The wave attenuation due to bottom friction and percolation of sea bed from the open sea to shallow water area depends primarily on the geological character and roughness landform of the bottom. The quantitative evaluation of the bottom friction and percolation by in-situ measurement was rarely reported up to now. Based on the Valeport MIDAS DWR Directional Wave Recorder, the in-situ method to measure the wave attenuation in the shallow water was put forward and applied in the field experiment. The measurement results showed that the method was reliable and appropriate for the measurement of bottom friction caused by sand or Coral Sea bed.
The wave characteristics in shallow water is quite different from the open sea due to the effect of the complex hydrodynamic behaviors such as refraction, diffraction, reflection, shoaling, climbing, breaking and etc. The wave energy in the shallow water is mostly smaller than that in the open sea. From the viewpoint of energy process, one part of the broken wave is transferred into the current parallel to the coastline and the other part of that induces the wave set-up and set-down in the crossshore direction. In the shallow water, the attenuated wave continues to propagate with the currents approaching to the shoreline. As for the design of structures on coast such as trestle bridge, breakwater and beach nourishment, the characteristics of wave and currents in the shallow water plays an important role in structure safety of constructions. The geological character and roughness landform of the bottom have the great influence on the propagation of waves and currents. Especially in the shallow water, the energy dissipation of waves and currents takes place primarily due to the bottom friction. However the results of in-situ measurement of bottom friction in the shallow water can be rarely seen, up until now. The study of bottom friction at Ala Moana Reef, Honolulu, USA and Australia's Great Barrier Reefs using buoys in the twenty century 80's were carried out (Gerritsen, 1980; Young, 1989). The coefficient of bottom friction was acquired employing the governing equations of wave energy flux, neglecting the influence of the currents. The wave attenuation character was acquired using S4ADW directional wave recorder at Yongshu Reefs, South China Sea (Li, 2003). However the wave attenuation coefficients due to bottom friction are not investigated furthermore. Numerical simulation and model experiments on bottom friction can be seen in most references. Almost all of the theoretical models are related with the turbulent wave boundary layers (Trowbridge, 1984; Trowbridge, 1984; Aydin, 1998; Hsu, 1997). In water wave basin or wave flume (Kamphuis, 1975; Haritonidis, 1989; Xu, 2015; Tian, 2016), the bottom shear stress is expressed by the product of the friction coefficient with the square of characteristic velocity of bottom. The bottom friction coefficient can be deduced indirectly by measuring the bottom shear stress (Hamid, 2003).
