Study on the stability of C.C. Blocks as revetment material against wave attack

Abstract

This study was done to check the validity of the formulae widely used for design of . cement concrete (c.c.) block against wave erosion. Experiments have been carried out in a 21.34 m long and 0.76 m wide glass sided laboratory flume of the Hydraulic and River Engineering Laboratory under the Department of Water Resources Engineering, BUET, Dhaka. One of the main tasks of the present study 'Yas to set up the equipment and calibrate the measuring instruments properly for useful collection of representative data. For. this purpose a major part of the time had to spend specially for separating reflection component of the incident wave and to set up data acquisition system with high scanning rate. Experiments have been done with two types of c.c. block sizes for I: 1.5 and 1:3 bank slope. The prototype size of blocks were 400mm x 400mm x 250mm and 350 mm x 350mm x 200mm. Considering available height of the flume the prototype parameters like water depth, wave' height, block sizes were scaled down to I :20(prototype: laboratory standard) scale ratio. Preliminary analysis showed that laboratory representation of the prototype data with this scale ratio represented stronger structure than actual ones. This is because that the clamping and friction between blocks and the friction between blocks and geotextiles cannot be scaled down in the laboratory. This led the subsequent runs to conduct with 1:10 scale ratio. Comparing the laboratory results with the Pilarczyk( 1990) formula the agreement was found satisfactory. Though the clamping and friction between blocks could not be represented even with I: 10 scale the results were found well within the design boundary defined by Schiereck(200 I) on theoretical basis and 1: I prototype experimental results. It was not fully appropriate to compare Hudson (1961) and Van der Meer (1988) formula with the placed blocks system. But comparison have also been made in order to get an idea about the influence of the parameters suggested by them. Analysis for wave run up has also been done. The experimental runs carried out wiht I: 10 scale ratio were compared with Pilarczyk (1998) formula of wave run up and I the agreement was found very satisfactory for smaller surf parameter (~<2.0). For the runs with 1:20 scale ratio the agreement was far way from satisfactory. Influence of wave frequency (wave period) in selection of c.c. blocks has also been studied by incorporating wave period in the parameters of Hudson formula and through dimensional analysis a power equation was developed.