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.
