Breakwater Heads Research Articles

MAXIMUM MOMENTUM FLUX FOR STABILITY ANALYSIS OF MODEL AND PROTOTYPE BREAKWATERS

None of the established formulae for breakwater armour stone stability, Hudson (1958) or van der Meer (1987), explicitly account for the water depth at the toe of the structure. More recently, Hughes (2004), Melby and Hughes (2004), Melby and Kobayashi (2011) have developed equations for stability and runup utilising the concept of wave momentum flux which explicitly accounts for the water depth of the wave probe(s) in close proximity to the structure. The equations are adapted to three forms; namely (1) linear, (2) extended-linear and (3) non-linear. In the paper linearity is assessed by using the Ursell number at each probe depth. Also, due to the placement of probes at various depths in MHL’s 2D wave flume it is possible to correlate the linearity of the wave measurement for the same time series and subsequently test the appropriateness of the momentum flux equation applied for assessment of stability and runup. The stability and runup data from 43 2D physical model tests where stability was previously assessed using van der Meer’s and Hudson’s equations are assessed using the momentum flux equations and an evaluation of the results has been made. It was found that the estimation of notional permeability and selection of the use of the plunging or surging formulae was critical to obtaining a closer match between measurement and prediction. The equations were also utilised in conjunction with numerical models to evaluate the armour size for repair of two breakwater heads in South Camden Haven and Bellambi. The maximum momentum flux equations were found to perform satisfactorily at these locations where the Ursell numbers were found to be high and the waves non-linear.

CHANNEL CONCENTRATION AND ITS IMPACT ON NEARSHORE WAVE CONDITIONS

Channel concentration is an important wave process in coastal regions. It occurs when a shallow water wave approaches a deep channel at a shallow angle, producing a localized region of increased wave height at the edge of the channel, and the early onset of wave breaking. Channel concentration can occur at dredge channels, so is of particular significance to port entrances and surrounding infrastructure, especially for breakwater heads. It also occurs at natural features such as submarine canyons, particularly where the canyon is perpendicular to the coastline. In contrast to refraction or channel reflection, channel concentration is not well documented in the available literature. While studies show wave concentration as a significant coastal process, there is insufficient information on the process to provide general design guidance for its consideration in harbour projects. As such, there is still the need to use design tools such as physical modelling to ensure the site specific wave conditions are characterised correctly.

Seabed liquefaction around breakwater heads at a river mouth: An integrated 3D model

In this paper, a 3D model is developed to investigate the foundation stability around breakwaters at a river mouth, where both ocean waves and river currents are considered. The present model consists of flow and seabed sub-models. In the flow sub-model, the VARANS equations is used to describe hydrodynamic process, while the dynamic u−p approximation is employed in the seabed sub-model. Based on the integrated model, both oscillatory and residual liquefaction phenomenon is investigated by adopting the poro-elastic and poro-elastoplastic seabed models, respectively. The numerical results reveal that the breakwaters at the river mouth with the river current significantly alter the hydrodynamic properties around breakwater heads. Furthermore, the dynamic soil responses and liquefaction zones predicted by the poro-elastic and poro-elastoplastic seabed models show the completely different development trend, which the latter seabed has more intense and severe reactions. The parametric study indicates that the breakwater foundation is more likely to be liquefied in a loosely deposited soil under larger waves. The river currents can exacerbate the liquefaction condition in the region near breakwater heads. In addition, the pore pressure is smaller in front of a milder slope of breakwater flank and a round breakwater head with a smooth slope.

Numerical study for wave-induced oscillatory pore pressures and liquefaction around impermeable slope breakwater heads

The problem of wave-structure-seabed interactions (WSSI) around impermeable slope breakwater head is numerically investigated with a three-dimensional (3D) integrated model. The Reynolds-averaged Navier-Stokes (RANS) equations are adopted to simulate the wave-induced fluid motion, and Biot's theory for poro-elastic medium is employed to describe the seabed behaviour under wave loading. A calculation scheme is established to integrate both wave motion and seabed response. The numerical results reveal that wave-induced flow field in the vicinity of breakwater heads is significantly disturbed by the existence of the structure, leading to wave reflection, diffraction and overtopping. Furthermore, the wave-induced negative pore pressure and liquefaction near the front of the breakwater heads is significant. The parametric study concludes that the increase of breakwater slope intensifies the seabed response and liquefaction around the breakwater head, and therefore it is proposed to design breakwaters with a mild slope.

