| Abstract: | Stability of loose rock riprap used to protect stream banks from erosive forces because of 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. Comparison to two other commonly used riprap‐sizing methods by means of the Hanssen–Kuipers skill score and the equitable threat score shows that the particle stability procedure provides significantly better damage predictions and, for this reason, is shown to be more accurate. Based on the onsite measurements, safety factors that provide increasing levels of security against failure are suggested for use in calculating the size of loose rock riprap needed to protect stream banks against erosion by currents. Copyright © 2011 John Wiley & Sons, Ltd. |
