Morphometric Analysis of Kala Oya River Basin, Sri Lanka Using Geographical Information Systems

N.S. Withanage, N.D.K.Dayawansa, Ranjith Premalal De Silva , R.M.C.W.M. Rathnayake

Morphological characteristics of river basins are assessed widely by means of morphometric analysis which is a  mathematical quantification of different aspects of river basins. In the present study, Kala Oya River Basin of Sri Lanka was morphometrically analyzed to assess its flood characteristics based on the morphological characteristics with the help of Geographical Information Systems and the methods available in scientific literature. Further, the soil and topographical conditions like land use and terrain characteristics of the basin were also identified. Results of the study revealed that the Kala Oya River Basin has a 6th order river network according to Strahler’s classification with a dendritic drainage pattern and coarse drainage texture. The obtained low values of bifurcation ratio, circularity ratio, elongation ratio and form factor values revealed that a lower and extended peak flow for a longer duration would result from the basin. The drainage density, stream frequency and drainage intensity values indicated that the basin has highly permeable soil, good vegetation cover and lower relief where a low rate of runoff could be resulted reducing the risk for both soil erosion and flooding. The analyzed relief aspects revealed that the basin is less susceptible for severe soil erosion. Confirming the results of the morphometric analysis, the basin area was with 85% of Reddish Brown Earth and Low Humic Gley Soils soil types which are permeable soils and 66% of vegetation cover and a lower relief as more than 95% of the basin was below 200 m and the highest point was also below 1000 m. However, the findings of rainfall analysis for the basin revealed that there were increasing patterns in receiving heavy rainfalls greater than 50 mm/day and maximum rainfall amounts during a day over last 30 years. Similarly, flood occurrences also showed increasing patterns with time. Thus, sustainable management plans should be made in advance to cope with the potential floods that can occur due to high rainfall events and/or due to the degradation of protective vegetation cover, although the studied Kala Oya River Basin is morphometrically capable enough to reduce the flood risk.