Impacts of vegetation changes on the hydraulic and sediment transport characteristics in Guandu mangrove wetland

Excessive mangrove spreading causes a significant impact upon the ecosystem and flood control operations in the Guandu mangrove wetland (Guandu Natural Reserve), Taiwan. Aerial photos, taken between 1978 and 1994, reveal that marsh habitats, dominated by Cyperus malaccensis Lam. and Phragmites communis (L.) Trin., have changed into a swamp habitat, dominated by Kandelia candel (L.) Druce. The coverage area of K. candel has increased from 0.04 ha in 1978 to 20.75 ha in 1994. The Kandelia habitat was more salty and located at a higher ground surface elevation than was the P communis habitat. Variations of the water surface elevation and reduction of the channel conveyance due to increase of the coverage area of K. candel (L.) Druce were also obtained in this study. A horizontal two-dimensional model, TABS-2, was applied in this study to simulate the hydraulic and sediment transport characteristics of this estuary wetland. Four cases with different removal ratios show that water surface elevation decreases as the removal ratio increases. When the removal ratio of Kandelia reaches 20%, variations of the water surface elevation in the wetland became insignificant. Significant sediment deposition occurs due to the extensive root network of Kandelia. The average deposition is about 33 mm during a 200-year return period flood event. Removal of Kandelia reduces the sediment deposition rate. When the removal ratio reaches 20%, the reduction in sediment deposition is about 5 mm. Considering the factors of flood protection and sediment deposition, the optimal removal ratio is between 10 and 20%. It also found that mangrove removal improves the ecological restoration of Uca (Thalassuca) Formosensis Rathbun, an endemic species of the fiddler crab in Taiwan. (C) 2004 Elsevier B.V. All rights reserved.