Numerical And Experimental Studies On Bragg Reflection Of Submerged Semi-circular Breakwaters

A breakwater is one of most common hydraulic structures in harbour engineering.The major functions of breakwaters include dissipating wave energy, preventingsediment transport, guiding currents and protecting shoreline. Also breakwaters arehighly important in stabilizing the waters in harbour basin and maintaining coastalconstruction. As one type of breakwaters, submerged breakwater is widely used. Itsstructural optimization and layout form are important for practical engineering. Thepurpose of this paper is to study the Bragg reflection of semi-circular breakwatersusing numerical and experimental methods.By using a simple Green function as the fundamental solution, a numericalmodel is developed by boundary element method (BEM) on the basis of potentialfluid theory. Both single and double semi-circular submerged bars under regular waveactions were examined in a water flume.The tests for double semi-circular bars included two cases. One obtained therelation curve of Kr versus2S/L by changing the space between the two bars withfixed period. The other one obtained the relation curve of Kr versus kh by changingperiod and keeping the space a constant.(Kr, reflection coefficient; S, space betweenthe centres of double bars; L, wave length; k, incident wave number; h, water depth).According to measured data, the effects of bar number (N), bar radius (R), distance ofbars (S), wave period (T) and wave breaking on the reflection coefficient (Kr) andtransmission coefficient (Kt) are examined.Some conclusions based on numerical and experimental results are obtained asfollows. In the relation curve of Kr versus2S/L, Bragg reflection appears periodicallywith mostly equal peak Kr values, and the intervals are independent of wave periodand wave steepness. In the relation curve of Kr versus kh, the position of peakmoves to larger period with the increase of distance between the two bars and thebandwidths of peaks decrease. For semi-circular submerged breakwaters with radius R=0.4m and the waveperiod T=1.2s, nearly full transmission occurs for all experimental cases. At this time,the semi-circular breakwater is of no use, and this should be avoided in practice.The breaking waves over the semi-circular submerged breakwaters, may belocated between plunging and surging breaker. The wave breaking has a few effect onthe Bragg reflection. However, the nonlinearity generated by breaking waves makethe transmitted wave field more complicated.