Estimating Direct Primary Benefits

The direct primary benefits from flood management projects accrue from the prevention of flood damage. This is determined by the annual average of flood damage minus the residual damage that may continue to take place even after the implementation of the flood management project— for instance a flood moderation reservoir, an embankment or a flood warning programme. Assessment of benefits can be brought about only if the assessment of damage prevented in monetary terms is made in a satisfactory manner. The assessment of the annual potential damage is made by averaging the flood damage incurred in the past. A correct idea of the average flood damage can be gained only if figures based on long periods are used; the longer the period the more reliable the average. Ideally weighted average damages can be obtained based on the damage-frequency curve obtained from past floods—their related frequencies and the actual damages recorded. This methodology is explained in the figure below.

If the time series used in the above analysis is long, the earlier damage estimates are likely to be less reliable because of likely changes in land use, cropping patterns, developmental activities etc. Floods of a similar magnitude could produce a different order of damage today, as compared to 50 years ago. This problem, however, can partly be taken care of by adjusting the past damage estimates for the land use change and increased productivity with the help of standard methods.

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