Abstract

Equilibrium leeside profiles inferred from sediment fallout patterns on an artificial slipface were compared across stability fields for subaqueous bedforms. Flow conditions appropriate to angular bedforms (ripples, dunes) yielded fallout patterns that translated into concave profiles, whereas runs conducted within the stability fields for flat beds yielded profiles that were convex in the vicinity of the brinkpoint. This correspondence occurs because flow separation is readily induced by an angular brinkpoint but is inhibited as the brinkpoint becomes increasingly obtuse to smoothly rounded. For runs just above threshold of motion, the upper part of the inferred leeside profiles becomes less strongly concave as grain size increases, changing to convex between D = 0.46 mm and D = 0.65 mm. This changeover occurs when the boundary is crossed between the ripple field and lower-flat-bed field. It suggests that a lower flat bed need not imply the disappearance of streamwise perturbations that initiate ripples but merely that incipient ripples are unable to develop the required angular profiles. For sands 0.65 mm and coarser, the change from lower flat bed to dunes (with increasing flow strength) occurs when the inferred lee profile changes from convex to concave. Because this change is not observed when sediment feed is sparse, it is attributed to the increased drag caused by tractive load. Within the field of angular bedforms (for all grain sizes), profiles diminish in concavity with increasing strength of flow. At the transition to upper-flat-bed conditions, the upper part of the lee profile becomes convex, implying that incipient bedforms should not be stable. This threshold evidently is reached when enough of the load is carried high enough in the flow to cause the position of maximum sediment outfall to shift downstream away from the brinkpoint.

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