic response of the membrane system.

An experimental study has been carried out on the forces from breaking waves on a tripod structure located on a shoal. The main purpose was to investigate both horizontal and vertical forces as well as overturning moments due to waves acting on the tripod. In addition, horizontal water particle velocities and crest elevations directly over the shoal have been measured. Nondimensional relations between the maximum depth average particle velocities over the shoal, the wave height, wave period, water depth, and wave crest elevation have been obtained. Further nondimensional relations between the wave crest elevation, wave height, wave period, and water depth have also been obtained. Finally, a simple wave force calculation method was developed that may be used to obtain wave forces on similar structures on other shoals that do not deviate significantly from the conditions used during the present tests.

A conditional simulation may be defined as a numerical method for embedding a deterministic sequence of waves or wave groups into a random wave simulation having a specified target variance spectrum. The conditional simulation may be performed in either the time or frequency domains. In the time domain, the procedure is direct and easy to understand, but it is time-consuming to compute. In the frequency domain, the procedure is more efficient when finite Fourier transform algorithms are used. The numerical stability of conditional simulation depends on the condition of a covariance matrix that depends on (1) the ratio of the length of the embedded wave sequence to the length of the unconditional sequence; (2) the compatibility of the variance of the embedded wave sequence with the variance of the target spectrum; and (3) the size of the discrete simulation time step Δt. Conditional simulations of water surface elevations were transformed by a finite Fourier transform interpolation algorithm to the motion of a planar wavemaker to generate conditional simulations in a 2D wave channel, and the comparisons between the measured laboratory waves and the numerical conditional simulations were good.

This paper presents a numerical model for simulating wave interaction with porous structures. The model calculates the mean flow outside of porous structures based on the Reynolds averaged Navier-Stokes equations. The corresponding turbulence field is modeled by an improved k-ε model. The flow in porous structures is described by the spatially averaged Navier-Stokes equations. The drag forces caused by the presence of a solid skeleton are modeled by the empirical linear and nonlinear frictional forms. The numerical model is first calibrated by simple experiments for flow passing through a porous dam with different porous media. Excellent agreements are obtained for the case using gravels with mean sizes of O(1 cm) to O(10 cm) as the materials for the porous dam. Reasonably good agreements are also obtained when small uniform glass beads with diameters of 3 mm are used. The calibrated numerical model is then employed to investigate the breaking wave overtopping a caisson breakwater, protected by a layer of armor units. Good agreements between numerical results and laboratory data are obtained in terms of both free surface displacement and overtopping rate. Different design scenarios are also studied numerically. The porous armor layer is effective in reducing the overtopping rate as well as in preventing the caisson breakwater from bottom scouring.

Hydrodynamic behavior of the different sand-grain-size fractions, transport paths, and advection speed were studied by two fluorescent tracer experiments at Regla Beach, in Chipiona, SW Spain. Sand tracers, with two different colors, were injected on a cross-shore profile. The coarse- and very-fine-sand fractions were moved seaward and northward from the injection points in the subtidal and intertidal zones. Medium- and fine-grain-size fractions remained longer. The sand advection speed (tracer centroid) was found to be a linear function of the speed of the longshore current, with a mean correlation coefficient of 0.016, similar to that found by previous authors in other beaches.