Journal of Waterway, Port, Coastal, and Ocean Engineering

Undertow and longshore current profiles outside of the surf zone were analyzed using a canonical situation involving an alongshore-uniform barred beach. Forcing mechanisms due to the presence of surface gravity waves are of interest. As is customary for surf zone studies, dynamics related to wave breaking and associated wave rollers were included. Also considered were the effects of the wave orbital velocities on the bottom and surface boundary layers. The effect of Earth’s rotation in the dynamics is included through two mechanisms: (1) the Coriolis-Stokes stress that results from the effect of Earth’s rotation on the wave orbital velocities, and (2) the Coriolis terms in the cross-shore and alongshore momentum equations. Model results qualitatively reproduce observed undertow profile shapes inside and just outside of the surf zone, as well as a few surf zone widths offshore and far offshore on the inner shelf. In particular, the results explain the surface-intensified undertow currents often observed outside the surf zone. The Coriolis-Stokes stress is not active inside the surf zone; but it is an important player, along with the other considered dynamics, in shaping the profiles outside of the surf zone. The model also suggests the presence of an alongshore current outside the surf zone that is generated owing to the effects of the Coriolis terms in the momentum equations. An analysis of the temporal spin-up behavior of the predictions suggest that circulation patterns strongly influenced by Earth’s rotation should be observable in the field within a few hours after the onset of wave activity.

A comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level changes on a natural beach. Data collected during the study are used to quantify the vertical profile of cross-shore and alongshore velocities and the importance of sheet flow sediment processes in the swash zone. The swash-zone boundary layer for cross-shore velocities is observed to generally occur over at least the lower 0.06 m of the water column. Alongshore velocities are often the same order of magnitude as the cross-shore velocities and are dominant near cross-shore flow reversal. Flows are often logarithmic in profile, but the instantaneous nature of the measurements renders application of the logarithmic model difficult. When valid, the logarithmic model enabled cross-shore shear stress estimates of up to $21.8\hspace{0.17em}\mathrm{N}\hspace{0.17em}{\mathrm{m}}^{-2}$ with maximum alongshore shear stress estimates of $12.3\hspace{0.17em}\mathrm{N}\hspace{0.17em}{\mathrm{m}}^{-2}$, further highlighting the potential importance of alongshore flows. Friction coefficient estimates, assuming a quadratic drag law, showed no statistical difference between onshore- and offshore-directed motion, with typical values of $0.023\pm 0.013$ ($\text{mean}\pm \text{SD}$). Sheet flow concentrations exceeded the maximum measured suspended sediment concentrations of approximately $400\hspace{0.17em}\text{kg}\hspace{0.17em}{\mathrm{m}}^{-3}$. Sediment loads in the sheet layer are up to 10 times larger than the sediment loads in the lower suspension layer. Simplified sheet flow sediment transport estimates are 3.6 times larger on average than those in the suspension layer. The latter two findings indicate the importance of sheet flow processes in the swash zone that are generally ignored.

Sheet flow sediment concentration profiles were measured in natural conditions for the first time as part of a comprehensive field study on swash-zone hydrodynamics and sediment transport. Three conductivity concentration profilers (CCPs) measured the sediment concentration profile in the sheet flow layer with a 1-mm resolution in the swash zone of a dissipative beach. This paper focuses on sheet flow during quasi-steady backwash events generated by infragravity motion when the effects of phase lags, surface-generated turbulence, and accelerations are small. The sheet flow sediment concentration profile has a linear shape in the lower section of the profile and a power-law shape in the upper section, with the transition occurring at sediment volume fractions of 0.20–0.30. The shape of the concentration profile is self-similar for measured sheet flow layer thicknesses ranging from 6 to 18 mm. Because of the self-similarity, a single concentration profile curve can be used to describe the normalized profile for the entire range of sheet thicknesses, leading to improved estimates of the sheet flow layer thickness in a simple analytical model. The sheet flow layer thickness and sheet load, the sediment mass mobilized in the sheet flow layer, are well correlated with the hydrodynamic forcing represented by the mobility number ($r^2=0.60$ for sheet thickness and $r^2=0.53$ for sheet load).

Posthurricane Ike surveys for almost 2,000 individual wood-framed houses were combined with high resolution numerical hindcasts to investigate the collapse limit state arising from overland waves and surge. For this storm, freeboard above wave crests, wave height, current velocity, and construction date were found to be the most important factors determining survival. Regressions show considerable skill in predicting collapse fragilities and will prove useful in assessing vulnerability. The relatively small range of wind speeds available in the data set did not allow accurate assessments of wind effects on collapse. Several ASCE and FEMA force estimates showed relatively low skill in modeling fragility, in large part because their breaking wave assumption was not accurate over much of the range. However, the use of best-estimate wave heights in force modeling greatly improved accuracy of fragility estimates. Despite the positive results of this study, numerous questions remain. It proved impossible to separate the effects of house age and construction methods. Although computed overland wave heights showed good correlation with fragility, comparison with the limited measured data available showed that wave heights were significantly overpredicted inland. Finally, more accurate standards for integrated and local loads on elevated houses will be necessary to perform detailed structural system-based analyses of fragility.