3D numerical model for wave-induced seabed response around breakwater heads

This paper presents a three-dimensional (3D) integrated numerical model where the wave-induced pore pressures in a porous seabed around breakwater heads were investigated. Unlike previous research, the Navier-Stokes equation is solved with internal wave generation for the flow model, while Biot's dynamic seabed behaviour is considered in the seabed model. With the present model, a parametric study was conducted to examine the effects of wave and soil characteristics and breakwater configuration on the wave-induced pore pressure around breakwater heads. Based on numerical examples, it was found that the wave-induced pore pressures at breakwater heads are greater than that beneath a breakwater. The wave-induced seabed response around breakwater heads become more important with: (i) a longer wave period; (ii) a seabed with higher permeability and degree of saturation; and (iii) larger angle between the incident waves and breakwater. Furthermore, the relative difference of wave-induced pore pressure between fully-dynamic and quasi-static solutions are larger at breakwater heads than that beneath a breakwater.

3D models for wave-induced pore pressures near breakwater heads

Wave-induced pore pressure is one of the important factors in the analysis of foundation stability around coastal structures. Existing models for the wave-induced seabed response around breakwater heads have been limited to poro-elastic soil behavior and de-coupled oscillatory and residual mechanisms for the rise in excess pore water pressure. To overcome the shortcoming of the existing models, in this study a new three-dimensional poro-elastoplastic model is established, in which both oscillatory and residual mechanisms can be simulated simultaneously. The reduced cases of the proposed model are verified with existing 2D experimental data available and a 3D poro-elastic analytical solution in front of a breakwater. With the proposed new model, a parametric study is conducted to investigate the relative differences of the predictions of the wave-induced pore pressure and liquefaction with poro-elastic and poro-elasto-plastic models. Based on numerical examples, it can be concluded that relative differences between elastic and elasto-plastic models are significantly affected by wave periods and water depths. Wave height significantly affects the development of residual pore pressure versus time. Plastic soil behavior plays an important role in a seabed of low permeability. Plastic soil behavior has more significant influence on wave-induced residual pore pressure than the amplitude of the oscillating pore pressure. Furthermore, poro-elastic analysis tends to under-estimate the size of the liquefaction regions around breakwater heads.

Response of a porous seabed around breakwater heads

The evaluation of wave-induced pore pressures and effective stresses in a porous seabed near a breakwater head is important for coastal engineers involved in the design of marine structures. Most previous studies have been limited to two-dimensional (2D) or three-dimensional (3D) cases in front of a breakwater. In this study, we focus on the problem near breakwater heads that consists of incident, reflected and diffracted waves. Both wave-induced oscillatory and residual liquefactions will be considered in our new models. The mistake in the previous work [Jeng, D.-S., 1996. Wave-induced liquefaction potential at the tip of a breakwater. Applied Ocean Research 18(5), 229–241] for oscillatory mechanism is corrected, while a new 3D boundary value problem describing residual mechanism is established. A parametric study is conducted to investigate the influences of several wave and soil parameters on wave-induced oscillatory and residual liquefactions around breakwater heads.

Topographically controlled, breaking-wave-induced macrovortices. Part 1. Widely separated breakwaters

In this and the companion paper (Part 2) we examine experimentally, computationally, and analytically the behaviour of breaking-wave-induced macrovortices during startup conditions. Widely separated breakwaters and rip current topographies are chosen as opposite ends of the parameter space. Part 1 examines generation mechanisms using phase-resolving and phase-averaged approximations, and suggests several simple predictive relations for general behaviour. Vortex trajectories and shedding periods for wave breaking on widely spaced breakwaters are also considered in detail. Results show broad agreement with theoretical trajectories. Predictions of vortex shedding periods on breakwater heads show excellent agreement with computations. Part 2 examines startup macrovortices on rip current topographies using computations and laboratory experiments, and changes in behaviour as the system transitions from wide to narrow gap width.

