Rainfall Intensity-Duration-Frequency relationship for Northern Ethiopia

Abstract

Intensity-Duration-Frequency (IDF) relationship provides essential information for the planning, design and evaluation of hydraulic structures, highways, urban drainage works and flood plan management. The objective of this paper is to develop IDF relationship that will enable estimation of intensity of rainfall (I) corresponding to any required rainfall duration (D) and frequency (F). Once this basic information of a given location is obtained, flood magnitude of various frequencies can he easily estimated which is essential for hydraulic and hydrologic works.

This paper describes the formulation of mathematical relationships of IDF developed for northern portion of Ethiopia (NE). The annual maximum rainfall magnitudes of varying duration were abstracted from rainfall charts and fitted to theoretical frequency distributions and then extrapolation of values for larger return periods were made. The analysis of rainfall intensities was expressed using the IDF equation of the generalized mathematical form.

The parameters of the mathematical form were generated for some selected eleven stations. The performance of the method was evaluated based on the historical and computed values of intensities using graphical and statistical methods. The results of this evaluation showed good agreement between the observed and the computed ones, implying that the method is reliable.

This IDF relationship, being an important hydrologic tool, will bridge the gap between the design need and the unavailability of design information especially in planning and design of water resources systems.