Effects of Bi-Modal Waves on Overtopping: Application of UK and Dutch Prediction Methods

Abstract

Sometimes dike sections or seawalls are partially protected by low-crested structures up to one kilometre in front of these coastal defences and more than one kilometre in length. Wave energy may be transmitted over these low-crested structures or dams and waves can penetrate through various openings. These waves have periods similar to the wave periods of the incident waves. Local wave growth of shorter period waves may become important if some fetch is present between dam and coastal defence. Wave spectra in front of the dike sections are often therefore bi-modal. Also sea and swell and/or irregularly shaped wave generation areas may give rise to these bi-modal or double peaked spectra. Until now, little was known about their influence on required dike heights. A series of physical model studies (conducted at a nominal scale of 1:20) were undertaken by HR Wallingford for the Flood and Coastal Defence with Emergencies Division of the UK Ministry of Agriculture, Fisheries and Food (MAFF). The intention was to provide information on wave breaking behaviour and the impact of bi-modal wave conditions on beaches and coastal structures. This research was described by Coates et al. and by Hawkes et al. Wave conditions and wave overtopping were measured for two or three water levels over different bed slopes of 1:50, 1:20 and 1:10. During a project for the Dutch Public Works Department — IJsselmeer District, Alkyon and Infram studied wave-structure behaviour over low-crested dams and the effect of bi-modal seas on required dike heights. As a part of this project Infram visited HR Wallingford and used above research to establish a method for predicting wave overtopping with bi-modal seas for Dutch applications. The research was described (partly) by Van der Meer et al. At that time it was recognised that the wave and wave overtopping data measured by HR Wallingford would be interesting to the Dutch Public Works Department for several reasons as they are responsible for establishment of wave boundary conditions and guidelines for safety assessment of the Dutch coastal defences. The actual work consisted of four parts and was described in full depth in Van der Meer et al. 1) re-analysis ot approximately 200 wave flume runs to obtain various time and frequency domain parameters. This work was performed by HR Wallingford 2) validation of the SWAN-model and analysis of wave breaking formulations. This work was performed by Alkyon and Delft Hydraulics 3) validation ot a model on shallow wave height statistics by Delft Hydraulics and 4) further analysis on wave overtopping by uni- and bi-modal seas, performed by Infram in co-operation with Delft Hydraulics Only the latter part, the wave overtopping analysis, is the subject of this paper.