Document Type : Research Article
Authors
1
PhD of Geography and Urban Planning, Department of Urban and Regional Planning, Faculty of Planning and Environmental Sciences, University of Tabriz, Tabriz, Iran
2
Professor of Geography and Urban Planning, Department of Urban and Regional Planning, Faculty of Planning and Environmental Sciences, University of Tabriz, Tabriz, Iran
Abstract
Urban flooding can lead to inundation and destruction of urban infrastructure, severe human and financial losses, and widespread socio-economic damages. Blue and green infrastructure plays a critical role in mitigating urban flood risk.
Therefore, the objective of this study was to evaluate the ecosystem service of urban flood risk mitigation in the Tabriz metropolis under a 2-year return period. In this research, Landsat satellite imagery, land use/land cover data, meteorological data, a biophysical table, GIS techniques, and the InVEST software were employed to assess the ecosystem service of urban flood risk reduction in Tabriz. The results showed that in 1984, under a 2-year return period and a 15-minute rainfall event, the volume of retained and infiltrated water and the economic benefit of the urban flood risk mitigation ecosystem service were 4.28 million m³ and US$10.87 million, respectively; for a 30-minute rainfall event, 3.17 million m³ and US$8.04 million; and for a 45-minute rainfall event, 2.58 million m³ and US$ 6.56 million. In 2002, the corresponding values were 4.37 million m³ and US$ 2.01 million for 15 minutes, 3.21 million m³ and US$14.71 million for 30 minutes, and 2.60 million m³ and US$11.94 million for 45 minutes. In 2022, the retained water volumes and economic benefits were 4.65 million m³ and US$33.25 million for 15 minutes, 6.15 million m³ and US$43.96 million for 30 minutes, and 5.20 million m³ and US$37.14 million for 45 minutes. The findings indicated that, due to the negligible extent of green infrastructure in the Tabriz metropolis, these land uses have played only a limited role in reducing urban flood risk. In the runoff retention and infiltration potential of Tabriz, land use/land cover exerted a greater influence than soil hydrologic groups. Overall, the results demonstrated that the ecosystem service of urban flood risk mitigation in Tabriz has been relatively significant.
Introduction
With global climate change and rapid urbanization, urban flooding has become increasingly frequent and severe, causing significant damage worldwide (Tellman et al., 2021). Flood-related disasters are among the most common and destructive natural hazards, leading to a growing proportion of populations at risk of flooding (Rentschler et al., 2023; Tellman et al., 2021). In densely populated urban areas, floods result in substantial human and economic losses, including infrastructure damage, reduced agricultural productivity, disrupted communication systems, and risks to human health and safety (Alderman et al., 2012; Peng & Zhang, 2022; Pham et al., 2021). Mitigating urban flooding is a critical component of urban planning and disaster risk reduction strategies. Traditional mitigation measures, often referred to as gray infrastructure, include urban drainage systems and flood pumping stations, which have been widely implemented to manage runoff and reduce flood-related impacts (Prudencio & Null, 2018; Qi et al., 2021; Zischg et al., 2017). However, these measures often require substantial financial investment and have limited effectiveness (Sohn et al., 2019). In contrast, green infrastructure solutions—such as green roofs, rain gardens, permeable pavements, urban green spaces, and urban forests—play a vital role in regulating stormwater runoff through ecosystem services (Ahiablame & Shakya, 2016; Qin et al., 2013).
These measures are environmentally sustainable, require minimal energy input, and are often cost-effective. In the metropolitan area of Tabriz, located in the Urmia Lake Basin, the city's topographic position on a plain and the presence of two major rivers, the Ajichay and Quri, have historically contributed to devastating floods. Rapid urban expansion, particularly following the Islamic Revolution, coupled with widespread migration and unplanned development, has exacerbated flood risks in Tabriz (Yazdani et al., 2018). Therefore, this study aims to evaluate the ecosystem services provided by green infrastructure in mitigating urban flood risk in the Tabriz metropolitan area.
