BED STABILITY AND BAR FORMATION IN MOUNTAIN STREAMS: EFFECTS OF DISCHARGE ON FLOW CONVERGENCE

High mountain streambed features such as boulders and stable particle clusters, immobile under normal flow conditions, significantly influence energy dissipation and bed stability within the channel reaches they occupy. This influence is measured through changes in the local velocity field and water-surface topography. Previous field experiments (Thorne and Furbish, 1996; Thorne, 1997) in a channel reach containing a large boulder positioned against one bank indicated the presence of a backwater zone upstream of the stable bed feature (SBF), shoaling and convergence of flow into the width constriction produced by the obstacle, and the presence of a well-developed low velocity zone in the lee of the SBF. The low velocity zone is a depositional zone and its dimensions depend on the manner in which the filament of high velocity expands upon exiting the width constriction. These studies further indicated, in the vicinity of the SBF, a pattern of flow "steering" that varies as a function of depth. Near-bed flows adjacent to the SBF accelerate through the constriction in a streamwise direction, while near-surface flows are directed around the obstacle toward the opposite bank downstream. The migration patterns of sedimentary particles moving through such a constriction are subject to these vertical variations in velocity.

The current study was performed to examine the influence of discharge on the differential steering developed as flow is forced around a stable object, and to analyze adjustments in the backwater zone and the low-velocity zone in particular. Two thousand velocity measurements were taken over four flows, with reached-averaged velocities ranging from 0.66 m/s to 1.26 m/s. A systematic variation in the cross-stream component of velocity with depth exists under all discharges studied, and illustrates how the flow field in the vicinity of a SBF may change during the passage of a flood wave. Sediment migration paths can be inferred from changes in the local flow field, and implications for deposition and bar formation are addressed.