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    Maintenance of pool morphology in a stream channel with a mobile bed requires hydraulic conditions at moderate to high flows that route bed load through the pool as it is delivered from upstream. Through field measurements of discharge, vertical velocity profiles, bed load transport, and streambed scour, fill, and grain-size distribution, we found that maintenance of a pool associated with a large, in-channel obstruction was more adequately explained by analogy to scour processes at bridge abutments than by the concept of velocity reversal. The cross-sectional area through the deepest part of the pool remained remarkably stable throughout a wide range in discharge magnitude. This occurred despite transport of significant quantities of bed load from upstream through the pool, well above and well below bankfull discharge and on rising as well as falling hydrograph limbs. There was no tendency for near-bed velocity or shear stress (averaged over the cross-sectional width of active bed load transport) at the pool centre to exceed that at the upstream riffle tail (pool head) or downstream riffle head (pool tail) as discharge increased up to at least 1.3 times bankful. Fractional rates indicated that at the pool centre significant bed load transport was initiated and approached 100 g m-1 s-1 at notably lower mean bed shear stress than at the pool head or tail. Furthermore, incipient motion analyses suggested that mean bed shear stress entrainment thresholds were lower at the pool centre than at the pool head or tail. These findings indicated that total entrainment force in the pool was underestimated by average bed shear stress alone. Through inference, rather than direct measurement, we concluded that, as at bridge abutments, turbulent effects generated by interaction of streamflow with the obstruction added a component of total entrainment force. We further inferred that this combination of mean bed shear stress and instantaneous turbulent force was responsible for entrainment and transport of sediment, thereby maintaining pool morphology, This conceptual model of pool maintenance through a combination of mean bed shear stress and large-scale turbulent force suggests that flow obstructions in gravel-bed streams may be a dominant factor, perhaps as important as mean hydraulic variables or caliber and volume of sediment supply, in controlling local channel morphology and local bed load dynamics.

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    Woodsmith, Richard D.; Hassan, Marwan A. 2005. Maintenance of an obstruction-forced pool in a gravel-bed channel: streamflow, channel morphology, and sediment transport. In: Catchment Dynamics and River Processes, Ch. 10: 169-196


    pool-riffle, scour and fill, bed load, flow obstructions, mountain streams

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