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Rainfall study is one of the alternative measure to solve flood problem in Bangkok
Metropolitan area. These study is concentrated on graphical recorded rainfall data that
could bring to synthetic storm pattern and maximum design discharge for drainage system.
Keifer and Chu method was selected among several approaches, had presented synthetic storm
which was considered on maximum rainfall intensity (" r " proportion) at
specific duration.
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" r", is a fraction of time between maximum antecedent intensity and total
storm duration, could be noticed into three types; First type was determined on actual
rainfall intensity peak of grouped rainfall data under their duration (rdu). Secondly was
determined on actual rainfall intensity peak of specific durations ( r (group) ). The last
type was determined on antecedent mass (ra).
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Five representative rainfall stations in Bangkok Metropolitan, were historical studied
thoroughly, are meteorological station (Bangkok), Ago-meteorological station (Bangkhen),
Ago-meteorological station (Bangna) and Royal Irrigation Department station (Samsen).
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It was shown that "r" value should be depended on rainfall duration (short
duration gives high "r" value while long duration decreases "r" value)
"r" of the first and second type ( rdu and r (group) ) have been corresponding
to each other but the third type was too dominated to synthetic storm pattern. Since its
principle came from actual rainfall intensity peak of grouped rainfall data under actual
rainfall duration, rdu was selected to synthetic storm pattern for Bangkok Metropolitan
design drainage system.
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Moreover, rdu of every station has good correlation to each other therefore, rdu of
meteorological station could be representative in Bangkok Metropolitan area.
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Synthetic storm pattern and actual storm pattern, were compared on equal rainfall depth,
gave a correspondent result of which storm pattern synthetic’s was generally higher than
the actual one.
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It could be concluded that Keifer and Chu method is safely suitable for urban drainage
system design.
