Stormwater management modelling for ungauged watershed in Matara municipality

Abstract

Most urban areas are having flood problems due to human activities. It is not easy to model urban watersheds since very often they are ungauged. This paper describes the modelling of an ungauged watershed in the Matara Municipal Council Area of Southern Sri Lanka using the free SWMM5 model of the EPA. Several field visits were conducted to capture the model parameters and flood scenario for model development and calibration. The watershed delineation was done using 1:10,000 topographic maps and canal slopes etc were determined using the elevation data of these maps. This watershed with an area of 1.12 km2 was modelled and calibration was done for the order of magnitude of the outputs. The model shows reasonable results with the actual conditions in the absence of gauge data This work demonstrates that urban stormwater model SWMM5 can be used in Matara Municipal Council area to a satisfactory level with adequate field data collection. However, for accurate results, it is necessary to suitably gauge the stream flows and carry out field and engineering surveys for the measurement of physical parameters. The model demonstrated the capability to identify engineering and stormwater management options. Model outputs revealed that cleaning of canals was one option for reducing the flooding at nodes. For the considered storm, a reduction of canal roughness from 0.2 to 0.015 reduced the flood peak by 56% and no flood is happening after introducing a rectangular detention unit which is having 200m2 floor area.