Showing papers on "Breakwater published in 1997"

Performance of Cage Floating Breakwater

Abstract: Conventional floating breakwaters are examined and the feasibility of developing a cage floating breakwater is explored. Earlier studies on floating breakwaters reveal that, to achieve a transmission coefficient (\iK\i\dt) less than 0.5, the breakwater width to wave length ratio (\IW/L\N) should necessarily be greater than 0.4 for most of the configurations. Recent studies on cost-effective floating breakwaters indicate that by fixing a row of pipes below the floating body, the \IW/L\N requirement can be reduced to 0.15 without any compromise in the performance. This concept has been adopted in developing a new configuration to serve as: (1) a floating breakwater; and (2) as a possible shallow water cage culture unit. Experiments were conducted to study the performance of the cage floating breakwater under wave and wave-current environment. The results on transmission and reflection coefficients are presented and compared with those reported in the literature. The variation of water surface oscillations and velocities within the cage, the effect of mooring line stiffness, and initial tension on transmission characteristics are also discussed.

Analytical Solutions of One-Line Model for Shoreline Change near Coastal Structures

Abstract: Analytical solutions provide a simple and economical means of quickly estimating qualitative and quantitative responses of the shoreline to a wide range of environmental and engineering conditions. This paper presents analytical solutions for shoreline evolution in the vicinity of coastal structures, including detached breakwaters, seawalls, and jetties with and without wave diffraction. The solution for a detached breakwater illustrates the growth of a salient with time behind the breakwater and the associated initial shoreline retreat at locations across from the breakwater tips. A simple solution describing flanking of a seawall is obtained by using two different solution areas where the longshore sand transport rate and breaking-wave angle vary. A similar technique is used to model diffraction downdrift of a groin or jetty by allowing the wave angle to vary with the distance alongshore according to a specified function or by employing a large number of solution areas. Cases are also presented for the accumulation updrift a groin and the shoreline response in a groin compartment with a breaking wave angle that varies sinusoidally in time. A solution for a single groin illustrates the impact of bypassing on the updrift accumulation.

New design methods for wave impact loading on vertical breakwaters and seawalls

Abstract: This paper discusses wave impacts on vertical and composite breakwaters and related coastal structures. It describes types of vertical walls in use, with particular reference to older walls that may be much more influenced by wave impacts. Methods to estimate wave forces are identified. Analysis of performance suggests that these under-predict wave impact loads, and cannot identify combinations of geometry and wave conditions which lead to impacts. Comprehensive 2-dimensional hydraulic model tests have been conducted using random waves to measure wave pressures (and other responses) on a wide range of simple and composite vertical walls. The test results have been used here to: • Identify the ranges of geometry and wave conditions which lead to wave impacts; • Develop a simple method to estimate wave forces under impact conditions. Analysis of % of impacts has defined a new design diagram to identify wave conditions and wall / mound geometries which cause impacts. These results are intended for engineers analysing vertical or composite walls in deep water, in harbours, or along the shoreline.

Bem-fem combined analysis of nonlinear interaction between wave and submerged breakwater

Abstract: A combined BEM-FEM model has been developed to study the nonlinear dynamic interaction between a submerged breakwater and waves. The resistance coefficients in the equations of motion inside the porous media have been experimentally determined based on measured values of the wave forces on spherical armor units in a submerged breakwater. Comparisons of the numerical model results with the experimental measurements indicate that this modification has improved the model accuracy in simulating the wave deformation and the energy dissipation due to a submerged breakwater. Results also show that the model gives good estimates for the wave kinematics inside and around the breakwater which are necessary to compute the stable armor stone weight.

