Stability of loose rock riprap used to protect stream banks from erosive forces due to flowing water is evaluated based on the ratio of static moments resisting overturning and those promoting overturning of a single rock particle. The ratio of moments defines a safety factor that describes the potential for riprap failure. The buoyant force acting on a particle is treated separately from the gravitational force, and is further split into components that resist and promote overturning. This approach provides consistency in reasoning throughout the formulation, which results in a particle safety factor that tends to unity as rock specific gravity approaches one. The safety factor formulation is tested using 38 onsite measurements of riprap-lined stream channels that have experienced floods approaching or exceeding the flow rates used to design the protective covers. Based on skill score assessments, the approach is shown to be a significantly more reliable riprap damage predictor than two other commonly used methods.
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