A generalized regional design storm rainfall model for Botswana
Design of drainage and dam structures involves a full understanding of the duration, magnitude and volume of peak flood flows anticipated. For gauged catchments a number of established flood frequency models and rainfall-runoff models are used widely. However, most planned developments for bridge or dam or any other water development structures in a catchment are located at a site in a stream where there are no discharge records. In a catchment, records of short duration rainfall data might not also be available in order to transform them to design flood. In this study, a generalized parametric model for rainfall intensity-duration-frequency (IDF) relations is developed for the estimation of rainfall extremes in Botswana. The model is calibrated on available records on rainfall IDF relationships that are in use by the Botswana Roads Department. The proposed formulation allows one to estimate the expected rainfall depth for a duration ranging from 5 minutes to 2 hours and beyond and for low and medium return periods (up to 50 years) in any location within Botswana. A simplified experiment is developed in order to assess the reliability of the proposed formulation, whose performances are tested also in comparison with other regionalization approaches recently proposed by the scientific literature. The proposed model reproduced the observed IDF both qualitatively and quantitatively with high values of R2 ranging between 92.9 to 99.9 % during model calibration and 92.3 to 99.8% during model verification. Given the lack of recorded continuous and short duration rainfall information, the model can be used to assist hydrologists in the design of drainage and other water control structures for ungauged sites in Botswana.
Keywords: Design storms, IDF model, Calibration, verification, parameter sensitivity analysis, Botswana