How debris‐flow composition affects bed erosion quantity and mechanisms: An experimental assessment

Understanding erosion and entrainment of material by debris flows is essential for predicting modelling debris-flow volume growth hazard potential. Recent advances in field, laboratory studies have distilled two driving forces behind erosion: impact shear forces. How these depend on composition interact remains unclear. Here, we experimentally investigate the effects processes a small-scale flume with loosely packed bed. We quantify gravel, clay solid fraction flow bed erosion. Erosion increased linearly gravel volume, decreased increasing fraction. was maximal around volumetric 0.075 (fraction total volume). Under varying fractions volumes positively related to both forces, while themselves are also correlated. Results further show that internal dynamics flows, quantified Bagnold Savage numbers, correlate erosional quantity. Impact became increasingly important grain size. The experiments showed abundance viscosity interstitial fluid affect dynamics, mechanisms magnitude. High inhibits mobility flow, movement individual grains transfer momentum impacts, therefore content possibly decreases pore pressures transport capacity, inhibiting erosion, despite high stresses Our results quantities may vary between contrasting composition, stress models volume-growth predictions be substantially improved including compositional effects.