Offshore Breakwater Research Articles

Mechanism of Deformation of Sand Bar Formed in Sanbyakken Region in Nakatsu Tidal Flat

Long-term changes in sand bar in Sanbyakken region in Nakatsu tidal flat were investigated through the comparison of the aerial photographs. Great changes have occurred in this sand bar in recent years, as aresult of increase in eastward longshore sand transport since 1998. The reason was studied in terms of change in wave field. Waves with the direction of N70deg E, which was determined from the shape of the sand bars on the tidal flat, were significantly sheltered due to the offshore breakwater of Nakatsu Port. Takingthe refraction effect of these diffracted waves into account, the tip of the sand bar was also sheltered by the waves. It was found that this caused increase in longshore sand transport.

Development and Validation of a Quasi-Three-Dimensional Coastal Area Morphological Model

A quasi-three-dimensional coastal area morphological (Q3DCAM) model has been developed using the process-based approach. It is for simulating complex multiscale coastal processes, primarily morphodynamic changes of the seabed. This software package has integrated three key submodels for simulating irregular wave deformations, nearshore currents, and morphological processes. The quasi-three-dimensional capability of the depth-averaged model has been developed to consider the vertical flow structure inside the surf zone and the cross-shore movement mechanisms of nearshore currents, e.g., undertow and mass flux. To this end, the calculations of the radiation stresses inside the surf zone have been improved by introducing the nonsinusoidal wave model for surface roller effects due to the breaking wave. To predict accurately the wave field and the morphological processes near coastal structures, the wave diffraction effects were included in a multidirectional spectral wave transformation model. The morphodynamic change was modeled by considering the sediment transport due to the combinations of waves and currents. These three submodels were validated by simulating three laboratory experimental cases in regard to: (1) irregular wave deformations over a shoal; (2) longshore currents in a wave basin; and (3) moveable bed evolutions around an offshore breakwater under attack of an incident wave. The numerical results of the morphological modeling confirmed that the Q3DCAM model consisting of the diffraction effects and the surface roller effects is capable of predicting waves, currents, and morphodynamic changes more accurately than before. Therefore, this validated model can be applied to simulate more realistic morphological processes in coastal zones including structures.

Numerical investigation on the dynamics of a vertical wall defenced by an offshore breakwater

The numerical and experimental investigations on the performance of an offshore-submerged breakwater in reducing the wave forces and wave run-up on vertical wall are presented. A two-dimensional finite-element model is employed to study the hydrodynamic performance of the submerged breakwater under the action of regular and random waves. The numerical prediction has been supported with experimental measurements. The wave forces and wave run-up on the vertical wall were measured for different breakwater configurations. The applicability of linear theoretical model in the prediction of wave forces on the wall by a submerged breakwater has been discussed.

Environmental impacts of shore-parallel breakwaters along Nagahama and Ohgata, District of Joetsu, Japan

A combination of sedimentological analysis of beach sands, field surveys and the assessment of the empirical methods for the breakwater design is used to investigate the impacts of offshore breakwaters on beach morphology along Nagahama and Ohgata. The results indicate that the nearshore sedimentation has been substantially disrupted due to the features formed behind the breakwaters stopping littoral transport, which led to sediment accretion on the updrift side of the breakwaters, and erosion on their downdrift side. Sedimentological analysis of beach sands made it possible to recognize the dominant process at the sections considered. In fact, the occurrence of strong offshore-going currents, the removal of finer mean grain size and the predominance of high-density heavy minerals led us to deduce that the two sections undergo a natural sedimentary imbalance.

Investigation of the effects of offshore breakwater parameters on sediment accumulation

Littoral sediment transport is the main reason for coastal erosion and accretion. Therefore, various types of structures are used in shore protection and littoral sediment trapping studies. Offshore breakwaters are one of these structures. Construction of offshore breakwaters is one of the main countermeasures against beach erosion. In this paper, offshore protection process is studied on the effect of offshore breakwater parameters (length, distance and gap), wave parameters (height, period and angle) and on sediment accumulation ratio, one researched in a physical model. In addition to the experimental studies, numerical model in which the formulae of the sediment discharge (i.e. the formulae of CERC and Kamphuis), was used was developed and employed. The results of the experimental and numerical studies were compared with each other.

Shoreline response to multi-functional artificial surfing reefs: A numerical and physical modelling study

The results of a series of 2DH numerical and 3D scaled physical modelling tests indicate that processes governing shoreline response to submerged structures, such as artificial surfing reefs, are different from those associated with emergent offshore breakwaters. Unlike the case of emergent offshore breakwaters, where shoreline accretion (salient development) is expected under all structural/environmental conditions, the principal mode of shoreline response to submerged structures can vary between erosive and accretive, depending on the offshore distance to the structure. The predominant wave incidence angle and structure crest level also have important implications on the magnitude of shoreline response, but not on the mode of shoreline response (i.e. erosion vs. accretion). Based on the results obtained here, a predictive empirical relationship is proposed as a preliminary engineering tool to assess shoreline response to submerged structures.

