Flood estimation

Every bridge, culvert, spillway, embankment and urban drain must pass a design flood. Underestimating it risks failure and loss of life; overestimating wastes money. Engineers choose a method according to the data available and the importance of the structure.

Empirical formulas, such as Dickens' and Ryves' formulas, relate peak flood to catchment area with a regional coefficient. They are quick but crude. The rational method, Q = C i A, suits small catchments, usually under a few square kilometres, such as urban drains and culverts: the design intensity is taken for a duration equal to the time of concentration. Unit hydrograph methods, with design storm rainfall, suit medium catchments and are widely used in India through regional flood estimation reports. Flood frequency analysis fits a statistical distribution (Gumbel, log-Pearson Type III and others) to the annual maximum floods recorded at a gauge, giving the flood for a chosen return period.

Return period T is the average interval between floods equal to or larger than a given size. A 100-year flood has a 1 percent chance of being exceeded in any one year, and a significant chance during a structure's life. Major dams use the probable maximum flood, from probable maximum precipitation, because failure would be catastrophic. Climate change and land-use change mean historical records may not represent the future, so sensitivity checks are increasingly expected.