Rainfall intensity for a particular frequency and duration is one of the most important parameter for the hydrologic design of dam, reservoirs, storm sewers, culverts and many other hydraulic structures. This can be obtained from Intensity-Duration-Frequency (IDF) relationship, which is determined by frequency analysis of rainfall data. The goal of this research is to develop rainfall intensity-duration frequency relationship for the data scarce urbanized Guma catchment in Freetown the capital city of Sierra Leone using the Gumbel and Pearson Type III distributions.
To achieve this goal, daily rainfall data for the period of 1991 - 2018 for the Guma catchment rainfall station was obtained from the Guma Valley Water Company and were converted to shorter duration (hourly) using the Indian Meteorological Department (IMD) method. Twenty-eight (28) maximum daily rainfall events were converted to hourly rainfall events.
Frequency analysis was conducted to develop the rainfall intensity-duration-frequency relationships using the Gumbel and Pearson Type III distributions. The Kolmogorov-Smirnov (K-S) goodness of fit test was utilized to determine which of the distributions have a better fit at 5% and 10% significant levels. The frequency analysis results shows that the Gumbel distribution gives higher intensity for all return periods and durations than the Pearson type III distribution. The result of the K-S goodness of fit test shows that all of the data fit the Gumbel distribution for different return periods (2, 5, 10, 25, 50 and 100 years) at the level of significance of , which yield , and , which yield , while the data do not give good fit using the Pearson Type III distribution at both levels of significance for the different return periods.
Keywords: Rainfall, Intensity-Duration-Frequency Relationships, Probability Distributions, Goodness of Fit Test.