The legacy of impact conditions in morphometrics of percussion marks on fluvial bedrock surfaces

Abstract : Percussion, or impact, marks are a common type of bedrock bedform found on many fluvial bedrock channels and have been attributed to bedload impact. Little is known about the conditions under which they form and how these affect morphology and dimensions of impact mark craters. We present data from a set of experiments exploring the formation of percussion marks by bedload impact under controlled conditions (impact velocity, angle, and particle diameter) by quartz spheres onto polished marble plates through a water interface. Particle impact causes impact craters consisting of a central depressed pit and a surrounding raised crater rim under all impact conditions. Data from 699 impact experiments show that crater rims are always circular and crater diameter (ϕc, in m) scales with the kinetic energy of the particle normal to the surface immediately prior to impact (K.E., in J) by the relationship K.E. = 2.48 × 107 ϕc3.188. We test this relationship on impact marks produced in a series of controlled flume experiments for a range of surface inclinations found in natural fluvial channel outcrops. Measurements of impact crater diameter were used to estimate K.E. using our empirical equation. Our model estimates very similar K.E. for impact craters produced in this quasinatural setting to those calculated from flume conditions when realistic values for mean impact velocity and mean impact angle are assumed. Applying this relationship to measurements of crater rim diameter in natural settings will allow the mapping of impact K.E. along and across channel reaches where these bedforms are found. Future numerical models of fluvial bedrock erosion based on impact K.E. could be field calibrated from measurements of percussion marks in marble channels or from installed marble slabs in other bedrock channel reaches.
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https://hal.univ-lorraine.fr/hal-01764513
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Submitted on : Thursday, April 12, 2018 - 10:37:41 AM
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Andrew Wilson, Jérôme Lavé. The legacy of impact conditions in morphometrics of percussion marks on fluvial bedrock surfaces. Geomorphology, Elsevier, 2013, 186, pp.174 - 180. ⟨10.1016/j.geomorph.2012.12.033⟩. ⟨hal-01764513⟩

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