Time-dependent failure analysis of large block size riprap as bank protection in mountain rivers

The protection of riverbanks with blocks, called riprap, is the most used method in alpine rivers to avoid uncontrolled lateral erosion. For rivers with significant bed slopes large boulders have to be used in order to withstand high flow forces. Such large boulders cannot be dumped anymore, like in the case of lowland rivers, but they have to be placed individually by machines because of their weight. Consequently the blocks are better interlocked even if a rough surface of the riprap is required. Thus a higher resistance of such individually placed or “compressed” riprap may be expected. The existing design methods have been developed for dumped riprap with relatively small block sizes. Dr. Mona Jafarnejad studied for the first time systematically the effect of compressed riprap, that means with individually placed blocks having a good interlocking, on the failure resistance. Based on a relative roughness and modified Froude number, Dr. Jafarnejad proposed an empirical relationship which can assess the limit between stable and failure conditions of the blocks and thus gives a criterion for the required minimum block size. Furthermore, and also for the first time, the time-dependant failure was analysed which is also not considered in existing design methods. Dimensionless empirical relationships between the time to failure and the bed shear stress as well as flow depth were developed. The results revealed that for a total failure of the compressed riprap a relatively high number of blocks have to be eroded. In loose dumped riprap, normally the erosion of a few blocks results in a fast failure knowing that the time to failure is very important in practice since the flood peaks have only a limited duration. Additionally Dr. Jafarnejad studied the effect of a second layer of individually placed riprap on the time-dependant failure process, which is novel. She developed also an empirical relationship, which takes into account the influence of such a second layer. For the same longitudinal channel slope and side bank slopes, the second layer stabilizes the section and postpones failure. The effect of a second layer is more significant for higher bed slopes and bank side slopes. Finally, compared to the traditional design methods, using a safety factor approach, Dr. Jafarnejad developed a probabilistic failure analysis method for riprap, which considers uncertainties of the design parameters and the future evolution of bed load transport under climate change.

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