Determining process of occasional flooding from channel hydrological characteristics of Chel Basin, North Bengal (India)

Abstract

Occasional flooding has a distinct impact on fluvial dynamics of a channel. It brings new thresholds of adjustment for hydraulic conditions of a channel. The task of estimation of peak discharge during occasional floods is very complex, as the water recedes very quickly. The flood and rainfall design techniques (Gumbel’s frequency distribution) are the relevant solutions for peak discharge estimation at an un-gauged station of measurement. This paper aims to estimate the volume of peak discharge during the phase of occasional floods at ‘t’ recurrence intervals (5, 10, 20, 50 and 100 years) which are 148.87221, 158.58408, 167.9592, 179.9475, and 218.8758 cumecs. The separate yield of peak runoffs from small 3rd-order basins has been estimated by rational method on 2 years (162.70 cumecs) and 10 years (212.60 cumecs) probabilities of recurrence. While deriving probable peak flood discharge, various temporal hydrological relationships regarding rainfall–runoff and stage–discharge have been established by curve-fitting method in linear and power regression models. The temporal study (2015–2017) reveals some interesting hydrological results, including (a) 1284.21–1522.22 Ha m day–1 channel runoff causes discharge greater than 150 cumecs, (b) effect of basin lag influences the runoff curve and (c) the water stage above 2.5 m gauge height indicates occasional floods. The rainfall as system input indicates high spatio-temporal variation ranges, between 200 and 5500 mm, as maximum annual average. This approach fundamentally assembles the rainfall–runoff–discharge trio relation in estimating the design for peak occasional flood discharge for Chel Basin.