Wave Refraction and Energy Attenuation Properties of Underwater Curved Breakwaters and Their Potential Application for Coastal Defence

Authors : Michael L. Hackett

Volume/Issue : Volume 8 - 2023, Issue 7 - July

Google Scholar : https://bit.ly/3TmGbDi

Scribd : https://tinyurl.com/ycyerzjv

DOI : https://doi.org/10.5281/zenodo.8265080

An investigation was carried out to determine the energy-dissipating effects of submerged curved planform breakwaters on water waves. The submerged breakwaters are of the shape of planoconvex and planoconcave optical lenses in planform. Physical modeling involving small-scale structures in a wave tank was done. The general concept is to make use of the refractive effects of the contours of these shapes to refract and cause converging or diverging of water waves so that they undergo superposition after passing over the shapes. When the waves superpose constructively their heights will increase according to the principle of superposition and, if the wave-breaking criteria for deep to shallow water are reached, breaking will occur and the wave energy will be dissipated in the lee of the submerged breakwaters. This will help to reduce wave impacts on coastal structures and on vulnerable shorelines by using an unobtrusive and aesthetic means of coastal protection. The research involved measuring the wave heights of incident waves in front of the submerged structures and the wave heights of transmitted waves in the lee. In this way, the incident and transmitted wave energy density can be obtained to determine the wave transformations that may have been caused by the breakwaters. Energy loss coefficients were also computed. Comparisons were done with the wave energy transformations produced by rectangular planform submerged breakwaters of comparable size. Generally, it was found that all three types of breakwaters dissipated wave energy, with the planoconcave ones performing the least and the rectangular ones the best. The planoconvex breakwaters performed comparably to the rectangular ones, showing only slightly smaller energy density dissipations and loss coefficients. So, it is possible for planoconvex breakwaters to replace rectangular ones and give comparable energy attenuation performance for a lower cost and fewer building materials.

Keywords : Absolute error of measurement – half the size of the smallest graduation interval on a measuring instrument and taken as the best possible accuracy. Crest width – distance from front to back of breakwater, usually rectangular in planform. Crest freeboard – for a submerged breakwater, distance from the top of the breakwater to still water level.  Platform depth – same as crest freeboard. Planform – the shape of an object as viewed from directly above it. Real focal point – point to which waves converge and meet at that point. Superposition – the crossing over of wave crests and troughs from different wave sources so that wave heights are either increased or decreased due to constructive and destructive interference.  Relative error – absolute error of a measurement divided by the value of the measurement expressed as a percentage of that value: relative error = (absolute error/measured valued)  100 %.  Virtual focal point – point from which waves appear to diverge as if coming from that point.  Wave series (WS) – a wave condition defined by an incident wave period and wave height as determined by the wave paddle frequency and amplitude.


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