Citation
Sivapiragasam, Ratna Rajah
(2002)
Frequency Analysis And Evaluation Of Short Duration Storms For Peninsular Malaysia.
Masters thesis, Universiti Putra Malaysia.
Abstract
The Department of Irrigation and Drainage (DID) published the Stormwater
Management Manual (SWMM) in year 2000. The Manual requires Intensity-Duration-
Frequency (IDF) values of low average recurrence interval (ARI) ranging from 1 month
to 12 months for the design of water quantity and quality control facilities. However,
these IDF values are currently not available because past analysis using annual maximum
series (AMS) of rainfall depths could only derive the IDF values for ARls of 2 years and
above.
The SWMM has recommended as an interim solution to use simple coefficients to
convert 2 year ARI rainfall intensity to obtain the 1,3,6 and 12 months ARls rainfall
intensity for short durations. The coefficients were derived by fitting Gumbel distribution
to the I hour duration rainfall depths obtained for the city of Ipoh and extrapolating the distribution to obtain the low ARIs . The coefficients need to be verified as they have
been recommended for use in any location of Malaysia for any duration of rainfall.
Twenty six rainfall stations distributed throughout Peninsular Malaysia were
chosen and monthly maximum series (MMS) rainfall depth for 1 5 minutes, 30 minutes, 1
hour, 3 hours, 6 hours and 1 2 hours durations were extracted from rainfall records
obtained from DID Hydrological Databank. For every station, frequency analysis was
performed using the Gumbel distribution and method of moments with Gringorten
Plotting Position formula to obtain rainfall intensity for 2, 3 , 6, 9, 1 2 , 1 5 and 1 8 months
ARI. Concurrently, the same frequency analysis was performed using the same durations
and length of data but by using AMS to obtain 2 years ARI rainfall intensities.
The coefficients obtained from this analysis differed significantly from those
recommended by the SWMM. As such, IDF curves were developed for the 26 rainfall
stations, and are suggested for use in SWMM. Further analysis was performed on the
MMS to determine the better method of estimate between the method of moment and the
method of L-moment by computing the standard error of estimation based on Random
Number Generation method. The findings were that the method of L-moment is a better
method of estimation generally. Identification of appropriate families/parent distribution
for various durations was determined from L-Moment Ratio Diagram. The study showed
that Pearson Type 3 was best fit for 1 5 minutes, and 30 minutes durations, Generalised
Normal curve for 1 hour duration and Generalised Extreme Value curve for 3 hours, 6
hours and 1 2 hours durations.
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