Three dimensional method for monitoring damage to dolos breakwaters

[摘要] ENGLISH ABSTRACT: Ports are frequently protected by breakwaters which are designed to withstand harshenvironmental conditions over long periods of time. However, in most cases, they graduallyor suddenly deteriorate before they reach their design life.In order to extend the design life of breakwaters, regular damage investigations andtimeous repairs are required. One of the important aspects is calculating the severity ofdamage to breakwaters. Traditionally, breakwater inspection has been done using aerialphotographs and underwater imaging to accomplish these investigations. Breakwatersare systematically divided into inspection stations and photographs are taken to inspectand assess the damage. The photographs are then visually compared with the previousinspection photographs to identify changes. This is difficult to achieve as it is not alwayspossible to get the helicopter exactly in the same position on every survey and highlydependent on the skill of the helicopter pilot. Another difficulty is taking underwaterimages near the breakwater due to poor visibility and wave breaking.A major disadvantage of the aerial photographic method is that it only provides a twodimensional (2D) picture of the breakwater above water. The inter-tidal zone on abreakwater slope is where damage usually occurs. It is difficult to capture data in thiszone, particularly due to wave action, which results in poor visibility. Ideally, a methodis required to capture and assess damage above and below water. High resolution laserscanning and acoustic technologies are available to inspect breakwaters above and belowwater. These are light detection and ranging (LIDAR) and multibeam echo soundersrespectively.The South African coastline has few significant embayments' that can provide naturalshelter from the high energy wave regime on the coastline. Consequently, South Africanports require protection in the form of heavy armoured breakwaters and seawalls. Maintenanceof these defences is critical for the safe functioning of these ports. Rapid assessmentof damage to breakwaters and seawalls which is cost e↵ective is necessary to ensurethat repairs are undertaken timeously. This study explores the use of LIDAR andmultibeam echo sounders, instead of the traditional aerial photography, to assess damageto breakwaters in general.Three dimensional (3D) data analysis using LIDAR and multibeam echo-soundings (asinvestigated in this thesis) is aimed at providing a more realistic method to quantifydamage above and below water rather than two dimensional (2D) photographs.The objective of this study was to evaluate the accuracy of a 3D method using the LIDARand multi-beam echo sounder data to quantify damage to dolos breakwaters above andbelow the water surface (including the inter-tidal zone). This was aimed towards thedevelopment of the 3D eroded volume method (3D method).The investigation began with an idealized undamaged model breakwater slope in alaboratory experiment comparing the repeatability of the 3D method with sequentialscans and the photographic method in an attempt to compare the deviation in repeatabilityof data captured. Thereafter three degrees of artificial damage were created to the slopeof the model breakwater in the form of settlement, intermediate damage and failure of alocalized area of the dolos armour layer.The 3D method was then applied to a 3D physical model study of the spur breakwater atthe root of the Table Bay main breakwater. Field data of the spur breakwater were thencaptured with the 3D method and compared with the most recent 2D aerial photographicsurvey. More than two decades of 2D aerial photography damage assessment data of thespur breakwater were available to establish a baseline.The 3D method data captured were compared with the visual 2D field data to determine the3D methods accuracy. The comparison of the cumulative damage progression monitoredby the visual 2D photographic method with that calculated by the 3D method, showedthat there is a good correlation between the two damage quantification methods. Thecumulative percentage damage obtained from the 2D visual analysis records has a meandi↵erence of 10% in comparison to the 3D method. The results of the 2D and 3D methodsdi↵er in cases of small settlements and larger displacements.This study has led to an advance in the quantitative 3D assessment of di↵erent degreesof damage to breakwaters with dolos armour units above and below water. The 3Dmethod used to assess the extent of damage to a breakwater was compared against the conventional visual method described by Phelp (1995). A marginal variation between thetwo methods was found, however the 3D method which quantifies 3D eroded volume isrecommended for future breakwater investigations to provide a better quantifiable aboveand below water assessment.

[发布机构] Stellenbosch University