Using high resolution laser scanning to indicate mechanisms of stabilisation under varying sub threshold flow exposures

Laser displacement scans were taken over an area of a 116 mm x 100 mm using a new rapid scan sensor yielding high resolution data (0.1 mm). Two laser scans were taken before and after a 1-16 hour period of antecedent flow run at shear stresses below that of sediment entrainment (stress history). The aim was to quantify any changes to both particle stability and surface topography. Data indicate that prolonged antecedent periods lead to increased particle stability by up to 14.50%. Specifically, uniform beds are more responsive to antecedency than bimodal beds. This appears to be related to the grain size distribution and its influence on the in situ repositioning of grains. It is suggested that uniform beds may have the freedom to rearrange to a greater extent due to larger pore spaces and poorer imbrication, whilst the finer particles within graded beds reduce pore space size and restrict in-situ particle movement of the larger grains. This is substantiated by grain orientation results showing that exposed surface particles pivot around an axis to reorientate into a more stable position through imbrication and streamlining in the direction of flow. Keywords: Stress history, Laser scanning, Surface organisation