Wave Transmission: Spectral Changes and Its Effects on Run-Up and Overtopping

Abstract

Most research work on wave transmission over low-crested structures has been concentrated on establishing the wave transmission coefficient, i.e. the ratio between transmitted and incident significant wave height. It is clear that such structures decrease the wave height, but there is more! Goda, Tanimoto et al. Raichlen et al. and Van. der Meer all conclude that also the mean period reduces to 0.4–1.0 of the incident mean period. The conclusion is that overtopping generates more waves. Further, Raichlen et al. and Lee give examples of measured spectra of transmitted waves. Both examples show the peak of the spectrum similar to the incident spectrum, but with much more energy at the higher frequencies. A good estimation of the wave height in front of a structure is required for design or assessment of such a structure. But also wave period and sometimes spectral shape may have influence on the design. Wave run-up, for example, depends largely on the wave period. In order to establish the required dike height for acceptable run-up, both wave height and period should be known. In situations where a low-crested structure in front of such a dike gives some protection, wave period and spectral changes should be studied. A local situation in the Netherlands was the reason for the research presented in this paper. Figure 1 gives a schematised layout. A large lake is situated on the west side (left side in the figure) and NW wind may generate waves up to a significant wave height of over 2 m. The dikes on the eastern side are partly protected by a system of various low-crested structures or dams, including some openings. The figure shows the results of a calculation on wave penetration. Such calculations were performed by Alkyon. Besides wave penetration, wave transmission is generated over the low-crested dams. The area is about 1 km by 2.5 km. This means that with a fetch of more than 1 km also locally generated (short) waves will be present during extreme conditions. The wave climate in front of the dikes can be described as a combination of wave penetration, wave transmission and locally generated wind waves. This paper deals with wave transmission only, but the effect on the total wave climate is discussed at the end.