Quarter-scale physical model experiments and a phase-resolving numerical model (FUNWAVE) were used to evaluate random wave attenuation through two types of synthetic vegetation. The experiment was performed with two peak periods, three water depths, and two stem densities. For each combination of parameters, free surface time series were collected at seven locations throughout the vegetation field and one location seaward of the vegetation. Each combination of wave conditions was evaluated for the following four different cases: Case A with no vegetation; Cases B and C with short and long specimens, respectively; and Case D with mixed vegetation. The wave height decay for each case was fit to two existing wave height attenuation prediction equations. The decay equations provided reasonable predictions for the normalized wave height attenuation, with an average root-mean-square error (RMSE) of 0.015. The linear combination of attenuation coefficients obtained for the cases of the individual plants provided a reasonable prediction of the attenuation coefficient for the cases of the combined, heterogeneous vegetation. FUNWAVE was used to model wave attenuation for these tests using a bottom friction factor calibrated for each run. The numerical attenuation followed the same trends as the measured data, with an average RMSE of 0.017. Similar to the physical model study, it was found that summing the calibrated model friction factors for the cases of the individual plants reasonably predicted the wave height attenuation for the cases of the combined vegetation with an average RMSE of 0.032.

Recently, major tsunami events have resulted in considerable damage to low-lying coastal developments as well as significant loss of life. This is owing to the fact that coastal communities and associated infrastructure are not able to withstand the extreme hydrodynamic forces induced by the overland progression of large tsunami waves. This paper reports the findings of a study, which is part of an ongoing comprehensive (hydraulic-structural engineering) research program initiated at the University of Ottawa in Canada. The goals of the latter are to contribute to the understanding and quantification of extreme hydrodynamic impacts on structural elements, and to the development of design guidelines for structures located in tsunami-prone areas. In this study, a single-phase three-dimensional (3D) weakly compressible smoothed-particle hydrodynamics (WCSPH) model is used to investigate the hydrodynamic forces, induced by the impact of rapidly advancing tsunami-like hydraulic bores, on a freestanding column of a square cross section. Numerical time histories of the water surface elevation and net base horizontal force acting on the column are compared with the results of large-scale physical experiments, conducted by the authors as part of an experimental component of this research program. The experiments were performed on the basis of analogies between tsunami bores and dam-break waves. The bore–structure interaction is analyzed in detail as the validated numerical model is subsequently applied to additionally investigate the influence of laboratory channel geometry on the results.

A Lagrangian numerical method called smoothed particle hydrodynamics (SPH) is used to analyze a simplified bar/rip channel system on a beach. Prior studies have shown that SPH models propagate water waves well, including breaking waves; here, it is shown that SPH also models the mean wave-induced nearshore circulation created by breaking waves. The model predictions are compared with the previous laboratory measurements and show good agreement, including the mean velocity profiles, mean surface elevation, and cross-shore velocity components over the rip channel. The alongshore variation of various components of radiation stress and the resulting alongshore force that acts as a feeder for the rip current are obtained from the numerical results.

This paper describes the heterogeneous packing (HEP) failure mode of breakwater armor. HEP reduces packing density in the armor layer near and above the mean water level and increases packing density below it. With HEP, armor units may move in the armor layer, although they are not actually extracted from it. Thus, when HEP occurs, armor-layer porosity is not constant, and measurements obtained with conventional methods may underestimate armor damage. In this paper, the Virtual Net method is proposed to calculate armor damage considering both armor-unit extraction and HEP. The Cubipod concrete armor unit is then described as a solution to the effects of HEP on conventional cubic block armor. The hydraulic stability of cube and Cubipod armor units was compared in two-dimensional laboratory experiments. Cube and Cubipod armor layers were tested in two wave flumes under nonbreaking and non-overtopping conditions. The hydraulic stability was higher for double-layer Cubipod armor than for single-layer Cubipod armor, which had a higher hydraulic stability than conventional double-layer cube armor.

The rheological properties of river and seabed sediments are important to describing the interactions of the currents and waves with the bed. This paper presents the findings from a study of the rheological properties of both sandy and cohesive (kaolin and mud) bed sediments under shear and mechanical vibration loads. It was found that the rheological properties of the sediments depended not only on the magnitude of the shear load but also on its duration. The sandy and cohesive sediments exhibited shear thinning and thixotropy for shear speeds greater than 1 revolution per minute, whereas the cohesive kaolin and mud exhibited antithixotropy for shear speeds lower than 1 revolution per minute. High-frequency vibration loads also significantly reduced the viscosity of the sediments, with the magnitude of the reduction increasing with the frequency of the vibration. The kaolin and mud were also fluidized by the vibration, and their recovery required a longer time compared with the sandy sediment. Some potential applications of these observations are highlighted in this paper.