Experimental Study on Wave Transmission and Reflection at Impermeable Submerged Breakwaters (original) (raw)
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Wave Transmission over Rubble-Mound Submerged Breakwaters
Journal of Marine Science and Engineering
This study focuses on the prediction of technical efficiency of narrow-crested submerged permeable rubble-mound breakwaters, in terms of wave attenuation. A number of existing formulae for estimating wave transmission coefficient for submerged breakwaters can be found in the literature, whereas in this work further improvement for that estimation has been achieved mainly through physical modelling. A series of 2D experiments under scale were conducted for regular and random waves providing data on wave transmission coefficient and respective wave breaking characteristics. A Boussinesq-type wave model capable of simulating wave propagation for regular waves over porous submerged breakwaters was also used in order to provide additional wave transmission information. Data analysis showed that wave breaking mechanism significantly affects wave energy dissipation and, therefore, wave breaking occurrence and type can be directly linked to wave transmission coefficient for a given structur...
Effects of submerged structure on rubble-mound breakwater: Experimental study
KSCE Journal of Civil Engineering, 2007
Breakwaters are generally constructed to protect the port and harbor facilities from dynamic forces of the ocean wave. However, stability problems of breakwaters have been issued due to the global warming, which causes the rising of sea level. A submerged structure, more effective method for resolving such problem, can be expected to improve stability and practicality of the breakwater. For several decades, many researches for interaction waves and submerged structure have been carried out by changing the wave parameter or submerged structure dimension. Based on results from laboratory experiments, the reflection coefficient and the run-up height of breakwaters are investigated for different widths and heights of the submerged structure with different distances between adjacent two breakwaters. We found that the reflection coefficient decreased as the wave steepness increased. We also found that the run-up height decreased because the wave breaking and reflection due to a submerged structure, the relative height of a submerged structure normalized by flow depth. Therefore, the dissipation of the incident wave energy plays an important role improving the stability of the rubble-mound breakwater more stable. The experimental outcomes can be a feasible design criteria of a submerged structure.
International Journal on Advanced Science, Engineering and Information Technology, 2021
Breakwater is used to break the wave energy that travels towards the beach. Part of the wave energy will be destroyed, transmitted, or reflected. The shape of breakwater affects the ability to break the wave energy. There are no researchers who have compared the effect of the shape of the breakwater structure on wave energy transmitted and reflected. This study aims to simulate the transmission coefficient (Kt) and reflection coefficient (Kr) of the breakwater in various forms. Physical models with a 1:100 scale model is used. A flap-type wave generator in a wave flume is used to generate the wave. Waves regularly move in one direction passed the model. The results showed an effect of the shape of the breakwater structure on Kt and Kr. By increasing the wave steepness (Hi/L), Kt's value will increase in the upright structure and decrease in the sloping structure. At the same time, the value of Kr tends to decrease when the wave steepness increases. The value of Kt is relatively ...
Oblique wave transmission through rough impermeable rubble mound submerged breakwaters
2010
There is a growing interest in the application of submerged rubble mound breakwaters as coastal defence structures. As their defensive ability highly depends on the amount of wave energy remaining at their lee side, the accurate prediction of the energy in the lee of such structures is of utmost importance. Past experiments have shown that the behaviour of rough permeable rubble mound structures under oblique wave attack was found to be significantly different from that of smooth impermeable low crested structures. This behavioural difference has led to the research objective of this present study which is to investigate oblique wave transmission by rough impermeable rubble mound submerged breakwaters using 3D physical model tests. This study intentionally uncouples two parameters, being the permeability of the core and the roughness of the breakwater. Analysing the data of this study shows that the permeability of the core has no visible influence on the structure's behaviour with respect to the influence of oblique wave attack. The fully permeable rough rubble mound breakwater behaves analogous to the fully impermeable rough rubble mound breakwater.
Wave transmission over low crested geotextile breakwater structures
2009
This paper summarises an investigation undertaken by the Water Research Laboratory and International Coastal Management into wave transmission for low crested geotextile breakwater structures. Structures such as these are being used as an alternative to rock structures or as interim coastal protection structures. These low crested (submerged) detached breakwater structures are typically implemented to provide a milder nearshore wave climate, and to maintain or increase the beach width in the lee of the breakwater. The structures tested were built using a single layer of geotextile mega containers, and ranged in cross section from a single mega container of approximately 4 m width, up to three mega containers measuring approximately 12 m in total cross section width. The two-dimensional physical modelling investigated the various breakwater structures under a range of monochromatic wave conditions, with wave periods varying from 5 to 10 seconds and wave heights ranging from small unbroken waves, up to depth limited breaking waves of approximately 2.5 m height. The test results determined that under shorter period wave conditions, a significant reduction in transmitted wave height was achieved even with a breakwater of only 4 m cross section width. Further reductions in transmitted wave conditions were recorded by increasing the breakwater width to 8 m, however, little additional reduction was achieved by further increasing the width to 12 m. During longer period wave conditions and slightly deeper submergence, the narrow 4 m breakwater width provided very little wave reduction, with increases in breakwater width to 8 m and 12 m providing more modest reductions in transmitted wave height. Following the physical modelling study, a range of empirical equations for wave transmission have been applied with the same hydraulic conditions as the physical model tests, and the predicted wave transmission compared to the physical model results.
