Multi-criteria prioritizing of Green Infrastructure Practices and their combinations to Control Runoff in Tehran Metropolitan

Urban development disturbs the natural infiltration of runoff by creating impermeable surfaces. With the increase of impermeable levels in the watersheds, the volume of runoff also increases and cities will be exposed to waterlogging and floods in the rainy season. Therefore, the level of groundwater resource is expected to have lowered gradually. Today’s main policy to face with this problem centers on using traditional flood systems like concrete channelization to convey runoff which increases the problem of groundwater recharge. So, it seems that Green Infrastructure Development (GID) could play a crucial role in the restoration of the disrupted hydrological cycle, lowering the runoff problem, and move toward towards urban sustainability.Materials and methods Study area Tehran is a metropolitan with an area of about 730 km2 and the population of more than 8.5 million people, located at the south part of Alborz Mountains. The city comprised of 22 districts. The middle districts and downtown areas are urbanized while the other districts have more open spaces and untouched lands. The slope of the most parts of the city is about 3-9 percent which is mainly from the north to the south. There are seven natural rivers streaming the city. These natural valley-rivers play important roles in natural ventilation and runoff conveying. Tehran has a diverse Land use. Apart from big green patches dispersed in the northern parts and the urban fringe areas, all patches of the city are manmade and impermeable. Tehran has faced rapid growth in the current decades. Rapid urbanization has worsened the natural hydrological cycle and put this megacity in jeopardy of water logging and flood. Flood risk is one of the second most important natural hazards in Tehran. The city is coated with impermeable surfaces like buildings, highways, roads, and parking lots. In rainy seasons rainfall rapidly turns into runoff flowing throughout the streets and finally leaves the urban watershed. Consequently, the level of groundwater supply is expected to have diminished.

The framework of this research has four steps. 1- Selection of relevant criteria and sub-criteria 2- Determining the weight of each of criteria and sub-criteria 2- reaching of decision matrix, 4- Prioritization of green infrastructure practices 3- Development of combination scenarios 4- Prioritization of combination scenarios. Each step is elaborated below.This step was done by reviewing scientific sources and interviewing experts and specialists. At this step, we tried to select criteria for multivariate decision models that are more repetitive in scientific texts and are in accordance with the geographical, climatic conditions and realities of the study area. Finally, the criteria in the process of introducing green infrastructure in Tehran with the aim of runoff control includes five main criteria namely: runoff quantity control, runoff quality control, cost, compatibility with city structure, and adaptation to the climate of the city.Expert panel and The Analytic Hierarchy Process (AHP) method were used to weight the selected criteria. In this regard, a questionnaire was first prepared in accordance with the standard structure of pairwise comparison, which is common in the hierarchical analysis method. Then, 15 faculty members, consulting engineers, and researchers in various fields related to the development of green infrastructure in urban areas were surveyed. Then the weight of criteria, sub-criteria, and inconsistency coefficients were calculated. Meanwhile, the responses of 6 participants had an inconsistency coefficient of more than one tenth and were excluded. Therefore, a total of 9 experts participated in the weighting process of the criteria. Expert Choice software was used in the process of calculating the weight of criteria and incompatibility coefficients.In order to compile the decision matrix and achieve the ability of the studied green infrastructure types, the review of scientific literature was performed. At this step, we referred to studies that examined the ability of green infrastructure in relation to each of the criteria.At this stage, the TOPSIS method was used to prioritize green infrastructure. TOPSIS is a multi-criteria decision-making method that is programmed based on the similarity of the solution to the ideal solution and the distance from the non-ideal solution. This technique can be used to rank, compare different options, select the best option and determine the distances between options. Based on the final decision matrix and the weight of the studied criteria, different forms of green infrastructure were prioritized. All calculations were performed to prioritize the options using R software and topsis package.Sensitivity analysis is performed to achieve assurance of results and to confirm that the weighting is non-biased. The method is that values are added or subtracted to each of the indicators and the model will be run again with the changes made and the results will be compared. In this study, PYSIS software was used to analyze the sensitivity of the results of this study. To ensure that the scores and weights were non-biased, sensitivity analysis was performed with 30% change in weights for 10,000 repetitions.To combine different forms of green infrastructure, five main functions of green infrastructure for runoff management were identified, including infiltration, storage, conveyance, irrigation, and rainwater collection. Based on these performances, different scenarios for combination of GI practices were identified and scored. The equation 1 was used for prioritizing the combination scenarios A= (S1 × D1)+(S2× D2)Where A is the final score of each compound, S1 is the final score of the green infrastructure expressed in the row, D1 is the green infrastructure score expressed in the row in the runoff management process, S2 is the green infrastructure score expressed in the column, D2 is the green infrastructure score in the column in the runoff management process.

Results revealed that permeable pavement with a score of 0.756 is the most suitable, while green atmosphere with 0.342 points was selected as the most unsuitable form of green infrastructure for the city of Tehran. Infiltration trenches, rain beds, rainwater harvesting system, detention ponds, bio-retention ponds, and green roofs were ranked second to seventh, respectively.The results of sensitivity analysis showed that with 30% change in coefficients and 10,000 times of model implementation, we did not see much change in the results of this research, and again permeable pavement has been introduced as the most appropriate option. The lack of change in the ranking of options in the sensitivity analysis indicated the low level of subjectivist bias.In the next step, in order to achieve appropriate scenarios, a combination of different forms of green infrastructure from Formula 1 was used and 112 possible combination scenarios were examined, scored and ranked. Finally, three preferred scenarios for each combination based on the hydrological performance were proposed.

In this study, a framework was proposed to use multi-objective prioritization of different forms of green infrastructure to control runoff in Tehran. The results showed that the permeable pavement is the most suitable form of green infrastructure for the city of Tehran, while the green swale is the most unsuitable. Infiltration trenches, rain gardens, rainwater collection reservoirs, ponds, biological ponds and green roofs were ranked second to seventh, respectively. Also, in order to achieve the appropriate composition patterns of green infrastructure, the functions of different types of green infrastructure were divided into categories including infiltration, storage, conveyance, irrigation, and rainwater collection, and the combined scenarios of this Infrastructures were examined. The results of this study proposed a suitable scenario of 30 green infrastructure combinations with different hydrological performance for development with different purposes.