| 20 YEARS RETURN PERIOD |
With the 20 years return period storm event, the results of model simulation( Figure 1) indicate that the inundated area of present situation cover 55% of the area. For other scenarios (A, B and C) it covers 43%, 64%, 63% respectively. In scenario-B, the total flood water volume was 19.59?106 m3 which was 2 times higher than the present situation. In scenario-C, the total of flood volume was lower than scenario-B. This can be explained by the distribution of discharge over time. In scenario-C the design rain storm with a 20 year return period (annual probability of occurrence 5%), the discharge was higher at the beginning but after two hours the discharge was lower than in scenario-B (see figure 1). This affected the total volume of the flood water in scenario-B.
|
 |
Figure 1 : The spatial distribution of maximum water depth of the three scenarios
and present land use with 20 years return period.
|
|
Figure 2 show the distribution of the flow velocity for the present situation and scenarios A, B and C. For most of the inundated area the flow velocity did not rise above 0.50 m/s. In scenario-B and C, The speeds of water flow were lower than 0.50 m/s cover 97% and 96% of flooded area respectively and cover 4% of the area with flow velocity higher than 50 centimeters per second. The high water flow velocities were found at the top of the downstream area (near the apex). In scenario-A, most of inundated area the flood flow velocity was lower than 30 centimeters per second. This means that, if the upland catchment became completely forest area, it will reduce the speed of the water flow in downstream area.
|
 |
Figure 2 : The spatial distribution of maximum flow velocity of the three scenarios
and present land use with 20 years return period.
|
|
|
|
Development of Methodologies for
Land Degradation Assessment Applied to
Land Use Planning in Thailand |
|
|
