Abstract:
Hydrological, ecological and biological cycles have been adversely affected due to the rapid
urbanization. In hydrological context increase of surface runoff is the major concern in many urban
cities across the globe. Most of the Sri Lanka's urban areas are also experiencing flash floods and
suburbs of Gall city have no exception. The main objectives of this study are to investigate flash
flood mechanism in Mode-ela basin, a major basin at Galle-suburbs and to suggest mitigation
techniques for extreme situations. Topographic and ground water profile data were collected and
mapped using GIS technique. Catchment discharge and drainage capacity of Mode-ela were
estimated using Rational and Manning's formulae respectively. It is evident that, because of
increase of imperviousness particularly within the lower part of the catchment, surface runoff is
exceeding the drainage capacity of the canal. Furthermore, the depth to the ground water table is
very low and natural infiltration is greatly restricted. Surface flow retardation, by any means, is
found to be the solution to mitigate the adverse effect of flash flood. The areal average runoff
coefficient was estimated at 0.31 based on land use classes. According to the investigations, it is
evident that the downstream part of the catchment is generating more surface runoff, which is
greater than the drainage capacity by a margin of 4.45 m3/s. 75 ha of 'homesteads' land (which is
situated above 30 m from MSL) has been identified as a prospective area to install artificial
storage/infiltration facilities. Few quantitative estimates have been proposed to support the above
stated retardation techniques. Regular cleaning and proper maintenance are appreciated to assist
the situation.
