Physical Model Studies on the Effect Coastal Vegetation on Wave Attenuation and Run-Up

Abstract

High rates of population growth, rapid urbanization along the coastal zone, rising sea levels and coastal flooding due to global warming and climate change represents some of the trends that affect the world at large. With an unprecedented rise in the frequency of natural calamities like cyclones, storm surges and tsunamis and the losses from such extreme events reaching an all-time high, it becomes a prime necessity to devise measures to defend our coasts. Evidences of coastal ecosystems in reducing the impacts of cyclones and tsunamis in Kendrapara (Odisha), Pichavaram (Tamil Nadu) and in other places, paved the way for researchers and administrators to realize that coastal habitats have an important role to play in risk reduction. A series of experiments are conducted in a two-dimensional monochromatic wave flume on 1:30 scaled models of different types of vegetation. To investigate the wave attenuation and beach inundation characteristics, the simulated vegetation models are subjected to incident waves of heights 0.08 m to 0.16 m and periods 1.4 s to 2 s, in water depths of 0.40 m and 0.45 m. The very first set of experimental runs conducted on submerged simulated vegetation, namely, seagrass and rigid vegetation reveals the dependence of wave height attenuation and beach inundation on meadow width and relative plant height. The vegetated meadow progressively interferes with the wave particle orbital velocities as the wave propagates through the meadow. The percentage reduction in wave heights ranges from 53.25% to 41.61% for the submerged seagrass model of width 2 m and from 62.65% to 46.71% for the submerged rigid vegetation model of width 2 m, for the highest relative plant height, h s /d = 0.525. Further, the beach inundation measured in terms of relative run-up (R u /H i ) varies from 0.861 to 0.534 and from 0.840 to 0.498, for the above two models with increase in wave steepness parameter, H i /gT 2 . The effect of height of emergence of the vegetation on wave height attenuation and further run-up on the beach is studied with emergent vegetation models of varying plant densities. For the emergent trunk model of width 2 m, and plant density, N = 107 trunks/m 2 , the percentage reduction in wave heights ranges from 39.47% to 33.83%; whereas, the relative wave run-up, R u /H i varies from 0.871 to 0.628, for h s /d = 1.25. As the plant density increases to 107 trunks/m 2 and 300 roots/m 2 , for the emergent trunk model with roots of width 2 m, the percentage reduction in wave heights ranges from 66.27% to 50.90% and R u /H i varies from 0.840 to 0.512, for h s /d = 1.25. The capability of individual habitats like seagrasses, coral reefs and mangroves as a natural barrier has been a topic of research interest since late 1900s. But it is still uncertain how these individual habitats complement each other in containing the brunt of these disasters and therefore tests are conducted with simulated heterogeneous vegetated models and their influence on wave attenuation. For the submerged heterogeneous model of width 4 m, the percentage reduction in wave heights varies from 67.50% to 51.25% and R u /H i ranges from 0.737 to 0.435, for h s /d = 0.525. The emergent heterogeneous model of width 4m and the compound heterogeneous model of width 6 m shows a variation of percentage reduction in wave heights from 70.00% to 52.50% and from 70.00% to 58.75%, respectively. Finally, experimental runs to test the effect of fragmented vegetated meadows on wave decay is taken up by introducing gaps in the vegetated meadow which may alter its hydrodynamics. For the fragmented heterogeneous vegetation model, the percentage reduction in wave heights increases with increase in gap width parameter, w gap /w from 0.125 to 0.375. The performance of all the vegetation types on wave decay is compared and based upon the results of this study, the optimum configuration of vegetated meadow, namely, fragmented heterogeneous vegetation model with the highest gap width parameter (w gap /w) of 0.375 is selected as the best model with highest percentage reduction in wave heights ranging from 76.25% to 66.88% and lowest values of R u /H i ranging from 0.498 to 0.254, for h s /d = 1.25.