Experimental investigation of monochromatic wave transformation characteristics over the coral reefs

Abstract

Due to the shallow water depth on the coral reef flat, most of the incident wave energy is dissipated by waves breaking on the frontal reef slope. However, extensive damages have been occasionally reported in low-lying coastal areas fronted by coral reefs during storms. It has long been known that waves breaking on a reef will cause wave set-up on the reef top. Wave set-up may be significant in determining water levels on coral reefs particularly in micro tidal environments. So it is important for the design and the stability of reef top structures. With the construction of artificial island in South China Sea, coral reefs are becoming a very popular research topic now. Laboratory experiments are carried out on an idealized platform reef in a wave flume located in the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian. The flume is 60 m long, 4 m wide and 2.5 m deep. The reef model consists of a reef face with a slope of 1:1 and a horizontal reef flat with 8.6 m in length. The reef flat is built 1.80 m high above the flume bottom. The water depth in front of the reef model is from 1.80 to 1.90 m, so the submergence of the reef top is from 0 to 10 cm. The water surface elevations and wave set-up over the reef top are measured using the wave gauges. The experimental results show that when waves propagate from deep water to the shallow water on the reef face and reef top, wave shoaling and wave breaking happen over the reef face or reef top. When ( η ¯ r + d r ) / H 0 > 0.7 , wave breaking occurs over the reef top, while for ( η ¯ r + d r ) / H 0 <0.7, wave breaking can be observed on the reef face. It should also be noticed that wave breaking may happen over the reef face when 0.7 ( η ¯ r + d r ) / H 0 As we expected wave set-up increases with the increasing wave heights and the decreasing submergence, while the wave periods have a complicated influence on the wave set-up. Our study indicates that wave set-up reaches the maximum value at T =1.75 s. The relationship between the relative wave set-up and relative submergence of the reef top is compared with the theoretical relationship of Gourlay and other available experimental data. In order to include the effect of wave-generated flow in Gourlay′s equation, a small value of the wave energy dissipation factor K p in the Gourlay′s equation is recommended. In our experiment, K p is different for the different submergence over the reef top. When ( η ¯ r + d r ) / H 0 0.5 or ( η ¯ r + d r ) / H 0 >0.8, K p is advised to be 0.35; while 0.5 ( η ¯ r + d r ) / H 0 0.8 , K p is suggested to be 0.50.