Showing papers on "Breakwater published in 1999"

Wave transmission at submerged rubblemound breakwaters

Abstract: Submerged rubblemound breakwaters are becoming more popular as a potential alternative to coastal protection measures where a moderate degree of energy transmission is acceptable. Such situations include areas where vegetative shore protection is existing or proposed or in the event that an existing shore protection structure has become damaged or under designed and a method is needed to reduce the incident wave energy. Although there have been previous investigations on the performance of submerged rubblemound breakwaters, there are only a few design equations available to the design engineer. Those available are based on a limited range of input design variables and as a result are insufficient in some cases. Physical model studies were performed at the Queen's University Coastal Engineering Research Laboratory (QUCERL) in Kingston, Canada to assess the performance of submerged rubblemound breakwaters under a wide range of design conditions in twodimensional (2-D) and three-dimensional (3-D) settings. The tests include a number of wide crested structures to provide data where previous investigations have not. The results show that the relative submergence, incident wave height and structure crest width are the most important design variables. A number of potential design equations were evaluated by statistical analysis methods. The proposed design equation fits the 2-D test data well and provides moderate agreement with the 3-D test results. Although physical testing is suggested for all design applications due to the complexity of site specific considerations, the proposed equation does provide a good preliminary design tool for submerged rubblemound breakwaters.

Nonlinear Wave, Composite Breakwater, and Seabed Dynamic Interaction

Abstract: The nonlinear dynamic interaction among water waves, composite breakwater and a sand seabed of finite thickness is studied experimentally and numerically in this work. Laboratory experiments were conducted to record the water surface levels around the breakwater and the dynamic pore-water pressure inside its base and seabed foundation. Two numerical models have been adapted to simulate the problem; boundary-element and finite-element method (BEM-FEM) and poro-elastic FEM models. The BEM-FEM model couples the wave field and the porous media governed by the fully nonlinear potential and modified Navier-Stokes equations, respectively. The BEM-FEM model predicts the waves transmitted through the porous media to the onshore side. The poro-elastic FEM model uses Biot's equations and runs under the effect of the surface pressure computed by the BEM-FEM model. It also considers the caisson deformation and computes the dynamic stresses in the poro-elastic media. The dynamic pore-water pressures computed by the por...

Sea level set-up behind detached breakwaters

Abstract: This paper reports some of the results of a theoretical and experimental study of submerged, detached, segmented, trapezoidal rubble mound breakwaters. It focuses on the phenomenon of sea level set-up which occurs at these structures. A theoretical analysis is presented which gives an explanation for the occurrence of set-up and the experimental results so far confirm the proposed relationship, which is given by Equation 4. Various other checks are presented in the paper, such as a demonstration that the set-up can be eliminated by pumping out from behind the breakwater and the measurement of a net drift velocity offshore in the core of the breakwater.

Water-Wave Diffraction by Thin Porous Breakwater

Abstract: The linearized theory of water waves is used to study the diffraction of an incident plane wave by a thin porous vertical breakwater of semi-infinite extent. Under the assumption that the wavelength is much greater than the breakwater thickness, the breakwater is replaced by a thin permeable barrier and a suitable boundary condition applied on the barrier. An explicit solution, found by the Wiener-Hopf technique, is presented for the barrier diffraction problem and then easily computable asymptotic forms are derived. Results are presented to illustrate the effects of porosity and variable angle of wave incidence.

Wave Forces on Submerged Semi-Circular Breakwater and Similar Structures

Abstract: —The results of design and experiment of a submerged semi-circular breakwater at the Yangtzeestuary show that the submerged structure will be unsafe when the general empirical wave force formulafor semi-circular breakwater is used in design.Therefore,a new calculation method for the wave forces act-ing on a submerged semi-circular structure is given in this paper,in which the wave force acting on the in-side circumference of semi-circular arch is included,and the phase modification coefficient in the generalempirical formula is adjusted as well.The new wave force calculation method has been verified by the re-sults of seven related physical model tests and adopted in the design of the south esturary jetty of the firststage project of Deep Channel Improvement Project of the Yangtze River Estuary,the total jetty length be-ing 17.5km.

Wave Overtopping of Rubble Mound Breakwaters

Abstract: A general expression for the overtopping discharge of a rubble mound breakwater has been derived utilising the general experience on this subject found in the bibliography and from the results of comprehensive series of model tests carried out at Danish Hydraulic Institute (DHI). The expression includes the important breakwater parameters (geometry of the breakwater profile) and the environmental parameters, and has been derived for applications with non-breaking waves in front of the structure. In research studies, model test series were carried out on pure rubble mound breakwater profiles with quarry rock as armour layer. Through results from projects carried out at DHI, the expression was extended to include different armour types and to describe the influence of a superstructure. The aim has been to set-up a reliable expression, which is simple, general and easy to use. Previously, predictions of overtopping discharges have been based either on expressions including empirical constants for different shapes of the breakwater profile, on very complicated expressions or on diagrams giving the overtopping discharges as function of the layout o fthe breakwater profile and the environmental conditions.

