This hydrological and hydraulic coupled models does not require specific knowledge, nor does it on any commercial modules. The HEC-RAS model is frequently used in a river flooding study (Adams III et al., 2018 Gao et al., 2018 Sleiman, 2018), but in the present study, we used and evaluated the HEC-RAS and HEC-Geo RAS extension of ArcGIS10.2 to simulate inundation extents in street and its surface drainage. HEC-Geo RAS (GIS-based) were also used for accurate optimization of the geometry characteristics for real visualization of flood areas. HEC-RAS can simulate both unsteady and steady state flow conditions, and it can be used to calculate inundation areas. HEC-RAS is a tow-dimensional (2D) model that developed by Hydrologic Engineering Center (HEC) (Staff, 2008) is one of the most popular model can coupled with SWMM model. Elliott & Trowsdale, 2007 Rossman, 2010) and coupled with other models, such as LISFLOOD-FP (Wu et al., 2017) and BreZo (Burns et al., 2015), to simulate urban inundation (Babaei et al., 2018 Chen et al., 2018a Elliott and Trowsdale, 2007).
The storm water management model (SWMM), is open-source model and powerful tool to urban drainage analysis, which was developed from 1969 to 1971 by the EPA (Rossman, 2004) has been used by researchers in various urban areas (A. Though, most of these models isn’t free, which limits their application. Various hydrological and hydraulic models have significant contribution to achieve this goal such as MIKE FLOOD (Löwe et al., 2017), ESTRYTUFLOW (Fewtrell et al., 2011), BREZO (Adeogun et al., 2015), SWM (Yu et al., 2015) and InfoWorks ICM (Russo et al., 2015). In recent years, some researchers have attempted to establish a more accurate relationship between rainfall – runoff and urban inundation (Bates et al., 2010 Lee, 2018 Li et al., 2009 Radice et al., 2017). Urban inundation hazard is associated with the physical characteristics of inundation such as extension of inundation, water level above street, volume of water flowing and its duration (Zhu et al., 2016). Moreover, urban inundation can completely hinder or limit the traffic systems function, and loss of communications and business opportunities is among its indirect consequences. Urban inundation due to any kind of inefficiency or defect of urban drainage systems causes considerable damage in buildings and other private and public infrastructure and is among destructive and the common natural hazards (Chen et al., 2018b Hammond et al., 2015 Price & Vojinovic, 2008 Tingsanchali, 2012). Complexities in the drainage infrastructures and urban areas have a natural influence on surface runoff that this runoff causes urban inundating (Chen et al., 2009 Jamali et al., 2018). This results in increased water pollution, hydrocarbons, heavy metals, pathogens and nutrients (Phillips et al., 2018 Pitt and Jr, 2001 Qin et al., 2016). Urbanized areas are accompanied by increase in impervious areas such as construction of drainage systems, roads, roofs, destruction of soils structure, and destruction of vegetation due to growth urban area (Hsu et al., 2000 Hung et al., 2018 Kamali et al., 2017 Shuster et al., 2005 Sillanpää & Koivusalo, 2015). In addition, the result of inundation hazard map indicates that 8.2% of the case study is determined as a high hazard zone. Simulated results can be used to develop urban inundation hazard forecasts. The results showed that the combined model (SWMM and HEC-RAS) was suitable to analyze urban inundation and determine inundation hazard zones on urban areas. Then, six factors such as distance from the main channel, slope, land use, drainage density, the main channel slope and elevation were selected to determine inundation hazard map using Analytic Hierarchy Process (AHP). The simulated runoff in the SWMM model is used as input to the HEC-RAS model and determines inundation hazard zones in 5, 25 and 50 return periods. In this study, the Storm Water Management Model (SWMM) is used to simulate the rainfall-runoff in the study area.
Therefore, to manage urban areas and prioritize regions to inundation elimination problems, the area most affected by inundation should be determined. Land use changes, increasing urbanization, unauthorized construction, inefficiency of sewage system and increased impervious surface in urban areas have significant impacts on inundation hazard. In recent years, rapid urbanization has led to change of rural areas into urban areas, and urban runoff will increase as the result of spread and growth of impervious areas. Extensive impervious area and the man-made streams are the characteristics of urban areas.