Using SWMM as a Tool for Floodplain Management in Ungauged Urban Watershed

Abstract

Mathematical modelling is a powerful tool for arriving at best water resources management. But it is necessary to possess gauged data for model calibration and verification. However most of the urban watersheds are not gauged and does not have high resolution terrain data when designing drainage infrastructure. There are many tools for urban stormwater modelling, but out of limited free tools, EPA SWMM5 has a good record of applicability in many locations elsewhere. The present work shows the applicability of SWMM5 for an urban land extent in the Matara Municipal Council area which is ungauged. This study used literature reported values and data collected during field visits to runoff modeling to obtain order of magnitude values with easy comparison of engineering alternatives for stormwater management. It is thought that possibility of using a systematic and extensive data collection program for parameter identification using simple field measurements and engineering judgment together with stakeholder information for historical record verification provides opportunity to initiate structured and solution oriented data collection programs and modelling. Kotuwegoda area with land extent of 0.475 km 2 and 22 subcatchments draining via 8 outlets was modeled for a single event. It was identified that the most sensitive parameter is channel roughness and that engineering options by means of changing channel roughness and incorporating detention storages, could reduce the peak flow by approximately 30% Keywords: Ungauged, Floodplain management, SWMM5, Urban storm water, Mathematical modelling 1. Introduction Water is a prime requirement for the existence of life however uncontrollable amounts of water can adversely affect the survival of living beings. In a tropical country like Sri Lanka, major water source is monsoonal rains. With significant rainfall in monsoon seasons, floods are very common. In the wet zone, frequency of flooding is in the range of two to three floods per annum. Floods not only lead to economic losses but also result in loss of human lives and this is considerably affected when the urban watersheds are flooded. Mathematical modelling of watersheds is a popular approach followed by engineers to identify solutions for flood problems [1]. Mathematical modelling of watersheds requires a suitable model for the representation of the phenomena that needs to be analysed, requires data for calibration and verification of the model outputs and sufficient resources both physical and financial for acquisition of the data and the model. In this context the best option for engineers would be to seek a model that is adequately detailed in describing the processes, sufficiently representing the extents, suitability, simple for data collection, spending the least amount of time, providing sufficiently accurate outputs which serve the purpose and affordable to available budgets. The EPA Storm Water Management Model (SWMM5) is a dynamic rainfall-runoff simulation model first developed in 1971 and used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas. The runoff component of SWMM5 operates on a collection of subcatchment areas that receive precipitation and generate runoff and pollutant loads. The routing portion of SWMM5 transports this runoff through a system of pipes, channels, storage/treatment devices, pumps, and regulators. SWMM tracks the quantity and quality of runoff generated within each