Long-Term Changes of Eelgrass, Zostera marina, by Using of Artificial Seeding behind Offshore Breakwater

Long-Term Changes of Eelgrass, Zostera marina, by Using of Artificial Seeding behind Offshore Breakwater

Investigation of detached breakwaters. Part I—hydrodynamics

This paper examines the hydrodynamic processes associated with detached breakwater schemes. It is based on a physical model study of the Elmer breakwater scheme, undertaken in the UK Coastal Research Facility. The main objectives were to investigate the influences on the hydrodynamics of wave directionality and directional spread, irregular bathymetry and breakwater permeability. In contrast to previous studies, it was found that one circulation cell was dominant across the middle of the bays for both random and monochromatic waves. Refraction due to the irregular bathymetry had a significant influence on the wave-induced currents behind the breakwaters. In addition, transmission through and overtopping of the breakwaters were responsible for the observed current patterns. Long-period waves were formed in the near-shore for multidirectional waves, resulting in long period oscillations of the currents. These complex hydrodynamic processes are not included in most current numerical models used in the design of offshore breakwater schemes. The paper concludes that a new generation of models is needed to incorporate these processes, but recommends the most appropriate techniques to use in the meantime.

瀬戸内海における海浜植物の人工養浜への定着に及ぼす離岸堤の影響

瀬戸内海における海浜植物の人工養浜への定着に及ぼす離岸堤の影響

荷役稼働率に基づく長周期波対策効果の評価手法

Long-period waves cause failure of cargo handling and break mooring ropes in many ports. One of the counter plans against long-period waves is to extend offshore breakwater in a port. It is an effective way to reduce long-period waves in a port.In this paper, we propose the method that evaluates such counter plans against long-period waves in terms of the cargo handling operation rate. We examined the difference of the cargo handling operation rate by changing evaluation indices, such as wave height, long-period wave height, and ship motion. Finally, we applied the proposed evaluation method to Hitachinaka port and investigated the effect of long-period wave counter plans.

導水板によるスリット構造物の長周期波消波特性

In many ports, long-period waves decrease the efficiency of cargo handling and sometimes cause cutting of mooring ropes. Possible counter measures against damages due to long-period waves are to forecast the surge of long waves, to extend the offshore breakwater, or to improve the mooring system. One of the most effective ways to reduce the long-period waves in a port is to construct the long-wave absorbing structures along breakwaters. In order to absorb long-period waves, however, the wave absorber requires long structures and may not fit in many existing ports.In this study, we propose a new slit-type long-wave absorbing structure whose length is shorter than other former long-wave absorbers. We performed hydraulic experiment and verified the overall performance of the present wave absorbing structure in terms of its wave reflection rate.

Resonance Induced by Edge Waves in Hua-Lien Harbor

This article first reviews previous numerical studies on the resonance problem of Hua-Lien Harbor. All the research leads to the conclusion that resonance can be stimulated by 2-D infragravity waves. However, a literature survey suggests that outside the harbor these plane infragravity waves are too small to excite violent water-body movement inside. On the other hand, 3-D infragravity waves trapped along the coastline, also known as edge waves, are more likely to exist outside the harbor and their effect needs to be thoroughly discussed. Based on previous measurements, the response of Hua-Lien Harbor is best simulated using edge waves of 160 and 140 second periods and their resonance mechanisms are analyzed. The former case has a longitudinal resonant mode and hence the amplitude in the inner harbor is large. The latter case has a transverse mode in the outer basin and hence only berths in the outer basin are influenced. These features are both consistent with field measurement. Therefore, it is very likely that edge waves are responsible for the resonance of Hua-Lien Harbor. Finally, based on observation after the construction of the present offshore breakwaters, a theory is proposed to explain the trapping of incident edge waves, and a measure to further reduce the resonance is discussed.

Designing Offshore Breakwaters Using Empirical Relationships: A Case Study from Norfolk, United Kingdom

This paper examines the applicability of several empirical relationships for the performance of shore-parallel breakwaters at Sea Palling in Norfolk, UK. The research presented here compliments the work of Axe et al. (1996), who applied the models of Pope and Dean (1986), Suh and Dalrymple (1987), Ahrens and Cox (1990) and McCormick (1993) and compared their predictions with observations of beach response to the Elmer offshore breakwater scheme in West Sussex, UK. When these models were applied to the Sea Palling breakwaters, they generally revealed large inconsistencies in the predicted beach response. It is suspected that the inability of these methods to correctly predict beach response at this location is most likely caused by the overriding influence of factors such as wave transmission, longshore drift and a large tidal range, over geometrical relationships in the scheme design.

