51
Statistical Methods or Flood Frequency Methods or Probability Methods:
Statistical methods are useful stools to predict floods when long term data on
stream flow and rainfall data are available. The stream flows vary from year
after year and the sequence of such stream flow are recorded and a
probable prediction can be made based on such available records. These
probability methods are more or less reliable when long term records say
100 years, are available, but may not be precise for short period of data. Flood
frequency or chance floods denote the likelihood or occurrence of flood. For
example a 1% flood frequency indicates the possibility of flood once in 100 years.
A 20% flood frequency there is likelihood of flood 20 times in 100 years.
Unit Hydrograph Method for estimating Flood Discharge
Hydrograph is a graphical representation of stream flow variation with time at a
particular point of stream. When a storm occurs there is an increase in the
discharge of the stream which varies with time. Initially there is an
increase in the discharge in the stream and is reflected in the rising limb of the
discharge curve. Hydrograph when plotted against time some valuable
information can be derived in respect of total run off due to the storm and
recharge to groundwater.
The unit Hydrograph follows the above principle to decipher the flood discharge.
A unit hydrograph is a hydrograph representing unit (1 cm or 1 inch) run off from
rainfall of some unit duration and specific aerial distribution. Unit duration refers
to the duration of a run off producing rainfall excess, that results in a unit
hydrograph or in other words a hydrograph depicting a duration of one unit, say,
3 hours, results in producing an excess rainfall of unit (1 cm or 1 inch). The unit
duration may vary from basin to basin or in different catchment areas,
depending upon its size.
