The Propagation Of Infragravity Waves Upon Harbor Entrance

The infragravity (surf-beat) waves with periods between30and300seconds are the dynamic factor for the evaluation of beach and the coastal engineering in coastal zone. A lot of research shows that the infragravity waves or surf beat has an important effect on sediment transport, harbor oscillations and ship motions. Therefore, the research on the infragravity waves has important implication.In our country, we have long coastlines and a great deal of various ports constructed or under construction. Although, we have achieved great progresses on the technique of design in harbor engineering, but the ability of defending surf beats for the breakwaters is very bad in harbor engineering recently. Once the infragravity waves propagate into the harbor, it will produce seriously negative impact on the waves in the harbor and the normal operation of boats, especially for the ports which are busy and important in business. In recent years, with the constructing of harbor and maritime projects, the effect of infragravity waves on the construction gets more and more attention.Many scholars, at home and abroad, have done a lot of research work on the generation mechanism, the numerical simulation of surf beat (infragravity waves) and the phenomenon of harbor oscillations caused by infragravity waves, but we have not see the research about the law of infragravity waves through the breakwaters, mostly the research on waves through the breakwater is the aspect of short waves. So, based on the situation above, we mainly research the propagation law of infragravity waves through the breakwaters in this text.The propagation of infragravity waves or surf beat around double, offshore and single breakwater is simulated by a Boussinesq wave model. The comparison between numerical results and theoretical solution verifies the validity and reliability of this numerical model. Through the analysis of infragravity waves and short waves, we can find that the diffraction coefficient of infragravity waves are larger behind double, offshore and single breakwater comparing to the short waves, which indicates that more energy of incident waves propagates into the harbor, so the breakwater is less effective to the long waves. Meanwhile, we also find that when the wave period becomes very large the distribution of the diffraction coefficient profile gets similar. Through the analysis of infragravity waves and short waves, we can find that the minimum value and maximum value in local area of the reflection coefficient before double,offshore and single breakwater is larger than the shore waves’. In summary, we can conclude that infragravity waves are worse for the mooring and sailing of boat and the stability of breakwater comparing to the short waves.