Showing papers on "Breakwater published in 1986"

Wave energy dissipation in arbitrarily shaped harbours of variable depth

Abstract: A finite element model for combined refraction-diffraction problems of linear water waves has been extended to include the effect of various dissipative mechanisms on wave excitation response in harbours of arbitrary shape and variable depth. Especially, the effects of bottom friction, partial absorption along the harbour, contours, and transmission through permeable breakwaters have been considered. Although, within the mild slope approximation, the model is valid for arbitrary wave lengths, in this paper its effectiveness for harbour design applications is demonstrated for long wave induced resonance phenomena. For this purpose a realistic harbour geometry has been selected. A hydraulic scale-model of this harbour enabled experimental verification of the computational results.

Erosion control device

Abstract: A modular breakwater device for placement on the floor in a body of water to alleviate erosion effects. There is a seaward face surface and a landward face surface. A plurality of passages extend between those surfaces permitting water to pass through them. The body has a continuous, uninterrupted lower surface. A number of legs extend from the lower surface and raise it off the floor and define a second passage beneath the lower surface and enables stable footing of the device on the beach floor. Details of the legs, the undersurface and the shaping of the seaward and landward surfaces appear.

Wave transmission across submerged near-surface breakwaters

Abstract: Wave transmission across a submerged breakwater at Santa Monica, California, is examined through a three-dimensional model test. The results agree with empirical criteria previously proposed by Tanaka (1976).

Effects of breakwater spacing on sand entrapment

Abstract: Both breakwater distance from the shoreline and the gap between successive breakwaters are shown in a model study to be important factors in determining the amount of sand that can be entrapped by ...

Two-dimensional empirical eigenfunction model for the analysis and prediction of beach profile changes

Abstract: A two-dimensional empirical eigenfunction model is proposed for the analysis and the prediction of beach profile change due to longshore and cross-shore sediment transports. Beach profile data from Redhill coast, Taiwan, measured every two months at 150 meters interval along the detached breakwaters are analyzed and the relative importance from two directions is investigated. Furthermore, by employing the method of Markov process and linear regression, a prediction model is formulated which takes into account the effect of breaking waves, bottom sediment and radiation stress of waves. This 2-D model is shown to be effective in the analysis and the prediction of beach changes near the coastal structures.

Side-Scan Sonar Investigation of Breakwaters at Calumet and Burns Harbors in Southern Lake Michigan

Abstract: Side-scan sonar surveys revealed the following features: 1. Many of the wood cribs in the Calumet Harbor breakwater have tilted or have been displaced, and reflections from behind the breakwater indicate gaps and voids. Fan-shaped deposits of coarse rubble on the harbor floor may be debris from the damaged cribs. 2. Both breakwaters are obviously exposed to a dynamic environment, as shown by fields of sand waves and ripples on the lakeward sides. 3. Dramatic sand waves occur in a gap between sections of the Calumet Harbor breakwater and may indicate the movement of a veneer of sand over an essentially clay harbor floor. 4. The Burns Harbor breakwater is purported to have been built on a thick sand mat, yet our data indicates that a featureless clay floor extends to the very base of the structure.

Beach fill at two coasts of different configurations

Abstract: Movements of borrow sand replenished at two coasts of different configurations are investigated by analyzing the topographic data which were surveyed periodically after the beach fill placements. One is a long straight beach and borrow sand was placed behind a submerged breakwater. Another is a pocket beach which has an arcshoreline with a groyne at one end and a headland at another. It is found that the amount of borrow sand moved in the longshore direction surpasses the amount of borrow sand transported in the cross-shore direction regardless of the shape of the coast. A clear correlation is also found between displacements of shoreline and changes of sectional areas. These results imply that the deformation of the artificially nourished beach and the dissipation rate of borrow sand can be predicted by the so-called one-line theory.