Full scale measurements of wave attenuation inside a rubble mound breakwater

Abstract: At the Zeebrugge harbour (Belgium) a cross-section of the N.W.-breakwater has been instrumented for the study of physical processes related to the behaviour of a prototype rubble mound breakwater in random wave conditions. Within the EC MAST programme (project MAS02-CT92-0023) this monitoring system has been reengineered and extended to a high-quality full scale data acquisition centre (Troch et al., 1995). The development of the prototype monitoring system to a world-wide unique system with respect to the infrastructure available at Zeebrugge, the instrumentation installed on site, and the data management developed, is briefly summarised. Filed measurements of wave attack in front of the breakwater, and pore pressure response inside the breakwater core, have been analysed in order to determine the hydraulic response of the full scale breakwater. Analysis results on wave run-up/run-down measurements, phreatic set-up calculations, and pore pressure wave attenuation are presented here in more detail.

Berm breakwater trunk exposed to oblique waves

Abstract: Comprehensive three-dimensional model tests with a berm breakwater roundhead and the adjacent trunk section were carried out at the Danish Hydraulic Institute (DHI) and at the Hydraulic & Coastal Engineering Laboratory of Aalborg University (AAU). This paper describes the influence of wave obliquity on the profile shape, on the initiation of longshore transport and on the longshore transport rate at the trunk section both during and after the profile reshaping, whereas the roundhead stability is described in Juhl et al (1996). Furthermore, tests were made for studying the influence of storm duration and of short crested waves. Equations for calculation of profile development and longshore transport rate under oblique wave attack are introduced.

Modelling the impact of detached breakwaters on the coast

Abstract: Detached breakwaters alter the nearshore wave climate and hence the wave-driven current and sediment transport patterns. They obstruct a part or all of the longshore sediment transport and because of this they play a role in the large-scale sediment budget of the coasts where they are applied. Their local effect on the coast is to form single or double salients or tombolos. Which of these beach shapes will develop may be important for the attractiveness of the beach, in terms of visual aspects and water quality. Besides having a function as part of a coastal management scheme, detached breakwaters with a jetty to the shore may be designed as a low-maintenance port. Obviously, one hopes that in such cases a salient rather than a tombolo will develop. Clearly, engineers need to have means to assess the bypass-characteristics and the local impact of detached breakwaters if they want to apply them in a responsible manner. Several methods are at their disposal. First, there are rules-of-thumb, based on field and lab experience. These are quite useful to get a first rough idea of the possible impacts, but generally do not give answers that are conclusive enough: a typical result is of the form that two design rules say you'll get a salient and three say you'll get a tombolo. The most common tools applied at present are coastline models. They assume that the coastal profile is more or less constant in shape, and that the longshore sediment transport is related to the local angle of incidence of the waves. Several commercially available packages have special options to assess the local wave climate behind detached breakwaters. Effects that are not included in the current and transport models, such as circulations induced by set-up gradients, can be simulated by adjusting transport parameters locally. Generally, a lot of calibration is required, and the predictive capability is often uncertain. The calibrated models often show nice comparisons with data, especially for the initial stages. The development towards equilibrium is generally not represented at all. Recently, more sophisticated "area models" have matured to the point that fully dynamic wave, current, transport and bed evolution simulations can be carried out over a period which is long enough to approach equilibrium conditions (see for instance Johnson et al., 1994). The advantages are obvious: these models explicitly take into account the most important processes, and therefore the amount of heuristic modelling is reduced substantially. In the light of the ongoing efforts of devising coastal area morphological models, the performance of DELFT HYDRAULICS morphological model is tested on the simple situation of a detached breakwater in a coastal area with parallel iso-bathi. This also furnishes a test for the DELFT3D system and has proven an opportunity of improving the model.

Evaluation of beach modelling techniques behind detached breakwaters

Abstract: This paper presents an evaluation of current design formulae for beach response to multiple breakwaters, comparing the predictions of the design guidelines with observations of beach response made during the large scale field experiment carried out at Elmer, on the UK south coast. The introduction describes typical shoreline response to breakwaters, the forcing mechanisms responsible and the characteristics of macro-tidal beaches. The remainder of the paper presents the empirical design tools, and the results of the evaluation.

