Design of Riverbank Riprap Using Large, Individually Placed Blocks

The protection of river banks in mountain rivers by riprap requires large blocks with weights typically exceeding 1 t. Thus, the blocks have to be placed individually. Such packed riprap has a relatively small spacing between the blocks. Consequently, the interlocking forces between the blocks are high, and the resistance of this protection to erosion is increased compared with traditional dumped riprap used for bank protection in low-grade rivers. Based on 98 systematical laboratory experiments, an adapted design method was developed for the case of riprap riverbank protection using large, individually placed blocks. Three different block sizes were tested by varying the longitudinal channel slope and bank inclination. For comparison, 34 tests were performed with dumped riprap. A design relationship is presented as a function of block size relative to the flow depth and a modified block Froude number considering the mean flow velocity. The additional resistance of riprap to erosion with large, individually placed blocks compared with dumped riprap is quantified. The proposed equations are compared with existing riprap sizing equations. The suggested method proved to be reliable in the assessment of the stability of packed riprap. Furthermore, it is shown that a second layer significantly delays riprap failure.