concepts of depth-area-duration analysis, frequency analysis, frequency of point rainfall, intensity-duration curves, maximum intensity-duration

1. • Once the sufficient rainfall records for the region are collected the
basic or raw data can be analysed and processed to produce useful
information in the form of curves or statistical values for use in the
planning of water resources development projects. Many hydrologic
problems require an analysis of time as well as areal distribution of
storm rainfall. Depth-Area-Duration (DAD) analysis of a storm is done
to determine the maximum amounts of rainfall within various
durations over areas of various sizes.

2. • 1. Examine the rainfall records of the region in which catchment area
under consideration is located. Also consider records of
meteorologically similar regions. From it prepare a list of most severe
storms with their dates of occurrence and duration.
• 2. For the listed severe storms prepare iso-hyetal maps and determine
the rainfall values over the area of each isohyet (rainfall contour).
• 3. Draw on a graph curves connecting area and rainfall values for
different durations say 1 day rainfall, 2 day rainfall, 3 days rainfall

4. • Although most severe storm in the listed storms may not have
occurred right over the catchment under consideration there is
possibility of such occurrence. So from DAD curves 1 day, 2 day, 3 day
rainfall depths for the catchment area of the proposed project are
read. These give the rainfall depths when the storms are centered
over the catchment.

5. • In hydrology, frequency analysis of station rainfall data is done for use
in design of bridges and culverts on highways, design of storm drains
etc. With the advancement of science of hydrology rainfall frequency
analysis is done using Gumble’s extreme-value distribution and
annual series data.
• Now the frequency analysis concept is applied on a seasonal basis
and for areal frequency. The rainfall records of deficient length have
to be extended by station year method. The results of frequency
analysis are plotted on the log-log paper.