The Experimental Study For The Effect Of Waves On The Twin-plate Breakwater

The twin-plates breakwater is a novel type of permeable breakwater.It is known that most of the wave energy is distributed near to the free surface.When propagating through the twin-plates beneath the free surface,the waves could be breaking and dissipating;meanwhile,the velocity of water particles and flow field could be changed,which leads to less energy flux passing through the breakwater.Comparing to conventional breakwaters,the twin-plate breakwater has the following advantages in engineering application,i.e.,fixed or floating types subject to the water depths;cost effective under deep water condition;permeable in the exchange of current and sediment,and maintaining the water quality;minimum loads on the foundation in the seabed etc.In almost all previous studies,the hydrodynamic characteristics of the twin-plates breakwater were investigated under the normal incident waves.In current study,we performed systematic investigation on the dissipating performance and wave loads of the twin-plates breakwater under random and freak waves with normal and oblique incidences.Based on experimental measurements,the hydrodynamic characteristics of the twin-plate breakwater has been investigated systematically under random and freak waves with normal and oblique incidences.The transmission,reflection,dissipation coefficient as well as the dynamic pressure and total vertical force were obtained,and it can be concluded:(1)The twin-plates breakwater works by converting the potential into kinetic energy,changing the energy flux direction and dissipating energy partially.Waves with oblique incidence is more capable in converting the potential into kinetic energy,and hence the total wave force and transmission coefficient under oblique wave are smaller compared to those under normal incident wave.(2)The transmission coefficient CT is coupled with the four parameters relative plate width,wave steepness,wave height and RNN.CT changes with relative plate width and wave steepness exponentially,and with wave height linearly,and with RNN logarithm.(3)The experimental data verified that RNN is an effective parameter to assess the performance of the twin-plates breakwater.The variation of the transmission coefficient can be estimated with simplified method by using RNN'(the extension parameter of RANN),which can replace the above mentioned four parameters.(4)Comparing to normal incidence,the oblique incidence leads to smaller transmission coefficient,i.e.the transmission coefficient with 15°?45° incidence is 5?15%smaller than that with normal incidence.Under oblique or freak wave action,the transmission versus the relative plate width,wave height and steepness are investigated by comparing to that under normal incident wave action.It is found that the variation of the transmission under freak wave has insignificant to that under random waves.(5)Comparing to random wave,the freak wave has significant effect on wave load of the breakwater.Based on the experimental results,the maximum pressure under freak wave can be 20?80%larger than that under random waves;the max uplifting force under freak wave is 1.06?2.45 times of that under random wave;the max vertical force with freak wave is 1.22?2.07 times of that under random wave.For the steep slope seabed or wave focusing topography,the freak wave should be taken into account.(6)With normal incidences,the maximum dynamic pressure was observed at the front part of the twin-plate breakwater x/B=0-0.2.In addition,large dynamic pressure was also observed at the rear part of the breakwater x/B =0.8?1.0.The characteristic of the pressure envelopes under the oblique incident wave is identical to that under random wave.(7)Based on the experimental results,empirical formula were presented to predict the transmission coefficient,maximum dynamic pressure and total vertical force.It has been verified that the prediction by the empirical formula is reliable with B/L=0.22?1.4,H/d=0.1?0.4,H/L=0.02?0.14 under the regular waves;B/L=0.23?0.66,Hs/d =0.1-0.3,Hs/L=0.01-0.07 under the irregular waves.