Application of SWMM for Urban Stormwater Management: A Case Study with Modelling Lai S.H., Ghani A. Ab., Zakaria N.A., Leow C.S., Chang C.K., Yusof M.F. River Engineering and Urban Drainage Research Centre (REDAC), University Sains Malaysia, Engineering Campus, 14300, Nibong Tebal, Penang, Malaysia Email: redaclai@eng.usm.my , Fax: 6-04-5941036 ABSTRACT This paper presents the design of stormwater facilities for a frequently flooded urban catchment according to the new Stormwater Management Manual produced by the Department of Irrigation and Drainage Malaysia (DID). Various measures are included in the design for quantity and quality control, including engineered waterway, outlet control, constructed wetland, pumping station, and etc. The proposed improvement works and design is simulated and analyzed using InfoWorks CS software, also presented is the simulated results for the existing drainage system in the catchment for comparison. The results obtained from calibrated model show that under the existing conditions, the whole catchment area is subjected to frequently flood of up to 2 m depth at downstream. Surprisingly, with some channel and culvert upgrading according to the criteria in SWMM, the results obtained show that the proposed design is able to handle the worse flooding scenarios (100-year ARI rain coincide with spring high tide), and therefore solving the flooding problems in the study area. The water quality in the study area is expected to be further improved with the construction of the designed wetland. 1 INTRODUCTION Urbanization and the increase in impervious surfaces typically associated with urban development have consistently been shown to result in degraded aquatic ecosystems. These effects are a function of increased stormwater runoff volumes, surface erosion and pollutants loading across a watershed due to the efficient routing of stormwater off impervious surfaces and into a storm sewer system that ultimately discharges into a receiving water body. These elevated volumes impact stream ecosystems through amplified flow rates which increase bed and bank erosion, rapid and efficient pollutant transport and an increase in nuisance flooding in urban watersheds. In Malaysia, frequent occurrences of flash flood in urban areas have resulted in an average loss of RM 100 million a year (DID, 2000). Department of irrigation and Drainage Malaysia (DID) estimates that RM 10 billion (reported in News Straits Times, June 22, 2000) is required to upgrade the conventional drainage system made up of concrete channels and channelized rivers to overcome the pollution and flash flood enigma. With the present conventional drainage system, new development means new and bigger monsoon concrete drains are required to be built at the downstream areas. Similarly the receiving river at the downstream end will need new flood mitigation project involving straightening, widening and deepening destroying the natural conditions including flora and fauna. Therefore there is a need to seek for a holistic and sustainable solution, not only to mitigate existing flood problems but also to prevent the occurrence of such problems in new area to be developed. In order to overcome the above problem, The Department of Irrigation and Drainage Malaysia (DID) produced a new urban drainage manual in 2000 (DID, 2000), known as Stormwater Management Manual or SWMM to account for the quantity and quality of stormwater runoff, and the amenity value of surface water in the urban environment. The new manual emphasized on the use of Best Management Practices (BMPs) such as “control at source” approach to achieve “Zero