Watershed similarity conditions for peakflow transposition - a study of river basins in the wet zone of Sri Lanka

Abstract

The majority of construction works in developing countries like Sri Lanka require engineers to work with ungauged watersheds. Such works often require extrapolation of peakflow from a gauged watershed using the different characteristics of each watershed. Though some attempts of peakflow transposition for different parts of the world are available, this has to be addressed for Sri Lankan watersheds. The annual peakflow of four main watersheds in the Wet Zone were studied to identify the feasibility of transposition. This study established a peak flow similarity index and three main criteria. It was identified that annual peakflows of similar watersheds can be transposed satisfactorily with the relationship that peakflow from a watershed is propotionate to the 0.8th power of its area. Peakflow transposition between similar watersheds within the same river basin indicated an average error of 35% whereas similar watersheds between river basins in the Wet Zone indicated an average error of 55%.