Study on effectiveness of coastal BIO-SHILD for reduction of the energy of high frequency surges

Abstract

Storm surges generated by cyclones 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. In this study, historical storm surges are analyzed and existing bio-shield protection is evaluated along the coastal belt of Bangladesh. Moreover, a laboratory experiment has been carried out and the result of a numerical model has been used to study the effectiveness of coastal bio-shield in reducing the energy of high frequency surges. In this study, wind speed of 25 cyclones that hit the country from 1960 to 2007 has been analyzed and found that the wind speed of cyclonic storm surges increases by 0.83 km/h each year. But the human death toll caused by storm surges shows a decreasing rate of 500 people per year because of the present growing successful institutional arrangement for disaster management. It is also found that there are only 60 km or 6% of forest belts of the 957 km total length of embankment of 49 sea-facing polders, much of which is already degraded. To study the effectiveness of coastal bio-shield in reducing the energy of high frequency surges, a laboratory experiment is 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. Bio-shields are represented by cylindrical bamboo sticks as rigid bio-shield and polyethylene (Polyethylene is 75 times more flexible than bamboo sticks) as flexible bio-shield whose diameter is 6 mm. The densities of bio-shield are 780 and 390 sticks per square meter and the porosities are 0.9779 and 0.9889 for spacing 25 mm c/c and 50 mm c/c of bio-shields, respectively. It is found for emerged conditions of bio-shield that the maximum wave height reduction is 71 % for bio-shield spacing of 50 mm c/c and that for submerged condition of bio-shield is 52 % for spacing of 50 mm c/c. These reductions of wave heights occur when bio-shield of2 m width (1 m Rigid + 1 m Flexible) are placed at 4 m distance from the embankment for wave period of2.0 sec. Moreover, a two-dimensional numerical model named Bay of Bengal model developed by Institute of Water Modeling is used to simulate the effect of coastal bio-shield in reducing the energy of storm surge SIDR along the Kuakata beach of Bangladesh. In this modeling, four simulations are performed for four different conditions including 74 cm of SLR. Surge levels and current speeds are measured at three specific locations on the Kuakata beach (polder-48). The model results demonstrate that up to 2 cm of surge height and 0.48 mls or 62 % of current speed are reduced by 200 m width of bio-shield when SLR is not considered. Again, the surge height and current speed are reduced by 3 cm and 0.46 mls or 59 % respectively by 200 m width of bio-shield when 74 cm of SLR is considered for the year 2100 according to 5th IPCC report. It is anticipated that these results will find broad application in coastal management and planning for areas with existing coastal bio-shield as well as for bio-shield restoration and establishment efforts.