On the stability of berm breakwaters in shallow and deep water

Abstract: The paper describes laboratory tests on berm breakwaters in shallow water, e.g. in water depths where waves might break before they break on the breakwater. Analysis have been made of test results for breakwaters in shallow and deep waters and a unified design equation is presented for the berm recession of berm breakwaters in shallow and deep water.

Wave Scattering by Submerged Vertical Plate-Type Breakwater Using Composite Bem

Abstract: In this paper, a composite boundary element method (Composite BEM) is formulated and applied to study the reflection coefficient of a submerged vertical plate-type breakwater, which is modeled as a thin or non-thickness structure. In the theory, two-dimensional motion with the wave crests parallel to the submerged breakwater and the 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 with other analytical solutions. In addition, numerical results of wave reflection for both a submerged rectangular dike and a vertical plate-type breakwater are presented and discussed.

Impact loadings on vertical walls in directional seas

Abstract: Recent research under PROVERBS has demonstrated that wave impact loads can cause damage or failure of caisson or blockwork breakwaters. Research studies of wave loadings, pulsating or impact, have generally used only 2 - dimensional wave flume experiments, and so most design methods are strongly biased towards 2-d . This paper describes results from 3-dimensional wave basin tests by Universities of Naples, Sheffield, and HR Wallingford in the UK Coastal Research Facility (CRF). The aim of the study was to quantify effects on wave pressures or forces of oblique or short-crested wave conditions on simple vertical or composite breakwaters. This analysis has focussed particularly on wave impact loadings as earlier 2-d tests at Wallingford showed them to be potentially severe for some combinations of foundation level and relative wave conditions. New reduction factors are presented.

Prototype measurements of the dynamic response of caisson breakwaters

Abstract: Prototype tests, aiming to describe the dynamics of caisson breakwater oscillations, were carried out at Genoa Voltri and at Brindisi Punta Riso. The rigid body natural frequencies and modes of oscillation of the caissons were evaluated as well as the rate of damping. The effect of the longitudinal array structure, never tested in physical models, was found extremely relevant; the Mass Spring Dash-pot model for an isolated caisson of Oumeraci & Kortenhaus (1994) was then conveniently updated in order to represent an array structure, showing finally good agreement with the recorded signals.

Proverbs: Probabilistic design tools for vertical breakwaters

Abstract: Final report and appendices of the European project Proverbs on tools for the design of vertical breakwaters (caisson type breakwaters) and similar hydraulic structures in the coastal zone It includes the loads (waves) as well as the strength of the structure (geotechnial aspects, structural aspects)

Wave run-up and overtopping: prototype versus scale models.

Abstract: The determination of the crest level is one of the most important points in the design of sloping coastal structures. The crest level of sloping coastal structures is governed by wave run-up and overtopping. Recent measurement results of run-up on prototype have indicated that wave run-up may be underpredicted by scale model tests. So full scale measurements of wave run-up is necessary. At the same time the overtopping discharge on a prototype breakwater will be measured. Within the MAST III - OPTICREST project, these measurements will be carried out at coastal structures at two locations: Zeebrugge (Belgium) and Petten (The Netherlands). The results obtained from these prototype measurements will be compared with those from scale model tests and will be used to calibrate numerical models. This paper also presents the contents of the OPTICREST project.

Wave forces on solid and perforated caisson breakwaters: comparison of field and laboratory measurements

Abstract: Following an old Italian tradition of prototype measurements of wave pressures at vertical breakwaters (Franco, 1994), a new twin recording station was set up and operated in 1992-1994 at Porto Torres (Sardinia, Italy) industrial harbour breakwater. In the framework of the MAST3-PROVERBS project an extensive analysis of the available data has been carried out. Within the same project 2D model tests have also been performed in order to investigate the relation between pressure distribution and overall forces on and under both plain and perforated (multichamber) caissons. The results have been compared with the available design formulae like Goda's (1985). Statistical distributions of the horizontal and uplift forces for both structure types have also been derived. The scope of the study has been to assess the reliability of the present design methods and to outline a more physically based approach especially for the perforated structure type.

Cost Comparison of Breakwater Types

Abstract: A cost comparison have been made between various types of breakwaters for a fictitious situation in water depths to a local maximum of -15 m and an adopted sea climate. The comparison is based on optimal total project costs, being the sum of costs of construction and capitalized damage during the service period. For these selected conditions the caisson breakwater and the rubble mound type provided with a single concrete armour layer appear most attractive. Composite type of breakwaters seem advantageous for water depths approximately below -20 m.

Design of Vertical Wall Caisson Breakwaters Using Partial Safety Factors

Abstract: The paper presents a new system for implementation of target reliability in caisson breakwater designs by means of partial safety factors. The development of the system is explained, and tables of partial safety factors are presented for important overall stability failure modes related to caisson structures placed on bedding layers and high rubble mound foundations with underlaying sand and clay subsoils.

