STABILITY OF RIPRAP AT BRIDGE PIERS

An experimental investigation was performed to determine the critical conditions for displacement of riprap placed on streambeds surrounding bridge piers. A functional relation between stability number, a dimensionless grouping of rock size and density to fluid velocity and density, and relative approach flow roughness, approach flow depth, riprap size, and placement depth was presented. Pier model tests conducted in a laboratory flume were used to determine the influence of each dimensionless parameter on the stability number. Analysis of the experimental results showed that the relative depth of placement and the relative rock size significantly affected the stability number for rectangular piers. A comparison of this data with similar data for round-nosed piers and other data for unobstructed flow conditions showed that rock sizes required for stability at piers must be substantially larger than those required for unobstructed flow. In addition, rock sizes for rectangular piers must be substantially larger than those for cylindrical piers for the same flow conditions. Design equations and future research were recommended.