Analysis of wave energy reduction and sediment stabilization by mangroves in Gazi Bay, Kenya

Abstract

Ndirangu MD, Chira RM, Wang’ondu V, Kairo JG. 2018. Wave energy reduction and sediment stabilization by mangroves in Gazi Bay, Kenya. Biodiversitas 19: 83-94. Mangrove forests provide natural protection to the coast by attenuating wave energy and stabilizing sediments. The efficiency of coastal protection is likely to decline with increased degradation and losses of mangrove forests. Nevertheless, there are few empirical studies to test these hypotheses. The objective of the study was to investigate how wave energy and sediment stabilization vary with tree density in a monospecies stand of mangroves (Sonneratia alba) at Gazi Bay, Kenya. Seven belt transects were randomly selected along 900 m stretches of shoreline with homogenous emergent wave energy. Structural parameters, including; tree density, pneumatophores density, and basal areas were quantified using two quadrat measuring 25 m by 20 m along each transect. Three intertidal stations, each with five sampling points were sampled for wave energy using uniform plaster of Paris clod cards. Sediment stability was measured using improvised sinking metal disks made from bicycle spokes, and sediment accretion was monitored by Surface Elevation Tables (SETs). All transects showed significant difference in pneumatophores density (F (2.39) =25.15), tree density (F (2.33) =24.79), and basal area (F (2.39) = 29.66). The wave energy sampled by tree stems between stations and the correlation of wave energy reduction against tree density/ha, pneumatophores density, and basal areas per m2 also showed a significant difference. Regression analysis showed a significant difference in the sediment stability against tree density (R2 = 61%) and basal areas (R2 = 72.8%) while there was no significant difference between sediment stability and pneumatophores density (R2= 47%). Regression analysis between mean sediment accretion rates against all parameters were not significant. This study will help the managers and the government on the merit of using mangroves as bio-shields in protecting coastlines against erosion and stabilizing sediment in the wake of much anticipated global changing climate and sea-level rise.