Offshore Breakwater Research Articles

BEACH CHANGES CAUSED BY CONSTRUCTION OF DETACHED BREAKWATERS NEAR THE END OF A POCKET BEACH

Beach changes triggered by the construction of detached breakwaters near the end of a pocket beach were investigated at the Tojo coast in Kamogawa City in Chiba Prefecture. This beach has an around 3.9km long stretch and is facing the Pacific Ocean. Shoreline changes were read from the aerial photographs and field observation was carried out. Sand accumulated behind detached breakwaters thanks to their wave dissipation effect. Simultaneously this caused beach erosion in the other area and countermeasures were needed to protect facilities. The cause is due to the lack of thinking on overall balance of sandy beach in a closed system of littoral drift. It is concluded that construction of offshore breakwaters in a pocket beach should be carefully done because of the impact to the surrounding coast.

A highly wave dissipation offshore breakwater

The aim of this paper is to develop an offshore breakwater, for which coefficients of both the wave reflection and transmission have low values. The breakwater is suggested to compose of n layers of porous materials with different porosities. A complex eigen function method is used in the theoretical analysis. Continuities of both mass flux and fluid pressure are assumed at interfaces between every two adjoining porous materials and at the interface between end materials and water region. Following a series of mathematical processes, the coefficients of the wave transmission and reflection along with the wave energy loss are calculated. The porosity of materials is varied in computations; and results are compared among structures composing of different layers of porous materials. A single layer offshore breakwater is shown to reduce simultaneously the coefficients of transmission and reflection only when the structure is very wide in the direction of wave propagation, and the structure material has a high porosity. A multilayer breakwater, however, can function well in reducing both coefficients at a much narrower width; structure having more layers can be more effective at narrower width. Finally, several experiments are conducted; theoretical computations and experimental results agree well.

Coastal protection of the isle of usedom — conceptional design of an offshore breakwater system at the streckelsberg, Baltic Sea

The Isle of Usedom is located in the South of the Baltic Sea on the border between Poland and Germany. The Streckelsberg is midway along the coastline of the island. The cliff coast area is extremely exposed to waves. Coastal protection works have been carried out here for the last 100 years, starting in 1895 with the construction of a revetment. All efforts could not solve the erosion problems in the long-term. Therefore, it was decided to install a new system of coastal protection measures in 1994. The conceptional design includes a system of three offshore breakwaters in combination with a beach nourishment system. To minimize downdrift erosion a groyne scheme will be additionally installed. The aim of the offshore breakwaters is to reduce the wave energy transformed into the sheltered area and to consequently reduce the longshore sediment transport. It is expected to support the natural development of a salient which will form the main part of the complete coastal protection system.

Intercomparison of coastal area morphodynamic models

Five different coastal area morphodynamic models have been set up to run on the same offshore breakwater layout and an intercomparison carried out on the hydrodynamic and morphodynamic output produced by each scheme. In addition, the predicted morphodynamics was checked against available laboratory and field data. It is concluded that the models are capable of producing realistic estimates for the dominant morphodynamic features associated with offshore breakwaters. Coupling of the wave, current and sediment transport components of each scheme is shown to yield bathymetry which attains a state of equilibrium, unlike models which are based on the initial transport field only.

Oblique wave diffraction by segmented offshore breakwaters

This paper presents a theoretical model to examine oblique wave diffraction by a detached breakwater system consisting of an infinite row of regularly-spaced thin, impermeable structures located in water of uniform depth. The fluid is assumed incompressible and inviscid and to undergo irrotational motion. Wave heights are assumed to be sufficiently small such that linear wave theory is applicable. The eigenfunction expansion solution of Dalrymple and Martin (1990) for normal wave incidence on this breakwater geometry is modified herein to study oblique wave effects. Numerical results, in the form of contour maps of the relative wave height behind the structure, or complex reflection coefficients, are presented for a range of wave and breakwater parameters. The accuracy of the present model is verified by a comparison with existing results for the limiting cases of an isolated detached breakwater, and a breakwater with a single gap. Also, for the multi-gap breakwater, the present solution is further verified for both normal and oblique wave incidence with results in the open literature.

A note on beach changes due to wind waves in Lake Shinji

Beach changes of Torigasaki Area in Lake Shinji Located in Shimane Prefecture wereinvestigated. On this beach a cuspate foreland was formed behind an offshore breakwater built for protecting cultivation facilities of shells against wave action. Aerial photogaraphs were compared to investigate tong-term shoreline changes of this cuspate foreland. Seasonal, cyclic beach changes due to the change in wave direction corresponding to the wind direction were found at the head of the foreland. Grain size distribution around the foreland was measured.