Beach response to shore-parallel breakwaters at Sea Palling, Norfolk, UK

The effects of four offshore breakwaters built between 1993 and 1995 at Sea Palling, Norfolk, UK, on beach morphology are discussed. The structures were built during the first phase of a multi-phase construction programme and are expected to provide long-term protection for Sea Palling and 6000 ha of low-lying land against tidal inundation through the provision of a wide beach in front of the existing seawall and dunes. The results presented here span all stages of breakwater construction during Phase 1. Beach level changes were monitored during the period from 1995 to 1999. A Geographical Information System (GIS) was used to calculate differential surfaces between surveys in order to investigate local patterns of accretion and erosion and to determine volume changes between consecutive surveys. Additional data collected by the Environment Agency since 1991 were analysed to investigate beach and bathymetry changes prior to the construction of the breakwaters. In association with periodic nourishment programmes the scheme has been locally successful in retaining recharged sediment on the beach but the extent of the salients prevents to a great extent the south-eastward littoral drift. As a result, effects on the beaches, both down- and up-drift of the breakwaters, have been considerable. The continuing increase in beach volume indicated that by January 1999 equilibrium had not been reached.

波の遮蔽構造物建設に起因して阿字ケ浦・那珂海岸で進みつつある大規模侵食

Large-scale beach erosion and accretion triggered by the construction of an offshore breakwater on the Ajigaura and Naka coasts in Ibaraki Prefecture was investigated through field observation and analysis of aerial photographs.The shoreline retreated largely outside the wave shadow zone, leaving gravels on the foreshorezone by selective erosion of bed materials and devastating famous bathing beach of mild slope. In contrast, in the wave shadow zone, a large amount of fine sand accumulated. The measured shoreline configuration was well predicte by the Hsu model to forecast a stable shoreline. On the Naka coast, the same phenomenon was occurred and seawall was heavily damaged.

PRACTICAL COUNTERMEASURE TO LONG PERIOD WAVE IN HARBOR

Long period waves with period of 30 to 300s causes the troubles in loading and un-loading of large cargo vessels and breaks of mooring ropes. Such damages are amplified by the resonance of mooring system composed of vessel and hawser to long period waves because the natural oscillation period of the system agrees with that of waves. The countermeasures represented by the extension of offshore breakwaters, improvement of mooring system and establishment to warning system have been proposed to prevent the long period wave turbulence.In the paper, the practical countermeasures employing the extension of offshore breakwater and development of long period wave absorber are proposed. Numerical modeling is carried out to evaluate the effects of countermeasures to reduce long period wave heights in harbor. The simulated results demonstrate that offshore breakwaters extended appropriately may reduce the incident energy of long period waves and may increase the rate of effective handling days and that double slit caisson sea wall is applicable to reduce the energy of reflected long period waves.

EXPERIMENTS FOR OPTIMIZATION OF SUBMERGED BREAKWATER CONFIGURATION OFF WEST COAST OF NIIGATA

The new concept of integrated and extensive shore defense system combining submerged breakwaters, jetties and beach nourishment has been studied for the erosion-prone west coast of Niigata Prefecture to replace the conventional shore protection system consisting primarily of offshore breakwaters in shallow waters for wave interception.This paper deals with two-dimensional experiments conducted on the multi-row submerged breakwaters proposed for construction off the west coast of Niigata Prefecture to analyze their crown width, crown depth, installation spacing, and other relevant design parameters so as to select an optimum breakwater configuration which meets wave heights that will not disturb sediments inside the submerged breakwaters.

Boundary element modeling of multidirectional random waves in a harbor with partially reflecting boundaries

A numerical model is presented for the prediction of the wave field due to the diffraction of directional random waves in a harbor of arbitrary shape with partially reflecting boundaries. The water depth is assumed uniform and the method is based upon the superposition of diffraction solutions for monochromatic waves obtained by a two-dimensional boundary integral equation approach. The incident wave conditions are specified using a discrete form of the Mitsuyasu directional spectrum. The present numerical model has been validated through comparisons with previous experimental data and theoretical results for both regular and random wave diffraction by offshore breakwaters and in harbors. Good agreement was obtained in all cases. Based on these comparisons it is concluded that the present numerical model is an accurate and efficient tool to predict the wave field inside a harbor or around a breakwater in many practical applications.

EFFECTIVENESS OF AN ARTIFICIAL TRENCH FOR HARBOR TRANQUILITY

In order to reduce wave heights in a harbor, an artificial trench installed in front of theharbor entrance was newly proposed as alternative works of detached and offshore breakwaters. Based on the wave refraction mechanism by water depth variations, change of wave propagation direction by the artificial trench is realized. Several factors regarding artificial trenches, such as locations, sizes, plane figures and so on, were examined theoretically by the numerical method of analysis. For the comparison, effectiveness of a rectangular resonator for harbor tranquility was also examined. It was confirmed that artificial trenches with a circular and rectangular figures are effective for reducing incoming waves to the harbors, especially for normal incidence of waves to that entrance.

拡張型Boussinesq方程式の沿岸波浪場・海浜流場への適用性に関する研究

The modified Boussinesq equation model which was proposed by Madsen et al was applied for wave motion and nearshore current fields and its applicability was discussed. Numerical simulations of the circular reef, half-unlimited breakwater and beach topographies with an offshore breakwater were performed The accuracy of this model was verified thorough comparison with laboratory experiments and previous theory. Furthermore, numerical analysis of wave transformation for typhoon No.9918 in the vicinity of the Kumamoto New Port were carried out Results demonstrated that wave height increased in the estuary of Shirakawa river and the velocity scale of nearshore currents was as large as that of tidal currents.