Urban Flood Estimation and Evaluation of the Performance of an Urban Drainage System in a Semi-Arid Urban Area Using SWMM Ali Moafi Rabori 1 , Reza Ghazavi 2* ABSTRACT: Estimation of urban runoff peak and volume is a fundamental step in determining the transferring capacity of urban drainage systems. The main aim of this study was to present an application of the Storm Water Management Model (SWMM) in order to estimate urban flooding of a semi-arid area (Zanjan city in the northwest of Iran). The performance of an urban drainage system in the study area was also investigated. According to the results, SWMM is an effective tool for urban flood estimation in a semi-arid area. In this study, urban peak flow was simulated via a calibrated model with acceptable accuracy. Based on the results of the model simulation, the capacity of the main canals in the study area is sufficient for peak runoff transferring for a design storm with 50 year return periods, without retrofitting. Whereas, based on local observa- tion and model results, localized and surface flooding can be observed in some urban areas. Water Environ. Res., 90, 2075 (2018). KEYWORDS: flood, design storm, storm water, drainage system, urban runoff management, SWMM. doi:10.2175/106143017X15131012188213 Introduction It is expected that some 70% of the world’s population will live in urban areas in 2050 (UN, 2008). Urbanization has serious effects on the quantity and quality of urban runoff. In urban areas, natural streams have changed in the artificial drainage network (Antrop, 2004; Haase, 2009) and the natural and rural area has changed to impervious surfaces. Consequently, watershed infiltration, groundwater recharge, and evapotrans- piration should decrease (Klocking and Haberlandt, 2002; Rose and Peters, 2001), whereas, the volume and peak flow of stream water should increase. The increased volume and peak flow of storm water discharges may cause problems, such as flooding and erosion (Dietz and Clausen, 2008; Schoonover et al., 2006; Wang et al., 2005). Some researchers suggest that urban development will reduce groundwater recharge via reduction in permeable areas (Brett et al., 2005; Collin and Melloul, 2003; Schoonover et al., 2006), whereas, some others indicate that urbanization will increase groundwater recharge because of urban water infrastructure leakage and reduction in evapo- transpiration (Howard, 2002; Lerner, 2002). In urban areas there is also a variety of pollutant sources. Thus, urban storm water may in some cases be a significant source of water pollution to receiving waters (Huong and Pathirana, 2013; Pyke et al., 2011; White and Greer, 2006). Proper management of water resources in urban areas requires consistent prediction of water quality and quantity. Information on precipitation, evaporation, infiltration, runoff, and water quality is needed for such prediction. Rainfall regime in arid and semi-arid areas is characterized by low, irregular, and unpredictable precipitation (Fengxiang, 2007; Ghazavi et al., 2010). Consequently, storm water in an urban area has a random nature and short duration. In such areas, hydrological data collection is difficult because of the high spatial variability of catchment factors, long dry period between two continual rain storms, and the high spatial and temporal variations of rainfall and storm water runoff (Osborn and Hickok, 1968; Osborn et al., 1979). Estimation of peak and volume of runoff is of fundamental importance in order to determine the flood magnitude, and thus design urban runoff management structures in terms of 1 PhD student, Department of Watershed Management, Faculty of Natural Resources, University of Kashan, Iran. 2 Associate Professor, Department of Watershed Management, Faculty of Natural Resources, University of Kashan, Iran. * Associate Professor, Department of Watershed Management, Faculty of Natural Resources, University of Kashan, Iran; e-mail: ghazavi@kashanu.ac.ir. WATER ENVIRONMENT RESEARCH  December 2018 2075