Low crested breakwaters are used to protect beaches from wave action. A series of large scale 3D laboratory experiments were carried out to investigate the effects in wave characteristics (disturbance) around a system of two non-parallel, permeable, low crested breakwaters by oblique wave incidence. The transmission and reflection coefficients were calculated and compared to existing formulae. The existing formulae, for both phenomena, transmission and reflection, had good relation in some cases. The angle of wave incidence did not show important influence for the wave transmission while is affected the wave reflection. The influence of wave period must be investigated in wave transmission and the influence of wave height in wave reflection.
Hydraulic Behaviour of Submerged Breakwaters: a Case Study
New forms of coastal structures are being investigated nowadays, catering also for the aesthetic value of the nearshore landscape. Among those structures, the submerged breakwater is becoming attractive for obvious reasons. The wave transmission coefficient associated with the latter has been studied extensively in the past. However, an aspect not thoroughly investigated so far is the effect of the structure porosity on the above coefficient. In this paper a review of the transmission coefficient over submerged breakwaters is given, based on application of empirical formulas and numerical models to a case study. Apart from the porosity, a second parameter was investigated, namely the friction along the breakwater perimeter. It was found that porosity can have a significant effect on the transmission coefficient, and that it can be adequately described by one of the wave models tested and by an empirical formula. The bed friction was found to have a smaller effect on the wave transmission coefficient than permeability has.
Experimental analysis on a low crested rubble mound breakwater
7th International Symposium on Hydraulic Structures (ISHS 2018), 2018
Coastal structures are usually designed for shoreline protection and defense of recreational activities. Nowadays low crested breakwaters -rubble barriers constructed with their crest level close to the still water depth - are preferred than emerged ones, since they guarantee better water circulation, less maintenance costs and greater environmental harmonization. In the present study, the flow induced by waves around a physical model of a detached low crested rubble mound breakwater is investigated experimentally. The physical model of scale factor 1/30, was designed in such a way so it resembles part of a system of detached breakwaters located parallel to the shoreline, in a coast of constant slope 1/15, assuming Froude similarity. For the design of the rock armor layer of the physical model, the van der Meer's hydraulic stability formula was applied. Two wave conditions were examined: one with an offshore wave height of 2 m(Case A) and one with the maximum annual characteristic offshore wave height(Case B), as they were calculated in prototype scale. Measurements include surface elevation time series, as well as three-dimensional velocity time series of the flow around the physical model. Results include flow patterns on the seaward and leeward side of the breakwater for both wave conditions, as well as transmission and reflection coefficients. Along the leeward side, the current profiles have an offshore direction close to the bottom and a shoreward direction close to the free surface where the reduction of the water depth induced an acceleration of the flow, influenced by the overtopping: the excess of water in the leeward zone flows back through the gap. The data for the transmission and reflection coefficients were compared with literature equations. The comparison revealed that the literature equations tended to underestimate the transmission coefficient due to the critical condition represented by a zero free-board breakwater. On the other hand, a good agreement with the predicted formulas was found for the reflection coefficient.
On Formulae for Wave Transmission at Submerged and Low-Crested Breakwaters
Journal of Marine Science and Engineering
Submerged and low-crested breakwaters are nearshore barriers with an underwater or slightly emergent crest, designed to reduce the energy of wave attacks and, consequently, to protect the coast from erosion and flooding. Their performance in reducing the wave energy can be evaluated by the value of the wave transmission coefficient, which thus requires accurate prediction. In the last few decades, several experimental investigations allowed the development of several formulae to predict this coefficient that agreed well within the given range of validity. In the present study, a comprehensive review of the existing formulae has been reported and the influence of input design variables has been highlighted. Moreover, an extensive set of experimental data has been collected and critically examined and re-analyzed to obtain a homogenous up-to-date database. Special attention has been addressed to the assessment of the reliability of each existing formula for and to evaluate its perform...