Bio-Shields: Wave Energy Reduction And Sediment Stabilization By Mangroves In Gazi Bay, Kenya

Abstract

Mangrove forests provide natural coastal protection by attenuating wave energy and stabilizing sediments. Efficiency of coastal protection is likely to decline with increased losses and degradation of mangrove forests; but 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, 80m long belt transects set perpendicularly to the shoreline were randomly selected along 900 m stretch of shoreline with homogenous emergent wave energy. Transects were positioned to give the widest possible range in remaining tree density, from transect without tree cover (control) to the one with highest tree density of 680 trees/ha. Structural parameters, including; tree density (stems/ hectare), pneumatophores density (ind./ m2), and basal areas (m2) were quantified using 2 quadrat measuring 25 m by 20 m along each transect. Three intertidal stations A, B and C were systematically positioned across each transect. Stations A’s were 20 m before the mangroves on the seaward side while station B, and C were placed in the forest and were 30 m apart. Each station had 5 sampling points which were sampled for wave energy using uniform plaster of Paris clod cards. Sediment stability was measured at 10 points per station using improvised sinking metal disks made from bicycle spokes. Sediment accretion was monitored by Surface Elevation Tables (SETs), positioned at station B. The accretion rate was measured at 10 replicate point per transect once per month for 8 months using a standard ruler. All transects showed significant difference in pneumatophores density (F (2, 39) =25.15, p = 0.000), tree density (F (2, 33) =24.79, p = 0.000), and basal area (F (2, 39)= 29.66,p = 0.01). One way analysis of variance (ANOVA) showed a significant difference in wave energy sampled by tree stems between stations A, B and C (F (2, 18) = 10.92, p = 0.001). Correlation of wave energy reduction against tree density/ha showed significant difference between stations (r2 = -0.594, p = 0.000), where wave energy reduced with increase in tree cover. Similar test also showed significant difference in wave energy reduction against pneumatophores density/ m2 (r2 = -0.794, p = 0.000) and basal areas per m2 (r2 = -0.451, p = 0.000). Regression analysis showed a significant difference in sediment stability against tree density (R2 = 61 %, P = 0.028) and basal areas (R2 = 72.8 %, p = 0.015) whilst there was no significant difference between sediment stability and pneumatophores density (R2= 47%, p = 0.089). Regression analysis between mean sediment accretion rates against tree density/ (R2 = 7.2%, p = 0.608), basal areas (R2 = 8.9 %, p = 0.566) and pneumatophores density/m2 (R2= 11.1, p= 0.415) were not significant. The 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.