Vertical walls and low reflection alternatives - Results of wave flume tests

McBride, M. and Allsop, W. and Besley, P. and Colombo, D. and Madurini, L. (1995) Vertical walls and low reflection alternatives - Results of wave flume tests. Technical Report. HR Wallingford.

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Abstract

Waves reflected from vertical breakwaters or other highly reflective structures forming port entrances can cause serious problems to vessel navigation. This often leads to port closures as vessels are unable or unwilling to enter or leave the port. This report describes all aspects of a 2D physical model study. This study was initiated to examine the reflection and overtopping performance of various 'low reflection' structures under irregular wave attack. It is these types of structures which can be used in the modification or construction of port entrances to reduce wave reflections, and so improve conditions for vessel navigation. Four main structures were tested consisting of a smooth vertical wall, single and double chamber wave screens and rock armour slopes. Various arrangements of these structures were tested to identify the optimum configurations. The study found that the use of single and double chamber wave screens is effective in reducing wave reflections and overtopping. From the analysis of these results, an empirical formula was derived which can be used to describe the reflection performance of such structures, within practical engineering limits. This report also describes a new technique to examine the effect of 'low reflection' structures on wave conditions. This was carried out through the analysis of local wave steepness. Previously a description of the reflection performance and overtopping of a structure was used in the assessment of the likely effect of wave reflections, and hence the relative changes in wave height, on vessel navigation. However, vessel navigation problems are not only related to wave height as the wave period is also significant in the creation of hazardous conditions. This new technique enables both relative changes in wave height and wave period to be considered, allowing a better assessment of the effect on vessel navigation to be made.

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