Although erosion during high-energy passage of a pyroclastic density current (PDC) causes great damage, analyses of the effects of such erosion are sparse in scientific literature compared to observations and interpretations of depositional processes. In this paper, we review observations of surfaces where PDCs have eroded sets of grooves that provide information on the erosion process. We postulate that in some cases, the grooves were carved by streamwise vortices in the boundary layer of the PDC and review possible fluid dynamic instabilities that can give rise to such vortices. For the prominent grooves at Volcán Bárcena, Mexico, we propose that a fluid dynamic instability, which we dub the “groovy instability,” occurred and caused formation of erosive counter-rotating vortices. This instability occurs when the particle concentration boundary layer thickness, δ_c, is larger than the velocity (shear) boundary layer thickness, δ_u, i.e., L=δ_c /δ_u>1. In subaqueous turbidity currents, these vortices have a typical wavelength of ~25*δ_c. If this relation is applied to the grooves formed on Volcán Bárcena, the inferred particle concentration boundary layer is estimated to have been <1 m thick. We postulate that a transition between erosion of grooves and deposition of dunes at Volcán Bárcena occurred when hydraulically supercritical flow on the upper flanks changed to subcritical flow about halfway down the mountain. We call attention to boundary layer dynamics in erosive pyroclastic density currents at a dimension that is difficult to scale quantitatively in laboratory experiments and is usually not resolved computationally and to the need for incorporating such dynamics into models of PDC dynamics.

尽管火山碎屑密度电流（PDC）在高能通过时会造成严重破坏，但与沉积过程的观察和解释相比，科学文献中对这种腐蚀作用的分析很少。在本文中，我们回顾了PDC腐蚀了沟槽的表面的观察结果，这些沟槽提供了有关腐蚀过程的信息。我们假设在某些情况下，凹槽是在PDC的边界层中由沿流方向的涡流雕刻而成的，并研究了可能引起这种涡流的流体动力学不稳定性。对于墨西哥沃尔坎·巴塞纳（VolcánBárcena）的突出凹槽，我们建议发生流体动力学不稳定性，我们将其称为“槽型不稳定性”，并引起侵蚀性反向旋转涡流的形成。_Ç，比速度（剪切）边界层厚度大，δ _ü，即，L =δ _Ç /δ _ü > 1。在水下浊流，这些涡流具有〜25 *δ的典型波长_Ç。如果将此关系应用于在VolcánBárcena上形成的凹槽，则推断的粒子浓度边界层估计小于1 m厚。我们假设VolkánBárcena的沟侵蚀和沙丘沉积之间发生了过渡，这是因为上侧的水力超临界流在下山的一半左右变为亚临界流。我们呼吁人们注意侵蚀性火山碎屑密度流中的边界层动力学，其尺寸很难在实验室实验中进行定量缩放，并且通常无法通过计算解决，并且需要将这种动力学纳入PDC动力学模型中。