Falling water levels increasingly affect shorelines in closed and semi-enclosed basins, yet their geomorphic impacts remain underexplored—particularly under accelerating climate change. The widely used Bruun rule often overestimates shoreline advancement during water-level fall because it assumes full sediment retention within the active profile. This study introduces a new index, the Compatibility Index (CI), and integrates it into the Bruun rule to quantify how much eroded sediment is stable and suitable for upper-beach rebuilding. Four CI methods were evaluated: three based on grain-size mean and sorting in the phi scale (KCI, DCI, JCI) and one using the full grain-size distribution in millimeters (PCI). Field data from four southern Caspian Sea stations (2013–2021) covering diverse sediment and slope conditions were applied. Incorporating CI reduced shoreline prediction error by up to 66%, lowering the root-mean-square error (RMSE) from 79 m to 25 m. These results demonstrate that the CI-integrated Bruun rule offers a practical tool for shoreline modeling and supports more reliable coastal planning in regions facing water-level fall.