Prioritization of Low-Impact Development Methods for Quality Management of Urban Surface Runoff Using the TOPSIS and SAW Method in Gorgan City

In recent years, due to the rapid growth of the population, the increasing development of urbanization and the industrialization of societies, as a result, the hydrology of urban watersheds has been accompanied by many changes. These changes over time have caused floods and inundation of roads in urban areas, and on the other hand, flooding in these areas due to the density of population and facilities can cause severe damages and reduce the quality of runoff. New approaches have been proposed to manage runoff and prevent flooding and water pollution, and in this case, the development of low-impact LID is one of these approaches. The purpose of using this technique is to simulate the hydrological conditions of the urban area before development. For this purpose, in this research, the evaluation of quality parameters (BOD, TP, TN) was investigated using the best low-impact development management practices (LID/BMPs) in Gorgan city.

In the first stage, information related to intensity-duration-rainfall curves, urban and land use maps, and urban development maps were collected for Gorgan urban watershed. EPA-SWMM model was used to create a hydrological model in Gorgan city and investigate the effect of LID on runoff. According to the urban development of the region, six options of green roof, biological system, garden waterway, rain, infiltration trench and permeable covers, with a 6-hour rainstorm were considered with a return period of 2, 5 and 10 years for qualitative analysis using parameters Nitrate, phosphate and BOD.

The results of the calibration and validation of the model based on the Nash-Sutcliffe criterion more than 0.5 showed that the SWMM model has the required accuracy for simulating the quality of urban runoff. The results showed that, respectively, the green roof system, rain barrel and permeable covers have the best performance in improving the quality of surface runoff in the region by reducing surface runoff pollutants compared to the existing situation. The use of low-impact development methods in the region depends on various factors, including environmental, economic and efficiency factors. The results indicate that all the LID scenarios selected in the research were effective in improving the quality of surface runoff, but each of the scenarios showed different capabilities.

Conclusion and Suggestions:  
Even though all the LID scenarios examined in this research were effective in controlling surface runoff and improving its quality, but with the implementation of the green roof, rain barrel and permeable covers, the amounts of phosphate, nitrate and BOD compared to the existing situation and other scenarios has decreased further. As a result, the impact of green roof management measures, rain barrel and permeable cover has been greater. Finally, it is suggested that in order to improve the quantity and quality of surface runoff, these methods should be considered in the implementation priorities.