A physically based engineering model has been developed at the Laboratory of Hydraulic Constructions for the evaluation of the ultimate scour depth of a jointed rock mass due to high-velocity jet impact. The model is based on experimental measurements and numerical simulations of water pressure fluctuations at plunge pool bottoms and inside underlying rock joints. The water pressures inside the joints revealed to be of highly transient nature, governed by a cyclic change between high peak pressures and low near-atmospheric pressures. Hence, the new engineering model, called the Comprehensive Fracture Mechanics (CFM) model, uses a simplified Linear Elastic Fracture Mechanics (LEFM) approach to express the erosion resistance of the rock mass. In the following, this model has been compared with Annandale’s Erodibility Index (EI) method, which is actually a widely used semi-empirical model for prediction of the ultimate scour depth in jointed rock. A systematic comparison between the two methods allowed to express the sensibility of the different parameters of the new physically based model and points out the promising character of its results.