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 result in severe damage to infrastructure, significant loss of life and property, and substantial economic and social impacts. Blue and green infrastructure plays a critical role in mitigating urban flood risk. This study evaluates the contribution of ecosystem services to flood risk mitigation in the Tabriz metropolitan area for a 2-year return period. In this study using Landsat satellite imagery, land use/land cover data, meteorological information, biophysical tables, Geographic Information Systems (GIS), and the InVEST software, assessed the ecosystem service of urban flood risk mitigation in Tabriz. The results indicate that in 1984, for a 2-year return period, the volume of water absorbed and retained, along with the economic value of ecosystem services for flood mitigation, was 4.28 million cubic meters and $10.87 million for a 15-minute rainfall event, 3.17 million cubic meters and $8.04 million for a 30-minute rainfall event, and 2.58 million cubic meters and $6.56 million for a 45-minute rainfall event. In 2002, the corresponding values were 4.37 million cubic meters and $2.01 million (15-minute rainfall), 3.21 million cubic meters and $14.71 million (30-minute rainfall), and 2.60 million cubic meters and $11.94 million (45-minute rainfall). In 2022, the values were 4.65 million cubic meters and $33.25 million (15-minute rainfall), 6.15 million cubic meters and $43.96 million (30-minute rainfall), and 5.20 million cubic meters and $37.14 million (45-minute rainfall). 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 mitigating urban flood risk. In the potential for absorption and retention of runoff in Tabriz, 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.
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:
1. Runoff volume, presented as raster data.
2. Runoff absorption and retention (in millimeters), expressed as a percentage of rainfall in a raster file.
3. Runoff retention volume (in cubic meters), depicted in a raster file.
4. Flood risk, summarized in a descriptive table and represented through vector and raster data, enabling identification of spatial variations within defined local boundaries.
5. Monetary assessment of flood-related damages.
6. 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) situation in Tabriz city indicates that the area of impervious surfaces has been continuously increasing across all three periods (1984, 2002, and 2022), with the exception of 2022, which saw a slight decreasing trend. Specifically, residential land use increased by approximately 16.54% from 1984 to 2002, but then experienced a decrease of about 0.50% from 2002 to 2022. This increase in impervious surfaces has reduced the capacity of most districts in the Tabriz metropolitan area to absorb and retain runoff, thereby heightening the risk of urban flooding. Continued expansion of impervious surfaces in the future could further exacerbate flooding risks, potentially submerging urban infrastructure, increasing economic losses, and threatening the stability of Tabriz.
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 cubic meters valued at $10.87 million; for a 30-minute rainfall, 3.17 million cubic meters valued at $8.04 million; and for a 45-minute rainfall, 2.58 million cubic meters valued at $6.56 million. In 2002, the corresponding values were: for a 15-minute rainfall, 4.37 million cubic meters valued at $2.01 million; for a 30-minute rainfall, 3.21 million cubic meters valued at $14.71 million; and for a 45-minute rainfall, 2.60 million cubic meters valued at $11.94 million. In 2022, the values were: for a 15-minute rainfall, 4.65 million cubic meters valued at $33.25 million; for a 30-minute rainfall, 6.15 million cubic meters valued at $43.96 million; and for a 45-minute rainfall, 5.20 million cubic meters valued at $37.14 million.
Conclusions
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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