Effects of short-crested waves on the scouring around the breakwater

Abstract: The aim of this research is to investigate quaiitatively the influence of short-crested waves on the scouring around the breakwater through mainly some laboratory studies. At first, we succeed to observe clearly in laboratory some sedimentary bed forms such as troughs, holes, triangle bars and longitudinal bars under short-crested wave actions. Then we elucidate the associstion of their formation mechanism with theory of short-crested waves, also indicate its effect on scouring at the toe of breakwaters. In addition, it is shown that the breaking wave height of short-crested waves is certainly higher than that of twodimensional standing waves. Finally we demonstrate some facts of failure of breakwater caused by short-crested wave breaking basing on some field results.

On time delays in the nearshore zone between onshore and longshore currents at incident wave frequencies

Abstract: Observations of horizontal velocities just outside the surf zone on two natural beaches show unexpected time lags between the onshore/offshore currents and the longshore currents at incident wave frequencies. These time lags, typically around 20 s, appear to have been first noticed by Katoh (1981) in data from a beach in Japan. A number of possible hypotheses to explain these time lags are examined. Of these, propagation of secondary waves formed by reflection or diffraction at a point alongshore from the measurement site seems most able to explain the observations. This suggests a significance for reflected/diffracted waves, some distance from breakwaters or headlands, which has not previously been recognized in field studies. However, a number of questions remain that will only be resolved by further field data, particularly from beaches far removed from end effects.

Beach development between headland breakwaters in a low wave energy environment, pasir ris, singapore

Abstract: One of the function of the offshore breakwater is to protect the coast from wave action. By dissipating the wave energy along its entire length, the breakwater causes sediments in its lee to deposit and a shore salient is formed. If the offshore breakwater are placed in a series along a coast with a gentle offshore slope and a substantial littoral drift tombolo will form behind the breakwaters between which bays will be sculptured by waves to form stable shapes (1). These attached breakwater would thus form a series of artificial headlands. In nature, beaches between headlands are influenced by the position of the headlands. Where the headlands are closely spaced and a limited sediment supply exists, small pockets beaches are formed. Where the headlands are far apart and an adequate sediment supply exists, long and wide beaches are formed. Generally, between these two extremes most beaches between natural headlands take a shape that is related to the predominant wave approach; on the downcoast sector is a long and straight beach, while on the upcoast end is curbed beach. Silvester (2) in his model study established a relationship between the logarithmic spiral constant ( °^ ) and the angle of predominant wave approach ( & ). A quasi-permanent shape was reached when waves broke simultaneously around the model bay. As it is difficult to measure the curve sector in nature, Silvester and Ho (3) suggested the use of an indentation ratio to relate the bay's shape to wave approach.

Wave trapping by breakwaters

Abstract: The refraction of water waves around the heads of breakwaters can lead to large wave heights along the sheltered side of the structure. These waves are shown to be edge waves, trapped on the breakwater, based on the comparison of laboratory data and linear edge wave theory.

Diffraction patterns near narrow breakwater gaps

Abstract: Dimensionless presentations of wave diffraction through a narrow breakwater gap have been computed from the analytical solution of Morse and Rubenstein. Particular attention is given to narrow gaps, typical of smaller harbors, and to the wave field near the gap, where available details are approximate and most sparse. These solutions are compared with frequently-used approximate solutions from superposition of partial Sommerfeld solutions for a semi- infinite breakwater. The superposition approximation is extended to angled incidence for wide breakwater gaps and generally to non- aligned breakwaters.

Wave interaction with moored sloping breakwater

Abstract: Interaction of periodic waves with a moored inclined floating breakwater has been studied theoretically and numerically. The floating breakwater is inclined at a well defined angle with the sea bottom; its seaward end in protruding above the water surface. In static equilibrium, without incoming waves, the body weight, the buoyance force, and the restoring forces from the mooring lines which are modeled using linear springs keep the breakwater at a fixed angle. The theoretical formulation is based on a suitable variational principle. For the numerical solution a combination of finite element approximation as well as eigen function expansion technique is used. The result is obtained in terms of wave transmission and reflection coefficient as well as the sway, heave and roll motion of the breakwater. The sensitivity of the solution on the parameters such as the bottom gap size, angle of inclination, and the mooring line stiffness are investigated over a range of incident wave transmission coefficient for dimensionless wave number hk > 0.60 ( k is wave number, h is the water depth). The results suggest that a certain degree of sheltering effect can be realized by employing this type of sloping breakwater.

