Effect of a second layer on the time to failure of compressed riprap as mountain riverbank protection

Recently, Jafarnejad, Franca, Pfister, and Schleiss [2017. Time-based failure analysis of compressed riverbank riprap. Journal of Hydraulic Research, 55(2), 224–235. doi: 10.1080/00221686.2016.1212940] presented an experimental study on the stability of compressed riprap, which means that it is composed of individually placed blocks with a single layer as riverbank protection. The relationship between the time to failure and dimensionless bed shear stress was presented. Here, the authors complement the previous investigation with results regarding the effect of a second riprap layer. In total, 49 tests (28 tests with one and 21 tests with two riprap layers) were performed using a single block size, three longitudinal channel slopes and three different riprap bank inclinations. Under similar conditions, the second layer can significantly delay the time to failure, whereas the block erosion rate was found to increase. The second layer has a more stabilizing role when the bank angle of the riprap is approaching the angle of repose of the blocks.