Closure to “Influence of erosion on piping in terms of field conditions” by GIJS HOFFMANS, J. Hydraulic Res. 59(3), 512–522. 2020. https://doi.org/10.1080/00221686.2020.1786741

required on the distribution of flow to the pipe and polder or the magnitude of pipe discharges. Second, Bligh’s cases include a significant vertical seepage path, which makes it questionable to compare Bligh’s design rule directly to a model based on horizontal pipes. For many field conditions, the SD model predicts a higher strength (Hc) than Sellmeijer et al. (2011). The introduction of the Shields diagram by Hoffmans and Van Rijn (2018) to model the pipe resistance is considered as a step forward. On the other hand, the analyses show that the SD model assumptions regarding the distribution of groundwater flow to the pipe and the polder determine the scaling effects and hence the higher Hc for field conditions. The DgFlow simulations presented here do not support those assumptions. This Discussion also reveals the need for systematic experimental research into scale effects of backward erosion piping, not only effects on the average gradient but also on pipe geometry and pipe flow conditions.