Sediment transport around a mound breakwater: the toe erosion problem

Abstract: The combined action of waves (incident and reflected) and the along breakwater current driven by wave energy dissipation inside an infinitely long porous breakwater under oblique wave attack (Baquerizo and Losada, 1998), is taken into account to study the formation of bars in front of the structure and the sediment transport at the toe of it. The main features of the bars are explained in terms of the incident wave characteristics, remarking the wave incidence angle dependence, and the structural and hydrodynamic properties of the breakwater. Results may explain the existence of erosion/deposition patterns at the toe of the structure.

Applicability of a quasi-three dimensional numerical model to nearshore currents

Abstract: This paper presents a quasi-three dimensional numerical model of wave-induced current due to wave breaking so as to make it applicable to the coastal region with the coastal structures. First, applicability of the present model to undertow velocity and longshore current field was investigated. Secondly, laboratory tests were carried out to examine the characteristics of nearshore currents around the detached breakwater. Thirdly, nearshore currents around the breakwater were computed using the present numerical model. Finally, the results of computation were compared with those of laboratory tests and the applicability of the numerical model was discussed.

Wave impacts on caisson breakwaters situated in multidirectionally breaking seas

Abstract: The paper concerns horizontal wave forces on caisson breakwaters in multidirectional breaking seas. It is based on model tests performed in a 3D wave tank at Hydraulics and Coastal Engineering Laboratory, Aalborg University. The measured horizontal wave forces are compared to the Goda force. Good agreement with the Goda formula is found for waves not breaking directly on the structure, while increasing degree of breaking on the structure results in forces of up to 50 % higher than the Goda force. Furthermore, the reduction of the horizontal wave force on long structures due to the non full correlation of the wave pressure along the structure is investigated. A formula for the force reduction factor based on cross correlation coefficients is given as a function of the mean wave direction and the wave spreading.

Hydraulic performance of a high mound composite breakwater

Abstract: The hydraulic performance of an innovative structure based on the concept of a high mound composite breakwater has been investigated in large-scale hydraulic model tests in the Large Wave Flume (GWK) of the Coastal Research Center (FZK), Germany. The experimental results concerning wave breaking, wave transmission and wave reflection are presented.

Digital imaging processing techniques for the aerial field monitoring of harbour breakwaters

Abstract: The entrances to the six largest ports in South Africa are protected by rubble mound breakwaters, which have Dolos armouring. PORTNET, the national Port Authority, have commissioned the CSIR to conduct detailed monitoring of existing rubble mound breakwaters including records of the wave conditions to which they are subjected in service. This paper presents the image processing techniques used to monitor breakwater damage as well as the wave field causing the damage. Wider applications of these techniques have been commissioned during pipeline outfall construction as well as during the design & modelling stage of a proposed new harbour. The image processing techniques have been used to measure both breakwater settling and damage as well as moored ship motion for competing breakwater designs constructed as in scale models, of proposed harbours at the CSIR's physical modelling facilities in Stellenbosch.. The image processing techniques developed by TECHTRIX International are reported here.

Model wave impulse loads on caisson breakwaters: aeration, scale and structural response

Abstract: The prediction of wave impact loads on prototype caisson breakwaters from the results of physical model tests is considered, with particular attention given to the effects of air, breaker shape, structural response and scale. A review of related literature is presented from which it is concluded that the different aspects of the problem may be related through the force impulse. Large scale soli ton impacts are used to show the importance of entrapped air in determining the form of the load time history. Small scale waves with artificially high levels of entrained air and highly controlled drop impacts are used to show and quantify an inverse relationship between entrained air and impact load maxima. Specially developed aeration probes and analysis techniques are used to show the influence of entrapped air on pressure maxima and quantify entrained air levels in small scale fresh water breaking waves. A definition of the force impulse is proposed and used to investigate its variation with breaker shape. The impulse magnitude is shown to be relatively invariant for regular wave impacts compared to a large scatter in impulse form. A numerical model of caisson dynamics is used to predict structural motion and to calculate a series of dynamic amplification factors. The prediction of structural response to obtain effective static loads through the use of these factors is investigated and achieved through the adoption of an 'equivalent impulse' concept. The scatter in impulse form is found to cause large variations in effective static loads between nominally identical impact events. The equivalent impulse concept is used to solve this problem. A comparison is made between the form and magnitude of the force impulses of the small and large scale waves. The results indicate that the impulse magnitude may be relatively free of scale effects. An example is given in which the results of a small scale test are interpreted, scaled and processed to account for the effects of entrained air and structural response in order to predict large scale effective static loads. These are shown to compare well with predictions made using measured large scale force time histories and the numerical caisson model.

A statistical tool for breakwater design

Abstract: This article presents a methodology for the design of breakwater dikes which includes wave estimation, evaluation of the runup at the dike, calculation of the dynamic and pseudohidrostatic forces operating on the crown wall and finally the estimation of the stability of the armour units in the main layer. The methodology presented is a development of existing experience in the evaluation of the effects produced by regular or monochromatic waves on structures (stability of pieces, induced forces and flow on the slope). This experience is extended to irregular or spectral waves via statistical distribution of waves, taking account of parameters such as root mean square or significant wave height, mean or peak period and water depth. The application of the methodology described here has given satisfactory results compared with those of other methodologies and experimental data from various researchers.