<p>Earthen dams are essential for water supply; however, recent failures due to internal erosion have underscored the need for efficient drainage filter designs. Erosion, a major cause of earthen dam failure, rapidly increases toward the sloping end. Specifically, the sandy drainage filter with a square-shaped end at the downstream slope exacerbates this erosion problem owing to rapid flow concentration. The filter end has an inherent risk of erosion owing to its geometric configuration. Therefore, in this study, the geometric shape of the sandy filter end is improved to a fan shape to address the flow concentration-induced erosion problem. Numerical analysis based on various permeability coefficients and fan shape conditions indicates that the optimal fan-shaped end enhances drainage performance and significantly reduces erosion risk, even with minimal expansion. This result demonstrates that by changing the shape of the filter end to a fan shape, both drainage performance and erosion risk can be improved, even with a reduced amount of sand in the horizontal filter, compared to the existing horizontal filter design.</p>