Beach change around detached breakwaters due to artificial nourishment of bypassed sand

Abstract: Field experiment of sand bypassing was conducted on the Shimoni—ikawa Coast in Toyama Bay. The materials of 5900 m3 dredged on the updrift coast of the harbor were thrown into behind the detached breakwater. After the nourishment detailed surveys of the bottom topography around the breakwaters, wave observations and tracer tests using colored concrete blocks were carried out in order to investigate the movement of the nourished sand. Temporal and spatial changes of the shoreline positions and the sectional area of the beach in the shoreward zone of the detached breakwater are examined. It is found through the field experiment that the materials nourished behind the detached breakwater were carried slowly in the longshore direction by the westward littoral drift, dominating on the coast, without the outflow of sand through the openings of the breakwaters. It is concluded that the detached breakwater is useful to control the on—offshore sand movement and to retain the sand behind the detached breakwater.

On the scattering of concrete armour units of detached breakwaters due to waves

Abstract: On the Fuji Coast 15 detached breakwaters and 17 wave dissipating breakwaters have been constructed to prevent beach erosion. By virtue of the construction of the breakwaters the retreat rate of the shoreline position has decreased. However a large number of concrete blocks have been scattered due to waves. This study investigates the actual circumstances on the scattering of the concrete armour units of the detached breakwaters through the field observations on the Fuji Coast located in Suruga Bay facing the Pacific Ocean. The change rate of the plane area of the breakwater is examined from the aerial photographs, and the relationships among the parameter, the depth at the offshore foot of the breakwater and the number of the removed concrete armour units are investigated.

Prototype experience with rubble mound breakwaters

Abstract: At the 19th ICE Conference in Houston in 1984 an alternative concept for the design of rubble mound breakwaters was introduced. This concept has the objective of providing a least cost structure by optimizing the use of locally available materials and utilizing simple construction procedures. Contractors' bids demonstrated that significant cost savings could be achieved, when compared to the cost of traditional designs. Considerable prototype experience has now been obtained with this concept of breakwaters. Breakwaters have been built using the concept in Canada, the United States, and Iceland since 1984 and have been subjected to storms and ice action. Prototype observations have supported the performance predicted during the design process. In this paper surveys of a breakwater taken after construction and after storm action are presented. In addition to wave action, this breakwater has also been subjected to extensive ice action. The response of the breakwater has been monitored and observed and is discussed.

Cost-effective optimization of rubble-mound breakwater cross sections / by Orson P. Smith.

Abstract: Abstract : This report discusses design criteria, design procedures, and practical considerations involved in planning, design, and construction of rubble-mound breakwaters. Currently available methods for estimating rates of damage to breakwater armor and for predicting wave transmission characteristics are also described. Using information typically available today to most breakwater designers, a step-wide procedure is presented which can identify an optimum breakwater cross section, both in terms of structural integrity and functional performance.

Floating Breakwater Prototype Test Program: Seattle, Washington.

