The influence of armourstone shape and rounding on the stability of breakwater armour layers

Abstract

This is the second of two reports relating to a collaborative prograrmne of random wave flume tests to quantify the effect of rock shape on stability. The first report, SR 150, by Hydraulics Research, Wallingford, gives a summary of recent advances in design methods for rock armoured coastal structures and presents a detailed description of the test programme with an early interpretation of the results. This second report is a collation of the recent results from several parallel research studies carried out at Queen Mary College, the principal subject being armour stone shape and rounding .. Practical and theoretical developments in image analysis techniques are given for gross shape and surface texture measurement of prototype and model armour blocks. The five shape classes of armour used in the flume tests, namely tabular (including elongate), equant (cubic), fresh (angular), semi-round and very round were described using the new techniques. Samples of test material used in the development of Van deT MeeT's design equations were also subjected to the shape analysis techniques. A detailed discussion of Van der Meer's formulae is given in relation to the new flume test results and an indication of the relationships between rounding and instability is deduced, using a new shape parameter For the impermeable core conditions tested, stability is more sensitive to shape under surging wave than under plunging wave conditions Long-term degradation resulting in rounding of prototype armour stone is considered within a quantitative framework for different environmental conditions, initial block sizes, rock types and with reference to limited field data.