Effect of compressed riprap thickness on the stability of river banks

One of the common measures for river bank protection is the installation of riprap. There are several methods to design riprap appropriately, which are however generally limited to dumped medium size blocks. Nevertheless, an additional resistance against erosion can be achieved by individually placing blocks in one or several layers instead of dumping them arbitrarily. An experimental investigation has thus been performed to study the stability of large blocks which are compressed as a river bank protection. Tests were carried out including one layer of stones as well as two layers, to evaluate the influence of the riprap layering (e.g. riprap thickness) on the bank stability. The effect of the thickness on the stability of ripraps is investigated in a 10 m long and 1.2 m wide tilting flume, with a rough fixed bed. Riprap median particle size was D50 37 mm. Testing was conducted for channel longitudinal slopes 0.015 and 0.030 and riprap bank inclinations of 27, 31 and 35 degrees. The riprap was installed on the top of a wide grain size distribution filter. Supercritical flow conditions were considered, given the steep channel slope. The complete removal of the riprap in a section under a constant discharge was defined as the failure criterion. The riprap failure threshold discharge was determined based on the series of tests with duration of maximum 180 minutes. In each test, riprap transport rate was measured every minute while the stones were tracked by a video camera and collected in a sediment trap at the channel end. The time of total failure was defined by standard video-image processing techniques. A time based analysis of failure was performed and first results revealed that, for similar conditions, the second layer stabilizes the riprap significantly and delays the time for total failure. Nonetheless, transport rate was found to be increased in this latter situation. to overtopping and it corresponds to a rotationgravitational movement of the stones. In overtopping, the water soaks the riprap and the material behind it. Once the level of the water decreases, the water in the saturated part induces a steep pressure gradient and slide-slope of the riverbank riprap takes place (Jafarnejad et al. 2012 and 2013). There are different methods to design riprap resisting against direct block erosion. Some of them are discussed in Garcia (2007). Stevens et al. (1976) offered a safety factor based technique by considering the stability of individual blocks in riprap. They supposed that each block is stable if the several forces causing a possible displacement of a block represent less than the reaction caused by the submerged weight. Wittler and Abt (1988) adapted Stevens’ study by adding frictional and contact forces from contiguous blocks. Abt et al. (2008) studied the round-shaped riprap stabilization in overtopping flow as well. Froehlich (2011), Ulrich (1987) and Stevens et al. (1984) also considered the weight of the submerged rock as the only resisting force. Froehlich and Benson (1996) worked on wide angles of repose to refer the slope of embankment effect on riprap stability. They proposed a “particle angle of initial yield” which was