Effectiveness of an Array of Offshore Breakwaters for Irregular Waves

Wave transformation by an array of offshore breakwaters is examined in multi-directional irregular waves. Especially, concerning the ratio of an array pitch length to a representative wave length, plane wave interactions with the arrayed breakwaters are examined extensively. It is pointed out that the wave height variation behind the breakwater array changes from a long-crested pattern to a shortcrested one with increasing a pitch length of periodically arrayed breakwaters as compared to a wave length.

Oblique-Wave Diffraction by Noncollinear Segmented Offshore Breakwaters

A theoretical solution is presented for oblique-wave diffraction by a segmented offshore breakwater system consisting of an infinite array of equally spaced vertical barriers oriented at an arbitrary angle to the line joining their centers. The solution is obtained by a Green's function approach. The geometrical periodicity of the breakwater system reduces the problem to that of a singular integral equation for the potential difference across a single breakwater element. Zero-order reflection and transmission coefficients are defined in terms of the amplitudes of the asymptotic (propagating) diffracted waves traveling in the opposite direction to or the same direction as the incident waves, respectively. Numerical values of these coefficients are presented for a range of angles of wave incidence and breakwater orientation, for various breakwater geometric configurations. For certain parameter combinations, significant wave reflection can be achieved using this type of breakwater system.

The significance of earthquake-induced dynamic forces in coastal structures design

The dynamic forces of sea water and backfill soil acting on coastal embankments and the hydrodynamic forces on offshore breakwaters have been analyzed using a finite-difference scheme. A non- horizontal sea bottom is considered in the analysis. Both rigid and flexible embankments are included in the study. For a coastal embankment, the hydrodynamic pressure of sea water acting on the embankment face significantly increases as the slope of the sea bottom increases. On the other side of the embankment, the pore pressure and the interaction force between the soil and fluid will augment significantly when the backfill soil is compressed during earthquakes. When the sea-embankment-backfill soil interaction is included, the dynamic forces acting on a flexible embankment are much larger than those on a rigid one. The comparison of evaluating the sliding of a caisson in a case study of SPM by a conventional analysis and the present seismic analysis was made. The earthquake-induced dynamic pressures on both sides of the embankment (or breakwater) could be much larger than the force generated by the storm surge. The hydrodynamic (or seismic) analysis incorporating the effect of an earthquake should be included in the coastal embankment or offshore breakwater design, especially for the area with active earthquakes.

SHORELINE DIRECTIONAL WAVE SPECTRA. PART 2. INSTRUMENTATION AND FIELD MEASUREMENTS.

Keywords PLYMOUTH UNIVERSITY, BRIGHTON UNIVERSITY WAVES, SPECTRA, SHORELINE, DIRECTIONAL, INSTRUMENTATION, FIELD, MEASUREMENT, INSTRUMENTS, STORMS, EVENTS, MAJOR, ENERGY, DENSITY, MAXIMUM, LIKELIHOOD, PROBABILITY, DATABASES, COMPUTERS, DATA, INSHORE, MONITOR, CLIMATE, STUDIES, PREDICTION, NUMERICAL, TRANSFORMATION, MODELS, EFFECT, OFFSHORE, BREAKWATERS, SEA, BEACHES, EVOLUTION, SEVERE, GAUGES, ARRAYS, GEOMETRY, SENSORS, SYSTEMS, ACQUISITION, COLLECTION, WATER ELMER, SUSSEX UK... Show All

The significance of earthquake-induced dynamic forces in coastal structures design

The dynamic forces of sea water and backfill soil acting on coastal embankments and the hydrodynamic forces on offshore breakwaters have been analyzed using a finite-difference scheme. A non-horizontal sea bottom is considered in the analysis. Both rigid and flexible embankments are included in the study. For a coastal embankment, the hydrodynamic pressure of sea water acting on the embankment face significantly increases as the slope of the sea bottom increases. On the other side of the embankment, the pore pressure and the interaction force between the soil and fluid will augment significantly when the backfill soil is compressed during earthquakes. When the sea-embankment-backfill soil interaction is included, the dynamic forces acting on a flexible embankment are much larger than those on a rigid one. The comparison of evaluating the sliding of a cassion in a case study of SPM by a conventional analysis and the present seismic analysis was made. The earthquake-induced dynamic pressures on both sides of the embankment (or breakwater) could be much larger than the force generated by the storm surge. The hydrodynamic (or seismic) analysis incorporating the effect of an earthquake should be included in the coastal embankment or offshore breakwater design, especially for the area with active earthquakes.

THE NEW PORTO TORRES BREAKWATER, ITALY.

