<![CDATA[Flooding in the Krueng Langsa watershed not only causes inundation and damage to infrastructure, but also has the potential to reduce water quality due to the transport of pollutants from various sources. This study utilizes the HEC-RAS model to analyze the risk of water quality degradation due to flooding by integrating the results of flood modeling (depth, velocity, inundation area, hazard class H1–H6), mapping of pollutant sources, and water quality data. The novelty of this study lies in the integration of hydraulic flood simulation, pollutant source distribution, and water quality indicators into a spatially based flood pollution risk assessment framework for the Krueng Langsa watershed. The planned flood discharge was calculated using the Nakayasu HSS for the 5, 25, and 50-year return periods (Q5 = 370.24 m³/s; Q25 = 531.56 m³/s; Q50 = 595.51 m³/s). The results of the 1D/2D HEC-RAS modeling showed that the inundation area increased from 2,486 ha (Q5) to 3,580 ha (Q50), with a shift in hazard class from the predominance of H1–H2 (low) to an increase in H3–H6 (moderate–high). Model performance was evaluated through validation against observed flood characteristics, indicating that the HEC-RAS simulation reliably represented flood extent and hazard distribution within the study area. The risk index for water quality degradation was calculated using the formula R = B × T × K (weight of pollutant source, flood hazard level, and relative concentration of pollutants). In Q5, the high-risk area was only 174 ha (7.0% of inundation), while in Q50 it jumped to 568 ha (15.9%). High risk is concentrated on flat slopes with dendritic flow patterns in the mid–downstream segment, especially around densely populated settlements and oil palm plantation land. The main pollutant parameters include chlorine (0.24 ppm), copper (0.05 ppm), BOD, ammonia, and coliforms that exceed quality standards. The integrated HEC-RAS-based risk assessment approach proved reliable for identifying priority zones vulnerable to post-flood water quality degradation. HEC-RAS integration and pollution risk analysis are effective in mapping priority areas for post-flood mitigation, emergency clean water provision, and control of pollutant sources in the Krueng Langsa watershed. These findings contribute to the development of integrated flood and water quality management strategies in tropical watershed environments.]]>
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