Search

Distributed Acoustic Sensing (DAS) is a new method for recording oceanographic processes using seafloor fiber-optic cables, such as telecommunication cables. DAS returns spatially distributed measurements of cable strain, which can be related to hydrodynamic pressure, turning a submarine cable into a dense sampling array. A reinforced fiber-optic cable was installed in the cross-shore from the dune toe to ∼15-m-water depth at the USACE Field Research Facility in Duck, NC to quantitatively compare DAS strain to co-located pressure sensors. We develop a methodology for transferring DAS strain to dynamic pressure and evaluate the performance of DAS to measure shallow- and intermediate-water waves in the incident band (0.04–0.4 Hz). A frequency-dependent empirical transfer function from DAS strain to dynamic pressure at the seabed is derived from the ratio of strain and pressure power spectra. DAS-derived significant wave heights and peak periods were consistent with pressure sensors (typical rmse = 0.2 m and 1 s) over a wide range of dynamic conditions (0.24–4 m wave heights and 3–18 s periods). DAS data were input into the cBathy algorithm to calculate cross-shore bathymetric profiles and were used to calculate cross-shore wave reflection. Preliminary applications of DAS to record wave splitting and shoreline position were explored. With this field data, we demonstrate new applications of DAS for measuring nearshore processes and encourage further exploration. The promising potential of DAS lies in the near-bed data volume provided, real-time capability, and ability to sample in any weather or light. • Distributed Acoustic Sensing can measure nearshore ocean waves. • An empirical transfer function is proposed to convert cable strain to pressure. • DAS-derived pressure provides a wide resolution for ocean analyses. • DAS is promising for nearshore monitoring on existing or custom fiber-optic cables. [ABSTRACT FROM AUTHOR]

Abstract The seabed is sloped in shallow coastal regions, where strong nonlinear shoaling waves can generate sheet-flow sediment transport. Both wave nonlinearity and bottom slope can lead to a wave-averaged (net) sediment transport rate, but the bottom slope effect is often overlooked in existing sediment transport models. A full-scale experimental study of sheet-flow sediment transport under the oscillatory flows with nonlinear features (skewness or asymmetry) was conducted in an inclinable oscillatory water tunnel, so the bottom-slope-induced net transport rate in the context of nonlinear waves was experimentally investigated. The tests cover a variety of nonlinear wave shape, bottom slope (0 to 2. 5 ∘ ) and two sediment diameters (0.24 and 0.51 mm). Since the downslope direction is the "offshore" direction in our tests, it is found that bottom slope reduces the net "onshore" transport rate due to wave nonlinearity. The slope contribution to the net transport rate is evaluated by taking the difference between the measured net transport rate from a sloping-bed experiment and the measured net transport rate from the corresponding horizontal-bed experiment. The results suggest that slope-induced net transport rate is insensitive to wave nonlinearity, and its variation with bottom slope can be considered linear in the context of predicting net transport rate. Comparisons between this study and a preceding sinusoidal-flow study of Yuan et al. (2017b) suggest that nonlinear waves can be approximated as sinusoidal flows for estimating the slope contribution to the net transport rate. Subsequently, an empirical model for predicting net sheet-flow transport rate due to bottom slope is established based on dimensional analysis. This model can be simply added to existing sheet-flow sediment transport models to improve their applicability in coastal regions. Highlights • Presented experiments on the sheet-flow sediment transport under skewed and asymmetric oscillatory flows above a sloping bed. • Test results show that the slope-induced net transport rate is insensitive to nonlinear wave shape. • Proposed an empirical model for slope's contribution in net sheet-flow sediment transport rate. [ABSTRACT FROM AUTHOR]

Abstract Waves are coexisting with currents in coastal zones; nevertheless, previous experimental studies for excess pore-pressure responses in a porous seabed were predominantly limited to the wave-only condition. In this study, the combined wave-current induced excess pore-pressures in a sandy seabed were experimentally simulated with a specially-designed flume, which can concurrently generate periodic waves and a following/opposing co-directional current. The effect of a current on the wave profile is firstly examined. The wave steepness is decreased by a following current, but enhanced by an opposing current. Flume observations indicate that, under combined wave-current loading, the wave-induced pore-pressure is increased for the following-current case, but reduced for the opposing-current case. Such wave-current combination effect becomes more significant for shorter wave periods. The variation trend of the excess pore-pressure distribution in the present flume observations is consistent with that of the existing analytical solutions. Nevertheless, due to the existence of wave and/or current boundary layer and non-lineartiy of wave-current interactions as indicated by the flume observations, certain deviations exist between the flume results for excess pore-pressure and the analytical solutions, which can not be ignored especially for the opposing-current case. The effects of the boundary layer on the combined wave-current induced pore-pressures in the seabed are further highlighted by supplementary numerical simulations. A favorable prediction by the analytical solution would be expected for following-current cases and smaller pore-pressure amplitudes would be obtained for opposing-current cases. Highlights • Flume observations of combined wave-current induced excess pore-pressure in a sand-seabed. • Comparisons of wave-current induced pore-pressure between flume results and analytical solutions. • Effects of non-lineartiy of wave-current interactions on the excess pore-pressure responses. • Deviation between the flume results for excess pore-pressure and the analytical solutions has been identified. [ABSTRACT FROM AUTHOR]

