A stilling basin is a critical hydraulic structure designed to dissipate excess energy from high-velocity flow exiting a spillway, preventing erosion to downstream channels. Despite its significant role in dam safety, lining damage of stilling basins occurs frequently, due to gaps and offsets between slabs and undersized underdrains. This study employs a CFD approach to examine how these factors affect flow dynamics and uplift forces. Different scenarios are examined, combining varying gap widths, offset heights, and vent configurations, under three flow rates for each. Results reveal that gap width minimally influences uplift forces. Offset heights considerably enhance upliftpressures, with a 10% increase when offset height doubles from 1.5 cm to 3 cm. Venting reduces uplift pressures effectively by facilitating water escape beneath slabs, with larger vent sizes yielding negative uplift pressures. However, venting intensifies pressure fluctuations, with the pressure coefficient rising substantially, particularly at higher flow rates. This study contributes to a deeper understanding of damage mechanisms and offers valuable insights for upgrading and rehabilitating such structures.