Numerical modeling of bed evolution behind a detached breakwater

Abstract: The sedimentological impact of waves on a sandy beach with a detached breakwater is simulated using a compound system of models. The results are satisfying since a salient could be generated behind the structure. They are in agreement with bed evolutions surveyed in experimental facilities and in nature. A quantitative analysis of the results performed in the framework of the working group 'Coastal Area Modelling' of the project MAST G8M shows that the volume of accretion computed here is in good agreement with the volumes obtained by other models and empirical formula.

Roundhead Stability of Berm Breakwaters

Abstract: Three-dimensional (3D) model tests were carried out for studying the stability of a berm breakwater roundhead and the adjacent trunk section. The present paper describes the influence of the wave incidence angle and wave steepness on the roundhead stability. The test results are described in terms of profile development, recession of the berm, eroded and deposited volumes and the transport of stones during reshaping. Results from analysis of the influence of wave obliquity on profile shape, initiation of longshore transport and longshore transport rate at the trunk section are presented in Alikhani et al (1996).

Interactions in the stability of toe-berm and main-armour for rubble-mound breakwaters : an experimental study

Abstract: This experimental study is concerned with one particular aspect of possible failure modes for a rubble-mound breakwater : the interactions between the toe-berm and the main-armour of the breakwater. Through a series of laboratory tests in a wave basin under long-crested and short-crested waves, we investigate the mutual influence of both these component parts of the breakwater on its general stability. The effects of several wave parameters are examined for four sizes of toe-berm stones. For the trunk section, experimental results are found to compare quite satisfactorily with existing design formulas both for the main-armour and the toeberm. As a general trend from the tests results, the interaction processes appear to have only moderate effect. Their major feature is an increase of damage to the armour when the toe-berm is unstable. On the opposite, minor effects of mainarmour on toe-berm stability were observed. In particular, the "toe-berm armouring process" (by units falling from the armour) appears to occur only marginally and under precise conditions.

Control of wave propagation angle by tapered-submerged breakwater

Abstract: A new type of submerged breakwater with a triangular plane form which modifies the wave propagation angle is proposed. Its performance is demonstrated and assessed analytically. Preliminary movable bed experiments were conducted. The breakwater was installed in front of the beach to modify the wave propagation angle in the offshore zone. Longshore current and littoral drift were driven through the effect of wave angle modification.

Breakwater Damage in Okushiri Port Due to the Hokkaido Nansei-Oki Earthquake Tsunami

Abstract: A tsunami caused by the Hokkaido-Nansei-oki earthquake struck Okushiri Island and the southwest coast of Hokkaido damaging many port facilities. This paper describes typical breakwater damage at Okushiri Port and the applicability of Tanimoto's formula for tsunami wave forces on vertical walls based on prototype failure analyses and the result of numerical simulations. The armor stability and the rubble mound scourings were examined from the results of 3-D model tests at a scale of 1/15. The damage patterns at the narrow opening of breakwaters were reproduced by the tests. The necessary weight of armor blocks and foot-protection blocks for breakwater heads were calculated by the C.E.R.C. formula using averaged flow speed at the peak of tsunami.

Nonlinear wave transformation due to a submerged breakwater

Abstract: A nonlinear model of wave transformation due to a submerged breakwater is developed on the basis of the nonlinear mild-slope equations. Numerical computation shows that significant amount of wave energy can be transferred from the fundamental component to higher harmonics by adjusting configuration of the submerged breakwater. In case of oblique incidence, wave direction as well as wave period changes due to the breakwater. These results implies the possibility to control wave period and direction as well as wave height.

Anti-scour device and method for scour prevention

Abstract: A device and method for preventing the scouring of sand from the area adjacent to the beachward (shoreward) side of a breakwater. The device is made from flexible, non-corrosive, marine resistant material having a cavity filled with a non-buoyant material. The device is placed on the seabed adjacent to the beachward (shoreward) side of the breakwater and is coupled to the beachward (shoreward) bottom edge of the breakwater. Filter material may be placed between the device and the seabed to further prevent scouring of sand between the non-buoyant material.