Urban reservoirs are crucial for municipal water supplies, governed by global standards, such as ACWWA (Canada), USEPA (United States), and the European Drinking Water Directive (2023). This study applies computational fluid dynamics (CFD) to improve water quality management in urban reservoirs. Utilizing OpenFOAM® for simulations, we focus on water age and chlorine distribution. Our model features a dynamic mesh for accurate fluid dynamics simulation across varying water levels and employs the k–ω SST turbulence model to effectively capture surface-related flows. We identified areas within the reservoir with poor circulation and proposed structural optimization strategies, including the addition of pumps to enhance water recirculation and the removal of internal barriers to improve flow. Operational adjustments were also made to refine inflow and outflow regimes, supporting the structural modifications. These interventions led to fresher water, as indicated by reduced water age, and improved chlorine management through enhanced circulation. This research underscores CFD’s pivotal role in modern urban water management, offering detailed insights into water system complexities and facilitating structural and operational enhancements for public safety.