By development of urbanization in metropolitan and reducing permeable surfaces, problems occur during precipitation including the increase of runoff volume and high level of pollution in canals and streams. One of the modern and effective ways for management of urban runoff quantity and quality is the application of Best Management Practices (BMPs) which are widely used for reduction of non-point source pollution and runoff peak flow. BMPs for runoff consists of structural and non-structural types. Structural practices include physical measures and executive projects such as ponds, vegetated swales, bioretention cell systems, permeable pavements and infiltration trenches which are developed with the aim of runoff impacts reduction. Non-structural practices are the measures done through law enforcement and educational programs. The main challenge for designers and decision makers is the selection of optimum combination of practices among various available alternatives. The best choice should be cost effective, accessible and executable in the specified region. The aim of this research was to determine the best choice of scenario for urban runoff pollution reduction which is cost-effective. In this regard, Strom Water Management Model (SWMM) was used to assess the defined scenarios and BMPs which are applied in a subbasin of Tehran metropolitan. Strom Water Management Model (SWMM) is a dynamic model used for simulation of precipitation- runoff i.e. water quantity and quality of runoff from a precipitation event and long-term simulation of runoff in developed urban areas. The practices used in this study include four systems of vegetated swales, bioretention cell systems, permeable pavements and infiltration trenches. Vegetated swales are wide shallow channels that are covered by plants and are designed to slow down the runoff and increase the infiltration and pollutant removal while directing the runoff. Permeable pavement consists of a permeable surface that provides infiltration and storage of runoff and a pavement for traffic. Infiltration trench is a drilling groove that is filled with crumbs and forms a subsurface pond. Bioretention cells systems consist of a biological surface layer, a soil sand layer and retention layer which applies the chemical, physical and biological features of plants, microbes and soil for pollutants removal from runoff.
Using SWMM model critical subbasins are identified and the effectiveness of BMPs are assessed. Afterward, scenarios are defined based on the acquaintance gained about the performance of subbasins and BMPs through modeling and sensitivity analysis. For water quality simulation in the model, total suspended solids (TSS) and chemical oxygen demand (COD) are selected. For the case study region, four types of landuses, including residential with low and high density, street and highway are defined. Return periods for the flood are considered 2, 10 and 100 years. Dynamic wave method was used in hydraulic routing and Horton method was used for modeling the infiltration. The soil type of the case study area was sandy loam and its parameters such as hydraulic conductivity and permeability were necessary for Horton method. To model the pollutant washing Event Mean Concentration method (EMC) was used. The basic value and the range of EMC for TSS and COD were chosen from EPA report and literatures. To determine the best combination of BMPs in subbasins, various scenarios are defined. In this regard, firstly six basic scenarios, including five structural and one non-structural BMPs were applied in sensitive subbasin to assess their effectiveness in runoff peak flow reduction and pollutant removal. Afterward, 15 scenarios were defined based on the previous six scenarios and their combinations. The scenarios were simulated in SWMM and the results were assessed and compared based on an index defined as the ratio of percent removal to the cost of scenario. This index was called optimality index which shows the cost effectiveness of each scenario.
The results showed that applying these BMPs can cause a considerable reduction in runoff flow and pollution load. The subbasins characteristics which have the greatest impacts on runoff water quantity and quality are three factors, including surface permeability, percent of runoff routing to the pervious surface and land use. The first two factors are used in designing and defining the non-structural BMP. Thus, increasing these two quantities are suggested as applying non- structural practices. The third factor (landuse) is used for determination of critical subbasins. Comparing the results of BMPs performance using the optimality index showed that the infiltration trenches have a better performance due to higher optimality index. Also in the scenario that the combination of all four structural BMPs were used, 70% of TSS removal with the cost of 16.4 billion Rials was obtained. In general, the BMPs that have a higher optimality index, like infiltration trenches, have a better performance, but the executive factors and the existence of suitable space are involved as limiting factors.