Stepped Submerged Offshore Breakwaters for Wave Energy Dissipation

The present work investigates the water wave interaction with bottom-standing thick porous trapezoidal-shaped structures and shore-side vertical rigid wall in the presence of uniform ocean currents. This study has been done to understand the impact of different physical parameters like friction, porosity, and ocean currents along with different structural parameters (width and height) on different phenomena like wave energy reflection, wave forces, wave energy dissipation, etc. The quadratic boundary element method-based numerical technique has been used to solve the boundary value problem. The structural porosity is modeled using Sollitt and Cross’s model of water wave interaction with thick porous structures. Several results associated with the wave energy reflection and energy dissipation have been analyzed. Also, the wave force exerted by the incoming waves has been investigated to check the stability and sustainability of the right vertical rigid wall and porous structure. The Doppler-shift effect is observed in wave transformation characteristics due to the presence of ocean currents. The impact of following and opposing ocean currents can be seen in the graph of wave energy reflection, dissipation, and wave forces. The periodic patterns can be observed clearly in wave characteristics like wave energy reflection, dissipation, and wave forces when plotted against the non-dimensional separation gap between the porous breakwater and shore-side rigid seawall.

Quantification of the turbulent kinetic energy dissipation rate in the water column, ε, is very important for assessing nutrient uptake rates of corals and therefore the health of coral reef lagoon systems. However, the availability of such data is limited. Recently, at Lady Elliot Island (LEI), Australia, we showed that there was a strong correlation between in situ measurements of surface‐wave energy dissipation and ε. Previously, Reineman et al. (2009), we showed that a small airborne scanning lidar system could measure the surface wavefield remotely. Here we present measurements demonstrating the use of the same airborne lidar to remotely measure surface wave energy fluxes and dissipation and thereby estimate ε in the LEI reef‐lagoon system. The wave energy flux and wave dissipation rate across the fore reef and into the lagoon are determined from the airborne measurements of the wavefield. Using these techniques, observed spatial profiles of energy flux and wave energy dissipation rates over the LEI reef‐lagoon system are presented. The results show that the high lidar backscatter intensity and point density coming from the high reflectivity of the foam from depth‐limited breaking waves coincides with the high wave‐energy dissipation rates. Good correlations between the airborne measurements and in situ observations demonstrate that it is feasible to apply airborne lidar systems for large‐scale, long‐term studies in monitoring important physical processes in coral reef environments. When added to other airborne techniques, the opportunities for efficient monitoring of large reef systems may be expanded significantly.

The widely accepted assumption that the equilibrium beach profile in the surf zone corresponds with uniform wave-energy dissipation per unit volume is directly examined in six cases from the large-scale SUPERTANK laboratory experiment. Under irregular waves, the pattern of wave-energy dissipation across a large portion of the surf zone became relatively uniform as the beach profile evolved toward equilibrium. Rates of wave-energy dissipation across a near-equilibrium profile calculated from wave decay in the surf zone support the prediction derived by Dean (1977). Substantially different equilibrium beach-profile shapes and wave-energy dissipation rates and patterns were generated for regular waves as compared to irregular waves of similar statistical significant wave height and spectral peak period. Large deviation of wave-energy dissipation from the equilibrium rate occurred at areas on the beach profile with active net cross-shore sediment transport and substantial sedimentation and erosion. T...

The dissipation of wave energy in the soil beneath a footing results from (a) dispersion of wave energy outward from the footing and (b) damping by energy losses within the soil. Following a brief illustration of a method for evaluating dispersion damping, the paper is primarily directed toward establishing numerical values of internal damping in granular soils. Laboratory tests using the vibration decay method permitted evaluation of the effects of confining pressure, amplitude of vibration, degree of saturation, and grain characteristics for four granular materials. The values of logarithmic decrement varies from 0.02 to approximately 0.20 for the various materials and test conditions used. It was found that the variation of logarithmic decrement with amplitude of vibration was appreciably affected by the degree of saturation and the grain characteristics of the material. The test results for a fine-grained quartz showed a marked dependence on time of loading and stress history, with the added complication that higher vibration amplitudes seemed to shake out the beneficial effects of prestress.

Surface wave energy and dissipation are observed across the surf zone. Utilizing the concept of surface rollers, a new scaling is introduced to obtain the energy flux and dissipation related to rollers from Doppler velocities measured by a shore‐based X‐band marine radar. The dissipation of wave energy and hence the transformation of the incoming wave height (or energy) is derived using the coupled wave and roller energy balance equations. Results are compared to in‐situ wave measurements obtained from a wave rider buoy and two bottom mounted pressure wave gauges. A good performance in reproducing the significant wave height is found yielding an overall root‐mean‐square error of 0.22 m and a bias of −0.12 m. This is comparable to the skill of numerical wave models. In contrast to wave models, however, the radar observations of the wave and roller energy flux and dissipation neither require knowledge of the bathymetry nor the incident wave height. Along a 1.5 km long cross‐shore transect on a double‐barred, sandy beach in the southern North Sea, the highest dissipation rates are observed at the inner bar over a relatively short distance of less than 100 m. During the peak of a medium‐severe storm event with significant wave heights over 3 m, about 50% of the incident wave energy flux is dissipated at the outer bar.

