| Abstract | A Computer Assisted System for Flood Frequency Analysis, called
CASFFA, was designed and implemented in this study. It consists of
two components, the first, called DPU, is used for data preparation,
and the second, called FFAS, carries out the required steps in flood
frequency analysis on the data provided by the DPU. The developed
system was used to analyse flood data in Thailand for both at-site
and regional approaches.
In the analysis, three popular distributions, the Generalized
Extreme Value (GEV), the Log Logistic (LLG) and the Three Parameter
Log Normal (LN3), were adopted for use along with three regional
procedures, namely ; the Station Year Index, the Wallis and the
Lettenmaier procedure. Three homogeneous regions, namely the
Northern, Chi and Mun were identified by means of the Krusal -
Wallis test.
For the Northern region, the Lettenmaier regional procedure
based on GEV fitted by the unbiased PWM estimators (GEV/PWM) is
usually the best in terms of the standard deviation and the relative
bias while the Wallis regional procedure used along with the LLG
fitted by the plotting position estimators (LLG/PLOT) is the best
in terms of the percent standard error. For both Chi and Mun regions,
the Wallis regional procedure is found to be better in terms of the
standard deviation when it is used along with the LLG/PLOT procedure
and is better in terms of the percent standard error when it is
used along with the LLG/PWM while the station year index regional
procedure based on the LLG/ PWM is best in terms of the relative
bias. It was also found that in most cases, the regional approach
provides better estimation for the T- year event than does the at-site
approach. |