Rock riprap is a layer of loose rock used to protect soil from the erosive or scouring flows of water. Rock riprap is sometimes used for bank protection or bed stabilization in stream restoration projects where other erosion control techniques are not appropriate.
In order for the rock riprap to properly function, installing rock of good quality and the proper size gradation is important. Design criteria for rock riprap are listed in the Environmental Criteria Manual (ECM) Section 1.4.6. Rock riprap materials and construction methods are described in City of Austin Standard Specification 591S.
The Watershed Protection Department recommends a field gradation test method for use during construction projects to verify that the specified rock will be installed. The gradation test method is described in the following documents, and an example analysis worksheet is provided. However, engineers may use an alternate preferred method conforming to ECM Section 1.4.6.