Journal of Watershed Management Research

Extended Abstract
Background: Floods are among the most devastating and recurrent natural hazards worldwide, and their frequency and severity are expected to increase due to climate change, urban expansion, and land-use alterations. In riverine environments, flood events pose serious threats to infrastructure, ecosystems, and local livelihoods, especially in developing regions where flood mitigation systems may be insufficient. In northern Iran, aquaculture—particularly cold-water fish farming has grown significantly along riverbanks due to the availability of clean and consistent water resources. However, many of these fish farms are located within active floodplains, exposing them to considerable flood risk. The Haraz River, a high-discharge perennial river in Mazandaran Province, is a key economic and ecological artery and hosts over 30 operational fish farms. Despite its importance, the flood hazard associated with different return periods along its reaches has not been adequately quantified. This study aims to perform a detailed hydraulic simulation of the Haraz River using HEC-RAS integrated with GIS tools to delineate flood-prone zones and assess the vulnerability of adjacent fish farms under multiple flood scenarios. The findings aim to support sustainable land-use planning and inform flood risk mitigation strategies in the region.
Methods: The study focused on three selected reaches of the Haraz River characterized by the highest density of fish farming activities. These reaches were surveyed and mapped using a high-resolution Digital Elevation Model (DEM) with 2-meter spatial resolution, derived from topographic and remote sensing data. The hydraulic modeling was conducted using HEC-RAS version 6.5, a one-dimensional steady-flow model widely used for simulating open channel hydraulics. Key inputs included channel geometry, bankfull cross-sections, flow path centerlines, and bank stations, which were extracted and digitized in the GIS environment. Manning’s roughness coefficients for the channel bed and banks were assigned based on field observations and hydrologic reports from regional water authorities.
Discharge values corresponding to return periods of 2, 10, 25, 50, and 100 years were obtained from the “After the Spring” gauging station located upstream. These flow rates were incorporated as upstream boundary conditions in the HEC-RAS model. Simulated hydraulic parameters included water surface elevations, flow velocities, cross-sectional profiles, and energy grade lines. The RAS Mapper utility was used to convert the model results into spatial flood depth grids, inundation extents, and flood hazard zones in a GIS environment, enabling quantitative overlay with the geolocations of existing fish farms.
Results: The hydraulic simulations revealed substantial spatial variability in flood impacts across the three study reaches. In Reach 1 (3,500 m), the floodplain was relatively narrow, and six fish farms situated further from the main channel exhibited minimal inundation. Under a 100-year flood event, only 16.9% of the farm area was affected, whereas the 2-year flood impacted only 8%. This was attributed to favorable terrain conditions and proper siting practices.
In contrast, Reach 2 (3,000 m) displayed significantly higher flood susceptibility. Seven fish farms in this segment were located close to the riverbanks, and the simulations indicated that up to 53.7% of their area could be inundated during a 100-year event. Even the 2-year flood affected 45.8% of these farms, highlighting the high baseline vulnerability in this zone.
Reach 3 (1,400 m) demonstrated moderate flood risk, with inundation areas ranging from 26.5% for the 2-year event to 33.3% for the 100-year scenario. The confined cross-sectional geometry and lack of natural levees in this reach contributed to the observed flood propagation.
Overall, the cumulative area of fish farms across the three reaches was 137,180 square meters. In the 100-year return period scenario, 45,713 square meters (approximately 33.3%) were inundated, underscoring the severity of flood exposure. The results confirm a direct correlation between return period and both the spatial extent and depth of inundation.
The study emphasized the critical role of topography, channel morphology, and land use in determining flood risk. Improper siting of aquaculture facilities within active floodplains significantly increases vulnerability to even low-frequency flood events. The lack of protective levees or floodwalls in Reach 2 and parts of Reach 3 exacerbated the situation. Moreover, anthropogenic modifications, such as unregulated construction and riverbank encroachment, were identified as aggravating factors. These interventions reduce the effective flow capacity of the river, raise water surface elevations, and trigger out-of-bank flow, particularly during peak discharge events.
In contrast, Reach 1 benefited from adherence to buffer zones and implementation of minor structural mitigation, such as berms and retaining walls. The comparative safety of farms in this reach validates the importance of incorporating hydraulic flood zoning into aquaculture planning. The use of HEC-RAS, combined with GIS spatial analysis, provided robust outputs that align with observed flood patterns and historical damage records in the region. Such tools offer valuable insight into flood dynamics and can support proactive planning, especially in flood-vulnerable watersheds with economic assets at stake.
Conclusion: This study demonstrates that more than one-third of the total area of fish farms along the Haraz River is at risk of inundation during high-return-period flood events. The HEC-RAS-GIS framework proved highly effective in simulating flow behavior, delineating flood zones, and quantifying flood risk to aquaculture operations. The outputs provide actionable guidance for river corridor management, aquaculture zoning, and disaster preparedness.
Key recommendations include:
Enforcing legal river setback distances during site selection and permitting of new fish farms.
Retrofitting existing vulnerable farms with engineered embankments and elevation adjustments.
Integrating flood hazard maps into early warning systems and emergency planning.
Monitoring and restricting development in sensitive floodplain zones to maintain hydraulic connectivity.
The findings support the integration of hydrodynamic modeling into regional planning efforts aimed at reducing the socioeconomic impacts of floods. Future research should explore coupling hydraulic simulations with economic loss modeling to estimate cost-benefit ratios of different flood mitigation strategies.