Fringing reefs play an important role in protecting the coastal area by inducing wave breaking and wave energy dissipation. However, modeling of wave transformation and energy dissipation on this topography is still difficult due to the unique structure. In the present study, two-dimensional laboratory experiments were conducted to investigate the cross-shore variations of wave transformation, setup, and breaking phenomena over an idealized fringing reef with the 1/40 reef slope and to verify the Boussinesq model under monochromatic wave conditions. One-layer and two-layer model configurations of the Boussinesq model were used to figure out the model capability. Both models predicted well (r2 > 0.8) the cross-shore variation of the wave heights, crests, troughs, and setups when the nonlinearity is not too high (A0/h0 < 0.07 in this study). However, as the wave nonlinearity and steepness increase, the one-layer model showed problems in prediction and stability due to the error on the vertical profile of fluid velocity. The results in this study revealed that one-layer model is not suitable in the highly nonlinear wave condition over a fringing reef bathymetry. This data set can contribute to the numerical model verification.

This paper presents laboratory experiments of wave-driven hydrodynamics in a three-dimensional laboratory model of constructed coastal wetlands. The simulated wetland plants were placed on the tops of conically-shaped mounds, such that the laboratory model was dynamically similar to marsh mounds constructed in Dalehite Cove in Galveston Bay, Texas. Three marsh mounds were placed in the three-dimensional wave basin of the Haynes Coastal Engineering Laboratory at Texas A&M University, with the center of the central wetland mound located in the center of the tank along a plane of symmetry in the alongshore direction. The experiments included two water depths, corresponding to emergent and submerged vegetation, and four wave conditions, typical of wind-driven waves and ocean swell. The wave conditions were designed so that the waves would break on the offshore slope of the wetland mounds, sending a strong swash current through the vegetated patches. Three different spacings between the wetland mounds were tested. To understand the effects of vegetation, all experiments were repeated with and without simulated plants. Measurements were made throughout the nearshore region surrounding the wetland mounds using a dense array of acoustic Doppler velocimeters and capacitance wave gauges. These data were analyzed to quantify the significant wave height, phase average wave field and phase lags, wave energy dissipation over the vegetated patches, mean surface water levels, and the near-bottom current field. The significant wave height and energy dissipation results demonstrated that the bathymetry is the dominant mechanism for wave attenuation for this design. The presence of plants primarily increases the rate of wave attenuation through the vegetation and causes a blockage effect on flow through the vegetation. The nearshore circulation is most evident in the water level and velocity data. In the narrowest mound spacing, flow is obstructed in the channel between mounds by the mound slope and forced over the wetlands. The close mound spacing also retains water in the nearshore, resulting in a large setup and lower flows through the channel. As the spacing increases, flow is less obstructed in the channel. This allows for more refraction of waves off the mounds and deflection of flow around the plant patches, yielding higher recirculating flow through the channel between mounds. An optimal balance of unobstructed flow in the channel, wave dissipation over the mounds, and modest setup in the nearshore results when the edge-to-edge plant spacing is equal to the mound base diameter.

Coastal areas are an apprized environment by society that will continue to expand rapidly. Traditional coastal protection structures are commonly deployed to protect coastal areas endangered by natural extreme weather events. However, due to their limited efficiency and very high costs, more efficient and sustainable strategies to deal with coastal erosion are imperative. This research work focuses on the assessment of engineering solutions to mitigate and delay coastal erosion. Three different structure geometries (triangular prism shape, single detached breakwater and group of two detached breakwaters) are analysed on a realistic bathymetry, using a combination of numerical models (SWAN and XBeach) to study the influence of those structures on the coastal hydro- and morphodynamics. SWAN was used for hydrodynamics and XBeach for hydrodynamics and morphodynamics assessments. In addition, a comparison between SWAN and XBeach hydrodynamics results was also performed. Structures considered in this study have regular shaped geometries, and are characterized in terms of their efficiency regarding wave height and wave energy dissipation considering different wave regimes and performance in terms of long-term beach morphodynamic impact (sediments accumulation and erosion). The analysis is concentrated in two scenarios, one for low and the other for highly energetic hydrodynamics (the most challenging to coastal zones defence). The obtained results allowed classifying their performance in terms of the impact on wave energy and wave height dissipation, and sediment erosion/deposition patterns.

