Abstract—This study uses two of numerical modeling approaches to represent the urban catchment, and assess the flood risk of an urban catchment; hydrological surface routing approach and hydraulic 2D surface routing approach. Both approaches were combined with 1D hydraulic stormwater drainage network. XPSWMM was used as the modeling tool. This comparison leads to select most suitable approach to assess the possible flood inundations in the study area. Comparison of results of two approaches shows that the both approaches are suitable to represent urban catchment’s hydrological behavior, but the results of hydrological surface routing are more close to observation data. Model was calibrated to one sub catchment and used to generate flood vulnerability maps for whole catchment. These maps will be used as water sensitive urban design guide lines in the future developments of the area, while structural and non structural measures are to be implemented. Index Terms—Flooding, hydrological modeling, stormwater, urban environment I. INTRODUCTION Urban catchment hydrology needs more attention with the non riverine urban flood hazards. Such hazards are intensified by the rapid urbanization processes throughout the world, which are inevitable with the increasing population and the resource scarcity. These floods in urban environments have caused large societal and financial consequences with the majority of world population centralized to the urban cities [1]. Due to the higher costs of controlling flood drainage after land has been urbanized, reference [2] showed more efficient flood preventive control is possible by predicting the impacts from the potential urban developments, and planning control measures in early stages. Numerical modeling is a widely used method of assessing urban hydrology and generates results for possible stormwater runoff hydrographs with the land-use variation due to potential urbanization; thereby predicting the possible flood hazards. There are number of models have been implemented in the history for the stormwater runoff quantity Manuscript received September 30, 2011, revised October 3, 2011. This work was supported in part by the City of Gosnells in Western Australia. Authors would like to acknowledge their support. A. P. Basnayaka, R. Sarukkalige are with Department of Civil Engineering, Curtin University, GPO Box U1987, Perth, Australia. (e-mail: a.basnayak@postgrad.curtin.edu.au; r.sarukkalige@curtin.edu.au). D.R.I.B. Werellagama served as a senior lecturer of the Department of Civil Engineering University of Peradeniya, Sri Lanka (e-mail: indukaw@gmail.com) and presently works as an Environmental Consultant in Auckland, New Zealand. analysis (e.g. SWMM [3]; Mouse [4]; Hydroworks [5]) and some are further developed as urban stormwater models [6]. Reference [7] and [8] cite that the representation of the urban hydrology within numerical hydrological models is done by many approaches throughout the recent history. With the inadequacy of common runoff catchment models and approaches to analyze the urban catchments with its complexities, combination of 1D and 2D models, different methods of representation of the urban concepts such as dual drainage systems [9], GIS and raster based flood modeling approaches by using LIDAR data and aerial photography (e.g. [1]; [10]; [11]) have been studied in the recent history. Land use and land use management changes affect the hydrology, which will create the flood hazards [12]. Urban infrastructures such as roads, bridges, pavements and car parks, commercial and residential buildings, reduce the impervious area of the land and hinder infiltration, to increase the surface runoff. Also these paved surfaces together with urban drainage systems consisting of underground drains, manmade channels, manholes and gutters increase the rate of runoff through the drainage which demands comparatively large scaled stormwater management systems, and tends to flood the urban area, where no such facilities are provided [13]. Removal of vegetation cover for the urban land development reduces the evapo-transpiration, and leads stormwater to be retained in the surface for longer duration. All these changes of urban hydrology and their consequences of higher volume and increased peak flows within short durations cause the urban land to be inundated and affect the urban life and properties adversely. This study is carried out to assess such growing urban catchment’s drainage by modeling the urban hydrology within a numerical model. Two different approaches, ‘hydrological approach’ and ‘hydraulic approach’ have been considered to represent the urban hydrology. Basic difference of two approaches is the way they route the surface runoff. This comparison has been done based on the calibrated and verified modeling results for one sub catchment, where observation data is available. Then the most suitable approach to represent the urban characteristics of the area was selected and used to generate the critical duration flood inundation maps for the whole catchment. The study area is being developing rapidly and current land sub division plans tend to further increase flood vulnerable areas by land use changes and newly man made drainages changing the natural hydrology. The results of this study will be used to assess the current stormwater drainage and predict possible flood vulnerable areas and flood depths, and to applying the local government’s water sensitive urban design guide lines to future developments [14]. Numerical Modeling of Flood Vulnerability in Urban Catchments for Flood Forecasting Amila P. Basnayaka, Ranjan Sarukkalige, and Induka Werellagama International Journal of Environmental Science and Development, Vol. 2, No. 5, October 2011 383