A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering.

Thesis (M.Eng.) - Asian Institute of Technology, 2000

Failure of hydraulic structures results in damage of properties and endangers human life. The estimates of Probable Maximum Precipitation (PMP) can be useful in the selection of hydrological design criteria for hydraulic structures because they give the upper limit of rainfall depth that the atmosphere can produce. Generalized estimates of PMP have been determined for the Eastern Part of Thailand. The simple moisture maximization and transposition is applied to the grid points. Neither three hourly nor daily dew point temperature data are available before the year I 98 I, the mean monthly dew point temperatures were used in determining the representative storm dew point temperatures. The representative storm dew point temperatures determined from the mean monthly dew point temperatures and determined from persisting I2-h dew point temperatures were checked that they have not much difference. The adjustment factor for moisture and elevation was considered in the present study while the adjustment factor for topography and the adjustment factor for the distance from moisture source were neglected. The adjustment factor for topography was neglected because the storms which moved through the study area came from multi-direction. Also it was not clear that the mountain ridges, which lay against the Southwest Monsoon, which flows through the Gulf of Thailand, had the influence on the moisture. The adjustment factor for the distance from moisture source was not considered because the study area is close to the Gulf of Thailand. The results of the estimates are presented in the series of isohyetal maps of I-day, 2- day, and 3-day PMP over 25, 1000 and 5000-km2 basin area. However, one can calculate PMP for a particular project by using the enveloping normalized depth-area-duration curves and the contour map of the adjustment factor for moisture and elevation. The magnitude of PMP was checked by comparing to the 100-y return period rainfall and to the highest observed rainfall. The ratios of PMP to I 00-y rainfall ranged from 1. 23 to 5.3 I (average 3.36) for 3-day rainfall, and ranged from I.43 to 5.78 (average 3.59) for I-day rainfall. The ratios of PMP to the highest observed rainfall ranged from 1.35 to 6.0 I (average 3.55) for 3-day rainfall, and ranged from 1.35 to 6.73 (average 3.75) for I-day rainfall.