AN ARRAY OF LARGE-SCALE EROSIONAL BEDFORMS: A DETAILED RECORD OF A GLACIAL LAKE OUTBURST FLOW ARCHITECTURE AND SPILLWAY EVOLUTION
Two-dimensional transverse ripples (relief ~1 m, width ~50 m) dominate the central scoured zone from the lake outlet to 10 km downstream. Sorted bedload boulders occur within the troughs and fine downstream. Longitudinal grooves and ridges (relief 1-2 m, length 400-2000 m) dominate from 10 km downstream to the end of the scoured zone (140 km downstream). Poorly sorted bedload boulders occur along the ridge crests whereas well-sorted smaller boulders are concentrated along the groove axes. In some places, the groove axes are beaded with elongate, low relief (<1m) depressions containing very high concentrations of well-sorted boulders. A narrow zone of longitudinal-elongate scour depressions flank the scoured upland.
Fluid kinematics changed with flood stage and position in the spillway. During earlier stages of the flood, fluid motion changed downstream from presumably more laminar in the outlet area to turbulent with regular patterns of separated flow forming rollers above erosional ripples in the transverse field to streamwise vortices above grooves and ridges in the downstream field. Fluid motion along the shallower flow margins was dominated by isolated and less well organized streamwise vortices and roller-vortex combinations. With time, vortices migrated upstream into the lower portion of the transverse bedform field. During the waning stage, entrenchment of the main spillway channel and decreased flow led to beaded depressions in groove-axes. Concurrently, ripple-trough runout channels formed with late-stage drainage into the newly created inner channel, which then conveyed most of the flow.