Chapter 5, written by Nadia Benahmed, Christophe Chevalier and Stéphane Bonelli, is concerned with flow erosion as it takes place in a pipe. In fact, this chapter thoroughly analyses the “Hole Erosion Test”, which was carried out for hydraulic structures in the United States and Australia, and recently in France. Several laboratory tests show the influence on erosion parameters of the clay nature, of the soil water content and of its density. Finally, this chapter emphasizes the importance of the coefficient of erosion needed to evaluate the safety of hydraulic structures in relation to the risks of failure caused by pipe flow with erosion.