| 50 YEARS RETURN PERIOD |
The results in table 8-10 indicate that for a storm with a return period 50 years (with increased rainfall intensity and rain volume). Under present situation, the average inundation depth was 1.72 meters but at some places a maximum depth of 3.82 was reached. The inundation flood area covers 62% of the total area (figure 1). The average maximum flood depth was similar in all scenarios with the exception of scenario-B which was higher. The total of flood volume was highest in scenario-B. This can be explained similarly as in section 8.1.3 for the design storm of 20 years return period. For scenario-A, the flood extent was almost similar with present situation. With intense rainfall, the downstream area trend to be flooded no matter what land cover in upstream catchment is.
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Figure 1 : The spatial distribution of maximum water depth of the three scenarios
and present land use with 50 years return period.
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Figure 2 show the distribution of the flow velocity for the present situation and scenarios A, B and C. For the most inundated area the flow velocity did not rise above 0.50 m/s. The speed of the water was higher than 50 centimeters per second which cover only in 7% of flooded area but not rise up above 1 meter per second. In scenario-B and C, the water speed was lower than 0.50 m/s and covers 93% of flooded area. The high water flow velocities were found at the top of downstream. In only 7% of flooded area in scenario-B and C, the speed of water flow was higher than 0.5 m/s but not rise above this stage. In scenario-A, most of inundate area the flood water velocity was lower than 30 centimeters per second. This means that- if the upstream catchment is under forest cover, it can reduce severe flash flood in the downstream territory.
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Figure 2 : The spatial distribution of maximum flow velocity of the three scenarios
and present land use with 50 years return period.
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Development of Methodologies for
Land Degradation Assessment Applied to
Land Use Planning in Thailand |
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