Retardation of breach growth under high flow velocities

Abstract

People all around the world live and work in low-lying areas. Low-lying areas have to be protected against high water levels in rivers and at sea by a water protection system (e.g. by dunes, dikes and barriers). Dikes are an important component within water protection systems. Generally, dikes are described as elongated naturally occurring or artificially constructed (earthen) structures, which prevent flooding of the hinterland. Dikes are mainly found along seas, estuaries, rivers, canals, lakes and water courses. Many dikes contain sand cores, which are covered by a protection layer (e.g. clay, asphalt etc.) to prevent erosion of the core. Unfortunately, there are times that one of the many known failure mechanisms of a dike causes (local) dike failure, exposing the sand core to the water (Rijkswaterstaat, 2006). A so-called initial breach is formed. Once the sand core is no longer fully covered by a protection layer, the sand core start to erode and the core is prone to fast breaching (Visser, 1998). The water flowing through the breach is eroding the sandy sediment and the breach keeps growing, which can have significant economical consequences and can also lead to loss of human life and animal life. Several options to retard the breaching process have been investigated by Lemmens (2014). Based on laboratory experiments, especially a mixture of sand and bentonite (in the core of the dike) seems to significantly slow down the breaching process. Thus far, this theory has only been tested for relatively low flow conditions in the order of 1 m/s. However, during the breaching process high flow conditions in the order of 2-10 m/s can be reached (Visser, 1998). In this study it has been investegated how bentonite would reduce the erosion velocity of sand under high flow velocity conditions. Erosion experiments were carried out in a tilting flume. In these tests, the volume percentage of bentonite additive, the diameter of the sand and the mean flow velocity were varied. All erosion experiments were performed under supercritical flow conditions. Falling head tests were executed to determine the effectiveness of adding bentonite to the core of a dike. Adding a certain amount of bentonite to sand certainly yields a high decrease in permeability. It appears that the most significant decrease in permeability happens with bentonite contents up to 6% of the volume. Results from the direct shear tests show no significant differences between sand mixtures and sand-bentonite mixtures with a bentonite content up to 10%. Hence, sand-bentonite mixtures with a bentonite volume content up to 10% still show sand-like behaviour. This indirectly indicates that the strength characteristics of a dike core will not be altered. The observed behaviour in the experiments enhances the development of an adapted erosion function. The erosion function of Van Rhee (2010) - including the effect of dilatancy and the permeability - has been adapted to the experimental data of the erosion tests to get reasonable accurate model results. The adapted erosion function has been implemented in the BRES-Visser model. The BRES-Visser model - which decomposes the breaching process in five different phases as proposed by Visser (1998) - calibrated with data from the Zwin’94 experiment, has been used to model the performance of the bentonite additive in the core of a dike. The effectiveness of the bentonite measure has also been tested in the Borssele case study. To conclude, sand-bentonite mixtures are able to significantly reduce the erosion velocity. The effects of dilatancy and a decrease in permeability have a large impact on the erosion velocity. This is already noticeable at very low percentages of added bentonite.