ارزیابی خدمت اکوسیستمی کاهش خطر سیلاب شهری با رویکرد دوره بازگشت بلندمدت (25 ساله)، مطالعه موردی: کلانشهر تبریز

Introduction

Urban districts, especially metropolitans, with increasing impervious surfaces, cause stormwater runoff from rain to flow on the ground surface instead of penetrating underground, thus causing urban hazards including urban floods and their negative and irreparable consequences. In the meantime, gray stormwater infrastructures, due to being old and expensive, lack the necessary capacities to comprehensively deal with urban flooding and its negative consequences, but green infrastructures, due to their low cost and provision of ecosystem regulation services, play an irreplaceable role in mitigating urban flood risk. Therefore, the purpose of this study is to evaluate the ecosystem service of mitigating urban flood risk in Tabriz metropolitan with a 25-year return period.



Methodology

The current research is descriptive-analytical in terms of method and has a developmental-applicative nature. The required information was collected using library, documentary, electronic sources, surveys and field observations. In this research, the urban flood risk mitigation model from the InVEST 3.12.0 software package has been used. This model is one of the models that mitigates the risk of urban flooding based on the vector map of the study area/watershed map of the study area, rainfall (in millimeters), land use/land cover map, soil hydrological group raster map, table Biophysical, vector map of built infrastructure, table of damage caused by urban flood estimates. Finally, this model estimates the result through the following raster and vector files:

1) It calculates the amount of runoff in the form of raster data.

2) It estimates the amount of absorption and retention of runoff (in millimeters) in the form of a raster file that shows the relative amount of rainfall (expressed as a percentage of rainfall).

3) It calculates the volume of runoff retention (in cubic meters) through a raster file.

4) It calculates the flood risk in the form of a descriptive table and the field of vector files and raster data that help to identify spatial changes in the specified local limits through the calculated values.

5) It estimates the damage caused by the flood through a monetary assessment.

6) It estimates the monetary value of the ecosystem service of urban flood risk reduction using the avoided damage cost method in the form of a descriptive table.



Results and discussion

The findings showed that in 1984, with a rainfall of 21.93 mm (0.000022 cubic meters) and a 15-minute rainfall and a 25-year return period, the amount of water absorbed and retained was 49 percent, the volume of water absorbed and retained was 6.77 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $17.19 million. With a 30-minute rainfall, the amount of water absorbed and retained was 62 percent, the volume of water absorbed and retained was 5.47 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $13.89 million. With a 45-minute rainfall, the amount of water absorbed and retained was 70 percent, the volume of water absorbed and retained was 4.72 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $11.99 million for the entire Tabriz metropolitan area. In 2002, with a rainfall of 14.07 mm (0.000014 cubic meters) and a 15-minute rainfall, the amount of water absorbed and retained was 51 percent, the volume of water absorbed and retained was 7 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $32.08 million. With a 30-minute rainfall, the amount of water absorbed and retained was 64 percent, the volume of water absorbed and retained was 5.62 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $25.78 million. With a 45-minute rainfall, the amount of water absorbed and retained was 72 percent, the volume of water absorbed and retained was 4.83 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was $22.16 million for the entire Tabriz metropolitan area. In 2022, with a rainfall of 10.78 mm (0.000011 cubic meters) and a 15-minute rainfall, the amount of water absorbed and retained was 57 percent, the volume of water absorbed and retained was 7.85 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was 56.08 million dollars. With a 30-minute rainfall, the amount of water absorbed and retained was 70 percent, the volume of water absorbed and retained was 6.15 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was 43.96 million dollars. With a 45-minute rainfall and a 25-year return period, the amount of water absorbed and retained was 77 percent, the volume of water absorbed and retained was 5.20 million cubic meters, and the ecosystem service benefit of mitigating urban flood risk was 37.14 million dollars for the entire Tabriz metropolitan area.



Conclusion

The results showed that with the increase in rainfall duration (from 15 to 30 and 45 minutes) during all three periods of 1984, 2002 and 2022, the amount of runoff absorption and retention and consequently the amount of ecosystem service benefit of reducing urban flood risk also decreased. The results also showed that districts with higher population density and consequently higher residential land use had the lowest potential for runoff absorption and retention. In Tabriz metropolitan, due to the small amount of land uses related to green infrastructure, including green space, agricultural lands, and pastures, which play a major role in mitigating urban flood risk, these land uses have not been able to play a significant role in mitigating urban flood risk. However, green infrastructure has played a greater role than water infrastructure in mitigating the risk of urban flooding in Tabriz. In the potential for runoff absorption and retention in Tabriz, land use/land cover has played a greater role than other factors, including the hydrological group of soil. Despite the low capacity of Tabriz's runoff absorption and retention, the role of ecosystem services in mitigating the risk of urban flooding in Tabriz remains significant. Without this role, Tabriz metropolitan would have to bear the high costs of potential flood risk.