Intensity, duration and frequency of rainstorms in Lokoja

Abstract

This study analyzed the Intensity, duration and frequency (IDF) of rainstorms in Lokoja. Daily rainfall data were collected from Nigerian Meteorological Agency (NiMET). These data were for fifty-five years (1955-2010).The Weibull’s equation was used to plot the return periods of the IDF. The data were fitted to gamma distribution, while the IDF curves were generated using the Gumbel’s distribution. Furthermore, a model expressing the relationship among intensity, duration and frequency of rainstorms for Lokoja was developed. Results showed that the highest annual rainfall in Lokoja in the last 35 years occurred in 1999 with a depth of 1767.1mm with a return period of 36 years while the lowest rainfall occurred in 1982 with an amount of 804.5mm and a return period of one year. The IDF model showed various rainstorm occurrences in Lokoja with their intensities. The highest rainfall intensities are those of relatively low duration (0.2 to 0.7hrs), the medium intensities for 1 to 3hrs durations and the lowest intensities for 6 to 24hrs durations). The highest intensity was that of 0.2 hour (226mm) with a return period of 100 years. The lowest intensity for 0.2 hour was 66.1mm with a return period of 1 year. The IDF curves also confirmed that rainfall intensity in Lokoja decreases with increasing time interval. This study therefore, will be useful in the design of hydrologic facilities like dams and drainage channels.

Keywords: Intensity, Duration, Frequency, Rainstorms and Return Period