Flood Frequency Analysis of the Rapti River Basin using Log Pearson Type-III and Gumbel Extreme Value-1 Methods

Flooding is a widespread, frequent and most disastrous natural hazard around the globe. Flood frequency analysis is a necessary step for floodplain zoning and planning of the dams, bridges, culverts, embankments and anti-erosion structures along or across a river. In the Rapti river basin, the flood frequency analysis has been performed using the annual maxima series of discharges recorded at Jalkundi and Birdghat (Gorakhpur) gauge and discharge (G/D) sites with the help of Log Pearson Type-III (LP-III) and Gumbel Extreme Value -1 (EV1) methods. The flood discharges at 25, 50, 100, 200, 500 and 1000 year return periods are lower at Birdghat than those at Jalkundi. Such a reverse scenario indicates storage of flood waters in the Rapti floodplain near and upstream of Birdghat G/D site. The Kolmogorov-Smirnov (K-S) and Anderson-Darling (A-D) tests have been applied on the annual maxima series of discharge data to identify the appropriate curve fitting models, namely LP-III and EV1. The goodness-of-fit analysis shows that the LP-III method is more appropriate for flood frequency analysis than the EV1 at above mentioned G/D sites.