Stability of Rounded Riprap in Overtopping Flow

Riprap design procedures often stipulate the use of angular shaped stone materials. However, angular shaped riprap materials are not always available nor economically feasible at all construction locales. Alluvial materials are often available at remote locations that yield relatively high quality materials, but the rock is usually rounded in shape. Currently, the stability of rounded riprap compared to angular riprap is unknown. A physical testing program was performed subjecting rounded rock to overtopping flow. Soil embankments 76 cm in height with slopes of 20%, 25%, and 30% were constructed in a recirculating flume. A geotextile was placed atop of the embankment, followed by a shallow (6 cm) filter/bedding material layer. Rounded riprap with median stone sizes of 2.4 to 9.9 cm (0.94 to 3.9 inches) was placed in layer thicknesses of 1.5 to 3 times the median stone size on the bedding. Flow was conveyed over 12 test configurations and slowly increased until the rock layer integrity was disrupted. The data analysis yielded a procedure through which the stable median rock size may be determined as a function of the degree of roundness, rock uniformity, slope, and unit discharge.