Introduction
Populous and developing cities in Iran, such as Tehran, face increasing flood risks. Climate change, increased intensity of sudden rainfall, urbanization and land use change, and lack of development of appropriate flood management infrastructure have led to an increase in the occurrence of flash floods. These floods can lead to widespread damage to urban infrastructure, disruption of transportation arteries, financial and human losses, environmental pollution and a reduction in the quality of life of citizens. Therefore, in order to effectively manage urban floods, it is essential to use appropriate methods and tools for flood hazard and risk analysis. In this regard, flood zoning is considered one of the important tools for analyzing flood risk and then planning to reduce it and increase urban resilience. This technique is more challenging in urban areas than in riverine areas or floodplains due to the density of buildings, streets, and other urban infrastructure. In this study, flood zoning was conducted for districts 21 and 22 of Tehran, which lack a suitable surface water collection network. As a novelty, we combined rainfall-runoff modeling with two-dimensional hydrologic modeling to analyze flood conditions in these districts.

Methods
This study used numerical methods to simulate rainfall-runoff processes and flood zoning for districts 21 and 22 of Tehran. The study was conducted in three steps: 1) Hydrological modeling for simulating precipitation-runoff processes using the hydrologic modeling system (HEC-HMS), 2) Two-dimensional hydraulic modeling to investigate flood flow propagation in urban areas using MIKE21 and FLO-2D models, 3) Analyzing the results and assessing the effects of flooding on urban infrastructure and transportation arteries.
In the first step, the HEC-HMS model was used to estimate the amount of runoff entering the urban area from the northern mountains of Tehran. Design rainfall with a 50-year return period was determined and trended for mountainous and urban areas. In the next step, the MIKE21 model was used to simulate surface flows resulting from runoff entering the city and the FLO-2D model was used to examine the distribution of excess precipitation over urban surfaces. The reason for using the FLO-2D model was that the MIKE21 model alone was not able to convert excess precipitation on urban surfaces into a flood zone. The equations used in hydraulic models include continuity and momentum equations. To investigate flood behavior and identify flood-prone areas, a triangular computational area with dimensions of 5 m2 for ​​rivers and canals and dimensions of 25 m2 in other areas was discretized and solved using numerical methods. The digital elevation model of the region was based on a combination of river and canal mapping data and GeoEye satellite images with a spatial resolution of 1m2.

Results
Modeling results showed that under 50-year rainfall conditions, 8% of Tehran’s 21st and 22nd districts will be affected by flooding. In these areas, the maximum flood depth was estimated at 11.8 meters in the Vardavard River, and the maximum flow speed was estimated at 4.5 meters per second at the beginning of Hashemzadeh Street (south of Kharrazi Highway). Based on the model results, the following routes will be particularly affected by flooding:
Major highways: Kharrazi, Fahmideh, Lashkari, Hamedani,and Fath highways will face severe runoff at several points; Tehran-Karaj Subway: In some areas, especially south of Arash Kamangir Park, there is a risk of flooding and disruption to the operation of the subway; Main boulevards: Hashemzadeh, Jangalban, Jozani and Tabiat boulevards, which are located around the Chitgar Lake, are among the main flood routes; Commercial and service centers: Farhang Fruit and Vegetable Market, Imam Hussein Mosque, Sharif University Town, and Vardavard Boulevard are among the areas that are within the flood zone.
In addition to the above, flooding due to a 50-year rainfall event will lead to waterlogging conditions in districts 21 and 22. This indicates that, due to the absence of a water collection network, the likelihood of flooding and the resulting damage increases dramatically.

Conclusion
To reduce the risk of flooding in districts 21 and 22 in Tehran, it is important to build a surface water collection system. The design of this system should be based on the topography, soil type and permeability of the area. By using high-precision data such as satellite images with high spatial resolution (e.g. world view satellite images), the accuracy of flood zones can be increased. Simulating flood conditions in case of implementing the surface water collection network and comparing it with the current situation can help to modify and optimize the proposed designs. Using the model prepared in the Tehran Flood forecasting and warning system helps citizens and urban institutions be aware of flood hazards and reduce their risk. Given the flood zones, urban development policies should be adjusted in a way that prevents the increase of construction in flood-prone areas.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research. 

Funding
This article was extracted from a research project funded by the Tehran Disaster Mitigation and Management Organization.

Authors' contributions
Visualization and software: Jafar Yazdi and Mohammad Shahsavandi; Conceptualization, methodology, validation, formal analysis, investigation, resources, data collection, writing the original draft, review, editing and supervision: Jafar Yazdi; Project administration: Ali Nasiri, Esmaeil Salimi and Morteza Delfan; Funding acquisition: Ali Nasiri, Esmaeil Salimi and Morteza Delfan.

Conflicts of interest
The authors declared no conflict of interest.



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