Mobility of patch sediment in gravel bed streams: Patch character and its implications for bedload

Gravel bed rivers may exhibit a pattern of decimetre to metre-scale patches that consist of fine bed material. These are distinct from larger patches or from longitudinal and transverse patches. They have characteristic locations, textures, fabrics, boundaries, microtopographies and exhibit intra-patch downstream coarsening. Their texture may be sandy, but can also be of granules or even sand-depleted fine to medium pebbles. These are mobilisable, open-structured and well-sorted comparatively fine sediments located in the lee of clasts. Their upstream borders are well defined by the protruding clasts, their microtopography is planar and flat and as the area of each increases, so does the extent of pocket filling and variance in patch size increase. Initiation of bedload in patchy gravel bed rivers is dependent not only on the average shear stress, but also, to a large extent, on the characteristics of the patches: their texture, their areal extent and their interconnectedness. The relations have not yet been fully explored. Patch character also affects bedload flux, with higher fluxes associated with finer, more expansive and interconnected patches. In poorly sorted bed materials, patches are the major source of bedload up to a level of shear stress that disrupts the entire bed.