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 tests and numerical simulations of water pressure fluctuations at plunge pool bottoms and inside artificially created under-lying 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. The new engineering model is composed of two sub-models, called the Comprehensive Fracture Mechanics (CFM) model, which uses a simplified Linear Elastic Fracture Mechanics (LEFM) approach to express the erosion resistance of the rock mass, and the Dynamic Impulsion (DI) model, which expresses the ejection of distinct rock blocks from their mass. In the following, these two sub-models are described more in detail.