Coastal protection function of mangrove ecosystems: a case study from Sri Lanka

Abstract

The Indian Ocean tsunami in 2004 revealed the inferiority of hard engineering solutions in coastal protection and provided sound evidence over the potential of coastal vegetation, particularly mangroves in protecting the coast against erosion, tropical storm surges, and occasional natural calamities like tsunamis. The present study was initiated therefore to determine the extent to which mangroves could be used to protect the coasts against damage by tsunamis. The structure of mangrove vegetation at Kirinda, Kalametiya, and Rekawa in Sri Lanka that resisted tsunami waves for varying extents in 2004 was studied in detail to discern the wave attenuation function of mangrove vegetation. Vegetation structure is a salient factor that contributes to reduction of the impact of natural disturbances to which mangrove ecosystems are vulnerable. The current study highlights that mangrove parameters such as canopy, trunk, and complex root system and wave parameters such incident wave height and inundation distance play vital role in mangroves-induced wave attenuation. The study found that tsunami run-up height and tsunami inundation distance were negatively correlated with tree volume, forest width and tree height whereas that was positively correlated with porosity of the mangrove vegetation. Depth of sedimentation caused by tsunami waves decreased across the mangrove vegetation from the proximal end (in relation to the advancing wave) to the distal end at all mangrove study sites indicating the progressive dissipation of energy of the wave. The mangrove plant communities comprised of Avicennia marina, Ceriops tagal, Excoecaria agallocha and, Rhizophora mucronata evidently, have served as natural coastal barriers and contribute to mitigate impacts of natural disturbances such as tsunamis and tropical storms. As such our study revealed that the effectiveness of coastal bio-shields is based on the make-up of the coastal forests.