Berm Breakwaters: a Modified Version

Abstract

The recession of two different berm breakwater design have been investigated. The first setup is a berm breakwater model of the original Sirevåg berm breakwater design. The second is very similar to the Sirevåg berm breakwater, but with class II instead of class I at the upper front of the berm. Both breakwaters have been subjected to the same storm progression model, consisting of 7 different steps containing a total of 18 000 waves. The storm progression model is based on measurements carried out close to the Sirevåg harbour,and have been used for several berm breakwater studies at NTNU the last decade.

The model of the original Sirevåg berm breakwater behaves to a large extent according to former studies carried out at the same test facilities. It thereby confirmes an apparent discrepancy between model and prototype behaviour observed during its ten year lifetime.

The tests show small differences between the two breakwater designs. Differences are found however found in the way they develop; The alternative breakwater reshapes quicker than the original Sirevåg berm breakwater model, but seems to develop a equilibrium profile towards the end of the the storm progression. This is due to the fact that the class I layer behind the class II gradually is revealed and act as a stronger cover for the already reshaped profile. The original Sirevåg berm breakwater model behaves completely opposite, as the class II layer is ultimately revealed providing less cover.

Comparisons between two different ways of measuring recession have been carried out.The traditional and much used definition of recession as the reshaping at the top of the berm is compared to method where the whole berm down to the bottown is analysed and measurements related to the size of the cover stones. Results indicate that the diffrences are small and no particular pattern is detected. However, as the alternative method have been utilized in all test studies regarding the Sirevåg berm breakwater at NTNU, most analysis is based on measurements utilizing this mehtod.

A part of the study has been to look at potential economical benefits from construction according to the alternative breakwater compared to the original Sirevåg berm breakwater.Though the extent of this analysis is very limited, the cost reductions are insignificant.It should however be pointed out that other effects than those making the basis for this calculation can come into play.

Different wave parameters measured during testing have been analysed and compared to the target values. Though deviations are present in all aspects, they are deemed acceptable taking into account the inherent scatter in berm breakwater model tests.