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Bangkok is located on the flat deltalic area of the The Choa Praya river. Bangkok always
has problem with flood. The polder system of the target area is recommended for preventing
external flood and the sell organized drainage network in polder is also recommended for
preventing flooding due to runoff generate from rainstorm. Due to the rapid development
with uncontrolable land use, most of the agricultural and unimproved area were changes to
residential, commercial and industrial area. Consequently this new land use pattern
increased the runoff rate though average rainfall stills maintained. This is effecting
existing drainage system that unable to cover with the excess drain water in time.
Therefore the problem of temporaly flooding is inevitable.
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Many method can be applied for solving the problem but the most effective one is to
provided the water detention storage in developing area. Two alternative approaches can be
adopted : public or private detention storage. In this study the private approach was
concentrated. The main criteria is that any developing area shall not increase the runoff
peak rate. Within the developing area, therefore, the excess runoff originated from
changing of land use pattern must be temporaly stored in their retention storage. The rate
of flow through throttle pipe to public sewer, must be lower than the flow rate before
development.
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Rainfall of 2 years and 5 return period were used for the purpose of analysis the size
of retention storage and investment. The study indicated that there is not significant
different, therefore, it is recommended to use 5 years rainfall as design criteria of
retention storage. The result of analysis using runoff coefficient before development 0.15
and effective depth of storage in retention area 1.0 meter, indicated that only 1.0 to 3.5
percent of the area, depend on land use development, will be reserved for constructed
retention storage. The typical of detention system with its connection to the existing
public drainage system was also recommended.
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