Porto Torres, a busy Mediterranean industrial harbour in northern Sardinia, has been significantly enhanced by a new 2.4 km breakwater incorporating a deep-water shipping quay. The 45 million project links up with an existing 1.7 km offshore breakwater using a unique combination of rubble mound and caisson walling techniques. This paper describes the design, model testing and construction of the project and reports on the success of its innovative wave-absorbing caisson wall design.

Proposal for a configuration of artificial reef as harmonized with fishery

In Japan, the improvement works of revetment and offshore breakwater have been promoted for coastal conservation. However, recently in the coastarea, there are occurring lots of problems such as increasing of oceaniction and draining of natural resources. To cope with such problems, it is inevitably required that the coastal structures are to be improved on a multi-purpose basis and on the other hand, the related facilities are to be improved in view of environment like fishery resources.In order to meet such social requirements, this Paper introduces a recommendable cross-section of the multi-purpose artificial reef which is observed most promising as a new model facilty for coastal conservation as well as fishery function.

SEA FRONT ENHANCEMENTS, HERNE BAY, KENT.

This paper reviews the provision of a new sea front, including formal and walled gardens, car parking, children's play area, public art items and extensive traffic calming works. The scheme followed the construction of the new sea defence for this part of the sea front at Herne Bay. The construction of a new offshore breakwater and the refacing of the existing sea wall are briefly described. The process of funding of the scheme from several agencies (including grant aid from the European Union) is reviewed. The enhancement works complete the regeneration of the town's sea front and have been the subject of much local praise. The key components of the success of the scheme are reviewed, including the highway improvements, street lighting, improvements and traffic calming measures (road narrowing, traffic calming humps and humped zebra crossings).

Wave Diffraction by Array of Thin‐Screen Breakwaters

The diffraction of water waves by an offshore breakwater consisting of an infinite row of equally spaced vertical screens is investigated theoretically. A Green’s function approach is utilized and the use of geometric periodicity reduces the problem to that of a singular integral equation for the potential difference across a single screen element. The integral equation is then solved numerically by expressing this potential difference as a series of Chebyshev polynomials with unknown coefficients. The reflected and transmitted wave systems each consist of a finite number of propagating wave components. The accuracy of the numerical scheme is verified based on energy considerations. A reflection coefficient is defined in terms of the amplitude of the asymptotic wave traveling in a direction opposite to that of the incident wave system. Reflection coefficients are presented for a variety of angles of wave incidence and screen width-to-spacing ratios. These results indicate that for certain parameter combinations significant wave reflection can be obtained utilizing this type of breakwater configuration.

REBUILDING OF SEA DEFENCES, CENTRAL PARADE, HERNE BAY, KENT.

This paper reviews the investigation, the design and the construction of the new sea defences fronting the central area of Herne Bay on the North Kent coast. The existing defences consisted of a small shingle beach backed by a concrete wall. The paper describes the coastal morphology, computer and physical modelling and the design work carried out. It also sets out the reasons for the selection of a sea defence scheme comprising a linked offshore breakwater with a larger shingle beach. The paper reviews the importance of public consultation and the role that this played in the final form of the design proposal; finally it gives a brief description of the construction work. (A)

Beaches downcoast of harbours in bays

In spite of beach berms being removed during each storm sequence the waterline may be retained constant either by a constant supply of littoral drift or static equilibrium bays being formed between headlands. The construction of salients behind offshore breakwaters may be at the expense of the adjoining coast, which resembles the ubiquitous bay shapes formed between headlands that can be either in dynamic or static equilibrium. This is the same silting situation in the lee of breakwaters extending from the coast where beach erosion results downcoast. Ports constructed upcoast of a bay in dynamic equilibrium can have their breakwaters so located that the bay can become statically stable. In the case of the bay being already in static equilibrium breakwater extension often causes massive accretion in its lee and concomitant erosion further downcoast, thus demanding prodigious expense for beach protection installations. Appropriate procedures are now suggested to minimise this undesirable effect of beach erosion associated with the construction and extension of breakwaters to ports upcoast of a bay.

企業による海洋科学 （７） 複合消波型構造物［マリンエース］の開発

企業による海洋科学 （７） 複合消波型構造物［マリンエース］の開発

Diffraction of Second-Order Long-Period Waves by Discontinuous Offshore Breakwaters

Diffraction of Second-Order Long-Period Waves by Discontinuous Offshore Breakwaters

Final results of R&D on new-type offshore breakwater

Design criteria of the new-type offshore breakwater to create “Marine Multi-purpose Zone” was studied in a period between 1986 and 1990. Eleven kinds of the new-type offshore breakwaters have been developed by the cooperative research with private sectors. Developed structures are briefly introduced and classified. Contents of Manual for planning of Marine-purpose Zone are summarized.