The present work proposes a morphological evolution model for sandy cuspate beaches, which depends on two parameters, swash excursion (S) and offshore wave steepness (H0/L0). The model was derived from observations of morphology and wave climate at Reñaca Beach in central Chile, comprising in-situ instrumentation, topographic surveys, wave modeling, and optical imagery. It was found that wave steepness alone was unable to explain the morphological changes observed on the beach. Rather, results indicate that swash excursion is better to distinguish between erosive and accretionary events, with swash excursion exceeding 29.5 m for more than 12 h being indicative of erosive events. While morphological changes are led by S, wave steepness correlates with the erosive potential of waves, especially when H0/L0 is greater than 0.017. Although derived for a specific beach under a limited range of conditions, this work adds to the understanding of the dynamic behavior of beaches exposed to persistent high-energy swells.