II. FLOOD LOSS REDUCTION AND REVIEW OF PAST EXPERIENCES
[ II-A | II-B | II-C
| II-D | II-E | II-F
A. Flood mitigation measures
[ A-1 | A-2 ]
1. Structural measures
Many conventional flood mitigation measures like flood control reservoirs, flood diversions or flood by-passes are not feasible within Bangladesh because of its extreme flat topography. The flood control measures in this floodplain country have been based mainly on the construction of earthen embankments parallel to the river banks. In general, the following flood control and drainage approaches have been exercised in Bangladesh.
Protection against monsoon river flooding: In general, schemes designed to provide protection during the monsoon aim to prevent spill of river waters by constructing embankments of heights greater than those of the annual maximum water levels along the rivers and to minimize internal flooding through the provision of appropriate drainage structures. Such measures are provided to protect agricultural lands and urban areas.
Protection against pre-monsoon river flooding: Protection from such river flooding is provided by constructing embankments of low height and is favored in deeply flooded areas. Such embankments obstruct pre-monsoon flash floods so as to ensure a safe winter rice harvest. These low-height embankments are designed to be overtopped during the monsoon and remain submerged for the entire monsoon season. These embankments are known as submersible embankments.
Gravity drainage to reduce rainfall flooding: The unwanted rain water from behind the embankment or from within the poldered area is evacuated mainly by gravity flow through drainage regulators and sluices incorporated in the embankments. The regulators and sluices prevent backflow from high river levels into the low-lying areas during the monsoon and drain the water from the area to be protected when the river water level gets below that on the inside. Gravity drainage is favored in highland and medium high land areas and in areas free from large variations in land level.
Pumped drainage to prevent rainfall flooding: Drainage by pumps is expensive and has been provided only in selected large flood control, drainage and irrigation projects to pump out accumulated rain water from the project area. Reversible pumps are generally used for both drainage and irrigation purposes with drainage rate requirements much higher than irrigation rates. Pumped drainage is provided in areas with a higher percentage of deep or moderate floods.
Dredging of rivers and canals: Dredging is done to increase the discharge capacity of rivers and canals. Loop cuts are applied for highly meandering rivers through the excavation of pilot channels. Due to the high cost, limited dredging is done only at critical locations like off-takes and confluences to remove sand bars for drainage improvement. Manual digging of canals is being practiced under the food-for-work programmes and is an important source of employment for the rural people.