Identification of factors affecting urban flood in Ilam City Watershed

Urban flood is the volume of water that is out of the capacity of the drainage network of the city and results in many problems and damages. Most researches around the world shows an increase in urban floods, especially metropolitan areas. The concentration of human population in urban areas with urbanization and changing the face of the earth's nature has caused an increase in impervious surfaces and change in hydrological cycle. This study was done to simulate runoff of Ilam City Catchment using of the SWMM hydrologic-hydraulic model. The model validation results showed that the simulated peak discharge and flow volume are in good correspondence with the observed value (BIAS= 3.25, RE= 0.065). According to the results obtained from the analysis of the sensitivity of the important parameters of the model, the factors affecting the flood occurrence in the study area include the inadequate size of urban channels, increase of impervious areas, coefficient of roughness of permeable areas, slope, area, equivalent width and roughness coefficient of impermeable areas. The results of the study show that sub-area 2 has the highest risk of flood due to land use changes, non-standard construction and inappropriate drainage network, which results in an increase in impervious surfaces, and in the absence of attention, the city of Ilam face severe flood risks due to the drainage network infrastructure and the lack of observance of the urbanization developments criteria. Also, the SWMM model has the accuracy required for urban flood simulation and it can be used for urban runoff management plans and designing superficial water collection and disposal networks.