Journal of Waterway, Port, Coastal, and Ocean Engineering

New remote sensing observations that demonstrate the presence of rip current plumes in X-band radar images are presented. The observations collected on the Outer Banks (Duck, North Carolina) show a regular sequence of low-tide, low-energy, morphologically driven rip currents over a 10-day period. The remote sensing data were corroborated by in situ current measurements that showed depth-averaged rip current velocities were $20–40\hspace{0.17em}\text{cm/s}$ whereas significant wave heights were $H_s=0.5–1\hspace{0.17em}\mathrm{m}$. Somewhat surprisingly, these low-energy rips have a surface signature that sometimes extends several surf zone widths from shore and persists for periods of several hours, which is in contrast with recent rip current observations obtained with Lagrangian drifters. These remote sensing observations provide a more synoptic picture of the rip current flow field and allow the identification of several rip events that were not captured by the in situ sensors and times of alongshore deflection of the rip flow outside the surf zone. These data also contain a rip outbreak event where four separate rips were imaged over a 1-km stretch of coast. For potential comparisons of the rip current signature across other radar platforms, an example of a simply calibrated radar image is also given. Finally, in situ observations of the vertical structure of the rip current flow are given, and a threshold offshore wind stress ($\text{>}0.02\hspace{0.17em}{\text{m/s}}^2$) is found to preclude the rip current imaging.

This paper focuses on the optimal location for dumping nourished sand on a barred coastline. This is done by investigating the short-term behavior of the cross-shore redistribution of nourished sediment on a breaker-bar profile in a two-dimensional vertical plane. This is achieved by the use of a complete numerical description of the surf-zone processes with respect to both hydrodynamics and sediment transport. The numerical model is based on the finite-volume approach with a free surface-tracking method, also known as the volume of fluid (VOF), and the sediment transport is calculated applying the Engelund and Fredsøe deterministic concept. The methodology is as follows: a Dean/Bruun equilibrium profile is exposed to regular waves (chosen as $H=1.3\hspace{0.17em}\mathrm{m}$ and $T=4.8\hspace{0.17em}\mathrm{s}$). These waves will form bars on the Dean/Bruun equilibrium profile, and the wave impact is continued until a quasi-steady behavior of the bars has been obtained, i.e., bars that have stopped growing in height and only migrate slowly offshore. Next, the profile is nourished. The resulting continued morphologic developments are compared with the development of the unnourished (reference) profile. Also, the effect of a net shoreward current over the profile is consider as a means of imposing the net recirculation resulting from longshore nonuniformities or the impact of wind-induced shear stresses. The impact of wave height, irregularity in incident waves, and the size of the sediment grains on the cross-shore distribution of suspended sediment transport rates are described. In addition, all profiles are seen to lose sediment seaward when no net shoreward current is enforced over the outermost breaker bar. Alternative strategies for nourishment are investigated, and it is seen that the shape on the nourishment seaward of the outermost bar can have both destructive and protective impacts on the original shoreward-located cross-shore profile.

This paper introduces the tide data acquisition and processing procedures in the Nationwide Ocean Wave Information Network for Ports and Harbors (NOWPHAS), especially those for global positioning system (GPS) buoys that measure water surface elevations using real-time kinematic GPS technology at water depths of 100–400 m and 10–20 km from the coastline. It then describes specific features of the 2010 Chile tsunami and the 2011 Tohoku tsunami, including foreshock and main shock profiles, which were measured along the Japanese coast by the NOWPHAS equipment. In the main shock of the 2011 Tohoku tsunami, the GPS buoy at a water depth of 204 m off Kamaishi Bay showed that the first wave crest consisted of primary slow and subsequent quick rises, and exceeded 6 m approximately 30 min after the earthquake occurrence. Some geometrically adjacent sets of GPS buoys, coastal wave gauges, and coastal tide gauges revealed the tsunami propagation and amplification near the coast.

Reliable simulation of onshore-offshore sandbar migration under various wave and current conditions has remained a challenging task over the last three decades because wave-undertow interaction in the surf zone has been neglected in the existing numerical models. This paper presents the development of an improved sandbar migration model using a phase- and depth-resolving modeling approach. This new model includes interactions between waves and undertow and an empirical time-dependent turbulent eddy viscosity formulation that accounts for the phase dependency of turbulence on flow velocity and acceleration. The authors demonstrate through extensive model-data comparisons that these enhancements resulted in significant improvements in the predictive capability of the cross-shore sandbar migration beneath moderate and energetic waves. The comparison showed wave-undertow interaction playing a crucial role in cross-shore sediment transport. Waves increased the undertow-induced suspended-load flux during offshore sandbar migration, and a weak undertow suppressed the wave-induced onshore bed-load transport during onshore sandbar migration. The proposed empirical time-dependent turbulent eddy viscosity significantly improved the prediction of onshore-directed bed-load transport during onshore sandbar migration.