This study derived an analytic solution for two progressive waves propagating on a sloping bottom. The solution of flow field in explicit form is obtained as a three-parameter function of rendering the two wave ordering parameter ε 1 , ε 2 and the bottom slope perturbed to the third order. The results can be simplified to a single propagating wave on sloping beach and wave–wave interaction in uniform depth. The wave number and angular frequency are related to water depth, bottom slope, and wave steepness and are also affected by another propagating wave, the increasing wave steepness or wave length of which will enhance the influence of the dispersion relation. The solutions include the wave–wave interaction, wave shoaling, nonlinearity, and the effect of bottom slope. Those are observed by means of the discussion of the free-surface displacement. The resonance caused by wave–wave interaction will occur in a specified water depth and the ratio of two wave numbers, leading to the singularity of free-surface displacement. [ABSTRACT FROM AUTHOR]

Abstract: In this study, an integrated model (PORO-WSSI II) for wave–seabed–structure interactions (WSSI) is developed by combining (i) the Volume-Averaged Reynolds-Averaged Navier–Stokes (VARANS) equations for wave motions in a fluid domain and the porous media flows in porous structures, and (ii) the dynamic Biot''s equations for a porous elastic seabed. The effects of the porous flow in seabed and marine structures, and the fluid exchange at the interface between the fluid domain and solid domain on wave characteristics are considered. The finite difference two-step projection method and the forward time difference method are adopted to solve the VARANS equations. The volume of fluid (VOF) method is applied to track water free-surface. The finite element method and the Generalized Newmark method are respectively adopted for the space discretization and time discretization of the dynamic Biot''s equations. A one-way integrating method is developed to integrate the VARANS equations with the dynamic Biot''s equations. Several experimental data available in the literature are used to validate the integrated model. An overall agreement between the numerical results and the experiment data indicates that the integrated model developed for the WSSI problem is highly reliable. The integrated model is then applied to investigate the dynamic response of a large-scale composite breakwater on a seabed, and the mechanism of WSSI. Numerical results indicate that there are intensive fluid exchanges between the water body and the seabed and strong seepage forces in the seabed under the ocean wave loading. The excessive upward seepage force leads to the liquefaction of the seabed in the region under wave trough. There is a liquefaction zone in the seabed close to the bottom corner of the rubble mound, which may lead to foundation instability of the composite breakwater. The parametric study indicates that the wave characteristics have a significant impact on the liquefaction properties (depth, width and area). [Copyright &y& Elsevier]

Abstract: Understanding of sea bottom boundary layer characteristics, especially bottom shear stress acting on the sea bed, is an important step needed in sediment transport modeling for practical application purposes. In the present study, a new generation system for bottom boundary layer under solitary wave is proposed. Applicability of this system is examined by comparing measured and numerical solution velocities. Moreover, transitional behavior from laminar to turbulence was investigated. It is concluded that the critical Reynolds number in the experiments shows good agreement with DNS result of Vittori and Blondeaux (2008) and laboratory data of Sumer et al. (2010), indicating validity of the generation system. Since the present generation system enables continuous measurement to obtain ensemble averaged quantities, it can be effectively utilized for future experimental studies on solitary wave boundary layers, including sediment transport experiments with movable bed. [Copyright &y& Elsevier]

Abstract: While the destruction caused by a tsunami can vary significantly owing to near- and onshore controls, we have only a limited quantitative understanding of how different local parameters influence the onshore response of tsunamis. Here, a numerical model based on the non-linear shallow water equations is first shown to agree well with analytical expressions developed for periodic long waves inundating over planar slopes. More than 13,000 simulations are then conducted to examine the effects variations in the wave characteristics, bed slopes, and bottom roughness have on maximum tsunami run-up and water velocity at the still water shoreline. While deviations from periodic waves and planar slopes affect the onshore dynamics, the details of these effects depend on a combination of factors. In general, the effects differ for breaking and non-breaking waves, and are related to the relative shift of the waves along the breaking–non-breaking wave continuum. Variations that shift waves toward increased breaking, such as steeper wave fronts, tend to increase the onshore impact of non-breaking waves, but decrease the impact of already breaking waves. The onshore impact of a tsunami composed of multiple waves can be different from that of a single wave tsunami, with the largest difference occurring on long, shallow onshore topographies. These results demonstrate that the onshore response of a tsunami is complex, and that using analytical expressions derived from simplified conditions may not always be appropriate. [Copyright &y& Elsevier]

The open-source code DualSPHysics is applied to simulate the interaction of sea waves with floating offshore structures, which are typically moored to the seabed, such as vessels, boats, floating breakwaters and wave energy converters (WECs). The goal is to develop a numerical tool that allows the study of the survivability of floating moored devices under highly energetic sea states, obtaining the optimum mooring layout to increase lifetime. The moorings are modelled by coupling DualSPHysics with MoorDyn, a lumped-mass mooring dynamics model. MoorDyn represents mooring line behaviour subject to axial elasticity, hydrodynamic forces in quiescent water, and vertical contact forces with the seabed. Calculated mooring tensions at the fairlead are added as external forces in order to compute the resulting response and motions of the floating structures in DualSPHysics. The coupled model has been validated against data from scale model tests generated during the experimental campaigns for the European MaRINET2 EsflOWC project. In order to evaluate the accuracy of the coupling implementation with the lumped-mass mooring model, free-surface elevation, motions of the floater and mooring tensions are numerically computed and compared to experimental data. Overall, the results demonstrate the accuracy of the coupling between DualSPHysics and MoorDyn to simulate the motion of a moored floating structure under the action of regular waves. Going forward, this modelling approach can be employed to simulate more complex floating structures such as floating wind turbines, buoys, WECs, offshore platforms, etc. • DualSPHysics and MoorDyn are coupled to simulate floating moored devices. • Motions of the floating structure and mooring tensions are in agreement with experimental data. • The proposed model will allow the study of the survivability of structures under extreme wave conditions. [ABSTRACT FROM AUTHOR]