Time Dependent Failure Analysis of Compressed Riprap as Riverbank Protection

Riprap is a common measure for protecting river banks against lateral erosion and several methods exist to design them. However, they are generally limited to dumped medium size rocks. If large rock blocks are required for stability reasons, they cannot be dumped anymore but they have to be placed individually. This gives additional resistance against flow erosion. The behavior of large rock blocks for alpine river embankments was so far rarely studied. Thus, an experimental investigation was conducted to investigate the stability of such compressed large blocks as river bank protection. Experiments were conducted in a 10 m long, 1 m wide flume with a rough fixed bed. Riprap was reproduced with uniform crushed stones having three different median block sizes of D50=0.037, 0.042 and 0.047m. Testing was undertaken for a Stream-Wise slope of 0.03 in supercritical flow conditions. Transversal slope of the riprap was 3.5V-5H: One layer of blocks as well as two layers was studied in order to investigate the influence of the riprap thickness. They were compressed and placed on the filter with a wide grain size distribution. The riprap failure threshold discharge was determined based on series of tests with duration of 3 hours. Riprap erosion rate was measured with a one minute frequency, by means of block tracking with three cameras and standard video-image processing techniques. Furthermore, the eroded rocks were collected and weighed in a sediment trap at the downstream end of the channel. The time of first total failure was recorded and a time based analysis of failure was performed. Total collapse of the blocks in a reach over the whole bank was defined as full failure. First results revealed that for similar unit discharge the rate of block erosion is significantly decreased by increasing the size of the riprap. The time of failure dépends also on the size of blocks. It could be observed that under the same conditions, the upper layer stabilized the protection system and delayed the time of total failure. Nevertheless, erosion rate of the upper layer of rock blocks increased.