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CHAPTER - V
FLOODS
Flood is almost an annual phenomenon in India in time of excess rainfall and
causes enormous hardship to the people in the affected areas. Besides
habitation and buildings, it also causes loss of crops. The nature of floods
varies from place to place depending upon the topography, the nature of natural
drainage or river systems. While flash flood occurs in the hilly areas due to
sudden increase in run off in the catchment areas due to excessive rains, water
in the plains overtops or breaches the embankment along the rivers inundating
the adjacent areas.
Estimation of maximum rate of run off or flood discharge is, therefore, utmost
importance for planning to prevent recurrence of flood in the flood affected
areas by creating suitable structures, such as embankments, spurs or gryones
etc.
The estimation of peak flow or flood can be made by one of the following
methods:
(a) physical inspection of past records
(b) flood discharge formulae
(c) flood frequency studies
(d) unit hydrographs
a) Past records of floods can be detected in the old structures
like monuments, temples etc. near the river bank. The Site
Engineers or Survey parties contact the local old people from
the villages and enquire about the maximum level of water
that occurred during the past years, may be 30 - 35 years
ago, and correlate with the markings left by the flood on the
old monuments or even trees. This level is taken as the High
Flood Level. The survey party then prepares the cross