Material and Methods
This study employs a descriptive-analytical methodology with a developmental-applicative focus. Data were collected from library resources, documentary records, electronic sources, surveys, and field observations. The research utilizes the urban flood risk mitigation model from the InVEST 3.12.0 software package. This model assesses urban flood risk mitigation based on inputs including a vector map of the study area (watershed), rainfall data (in millimeters), a land use/land cover map, a soil hydrological group raster map, a biophysical table, a vector map of built infrastructure, and a table estimating damages from urban flooding. The model generates the following outputs:
Runoff volume, presented as raster data.
Runoff absorption and retention (in millimeters), expressed as a percentage of rainfall in a raster file.
Runoff retention volume (in m3), depicted in a raster file.
Flood risk, summarized in a descriptive table and represented through vector and raster data, enabling identification of spatial variations within defined local boundaries.
Monetary assessment of flood-related damages.
Monetary valuation of the ecosystem service of urban flood risk mitigation, calculated using the avoided damage cost method and presented in a descriptive table.
Results and Discussion
Across the three study periods (1984, 2002, and 2022), the soil in most of the ten districts of the Tabriz metropolitan area, except for parts of Districts 4, 6, and 7 and minor portions of other districts, was predominantly loam and clay. These soil types exhibit moderate to high runoff potential, meaning that a significant portion of rainfall runoff flows over the surface rather than infiltrating the soil. Consequently, most districts in Tabriz are at elevated risk of urban flooding due to their high runoff potential.
The continuous urbanization, significant land-use changes, and increased construction and urban density in Tabriz over recent decades have resulted in a substantial increase in impervious surfaces. These surfaces, particularly those associated with high-density residential areas, have consistently contributed to the highest runoff volumes across all three periods. Green infrastructure, including green spaces, agricultural lands, and pastures, has played a more significant role in mitigating urban flood risk than water infrastructure in all three periods, with the exception of one instance (a 15-minute rainfall event in 2002).
An examination of the land use/land cover (LULC) in Tabriz city indicates that impervious surfaces increased continuously from 1984 to 2002, whereas a slight decrease was observed in 2022. Specifically, residential land use increased by approximately 16.54% from 1984 to 2002, followed by a decrease of about 0.50% from 2002 to 2022. This expansion of impervious surfaces has reduced the capacity of most districts in the Tabriz metropolitan area to absorb and retain runoff, thereby increasing the risk of urban flooding. Continued growth of impervious areas in the future could further exacerbate flooding, potentially inundating urban infrastructure, increasing economic losses, and threatening the stability of the city.
The findings reveal that in 1984, for a 2-year return period rainfall event, the volume of water absorbed and retained, along with the associated ecosystem service benefits for mitigating urban flood risk, were as follows: for a 15-minute rainfall, 4.28 million m³ valued at US$10.87 million; for a 30-minute rainfall, 3.17 million m³ valued at US$8.04 million; and for a 45-minute rainfall, 2.58 million m³ valued at US$6.56 million. In 2002, the corresponding values were: for a 15-minute rainfall, 4.37 million m³ valued at US$2.01 million; for a 30-minute rainfall, 3.21 million m³ valued at US$14.71 million; and for a 45-minute rainfall, 2.60 million m³ valued at US$11.94 million. In 2022, the values were: for a 15-minute rainfall, 4.65 million m³ valued at US$33.25 million; for a 30-minute rainfall, 6.15 million m³ valued at US$43.96 million; and for a 45-minute rainfall, 5.20 million m³ valued at US$37.14 million.
Conclusion
The results showed that in the metropolitan of Tabriz, due to the insignificance of green infrastructure, these land uses have not played a significant role in reducing urban flood risk. In the potential for absorption and retention of runoff, land use/land cover has played a more significant role compared to the hydrological soil group. Furthermore, the results suggest that ecosystem services have played a relatively significant role in mitigating urban flood risk in Tabriz.
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