Porous Breakwater

Harbours are considered to be the important transit locations to carry out the world trade through seaways, which need to be protected from the disturbances due to the incoming waves. This is a basic requirement for harbours in order to ensure smooth maneuvering of marine vehicles and berthing of vessels, failing which, larger strains are induced on its mooring lines resulting in their damage. Breakwaters are protective structures usually built offshore to protect beaches, bluffs, dunes or harbor areas from wave action. However, because offshore breakwaters are costly to build, they are seldom constructed to protect the natural features alone, but are generally constructed for navigational purposes also. Breakwaters can be either rigid in construction or floating. The rigid breakwaters, which are usually constructed of riprap or rock, have both beneficial and detrimental effects on the shore. All breakwaters eliminate wave action and thus prevent the free flow of sand along the coast and starve the downstream beaches. Floating breakwaters do not have the negative effect on sand movement, but cannot withstand extensive wave action and thus are impractical with present construction methods in many areas.

This study investigated how the porosity of submerged breakwaters affects wave transformations. Eight model geometries each with six different porosities, from 0.421 to 0.912, were also considered. Various porosities with different breakwater geometries were studied to yield more information about porosity effects on wave fields. This research was motivated by a concern for artificial habitats for marine life. These habitats appear to be porous submerged breakwaters. Therefore, understanding the influences of porosity on wave transformations facilitates the design of artificial habitats.

Experimental results reveal that the model width has little effect on wave reflection and transmission when the model heights are fixed. The transmission coefficient is maximum at a kh in the range from 1.3 to 2.0 and minimum at a kh around 0.7. The wave reflection maximum is at kh of near 0.5. The energy loss of the primary waves is maximum near kh = 0.81 and minimum when the porosity of the model is large. Porosity does affect wave transformation and its influence becomes significant as the heights of the models increase. For the range of porosities tested, wave energy loss from the primary harmonic was found to be almost constant at around 0.4 when kh >1.3