INFLUENCE OF SUBMERGED BREAKWATER HEAD ON DEFORMATION OF SUBMERGED BREAKWATER BODY

Deformation of submerged breakwater was examined by three-dimensional experiments to investigate theinfluence of the deformation of the head of submerged breakwater on the total damage of the breakwater. The erodedarea at the breakwater heads was larger than that at the breakwater trunk. The rubble stones moved from the upstreamhead of longshore current were accreted within one wave length from the end of the upstream head. Therefore, thereare two types of accretion at the leeside of the submerged breakawter depending on the ratio of the breakwater lengthto wave length. The difference in deformation between two-dimensional and three-dimensional experiments is large, especially at the upstream head of the breakwater.

Spatial and foraging behaviour, diet and morphogenesis of postlarvae and juveniles of Pempheris vanicolensis in the Gulf of Aqaba, Red Sea

Post‐settlement ecology, ontogenesis of morphology and behaviour were studied in juvenile Pempheris vanicolensis (Pempheridae, Teleostei) in the Gulf of Aqaba, Red Sea. Postflexion larvae of 5.6‐6.0 mm total length settled around breakwater heads and natural rocky outcrops of the subtidal zone, widely separated from adults dwelling in groups in coral reef crevices and caves. Settling juveniles formed groups of uniform age and foraged on zooplankton around their habitats. Their circadian behaviour was regulated by levels of illumination in the water, differing slightly in the various age groups. At sunset, juvenile P. vanicolensis migrated and dispersed in deeper water, returning before sunrise to their rocky habitats. Their patterns of foraging and migration behaviours were analysed. At 28.0‐32.0 mm total length and 500 mg body weight, the increase in height and length (H/L ratio) of the juvenile changed from monoto heterochronous, after which a rhomboid body shape, typical for this species, was formed. Concomitantly, the retina of the juvenile changed from a photopic to a scotopic one, the body became heavily pigmented, and the fish left the subtidal rocks, migrating to crevices in coral formations where they settled in close proximity to groups of adult P. vanicolensis.

Stability of low-crested rubble-mound breakwater heads : C. Vidal, M. A. Losada & E. P. D. Mansard, Journal of Waterway, Port, Coastal & Ocean Engineering — ASCE, 121(2), 1995, pp 114–122

Stability of low-crested rubble-mound breakwater heads : C. Vidal, M. A. Losada & E. P. D. Mansard, Journal of Waterway, Port, Coastal & Ocean Engineering — ASCE, 121(2), 1995, pp 114–122

Stability of Low-Crested Rubble-Mound Breakwater Heads

Three-dimensional physical models of low-crested, detached rubble-mound breakwaters were built, with their individual sections representing the following components of the breakwater: front slope, crest, back slope, total section (combination of the three previous sections), front head, and back head. The stability of the armor units in each of these sections was assessed, for different freeboards, using irregular waves. In a 1992 paper, Vidal et al. presented the stability criteria for the different trunk sections of this model breakwater. This paper focusses on test results from the head sections. A comparison between the stabilities of the head and trunk sections is also discussed. The stability of the back head (BH) section is found to be highly freeboard-dependent. For fully submerged conditions, the back head is one of the most stable sections of the breakwater. On the other hand, when the breakwater is nonovertopping, it is the least stable section. The front head (FH) section's behavior is similar...

Hydraulic performance of berm breakwater heads

In 1984, a berm breakwater built the same year on the island of St. Paul was completely destroyed in two serious storms. The breakwater roundhead was constructed with a profile similar to that of t...

