Study on the frequency of heavy rainfall in Huong Khe district, Ha Tinh province

Rainfall intensity, duration and frequency of 24 consecutive hours or longer (48h, 72h, 96h) are very essential for the assessment of flood risk and the design of the reservoirs and dams in Huong Khe district, Ha Tinh province. The analysis of flood-causing rainfall and the actual floods from 1990 to 2012 has shown that floods usually occur when: a) 24-hour continuous rainfall reacheds 710.6mm or more; b) Heavy rains which lasted longer than 24 hours and reached 548.9mm/24h to 630.2mm/48h or more; c) Heavy rains lasted from 72 hours to 96 hours and reached from 534.5mm/72h to 575.6mm/96h. The following conclusions have been drawn from analysis results of development of the empirical and theoretical exceedance frequencies of Pearson III distribution of 24h-96h heavy rainfall: a) All the theoretical and empirical frequency data have very high correlation coefficient from 0.891 (24h rainfall) to about 0.948 (72h-96h rainfall); b) For 24h rainfall, the actual rainfall of the empirical P of 13% to 26% is about 40mm lower than the theoretical rainfall, while the actual rainfall of the empirical P of 8.7% is about 80mm higher than the theoretical value, and that of the empirical P of 4.35% is about 175mm higher than the theoretical value; the actual rainfall at empirical P of 8.7% is corresponding to theoretical P of 4.5%, and actual rainfall at empirical P of 4.35% is corresponding to theoretical P of ~1%; c) For 48h and 72h rainfall, the empirical and theoretical frequency data are very close to each other for the P in the range of 8.7% to 30%, only empirical P of 4.35% is much far from theoretical one and corresponding to rainfall frequency of ~1%; d) For 96h rainfall, the empirical and theoretical frequency data are very close to each other for most P range, only empirical P of 8.7% and 4.35% are somehow far from theoretical ones and corresponding to rainfall of theoretical frequencies of ~4.5% and ~1%, respectively.