Introduction 
Rivers have long played a central and multifaceted role in shaping human civilizations and supporting natural ecosystems. As dynamic and life-sustaining components of the environment, rivers are critical sources of freshwater for drinking, irrigation, and industrial use, but also serve as natural corridors for transportation, biodiversity conservation, and ecological balance. Historically, many human settlements have been established near rivers due to the availability of water, fertile lands, and ease of access and trade. However, the very characteristics that make rivers vital—their variability and dynamic flow patterns—also make them sources of natural hazards, particularly flooding. This is especially pronounced in mountainous regions or areas subject to intense and irregular rainfall, where the risk of flash floods is significantly elevated. Floods are among the most frequent and destructive natural disasters globally, resulting in yearly loss of life, displacement, and economic damage. With the ongoing impacts of climate change, precipitation and runoff patterns have become increasingly unpredictable, amplifying the frequency and intensity of flood events. This heightened flood risk is especially problematic in regions where urban development has outpaced infrastructure planning or where governance systems cannot implement and enforce adequate water and land management strategies. In such contexts, vulnerability to flood disasters is markedly increased, particularly in areas where populations are concentrated and infrastructure is either aging or poorly constructed.
One of the most vulnerable zones in any riverine system is the downstream area of large dams. While dams are typically constructed to regulate water flow, generate hydroelectric power, and manage water supply, they can inadvertently contribute to increased flood risk under certain conditions, such as unexpected discharge, structural failure, or extreme weather events. Moreover, human encroachment into riverbeds and buffer zones further exacerbates vulnerability. Illegal construction, lack of zoning regulations, inadequate early warning systems, and low public awareness about flood risks are all contributing factors. The current study focuses on a six-kilometer stretch of the Karaj River located downstream of the Amir Kabir Dam, situated in Alborz Province within the urban limits of Karaj city. This area has been identified as particularly susceptible to sudden and severe flood events due to several compounding factors, including steep topography, high population density, and irregular water releases from the dam. In addition to these environmental and infrastructural vulnerabilities, social factors such as poverty, lack of education, and limited access to information further contribute to the region’s overall exposure to flood risks.
Materials and Methods
To comprehensively assess the vulnerability of human communities to flood hazards in the selected area, a mixed-methods approach was adopted, incorporating both quantitative and qualitative research methodologies. The study was executed in three phases: hydrological and statistical analysis, hydraulic modeling, and social-physical vulnerability assessment. In the first phase, historical hydrological data on the Karaj River’s peak instantaneous discharge rates were collected and analyzed. Flood frequency analysis was conducted using SMADA (Stormwater Management and Design Aid), a software for hydrological modeling and statistical analysis. This involved fitting various probability distributions to the discharge data to estimate flood magnitudes for different return periods, such as the 10-year, 50-year, and 100-year floods. The outputs from this phase provided critical input parameters for subsequent flood hazard modeling and risk assessments. The second phase involved the hydraulic simulation of flood behavior under different scenarios using the HEC-RAS (Hydrologic Engineering Center’s River Analysis System) software, a powerful and widely-used tool for modeling one-dimensional and two-dimensional surface water flow. Inputs for the model included a Digital Elevation Model (DEM), cross-sectional profiles of the river, hydrological discharge data from the previous phase, and relevant boundary conditions. The model enabled the generation of flood inundation maps for varying levels of flow intensity, clearly delineating flood-prone zones and allowing for precise spatial identification of areas at risk. Once flood hazard zones were mapped, the third phase focused on identifying and analyzing human settlements in at-risk areas. A total of 588 residential units were identified within the study buffer, with 72 directly located in high-risk zones. These were analyzed in detail based on location, construction type, usage status, and demographic composition. Further investigation included a physical vulnerability assessment of buildings, which took into account factors such as construction materials, building elevation, structural age, and resistance to water flow and moisture. To provide a more holistic understanding of vulnerability, social data were collected through structured questionnaires distributed to 136 households and semi-structured interviews with six technical experts and four professionals in the fields of urban planning, hydrology, and disaster management. Additionally, secondary data were obtained from statistical yearbooks, municipal records, and official zoning maps. The collected data were analyzed using statistical and spatial analysis techniques, and composite indices for social and physical vulnerability were developed to guide interpretation and policy recommendations.
Findings
The study’s findings reveal a high degree of social vulnerability in the study area. Residents were categorized into four vulnerability levels based on indicators, including educational attainment, income level, family size, past flood experience, social cohesion, and risk awareness. The results indicated that 9% of the population falls into the “very high” social vulnerability category, 25% into “high,” 44% into “moderate,” and 22% into “low.” Many community members lack adequate knowledge of flood risks and are not engaged in disaster preparedness or response planning. In terms of physical vulnerability, 44% of the buildings were found to have “low” sensitivity to floods, 33% “moderate,” 18% “high,” and 4% “very high.” High-sensitivity buildings are typically located closest to the river channel and often constructed with inadequate materials or design standards. Many of these buildings lack the necessary elevation above flood levels and are not designed to withstand water flow pressures during flooding events. The juxtaposition of these two dimensions—social and physical vulnerability—paints a concerning picture of the area’s overall flood risk. The findings highlight the convergence of environmental exposure, infrastructural weakness, and social fragility, compounding the region’s susceptibility to disaster.
Conclusion
The results of this study underscore the urgent need for comprehensive, multi-sectoral interventions to reduce flood risk and build community resilience in the Karaj River’s downstream zone. The current state of infrastructure, governance, and community preparedness is insufficient to cope with the scale and intensity of potential flood hazards. Several key factors have contributed to the region’s vulnerability, including weak enforcement of land use policies, low public awareness, unauthorized development in flood-prone areas, and the absence of real-time monitoring and warning systems.
To address these challenges, the study offers the following strategic recommendations:
1. Development and regular updating of flood hazard maps based on advanced hydrological and hydraulic modeling to support informed decision-making;
2. Enforcement of construction restrictions in high-risk areas, coupled with the introduction of robust legal frameworks to prevent unauthorized development;
3. Investment in flood management infrastructure, including stormwater drainage systems and retention basins to regulate and control runoff;
4. Implementation of community education and awareness programs focused on flood risk, emergency response, and resilient behaviors;
5. Strengthening of local capacity and community empowerment initiatives to enhance social cohesion and adaptive capacity in the face of natural hazards;
6. Deployment of early warning technologies and emergency communication systems tailored to local needs and capacities.
Ultimately, the analytical and methodological framework developed in this study can serve as a replicable model for flood risk assessment in other vulnerable regions across the country. Integrating numerical modeling, spatial analysis, and social assessment with participatory approaches offers a powerful foundation for designing more effective and inclusive disaster risk reduction strategies at both local and national levels.