Abstract:
Safety of the Water Resources Development infrastructure is fully assured only if they
are tested for the design flood event of appropriate design return period.
The literature review highlighted the design rainfall pattern as a critical factor for the estimation of
design flow. Alternate Block Method (ABM) is considered as a simple and consistent method for
estimating design rainfall pattern. The runoff hydrographs, peakflows and time to peak values from a
storm event based rainfall distribution and uniform rainfall distribution were compared with ABM
estimates for the sub catchments of Attanagalu Oya at four locations. The study revealed that ABM
results in the highest peak values compared to Storm Event based and Uniform rainfall distributions.
Larger catchment areas display hydrographs of similar shape for all three rainfall patterns. The
disparity between the peakflows from the three rainfall patterns becomes lower in larger catchments,
indicating 42%,58%,62% and 73% peakflow estimates of the ABM estimate, from smallest to largest
catchment, with storm event based distribution. The time to peak values from storm event based and
uniform intensity distributions indicate an irregular behaviour with the catchment area. In the absence
of a proper rainfall pattern that can be used confidently, ABM provides a better and reliable
methodology ensuring the safety of the structure. If the rainfall pattern is not selected carefully there
will be a threat to the safety of the structure.