This study examines oblique wave motion over multiple submerged porous bars in front of a vertical wall. Based on linear potential theory, an analytical solution for the present problem is developed using matched eigenfunction expansions. A complex dispersion relation is adopted to describe the wave elevation and energy dissipation over submerged porous bars. In the analytical solution, no limitations on the bar number, bar size, and spacing between adjacent bars are set. The convergence of the analytical solution is satisfactory, and the correctness of the analytical solution is confirmed by an independently developed multi-domain BEM (boundary element method) solution. Numerical examples are presented to examine the reflection and transmission coefficients of porous bars, C R and C T , respectively, for engineering applications. The calculation results show that when the sum of widths for all the porous bars is fixed, increasing the bar number can significantly improve the sheltering function of the bars. Increasing the bar height can cause more wave energy dissipation and lower C R and C T . The spacing between adjacent bars and the spacing between the last bar and the vertical wall are the key parameters affecting C R and C T . The proposed analytical method may be used to analyze the hydrodynamic performance of submerged porous bars in preliminary engineering designs.

Wave attenuation by a submerged circular porous membrane

Surf Wave Hydrodynamics in the Coastal Environment

In this paper, a multi-domain boundary element method (MBEM) is formulated and applied to study wave interaction with double vertical slotted walls, which are modeled as thin or non-thickness structures. Two-dimensional motion with wave crests parallel to the vertical slotted walls and linearized irrotational flow are assumed. The accuracy of the solution obtained using the numerical technique is demonstrated by comparing the numerical values with those obtained from experiments and from other analytical solutions. A comparison of the hydrodynamic performance of the breakwater with identical or different double vertical slotted walls is conducted. In addition, the numerical results of the wave reflection, transmission and energy dissipation for different relative permeable depths, chamber widths, and porosities are presented and discussed. Double vertical slotted walls with a longer rear wall are recommended because they more effectively suppressed wave energy at deeper submergence. The double vertical slotted walls also very effectively dissipate the incident wave energy. Our numerical results indicate that when the permeable middle part of the seaward (first) wall (dm_1/h = 0.6) and the permeable middle part of the leeward (second) wall (dm_2/h = 0.2) have different porosities of e_1 = 0.5 and e_2 = 0.3, respectively, the breakwater has a high reflection coefficient, a low transmission coefficient and the maximum energy dissipation coefficient. The maximum energy dissipation coefficient of 0.963 occurs at kh = 1.635.

Effect of thin vertical porous barrier with variable permeability on an obliquely incident wave train

The reduction of vibration by acoustic black holes (ABHs) with damping treatments can be achieved in two stages: energy focalization and energy dissipation. The energy focalization is mainly due to changes of the local thickness by slowing down the flexural wave speed and energy dissipation can be achieved by using viscoelastic damping materials. In structures with embedded ABHs, the damping effectiveness can depend significantly on the types of damping treatments. In this paper, 4 different damping treatments according to the types of attached region are considered in order to estimate the effectiveness of damping treatments as 1) a fully-covered unconstrained damping treatment, 2) a fully-covered constrained damping treatment, 3) a partially-covered unconstrained damping treatment and 4) a partially- covered constrained damping treatment as well as no damping treatment as reference data. In this study, the performance of damping treatments is explored using numerical simulations of three-dimensional thin plate embedded truncated ABH(s). The wave energy in the ABH, the normalized total energy and the focalization ratio are introduced to compare the effectiveness of the damping treatments. The numerical results show that the fully-covered constrained damping treatment provides the most effective configuration in terms of the wave energy in ABH and the normalized total energy.

Mangrove forests cover the shores of many tropical and sub-tropical coast lines. These trees are tolerant to saline environments which enables them to grow in the tidal zone. They are known for their often complex and impressive root system. From a Civil Engineering point of view, mangrove forests are interesting for since they reduce transmitted wave energy of incident waves and thus serve as a natural coastal protection. In Vietnam, mangroves grow in the deltas of the Red river Delta and Mekong River where they protect valuable aquacultural and agricultural lands. Although awareness on the value of mangrove forests has risen in recent years, there is still little known about how (much) waves are reduced in these forests and how to investigate this effect. This study deals with the physical processes involved in the dissipation of wave energy by vegetation and with wave measurements in mangrove forests. From the analysis of the physical processes, it followed that the dissipation of wave energy is relative to the vegetated cross-area exposed to waves and the third power of the amplitude of horizontal particle velocities. It also turned out that the effect of shoaling was very small on slopes, typical in mangrove forests, which makes calculation less complicated. This study also included a trip to Vietnam for wave measurements in a mangrove forest. This fieldwork was carried out together with the Hanoi Water Resource University. Unfortunately, no wave measurements were carried out successfully. However, the fieldwork did yield interesting information about mangrove vegetation and its habitat that is useful for future measurements.