Effect of Soil Mix on Overtopping Erosion

Overtopping is one of the major causes of dam and levee earthen embankment failures. The mechanism of breach caused by overtopping erosion is a complicated process but commonly modelled as either head-cut starting from the toe of the embankment slope progressing upward, or as surface erosion starting from the embankment crest. The two mechanisms are modelled for cohesive and non-cohesive soils, respectively. However, the distinction between the two types of soils from the erodibility aspect is not well established. In this paper, overtopping erosion of different soil mixes is addressed through sediments transport models. The paper presents results from small scale breach experiments on a levee section constructed from four compacted soil mixes of clayey and silty sand with and without fine gravel. Simulation of the small scale tests was performed using two dimensional shallow water (2DSW) hydrodynamic model and a sediment transport library (SEDLIB). The experiment results indicate that for silty sand mixes, erosion start at the crest and progresses downward, while erosion for the clayey sand mixes starts close to the toe and progresses upward. The presence of pea gravel size particles in the mixes slows down the erosion process and time to breach as they do not erode at the same rate as the finer particles. 2DSW-SEDLIB simulation results are presented and compared to the experimental results.