Identifying Effective Components on Applying Nature-based Runoff Management Approaches in Local Streets of Tehran by Cross Impact Analysis (CIA)

The current urban runoff management approach in Tehran is a structural approach which its main purpose is to collect and transfer runoff out of the city. However, nature-based approaches emphasize the maximum absorption as near as the source of runoff and apply interdisciplinary perspectives to achieve it. This research pursues two main goals in order to provide the basis for the application of nature-based approaches in the local streets of city of Tehran, Iran: 1) identifying of disciplines and main variables of the nature-based urban drainage approaches, and 2) identifying the role each variable plays in implementing these approaches in Tehran in the current situation. First, a theoretical model of the disciplines and variables has been develops based on literature review. Second, the role of each variable from the point of view of experts was investigated by using Cross Impact Analysis (CIA) method with the tool of MicMac software. The results show that in the current situation, ‘runoff speed and volume’ as an influential variable has the most important role on implementation of these new approaches in urban runoff management in Tehran local streets. And variables of ‘street profile’, ‘land use’, ‘activities’, and ‘planting’ as well as ‘economic feasibility’ are linkage variables. Other variables are either dependent or do not have high priority in the current situation. The results of the research place special emphasis on interdisciplinary teamwork between the specialties of water sciences and urban landscape as well as economics to provide the basis for the application of these approaches.IntroductionFlash Floods are intensified in Tehran and the change of natural permeable surfaces to impervious constructed areas is one of the main reasons. So, nature-base approaches investigated in this research. There are some different terms that represent nature-based approaches: Alternative Technique (AT), Sustainable Drainage Systems (SuDS), Sustainable Urban Drainage Systems (SUDS), Low Impact Development (LID), Best Management Practices (BMPs), Water Sensitive Urban Design (WSUD), Green Infrastructure (GI), Sponge City. Investigating show that although each of them pursue different objectives but all of them are the same in these two aspects: a) applying interdisciplinary viewpoint and b) trying to runoff usage in place it produced. Regarding to these, the research is focused to maximize runoff penetration in local streets as a place that its flow start from there. Two main goals of this paper are: 1) identifying of disciplines and main variables of the nature-based urban drainage approaches, and 2) identifying the role each variable plays in implementing these approaches in Tehran in the current situation. Figure 1 shows the research structure diagram.Figure 1Research Structure Diagram, Source: (Authors)Materials and MethodsIdentifying of disciplines and main variables of the nature-based urban drainage approaches by Expert Panel MethodDisciplines and main variables of the nature-based urban runoff management approaches were extracted from literature review of new approaches and guidelines and finalized by Expert Panel Method as follow:• Urban landscape: Street Profile, Adjacent Land Use, Activity, Planting;• Facility: Underground Utilities, Overhead Utilities;• Soil: Surface Permeability, Ground Sinkhole;• Water: Runoff Pathway, Runoff Speed and Volume;• Environment : Runoff Purification, Garbage; and• Socio-economy: Social Acceptance, Social Participation, Social Behavior, Security, Economic Feasibility.Then a theoretical model of the disciplines and variables was developed. (Figure 2)Figure 2Proposed theoretical model of the disciplines and variables of implementing nature-based urban drainage approachesCross Impact Analysis with MicMac To identifying of disciplines and main variables of the nature-based urban drainage approaches, and the role each variable plays in implementing these approaches in Tehran in the current situation, a questionnaire was designed based on the proposed theoretical model. To test the content validity of the questionnaire, Content Validity Ratio (CVR) was used. The formula of content validity ratio is CVR=(Ne - N/2)/(N/2), in which the Ne is the number of panelists indicating "essential" and N is the total number of panelists. The numeric value of content validity ratio is determined by Lawshe Table. In this research CVR=0.75 and the panel size was 8. Therefore, the questionnaire had face validity. 16 urban runoff drainage specialist from different disciplines answered the questionnaire. Then collected data analyzed by Cross Impact Analysis (CIA) method using MicMac software tool. Hence, the research approach of this paper is mixed qualitative and quantitative. Discussion of ResultsUsing MicMac software, the runoff management variables have been categorized into four quarters based on their influential and dependence power, as shown in Figure 3.Figure 3Cross-impact matrix multiplication applied to classification (MicMac) analysis of variables of implementation of landscape-based urban runoff management approach in local streets of TehranAnalyzing figure 3 indicates: • Influential variable (top left quarter): runoff speed and volume;• Linkage variables (top right quarter): street profile, land use, activities, planting, and economic feasibility;• Dependence variables (bottom right): Social Behavior, Security, and Surface Permeability;• Autonomous variables (bottom left): Ground Sinkhole, Underground Utilities, Overhead Utilities, Runoff Pathway, and Runoff Purification; and• Regulative variables (center): Garbage, Social Acceptance, and Social Participation.ConclusionThe cross impact analizing results which taken by use of experts viewpoints and technically derived by MicMac, shown that ‘runoff speed and volume’ as influential variable with other five variables ‘street profile’, ‘land use’, ‘activities’, ‘planting’, and ‘economic feasibility’ as linkage variables that play important role in functionality and stability of nature-based methods, are the main variables which should be attended for runoff management in Tehran’s local streets. The results asserted the other variables are not determinant and priority and don’t play and important role in present situation. Overlapping the result of cross impact analyze and presented theoretical model in this research shows that water science discipline, because of runoff speed and volume defined as influential variable, and urban landscape discipline, according to determinant role of its variables on stability and function of nature-based approach, are two core disciplines which play an important role in implementation of nature based approach in local streets urban drainage. Also the results emphasis on use of economical knowledge for feasibility study of applying the mentioned methods. However should be considered that the result of research is just embraced the local streets of Tehran and could not be generalized for other type of streets and roads and for other cities.