Estimating probable Peak Runoff for different Return Periods for each Catchment in
Greater Colombo River Basin
H. G. Nadeeka Thushari,
GIS Officer, IS & GIS Division, Urban Development Authority, Sethsiripaya,
Battaramulla, E-mail: nadee_tusha@hotmail.com / Natikee@uda.lk
Prof.Lasantha Manawadu,
Senior Lecturer, Department of Geograpphy, University of Colombo,
E-mail: lasan@geo.cmb.ac.lk
Abstract
The importance of accurate runoff quantification cannot be overstated. Estimates of peak rate of
runoff provide the basis for all decision making related to flood monitoring & managing, as well
as planning, designing, and construction of drainage facilities. Peak discharge is the greatest
amount of runoff coming out of the watershed of any one time. Intent of this paper is to provide a
reasonably dependable and consistent method of approximating the characteristics of urban
runoff in Sri Lanka. Therefore, this study aims to estimate the discharge values by analyzing
each catchment (flood plain) for different return periods in one of the urban catchments in Sri
Lanka, Greater Colombo River Basin. The flood discharge of different return period is derived
by a rational equation which is used by the hydrological engineers. The rational method is most
effective in urban areas with drainage areas of less than 200 acres and, is most commonly used
method of determining peak discharge from small drainage areas. Discharge information is
required in order to calculate the water surface profiles in any of the flood monitoring system.
The methodology used here is applied to the canal network in Greater Colombo River Basin.
Required spatial data layers are prepared in ArcGIS and other calculations were done using
Excel. Processed rainfall intensity values were used in this estimation. This paper will be
demonstrated that the Rational Method properly understood and applied can produce satisfactory
results for urban storm. Therefore, as a country govern by the often flood, the analysis results can
be applied to monitor flood and it’ll be helpful to make decision for flood disaster reduction.
KEY WORDS: Peak Runoff, Discharge, Return Period, Catchment, River Basin, Rational
Method, Rainfall Intercity.