II. FLOOD LOSS REDUCTION AND REVIEW OF PAST EXPERIENCES

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C. Methodology for assessment

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3. Selection of return period in planning protection level

The probability of river flooding at a particular location is assessed by performing frequency analysis of annual maximum peak-flood data at that location. Selection of a return period according to the Manual of Bangladesh Water Development Board (Bangladesh Water Development Board, 1996) for the design flood in determining the height of embankment is shown in Table 8. In general, a 100-year return period is considered appropriate for the major rivers, the Brahmaputra-Jamuna, the Ganges-Padma and the Meghna. A 20-year return period is adopted for the coastal embankments that are designed to prevent tidal inundations. Drainage regulators in the non-tidal areas are constructed taking into consideration a10-day rainfall for a 10-year return period, with a peak 20-year river stage return period to ensure effective drainage in the pre-monsoon season. For the post-monsoon season, the same rainfall period is adopted, with a 10-day average water level for a 10-year return period.

The return periods for the design events for flood protection and drainage structures are not selected on the basis of so called risk-based approach that the floods would be controlled up to the point where the additional costs of flood mitigation equalled the expected value of the remaining flood damages. Necessity of risk-based decision-making has been emphasized by Chowdhury et al. (1996). Selection of the level of flood protection and the capacity of drainage structures should be based on the minimization of the sum of annualized capital cost, expected risk-damage costs, and costs of operation and routine maintenance.

It is observed by Mott MacDonald and Others (1993b) under the Flood Action Plan Study that there is relatively little difference between crop damage which is avoided from protection up to a 20-year flood level, and damage avoided from lesser degrees of protection (10, 5 year return periods) in the north-west region of Bangladesh. However, it can be expected that the damage caused by a 20-year flood would be considerably greater than that caused by a 5-year flood. But the infrequency of the major events means that major damages in one year do not translate into major changes in the value of expected annual damage. The results also suggest that crop damage in years of moderate flooding may also be quite widespread if less dramatic than in high flood years. These results raise the question of the desirable level of protection to aim for, from both economic and other viewpoints.

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