Rock and Roll—It's Here to Stay: Sizing Loose Rock Riprap to Protect Stream Banks

Abstract

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. INTRODUCTION Individual rock particles on the surface of a layer of loose rock riprap tend to be dislodged more by rolling rather than by sliding. For this reason, stability of loose rock riprap used to protect stream banks from erosive forces due to flowing water is often evaluated based on the ratio of moments resisting overturning (MR) and those promoting overturning (MP) of a single rock particle caused by gravitational, hydrostatic, and hydrodynamic forces as follows: s R P f M M =∑ ∑ (1) that provides an index of particle stability, with ratios larger than unity indicating a stable condition. Departing from the conventional approach, an analysis of riprap stability is presented here treats the buoyant force acting on a particle separately from the gravitational force when calculating moments. Splitting the gravitational, buoyant, and hydrodynamic forces into components that promote and resist overturning, consistency in reasoning throughout the formulation is maintained. As a result, the particle safety factor tends to unity as rock specific gravity approaches one, as intuition suggests it should, which is in contrast to other riprap sizing methods that are also based on overturning moment analyses. Procedural steps are provided for calculating the safety factor of existing riprap covers, and also for calculating the minimum rock diameter for design of a 1603 World Environmental and Water Resources Congress 2012: Crossing Boundaries © ASCE 2012