海岸地帯管理ジャーナル

概要

Experimental Study on the Effectiveness of Emerged and Submerged Coastal Bio-Shield in Storm Surge Protection

Md. Mostafa Ali, Md. Arifur Rahman and Md. Ataur Rahman

Surges and waves induced by cyclones and other severe storms can cause devastating damage of property and loss of life in coastal areas of Bangladesh. Bio-shield in wetlands can reduce the energy of storm surges and cyclones. To study the effectiveness of emerged and submerged coastal bio-shield in storm surge protection, a laboratory experiment was carried out in a 22 m long, 0.75 m wide and 0.75 m deep wave flume in the Hydraulics and River Engineering Laboratory of Bangladesh University of Engineering and Technology (BUET). Experiments were carried out for investigating the performance of the bio-shields in submerged as well as emerged conditions. Bio-shields were represented by cylindrical bamboo sticks as rigid bio-shield and polyethylene as flexible bio-shield whose diameter was 6 mm. There were a total of 51 experimental runs covering 3 different wave periods of 1.6, 1.8 and 2.0 seconds. Time-series of water surface elevations for vegetated (with bio-shield) and non-vegetated (without bio-shield condition) were compared to identify the influence of bio-shield on wave height reduction. It is found for emerged and submerged conditions of bio-shield that the maximum wave height reductions are 71% and 52% at a location which is 3.5 m away from the embankment for wave periods of 2.0 sec at bio-shield spacing of 50 mm c/c and 25 mm c/c respectively. These reductions of wave heights occur when bio-shield of 2 m width (1 m Rigid + 1 m Flexible) are placed at 4 m distance from the embankment. Hence, the most effective position for maximum wave height reduction by both emerged and submerged bio-shield of 2 m width is 4 m distance from the embankment. It is anticipated that these results will find broad application in coastal management and planning for areas with existing coastal bio-shield as well for bio-shield restoration and establishment efforts.