Determination of the Most Important Parameters Affecting the Urban Runoff using SWMM Model (Case Study: Mashhad City, District 9)

Due to the rapid urbanization in Iran, different issues have emerged in its major cities such as street flooding, dispersal of dangerous chemicals and flood-related risks due to the lack of a proper drainage network and disorders in the flood channels; and, Mashhad is not exempt from these issues. The aim of this study is to determine the most important parameters affecting (area of impermeable surfaces has the highest impact, followed by the Manning roughness coefficient in permeable areas, the equivalent width, slope, height of storage in impermeable areas, and impermeable areas without surface storage) runoff generation in District 9 of Mashhad city and to evaluate the performance of the SWMM model. The SWMM model was used to simulate runoff and identify critical areas. In order to determine the duration of the design storm, time of concentration was measured and used as a surrogate. Sensitivity analysis showed that among the eight considered parameters, area of impermeable surfaces has the highest impact, followed by the Manning roughness coefficient in permeable areas, the equivalent width, slope, height of storage in impermeable areas, and impermeable areas without surface storage. The model calibration results showed a good agreement between the simulations of the four incidents with a NS higher than 0.5. The obtained NS confirms the SWMM model’s performance and accuracy. The RMSE was measured 0.6, 0.65, 0.58 and 0.91 respectively for the incidents dated 2015/1/10, 2015/2/15, 2015/2/21 and 2015/3/6, as well as 0.61 and 1.14 for the incidents dated 1394/1/7 and 1394/1/11, which suggest an acceptable model performance. Given the results, there is a good agreement between the simulated and actual runoff volumes which is an indication of the good performance of the SWMM model in predicting urban flood and the model could then be used in urban flood management projects and drainage network planning in Mashhad.