Effects of porous barriers such as coral reefs on coastal wave propagation

Observations following the Sumatra Tsunami in Sri Lanka have indicated significantly enhanced wave heights and water inundations in areas where coral poaching has been prevalent. It has been hypothesized [EOS, 86(33), 2005] that low-resistance paths created by coral removal have led to water jetting through them, while simultaneously reducing flow speeds in nearby coral-laden areas that offer higher bottom resistance to the flow. A laboratory experiment to verify this hypothesis is described in this paper, where corals are simulated using a submerged porous barrier made of a uniform array of rods that impose enhanced drag on the flow. The flow velocities pertinent to an oncoming solitary wave packet on a slope are measured in the presence and absence of the simulated uniform coral cover as well as with an opening (gap) in the coral canopy. It is shown that the coral canopy substantially decreases the flow velocity due to increase in the bottom drag coefficient, which is a strong function of the canopy porosity. The exit flow velocity from the gap is significantly higher compared to the surroundings, thus leading to jetting flow. The magnitude of jetting is a strong function of porosity, in addition to a suite of other parameters that accounts for waves, corals, water depth and gap size. The results support the notion that during isolated wave events the removal of natural barriers may cause local flow intensification, thus leading to adverse impacts on coastal assets and ecosystems in areas of barrier removal.