TN-7 : Study of depth-area-duration and depth-duration characteristics

Abstract

Some of the extreme storms in the contiguous areas of Saurashtra and Rajasthan forming part of the arid and semi-arid belt in India were analysed by the depth-area-duration and depth-duration techniques and their characteristics were examined. The depths of rainfall of these extreme storms were compared using the statistical estimates of rainfall over this region both over shorter and longer durations. The meteorological systems associated with these extreme rain storms have also been examined with a view to identify any common characteristic which could be useful for study of the consequent flood events.

The studies have broadly indicated that

(i) most of the severe storms in the Saurashtra and Rajasthan regions have occurred in the month of July; (ii) the extreme storms have occurred soon after the onset of monsoon over these regions namely June in Saurashtra and July in Rajasthan; (iii) the one day rain depths of the June 1983 storm over Saurashtra region have exceeded the 10000 year return period estimates of rainfall over the Saurashtra region; (iv) the rainfall over shorter durations ( less than 24 hour) have exceeded the 50 year return period estimates over their respective regions during the June 1983 storm over Saurashtra and July 1981 storm over Rajasthan; and (v) the storms of June 1983 and July 1981 have comparable magnitudes, though the July 1981 storm was relatively less intense & more localised than the June 1983 storm.

From the point of view of flash flood occurrence, the magnitude of the total storm depths of June 1983 and July 1981 storms and their intensities over shorter durations have far reaching implications in terms of the magnitude of peak flood as well as volume of water to be discharged. This indicates need for review of the design flood of the hydraulic structures, existing or under construction and those being planned especially those with smaller catchment areas.