WHAT IS HIDDEN IN HIDING FUNCTIONS? BED ARRANGEMENT IMPACTS ON SIZE-SELECTIVE TRANSPORT AND LANDSCAPE EVOLUTION (Invited Presentation)

Landscape evolution models rely on bed load transport equations, which often employ hiding functions to quantify the relative mobility of different grain sizes. Such hiding functions are assumed to be controlled by hiding effects; the relative mobility of small grains is reduced because they project less into the flow and have larger friction angles than coarse particles. Hiding effects occur at the scale of individual grains but hiding function exponents (b) are determined at the reach scale, and therefore the actual control of hiding effects on b is not clear. Grain scale measurements of projection and friction angles are also usually for particles placed on bed surfaces instead of naturally positioned in situ grains with packing and burial. Using laboratory flume experiments, we measured projection and resisting forces (quantifies friction angles) for in situ and surface particles. We then: 1) determined critical shear stresses using these variables and a force balance equation 2) fit b values using these critical stresses, and 3) compared b values obtained for in situ and surface particles to those from reach-scale bedload transport observations. In situ grains had larger critical shear stresses than surface particles because of their higher resistance forces and smaller projection values. Projection was relatively similar for small and large in situ grains, which caused relatively small hiding effects. Only such weak hiding effects from in situ sediment could explain the b values from reach-scale bedload measurements. Multiple regression analyses indicated that b was largely controlled by resisting force variations and not projection. Therefore, the relative mobility of coarse and fine grains in our experiments was mostly dictated by packing rather than hiding effects. The temporal and spatial variations in packing may influence bedload fluxes, downstream fining and landscape evolution.