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.
