Experiment on wave overtopping of a vertical seawall on coral reefs in large wave flume

Abstract

Coral reefs are widely distributed in the South China Sea, and its topography is different from the gentle slope of the coastal continental shelf, which has a great impact on wave propagation and deformation. At the same time, reefs are located in a harsh marine environment, so these factors having a profound effect on the stability of the backfilled sand island and the effective operation of other ancillary facilities. Seawall is an important protective structure in the protection works of reefs, and the wave overtopping of seawall plays a decisive role in the engineering safety of reefs. At present, there are few studies on the wave overtopping of reefs around the world, and the existing formula for calculating wave overtopping discharges over seawall on gentle slope terrain is not applicable to reefs terrain, so detailed research on wave overtopping of a vertical seawall on coral reefs is needed. A series of experiments are carried out in the Large Wave Flume (LWF) of Tianjin Research Institute of Water Transport Engineering (TIWTE), which is 456 m long, 12 m deep and 5 m wide. The maximum wave height generated by LWF is 3.5 m, and the wave period range is from 2 to 10 s. The model layout includes 3 parts: Deep-water zone, steep slope zone and platform zone. The depth of the deep-water zone is 5 m. The front slope is divided into 3 sections with slope 1:1, 1:4 and 1:15. The steep slope toe is 330 m away from the wave generator. The strongly non-linear physical process of wave transmission and breaking can be reproduced by this physical model. Based on Froude similarity criterion and a 1:15 scale model, the propagation and deformation process of regular waves on reef-top with seawall is analyzed. The effects of wave height, wave period, water depth and the distance between seawall and reef-edge on the mean overtopping discharge are studied. The results show that the mean overtopping discharge increases significantly with increasing of wave height and period, while it decreases with decreasing of water depth. However, the mean overtopping discharge is not the monotone function with distance. Mostly the mean overtopping discharge decreases with increasing distance. When wave period is longer than 13.56 s, the mean overtopping discharge decreases firstly, then increases a little, finally decreases with increasing distance. The dimensionless mean wave overtopping discharge increases as the wave steepness decreases for given wave height. The dimensionless mean wave overtopping discharge is found to be functions of relative freeboard height, the dimensionless mean wave overtopping discharge decreases as relative freeboard height increases for given wave conditions. On the basis of the experimental data, a formula considering the effects of wave height, wave period, water depth and the distance between seawall and reef-edge on the wave overtopping, is presented to estimate the mean wave overtopping discharge of a vertical seawall on coral reefs.