Abstract: : Because of increased interest in the use of floating breakwaters to provide wave protection, the US Army Corps of Engineers initiated the Floating Breakwater Prototype Test Program in February 1981. The objectives of the program, which utilized two types of breakwaters--a concrete box and a pipe-tire mat--was designed to answer several important engineering questions which included the following: determining the most efficient breakwater for a particular wave climate, predicting the forces that act upon structures and anchoring systems, determining the optimum construction materials, and providing a low-cost means of connecting or fendering the individual breakwater modules. After construction and mooring at an exposed site in Puget Sound, the breakwaters were monitored to evaluate their performance and structural response. The results are being compiled to aid designers of future floating breakwaters. (Author)

Permeation type breakwater wall

Abstract: PURPOSE: To effectively damp the energy of waves by providing a steel offing- side breakwater having a semi-circular grooved cross section which is opened toward the land side and a breakwater facing toward the offing side CONSTITUTION: A number of steel offing-side breakwater parts 1 having a semi-circular grooved type cross section which is opened toward the land side are set extentionally in the sideward direction rightangled to the flow of waves A number of steel offing-side breakwater parts 2 having a semi-circular grooved type cross section which is opened toward the offing side are set extentionally to the sideward direction rightangled to the flow of waves Both ends of the parts 1 and 2 are fixed to a steel holder 7 to make up a breakwater wall 9 The energy of waves can thus be damped and reflecting, permeating, and overflowing waves can be lessened COPYRIGHT: (C)1988,JPO&Japio

Analysis of Permeable Breakwaters

Abstract: This paper discusses the characteristics of hydraulic and water exchange due to the wave action against vertical slit-type breakwaters. The theoretical solution of the reflection and the transmission coefficients for the breakwater models is compared with the experimental results. Furthermore, the water exchange discharge and the water concentration change through the gap of the permeable breakwater are examined experimentally. Finally the most effective geometry of the cross section is suggested for the breakwaters used here.

Can we save on breakwaters by providing better mooring-fendering?

Abstract: The demand for deepwater berths in more exposed sea territories has prompted the installation of still larger breakwaters with extreme ends located in up to about 30m depth and the main part of the breakwater located in depths exceeding 20 meters. It may sometimes be difficult to justify the cost of such breakwaters and may therefore pay to find a cheaper alternative. In specific cases it might be possible to avoid the use of a breakwater altogether in favor of a berth--a pier only. This article is devoted to examining key literature on mooring/fendering systems to arrive at some conclusions on improving such systems as an alternative to breakwaters.

Effects of Permeable Breakwater on Shallow Wave Propagation

Abstract: Wave propagation is considerably affected by the existance of permeable breakwaters. The energy of the incident wave is partially reflected, partially transmitted and partially dissipated within the structure. The reflection-transmission properties of a breakwater depend both on wave dynamics and on the physical and geometrical characteristics of the structure. In this paper an analytical model is presented for simulation of shallow wave propagation through a pervious breakwater. The analysis is based on the principles of continuity and momentum balance. The wave is considered as linearized and frictional. Water motion within the structure is described by both Darcy and Dupuit-Forcheimer theories. The breakwater is assumed of rectangular shape and homogeneous. Comparison of the model with other theories and experimental data showed satisfactory agreement.

Study of statistical characteristics of irregular wave pressure on a composite breakwater

Abstract: Wave pressure is the most important external force for the design of breakwater. During recent years, there has been considerable development in the technology of vertical face breakwater; however, there is no reliable method to compute wave forces induced by irregular waves. The purpose of this study is to obtain statistical characteristics of irregular wave pressure distribution from the data of model tests. The results of this study shown that vertical face breakwater under the action of irregular waves, some waves are reflected, so that the next wave breaks a critical distance resulting in a rapidly rising shock pressure on the breakwater. On the average, the wave pressure increase with incoming wave height, but the maximum wave force does not necessarily occur for the largest wave height. It can be occurred for serval larger wave group in an appropiate phase composition. The irregular wave pressure distribution on the breakwater is quite uniform; the ratio of tested and calculated wave pressures decreases with the reduction of relative crest height of breakwater. Coda formula can predict the total horizontal force of the upper part of breakwater quite well except exetreme shock pressure occurred by non-breaking waves. Wave forces calculated by Miche-Rundgren and Nagai wave force formula are about 10% cummulated exceeding percentage of wave force obtained from model test.