Abstract:
Flash floods are becoming more and more common and frequent particularly in urban
areas all over the world. Unplanned development and alteration of existing land-use
are identified as major contributing factors for flash floods. Nevertheless, according
to climatologists there is a significant change in precipitation patterns due to climate
variability and rainfall extremes have been distorted. Sri Lanka has no exception with
regards to flash floods particularly in urban areas with lot of changes in relation to basin
characteristics as well as variability of climate/weather. City of Galle has had flash flood
experience due to inclement weather. Recently some research attempts have been made
to investigate the vulnerability against flash floods through simple hydrological modeling
approach and with extreme precipitation inputs. Design precipitation hyetographs have
been used as the system input for above hydrological investigations using available
Intensity-Duration-Frequency (IDF) relationships for required return period. However,
the available IDF curves for Galle meteorological station were developed based on the
precipitation data before 1980 and they do not represent the climatological changes that
would have taken place in the recent past. The objective of this study is to develop/update
IDF curves for Galle meteorological station using most recent precipitation data.
The data required for this study were rainfall depths for short durations from 15
minutes to several hours. As the digital data for Galle meteorological station are not available
(except daily data), all short duration data had to be extracted from rainfall pluviographs.
Several appropriate distribution functions were studied to obtain IDF curves.
Then the statistical method, the Type-I extreme-value distribution (Gumbel) was identified
as the most appropriate method to estimate the relevant parameters of the IDF model.
Recommendations were made for the future development of rainfall IDF curves in the
region both in terms of data and organizational requirements.