Effect of coral reefs on wave attenuation and erosion: Mnemba Island, Zanzibar.

Abstract

Global warming and sea level rise are some of the greatest threats humanity faces today with low-lying coral reef islands most vulnerable to these threats and their repercussions. This study focuses on the mechanisms that have resulted in the severe erosion of low-lying coral reef islands. Mnemba Island, a small island off the north-east coast of Zanzibar is used as a case study. It is a premium high value resort that is currently experiencing severe erosion. The erosion has resulted in the collapse of various structures and utilities littering the eastern beach with debris and fallen Casuarina trees which dominate the Island’s eastern border. Mnemba Island is a typical low-lying island surrounded by a coral reef platform. This study identified the key role these reefs play in wave attenuation and protecting the island from the large swell that surrounds the reef. The reef crest was found to attenuate wave heights by up to 70%. Overfishing and changes in the climate have resulted in the destruction of the coral reef communities that protect the island because of their increase hydraulic roughness and shallowness. Modelling shows that the impact of reduced bed roughness from coral degradation and just 10 cm sea level rise can result in up to 40% increase in wave height at the Island’s eastern beach. Coupled with the large tidal range and increased wave heights propagating over the reef platform, wave driven forces and currents are enhanced thus disturbing the quasi-equilibrium wave climate resulting in major erosion. The fine sediments are lifted into the water column and transported into deeper sections of the reef platform. The low wave energy in the following seasons is insufficient to overcome the critical shear stresses that lift the transported sediments back into the water column and hence, a net loss of sediment off the reef platform is experienced.