Development of short duration rainfall equation for design of urban drainage systems in Bangladesh

Abstract

The main objective of the study was development of an empirical equation to correlate annual daily maximum rainfall and the annual maximum rainfall of short durations ranging from 5 to 120 minutes. The equation was established and, based on this, a design chart was developed for direct use by the practicing engineers for estimation of short duration rainfall magnitude at any location within Bangladesh. In performing the study, frequency analysis was undertaken on annual maximum daily rainfall data from 25 stations of Bangladesh Meteorological Department (BMD) and 294 stations of Bangladesh Water Development Board (BWDB). Extreme Value Type I was found to be the most appropriate probability distribution function for frequency analysis of annual maximum rainfall in Bangladesh. A comparative study was undertaken between the annual maximum daily rainfall data obtained from two comparable sets of data from BMD and BWDB - the two sources of rainfall data in Bangladesh. The deviation of BWDB data from those of BMD was within acceptabie limits _ between -22.6% and +36% with a mean and standard deviation of 1.5% and 0.13 respectively. It was found that data from BWDB and BMD can be used interchangeably for most practical purposes. A coherent correlation was developed between daily rainfall and rainfall of short duration covering the whole of Bangladesh. The equation provided reasonable estimation of short duration rainfall potential within an error range of :t20% in 91% of the time. A design chart was developed that can be used by the practicing engineers for estimation of short duration rainfall magnitude anywhere within Bangladesh even if there is no rain gauge station. The correlation equation between daily rainfall and rainfall of short durations developed in this study can be used to prepare Intensity-Duration-Frequency curves of short rainfall for any loc\ltion where data on daily rainfall are available. Such curves were prepared for Dhaka, Chiltagong, Rajshahi, Khulna and Barisal - the five divisional towns of Bangladesh _ where most of the development works are concentrated. The correlation can also be used to prepare isohyetal maps of short duration rainfall for a given duration and return period for the whole of Bangladesh. Such a map of 1O-yr 1-hr rainfall was prepared and presented. Design tables were prepared for selection of vertical and horizontal pipe sizes in roof drainage systems. Such tables were prepared separately for the towns of Dhaka, Chiltagong, Rajshahi, Khulna and Barisal.