Development of Intensity Duration Frequency(IDF) Curves for Bahir Dar City from Daily Rainfall Data by Using Simple Scaling Method, Bahir Dar, Ethiopia.

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Addis Ababa


This research mainly dealt the development of intensity duration frequency curves for Bahir Dar city from annual maximum daily rainfall data by using simple scaling method. Regarding the rainfall data, 41 years of daily rainfall data and 4 years sub hourly and hourly rainfall data were collected from National Meteorological Agency (NMA) in Addis Ababa, Ethiopia. Hydrological data checking was conducted which were mainly testing for independence, stationary, homogeneity and outliers of the rainfall data. And the data were found independent, stationary, homogenous and with no outliers. There were missing data for 12 months in 1991, 2005 and 2014 and in-filling of the data were done using simple average method. By using easy fit software, it was possible to know that the annual maximum daily rainfall data to Bahir Dar city followed the general extreme value(GEV) distribution function and the three parameters of the distribution function were also determined. The annual maximum daily rainfall were found to be time scale invariant which helped to develop the intensity duration frequency relationships and curves using simple scaling method. The scale exponents of the scale factor was found constant in two ranges of durations and . Because of the distinction of the linearity of the slopes for these two durations, it was possible to detect the slopes difference at one hour duration. The automatically 15 minutes intervals recorded for 4 years (2012-2015) rainfall data were the back bone of the successful development of the curves for Bahir Dar city. The determination of the design daily rainfall for 2 years, 5 years, 10 years, 25 years, 50 years, and 100 years return periods was done. The comparison of the regional IDF curves and simple scaling method generated IDF curves showed that the intensity of the simple scaling method generated curves were higher value than the intensities generated using the regional IDF curves. Moreover, the 24 hour rainfall values calculated from simple scaling model fall above the design rainfall values calculated from observed rainfall data for different selected return periods using frequency factor method. In addition, the comparison of the regional IDF curves, observed IDF curves, and simple scaling IDF curves were also analyzed in order to understand and check the performance of simple scaling modeling with regards to the observed rainfall data. Based on this analysis, it was learnt that the intensities or rainfall values from simple scaling model fall above the values of the observed rainfall or intensity values. In contrary, the values from regional IDF curves fall below both observed rainfall and design rainfalls calculated values. This proved that simple scaling model performs better than the regional IDF curves. From the study, it was also possible to learn that the intensities at shorter rainfall durations are higher than the intensities at longer rainfall durations.



Development of intensity duration frequency