Low Frequency Waves Produced By Double Wave Groups

There is about two-thirds part world population living in littoral region for its abundant resources. In the coastal zone, low frequency waves such as infragravity waves and surf beat play an important role in sediment transport, harbour oscillations and ship motions. In present paper, low frequency waves (LFW) produced by double wave groups are studied experimentally and numerically. In addition, run-up of regular and irregular waves on different beach slopes is discussed.Firstly, we study the features of low frequency waves produced by double wave groups experimentally upon five plane beaches with the slopes of 1:20, 1:40, 1:60, 1:80 and 1:100. The experimental results indicate that the number of the reflected low frequency waves is two if the slope is greater than 1:80 while the number is single with the slope of 1:100. It also shows that the interval between two reflected low frequency wave crests almost equals to the duration of incident wave groups and wave troughs' interval relies on the duration of incident wave groups and beach slope while is independent of wave height. We find in the experiment that uprush times of wave groups is equal to the wave groups number and the interval between two uprushes is approximately the wave groups duration.Secondly, the numerical model for LFW produced by double wave groups is presented and computational results show good agreements with the experimental data. The governing equations are derived by time-averaging Eular equations over short wave period, and weighted-averaged flux (WAF) method is used to solve the equations. In addition, the dependence of reflected LFW amplitude on scaled friction factor (scaled by the beach slope) is investigated. We find that the LFW amplitude decrease with the increase of factor, especially when the factor is less than 1.0, the LFW amplitude is greatly dissipated. In our computation, we find that good agreements can be obtained with the value of scaled factor is 2.0.Finally, we discuss the run-up data of regular waves and irregular waves. Simultaneously, the seiching mode produced by regular waves in the wave flume is testified by solving shallow water wave equations (Wilson, 1972) and it is notable that water run-up produced by regular waves after the seiching attenuation agrees with the theoretic one.