INCIDENCE ANGLE DEPENDENT SEDIMENT ROUTING: A PROPOSED MECHANISM FOR FLUVIAL BEDFORM INTERACTIONS

The angle of incidence between mean flow direction and bedform crestlines represents a criterion for cross-stream sediment transport in aeolian and fluvial bedform fields. New observations from the North Loup river, NE show that beyond a threshold value classical grain saltation in a growing boundary layer - angle of repose avalanching dune migration gives way to lateral advection of grains along the lee slope. Larger values cause stream-wise helical vortices to separate from the lee slope. Helical flow and associated rapid downstream advection of sediment allows for the formation of stream-wise spurs. The size of stream-wise helical vortices and spur spacing scale with flow depth and co-evolve with all scales of bedform formation and migration. Sediment entrained in helical vortices may be incorporated into or bypass the next bedform. Helical vortices may also scour downstream bedforms or into the lee slope immediately upstream. Coupled, these two phenomena effectively map the fastest path for sediment through a bedform field and are proposed as a mechanism for bedform interactions.