Development of Intensity-Duration-Depth Relationships for Western Ghats in Karnataka, South India: A Pragmatic Approach for Utilizing Short Records

Abstract

Popular models currently available for estimation of design intensities in the wet mountainous Western Ghat areas of South India are not those based on analysis of local rainfall records. Possibilities of development of i-d-f curves applicable in the region are also remote due to the constraint of short length of records. The present work, taken up in this background, presents the novel idea of substituting daily rainfall depth for frequency in the popular i-d-f curves and furnishes a reliable model for the Western Ghat regions of the state of Karnataka. The procedure advocated for the development of the model relies on the fact that in areas characterized by nearly continuous falls and long spells of rain, as here, the intensities are well associated with total rainfall and hence the i-d relationships are functions of daily depth. This work proposes to use all the available subhourly data from different stations across supposedly homogenous regions and to use envelop curves to reduce sampling variations. i-d curves for individual days falling within specified ranges of daily depth are grouped together, and each group is represented by an envelop curve. Power functions, forced through constant exponents, are developed for the different envelop curves. The association between the varying parameter of such curves and the daily depth (D) is modeled by a power function, resulting in i-d-D relationships of the form i=a(Db/dc). The model applicable for the study area has been derived using intensity—duration data of 4–5 years from 33 stations spread across the study area, divided into four regions based on normal annual rainfall. The model has been validated and found reliable. The model results are discussed and used to test the validity of the method presently being followed in the region, the one advocated by the Central Water Commission (CWC). Pitfalls in the CWC method have been brought out. It is concluded that the model presented forms a reliable tool for estimating design intensities until a more authentic one is developed using further extensive data.