AGENERAL DIMENSIONLESS RAINFALL DEPTH-DURATION-FREQUENCY RELATIONSHIP

The frequencies of high-intensity rainfalls may be required for several engineering purposes. The analyzed and dependable rainfall intensities of durations up to 24 hrs with frequencies of up to 100 yr from 126 stations in Iran were used in this study. The results of this survey showed that (a) probability density function for coefficient of variation (CV) at each rain duration was approximately normal, (b) the ratios of rain depth with different frequencies to that of 10-yr frequency [P(T)/P(10)] was constant, averages of these constants are comparable with those in other countries, and P(T)/P(10) and rain duration are significantly related to each other at each frequency, (c) the ratio of rain depth with different durations to that of 1-hr duration [P(D)/P(60)] was constant, and in spite of good agreement between average of these empirical constants with those of other countries, it was shown each specified rain duration, (d) a dimensionless relationship was derived between P(D,T)/P(60,10) and two independent variables of return period and rainfall duration for climatological conditions of Iran .Therefore, intensity-durationfrequency (IDF) or depth-duration-frequency (DDF) relationship can be obtained at any point in Iran knowing the value 10-yr hourly rainfall.