Recent turbulence-resolving simulations of fine sediment transport in the oscillatory bottom boundary layer (OBBL) revealed the existence of a diverse range of flow regimes over muddy seabeds. Transitions between these flow regimes are caused by different degrees of sediment-induced stable density stratification in the OBBL. These transitions have critical implications for the role of wave resuspension in the delivery of fine sediment and hydrodynamic dissipation over muddy seabeds. This study further investigates the effect of Newtonian rheology, parameterized as a concentration-dependent effective viscosity, on turbulence modulation and the transition from turbulent to laminar states. Assuming small particle Stokes number, the equilibrium approximation is adopted to simplify the Eulerian two-phase flow governing equations. The resulting simplified equations are solved with a high-accuracy hybrid scheme in an idealized OBBL. A sixth-order centered compact finite difference is implemented in the vertical direction to solve the governing equations with a flow-dependent viscosity while the pseudospectral method is retained for two horizontal directions. At Stokes Reynolds number ${\mathsf{R}}_{\mathrm{\Delta }}=\mathrm{1,000}$, simulations reveal that when rheology is incorporated, the enhanced effective viscosity can further attenuate the flow turbulence in addition to the well-known sediment-induced stable density stratification. Through the enhanced viscosity, the velocity gradient very near the bed is significantly reduced, which leads to much weaker turbulent production and the onset of laminarization. Although the sediment-induced stable density stratification is known to cause laminarization of bottom boundary layer flows, our preliminary finding that the enhanced viscosity via rheological stresses encourages the flow laminarization may be useful in explaining the field-observed large wave dissipation rate over muddy seabeds during the waning stage of a storm.

The need for an improved harbor access resulted in the construction of two new rubble mound breakwaters in Ostend, Belgium. In the past, a specific dredging technique was used to maintain the required depth of the access channel to the harbor. This has resulted in a complex soil layering underneath the northern part of the new western breakwater with locally very thick soft soil layers. As an alternative for an expensive soil remediation, a very strong geotextile was included into the breakwater core, reinforcing the structure. In addition a staged construction was necessary, allowing the soft layers to consolidate and preventing undrained failure during construction. Because of the complexity of the situation, a numerical model was used to get an estimate of the settlements and the consolidation process during and after construction of the breakwater. Extensive field measurements of the settlements were performed during construction and are still performed at this moment (early 2013). Comparison of the settlement measurements with the calculations results in a better understanding of the consolidation process and allows improvement of the model, resulting in a more accurate settlement (and thus consolidation) prediction curve. Because of the improved insight in the consolidation of the soft layer, a substantial gain in the progress of the construction of the dam is achieved. This paper shows, based on a case study of a breakwater constructed on soft soil, that existing models used to predict settlements can be improved using field data, leading to a clear benefit regarding the construction phasing.

The design of breakwater armor units against tsunami attacks has received little attention in the past because of the comparative low frequency of these events and the rarity of structures designed specifically to withstand them. However, field surveys of recent events, such as the 2011 Tohoku Earthquake Tsunami and the Indian Ocean tsunami in 2004, have shown flaws in the design of protection structures. During these extreme events, many breakwaters suffered partial or catastrophic damage. Although it is to be expected that most normal structures fail because of such high-order events, practicing engineers need to possess tools to design certain important breakwaters that should not fail even during Level 2 events. In the future, research into the design of critical structures that only partially fail (i.e., resilient or tenacious structures) during very extreme Level 2 tsunami events should be a priority; in this sense, the present paper proposes a formula that allows the estimation of armor unit damage depending on the tsunami wave height.

The further development and application of the two-dimensional (2D) flow and sediment-modeling concept, formulated for the treatment of sediment mixtures, is addressed herein. The sediment domain is divided into subdomains for suspended, active-layer, and subsurface-strata sediments. Governing sediment equations are defined for each subdomain, including sediment exchange mechanisms between suspended sediment and active-layer sediment, as well as active-layer and subsurface sediment. The sediment mixture is represented through a suitable number of sediment size classes, and governing-equations’ terms are defined as size class–specific. Governing equations and associated numerical solutions are formulated within the general nonorthogonal curvilinear coordinate framework. A numerical solution is based on the fractional-step approach, using the characteristics method with higher-order interpolations for both flow and suspended-sediment advection steps. The numerical solution for equations describing active-layer and subsurface sediment is fully coupled, providing that the sum of all sediment size fractions is always equal to unity. Numerical results are compared with the results of field data measurements. The comparison strongly supports the inclusion of sediment mixtures in flow-sediment modeling.

Fluid mud is a non-Newtonian slurry of aggregated fine sediment present in numerous bays, estuaries, and lakes. It is a transitory state of mobile mud whose properties are relevant to the definitions and utility of hydrodynamic, acoustic, and, particularly, nautical depth in navigation channels. For estimation of the nautical depth up to which mud must be removed to keep its resistance against vessel propulsion low, processes underpinning fluid mud transport are briefly reviewed. Data from a coastal harbor are revisited to illustrate the role of shear rheometry as an essential component of the method for nautical depth estimation. Broadening the application to channels laden with organic muck often found in many ports, such as those in the southern United States, will require new data and understanding of the biomechanical properties of organic aggregates. Fluid mud dynamics is integral to such efforts.

Tsunami debris can place large demands on the structures it impacts. The types of potential debris and impact forces they generate are not well understood, and relatively little consideration is taken for the risk of tsunami debris strikes during structural design. A procedure is outlined to assess the site-specific potential for debris impact and its significance to structures. The procedure involves a categorization of potential debris based on fundamental characteristics. It includes an assessment of the ability and likelihood of debris transport, as constrained by the topography and the constructed environment. Data from aerial surveys and on-ground surveys after the March 2011 Tohoku tsunami are used to demonstrate and validate the considerations proposed for an assessment of debris and its transport. Instances of structural damage found during the site survey and that potentially resulted from debris strikes are reported. These cases are evaluated to correlate the proposed debris categories and impact demands with the observed structural damage. The velocities needed to cause the observed damage were determined using existing guidelines for debris impact forces. The procedure proposed here can be considered as a preliminary effort toward the development of a framework for risk assessment, including estimates of probable damage levels. Applications include both structural design and tsunami disaster planning.