<p>Extreme rainfall events increasingly threaten the safety of aging, small earth dams, often causing overtopping failures. This study proposes an overtopping prevention system featuring an L-shaped wall installed on the dam crest with an embedded core and filter. Four analysis-based dam models were examined: one without drainage, one with a horizontal drain, one with a vertical filter, and one combining the filter with the proposed L-shaped wall. Model analysis showed that dams designed with a filter exhibited distinct pore-water pressure reductions, and the model in which an L-shaped wall was added to the same filter design demonstrated pore-water pressure values that were 30–40% lower than those of the filter-designed dam even under rising water-level conditions. To validate the design, a large-scale, L-shaped physical model was constructed and tested. Although minor deformation was observed near the spillway, this was attributed to construction variability rather than structural deficiency, and the L-shaped model effectively controlled overtopping while improving internal drainage. Comparison of the scaled physical and the prototype numerical models demonstrated agreements (&gt; 95%) in seepage-line patterns, pore-pressure trends, and deformation behavior, demonstrating the reliability of the scaled model. In the prototype numerical model, slightly elevated pore pressure near the center of the vertical filter suggests that the narrow filter width may underestimate its drainage performance, indicating the need for a vertical-filter design that incorporates a modestly broader allowance. Even with this consideration, the L-shaped wall maintained stable performance under overtopping conditions, showing potential as a practical protection measure for aging small reservoirs.</p>