Environmental impact evaluation of the coastal embankment on surge flooding in Chittagong city

Abstract

This study was aimed at evaluating the environmental impact of the coastal embankment against surge flooding in Chittagong City. The evaluation was made through four major avenues: (I) assessing the flood risk associated with surge flooding; (2) evaluating the effectiveness of the embankment against surge flooding; (3) estimating the height required to protect the area from surge flooding; and (4) assessing the environmental consequences of the embankment. A mathematical model, capable of estimating the surge height, was used for flood risk assessment and for estimating embankment height required for surge protection. Evaluation of effectiveness of the embankment against surge flooding was made through estimating different types of damages that the embankment faces during monsoon and cyclonic surge propagation. The environmental consequences were assessed through extensive field investigations and questionnaire survey among 300 residents in the project area near the embankment. It was found that out of a total of 169 sq.km in Chittagong, 59, 82, 90 and 108 sq.km areas are subjected to surge flooding due to surge height with exceedence probabilities of 0.1, 0.05, 0.02 and 0.01, respectively. Estimated areas where damage to human lives are expected significantly are 48, 65, 72 and 86 sq.km with the 10,20,50, 100 yr. return periods, respectively. The embankment was constructed for the protection of agricultural lands from periodic saline water inundation during normal high tides and are found to be inadequate and ineffective against surge flooding during cyclones. The major reasons are: (I) inadequate height which resulted in frequent overtopping by surge waves during cyclones; (2) lack of compaction during construction of the embankment resulting in low shear strength during submersion; (3) inadequate slope protection work to resist monsoon wave action and surge wave, and normal run-off related erosion of the inner and outer slopes; (4) lack of maintenance. The estimated heights for surge wave protection with exceedence probabilities of 0.1, 0.05, 0.02 and 0.01 are 5.90, 7.08, 8.95 and 10.45 m, respectively above the coast line level at the period of cyclone. The embankment was found to cause long-term positive impacts on soil fertility and crop production, surface water salinity, cultivation of fisheries in the depressions, communication, public health, socio-economic conditions. The negative impacts include surface water drainage congestion during rainfall in some areas, reduction of surface water for irrigation, declination of openwater fisheries. However the embankment has induced overall positive impacts in the project area. Although, all age and professional groups reacted positively about the performance of the embankment on environment, it seemed to fail to show proper effectiveness against surge flooding during cyclone.