SEDIMENTOLOGICAL INFLUENCES OF DETACHED BREAKWATERS

Wave diffraction and refraction in the surroundings of a new detached breakwater induce currents strong enough to cause substantial local sediment transport and consequently morphologic changes, the main feature of which are sand spits or tombolos. As shown by field studies the morphologic changes cause an equilibrium state with some minor fluctuations due to changing wave climate. Model studies have also proved the existance of such an equilibrium state for any given geometry and sea state The present paper treats mainly this equilibrium state, but also attempts to explain the mechanism of sand transport characterizing the initial and transitional states. According to observations by the authors in small scale models, the transporting mechanism in the transitional state involves sea bottom erosion, especially near the breakwater heads outside the protected area, and transport of sand towards the shore of the sheltered area in the form of small migrating sand bars, which finally join the shore line and widen it. Results of experimental investigations conducted by the authors, for the particular case of high impervious breakwaters attacked by waves of normal incidence, as well as results of field and model studies given by others, are used to define relationships among the factors determining the equilibrium state and to base a new hypothesis regarding the equilibrium state. This hypothesis states that a morphologic and sedimentologic equilibrium is reached behind a detached breakwater, when the shape of the contour lines becomes such, that along the sheltered beach the diffracted waves have components of momentum opposed to the gradients of the mean sea level induced by radiation stress due to non uniform wave heights along the wave fronts approaching from both sides of the breakwater. The significant parameters characterizing the dimensions of the spit or tombolo in the equilibrium state are shown to be the relative length of the breakwater (compared to its distance from the original shore line), the relative distance from the original shore line (compared to the position of the breakers' line) and the relative height of the breakwater crest (above M.S.L, compared to the incident wave height).

Étude expérimentale systématique en vue de l'utilisation des modèles mathématiques pour l'étude de la diffraction pure de la Houle

I.-Subject of Investigation. In a paper to the IXth. Coastal Engineering Congress in Lisbon in 1964, two SOGREAH engineers, Mr. Barailler and Mr. Gaillard, described a number of applications of electronic computers for research on waves in coastal areas. lnvestigation with a physical scale model yields both a validity criterion for calculation and a interesting illustration. The present article covers the full results of a systematic pure wave diffraction study on a model and compares them with the corresponding computer-calculated data. It is in two parts, which discuss the following subjects: (i) Pure diffraction in a body of water in which there are no wave-reflecting obstacles. (ii) Pure wave diffraction in a body of water containing totally or partly reflecting obstacles. II.-Test Conditions. The tests were conducted in a constant depth of wate rand with the waves heading straight for the "harbour entrance," which was determined in the first part of the investigation by the following characteristic breakwater head configurations: Thin breakwater heads; Thick vertical-fronted breakwater heads; Thick conical-section breakwater heads. The second part of the investigation considered wave conditions in a body of water bounded by wave reflecting obstacles. III.-Results. Only the general findings of the investigation are given, as available space does not allow full sets of test results to be included and compared with calculated curves. Calculations allows neither for the thickness of the breakwaters to either side of the opening nor for the embankment slope at the considered point. The position and dimension of the "theoretical" opening therefore require very careful examination. Only position need be considered for a vertical-fronted brea1ewater. Comparison of various solutions shows that closest agreement is obtained for the opening coinciding with the breakwater centerline (case 2). Embankment slope has to be allowed for in the conical-section breakwater lead case, because of its "converging duct" effect. The "equivalent opening" is shorter than that measured at datum level, which is shown up by a shift in the measured maximum amplitude values towards the centerline. Provision has been made for the determination of this "equivalent length" on a computer. In the tests with reflecting obstacles the theoretical opening was assumed to lie in the plane of the inside breakwater face, as the calculation of wave diffraction with reflection requires basing on clearly-defined boundaries. In this case, breakwater head thickness could be allowed for by further subdivision of the considered region; computer tests are now in progress on this point. Conclusion. a) Diffraction without reflection: Comparison between physical and mathematical model results under the above conditions shows very satisfactory agreement for vertical-fronted breakwater heads. In the case of conical-section breakwater heads, agreement is satisfactory beyond a limit at about 1 1/2 wave lengths from the opening. Though absolute measured and computed amplitudes also agree well up to this limit, a shift in the maximum value positions is observe d, which it should be possible to correct by establishing the position and length of the "equivalent opening". b) Diffraction with reflection: Satisfactory agreement is achieved with both methods.