Influence of erosion on piping in terms of field conditions

The Shields–Darcy (SD) model by Hoffmans and Van Rijn (Citation2018) describes the resistance of hydraulic structures to backward erosion piping, which is a form of internal erosion. In the article being discussed, Hoffmans compares the SD model to the model by Sellmeijer et al. (Citation2011), focusing on field scales. This Discussion presents finite element simulations that deviate from Hoffmans’ conclusions that the model by Sellmeijer et al. (Citation2011) results in an unrealistically low critical gradient. As both the SD and Sellmeijer models fit reasonably well to laboratory experiments (Hoffmans & Van Rijn, Citation2018), extrapolation to field scales (say aquifer thickness D > 5 m, seepage length L > 10 m) is important, particularly since these models are used for the design of flood defences. Hoffmans addresses this issue by analysing the resistance as function of aquifer depth D. Hoffmans recommends checking the outcomes of the SD model with a mathematical piping model like that of Van Esch et al. (Citation2013).