Discuss the flood frequency method for estimation of peak flood.
Answers
Basic to all frequency analyses, is the concept that there is a collection of data,
called the ‘population’. For flood frequency studies, this population are taken as
the annual maximum flood occurring at a location on a river (called the site).
Since the river has flooded during the past years and is likely to go on flooding
over the coming years (unless something exceptional like drying up of the river
happens!), the recorded flood peak values which have been observed for a finite
number of years are only a sample of the total population. Here, ‘flood peak’
means the highest recorded discharge value for the river at any year. The
following assumptions are generally made for the data:
The sample is representative of the population. Thus, it is assumed that
though only a finite years’ data of peak flow has been recorded, the same
type of trend was always there and would continue to be so in future.
• The data are independent. That is, the peak flow data which has been
collected are independent of each other. Thus, the data set is assumed to
be random. In a random process, the value of the variant does not
depend on previous or next values.
Flood frequency analysis starts by checking the consistency of the data and
finding the presence of features such as trend, jump, etc. Trend is the gradual
shift in the sample data, either in the increasing or decreasing directions. This
may occurs due to human interference, like afforestation or deforestation of the
watershed. Jump means that one or a few of the data have exceptional values –
high or low, due to certain factors, like forest fire, earthquake, landslide, etc
which may change the river’s flow characteristics temporarily.