Riprap Design for Overtopping Flow

Near‐prototype flume studies were conducted in which riprap‐protected embankments were subjected to overtopping flows. Embankment slopes of 1, 2, 8, 10, and 20% were covered with riprap layers with median stone sizes of 1, 2, 4, 5, and/or 6 in. Each riprap layer was tested by slowly increasing the discharge to failure. Riprap design criteria for overtopping flows were developed for estimating incipient stone movement and riprap layer failure as a function of the unit discharge, stone shape, median stone size, and embankment slope. Incipient stone movement occurred at approximately 74% of the riprap layer failure unit discharge. It was determined that rounded shape stone should be oversized approximately 40% to provide comparable protection of an angular shape stone. Flow channelization was observed to occur at approximately 88% of the unit discharge at failure. A flow concentration factor of approximately 1 to 3 was introduced for sizing stone.