One wave gauge array is installed at the forward of the submerged breakwater to measure the directional spectrum of waves composed of incident and reflected waves. Eleven wave gauges are also installed around the submerged breakwater to measure wave heights and periods variation. Six current meters are installed to measure the velocity of water particle at surface and bottom layers. The measured current velocity is divided into steady and un-steady components. The steady component is defined as the average of the measured current velocity, and the un-steady as the difference between the measured velocity and the steady one.
Figure 1 Layout of model submerged breakwater and measurement instruments
3. CHARACTERISTIC OF WAVE
(1) Wave Height around Submerged Breakwater
Figure 2(a) shows the wave height variation around the submerged breakwater. The water depth at the forward of the submerged breakwater is very deep compared with the incident wave height. However, the wave heights at the measuring points from H3 to H5 are larger than the incident wave height. The increasing may be induced by the reflected waves from the submerged breakwater. The plunging wave breaker is observed at the slope top of the submerged breakwater. The wave height decreases on the submerged breakwater. The wave heights at the points from H9 to H11 are larger than those at from H5 to H7 because of diffracted waves.
Figure 2(b) shows the variation of the wave periods around the submerged breakwater. The wave periods at the points H6 and H8 are approximately equal to 0.5s. The waves diffracted by the submerged breakwater heads propagates ashore on the crown. Both overtopped incident waves and waves diffracted by the submerged breakwater heads are composed on the crown. The wave periods at the backward of the submerged breakwater (H9 to H11) are smaller than the incident wave period, because the phase of overtopped waves differs from that of diffracted.
Figure 2 Wave height and period around submerged breakwater (The incident wave height is 6cm)
(2) Experimental Formula for Wave Height Variation
It is necessary to estimate the approximate wave height on and behind the submerged breakwater with a simplified formula.
Suzuki et al.1) suggested the experimental formula for the transmission coefficient Kt of a submerged breakwater, in case that diffracted wave height is much smaller than transmitted one. The formula is given by
Kt=Ht/Hi=1-f1・tanh(B/Li)・tanh(Hi/d)・(Hi/Ls) (1)
where, Ht is the transmitted wave height, Hi the incident
