In urban watersheds, the increase in impervious levels, which due to physical development, leads to an increase in the volume of runoff, peak discharge, and a decrease in the amount of permeability and an increase in urban flooding .and this is despite the fact that most of the urban watersheds do not have hydrometric stations and runoff statistics, hence determining the hydrological response of urban watersheds is essential considering the complexity of the drainage system (Yarahmadi et al. 2018).

Ghadri Dehkordi et al. (2019) using the SWMM model to determine the ability to collect and extract runoff in Babolsar city, showed that the runoff coefficient is directly related to the return period, and larger rainfalls have a higher runoff coefficient.. Yarahamdi et al. (1400) in evaluating the effectiveness of the SWMM model in order to investigate the drainage network flood nodes under the influence of climate change in the east of the six municipalities of Tehran, showed that the surface runoff collection system of the research area needs to be improved and modified, the slope of the passages and dimensions It has channels to transfer runoff caused by floods . .Zhang et al. (2020) designed and implemented a framework based on SWMM for urban runoff management. The results showed that the model can provide calculations in real-time, consistently, quickly and accurately, and can be effectively used in real-time management of urban runoff. In a research, Brown et al. (2021) investigated the impact of runoff management in small cities and showed that land use changes lead to hydrological changes in the city and will be intensify by climate change.



Methodology

Khorramabad city with an area of 50.28 km2 is located in Lorestan province.The purpose of this research is to simulate runoff and determine the most important parameters affecting urban flooding in parts of Khorramabad city using the SWMM model. .First, while determining the runoff values for 24-hour rainfall with different return periods, the best statistical distribution according to the intensity-duration-frequency diagrams, the flood potential of Khorramabad city was investigated.and then simulation of infiltration by SCS method and soil information layer and By using parameters of equivalent width, slope percentage,percentage of impenetrable areas Manning roughness coefficient of permeable zones and impenetrable,Pond storage depth for permeable and impervious areas and percentage of impervious areas without pond storage, SWMM model sensitivity analysis was performed. Therefore, in this research, the SWMM model was investigated and evaluated in order to simulate urban runoff and determine the most important parameters affecting the occurrence of floods in parts of Khorramabad city.



Results and Discussion

The results showed that there is a significant relationship between the model efficiency evaluation index related to the estimation of simulated and observed runoff height.and the evaluation of the efficiency of the model by the Kling-Gupta index provided an acceptable value and the model for simulating the resulting rainfall and runoff compared to the measured rainfall and runoff, as well as presenting the hydrological and hydraulic characteristics of urban watersheds, has an acceptable efficiency.The results of the model calibration showed that there is a significant relationship between the runoff depth produced by the model and calculated by the SCS method.and the SWMM model can simulate the runoff coefficient and flood potential well by modeling infiltration in different ways. The results of the model calibration showed that there is a significant relationship between the runoff depth produced by the model and calculated by the SCS method. and the SWMM model can simulate the runoff coefficient and flood potential well by modeling infiltration in differe.. The results showed that in Khorramabad urban watershed, which has a higher percentage of impermeability, the potential of runoff and flooding is also higher.and the percentage of impervious areas has the greatest effect in creating peak discharges.The results show that the most sensitive parameters, in order of priority, include the percentage of impervious areas, the depth of pond storage in impervious areas,equivalent width,Manning's roughness coefficient in impervious areas subbasin slope, the depth of pond storage in permeable areas, Manning's roughness coefficient in permeable area and the percentage of impervious areas without pond storage.



Conclusion

Today, many hydrological and hydraulic simulation models have been designed in connection with the simulation of flow and runoff patterns In the field of urban runoff, one of the best is the SWMM model, which is specially designed. Since the city of Khorramabad was built along the Khorram River channel, it is always subject to urban flooding; Therefore, in this research, the SWMM model was investigated and evaluated in order to simulate urban runoff and determine the most important parameters affecting the occurrence of floods in parts of Khorramabad city. on the results, it can be said that the SWMM model can estimate the runoff coefficient and flood potential well by modeling infiltration in different ways.In this research, considering the lack of access to infiltration data, the SCS method was used to estimate infiltration and the results showed that impervious surfaces including residential areas, roofs,the roads of asphalt etc. have the largest share in the flood potential of the study area.Theresults showed that some channels and nodes of Khorramabad urban network do not behave properly and cause risky behavior in sensitive and busy and dense situations of the city which is suggested to be improved in order to solve the hydraulic defects ,In addition, it is possible to reduce the impervious area to a minimum by measures such as reducing private parking lots and building public and multi-story parking lots, preventing the construction of very wide and unprincipled sidewalks and roads, especially in residential areas.