7. Bedforms in coarse-grained channels Step-pool units Cluster bedforms Riffle-pool sequences

Step-pool units Usually observed along headwater segments Stream bed characterized by cobbles and boulders Bed organized into a sequence of steps and pools Overall, steep gradients, shallow, fast flows. Stable bed conditions (very little, if any, bedload transport under most flow conditions).

Lots of recent research on the hydraulics, morphology and stability of step-pool systems. Under most flow conditions, flow depth is less than size of largest clasts. Often observed when gradients are between about 2 and 4 degrees.

Cluster bedforms Form during the course of bedload transport Characterized by a clear longitudinal “sorting” pattern (organization of particles on the bed according to size). Largest particles (”obstacles”) play a major role in the formation of cluster bedforms. Ubiquitous (widespread) features in naturally-sorted coarse-grained rivers

Cluster bedforms Accumulation of coarse, imbricated pebbles in the front of obstacles during bedload transport Fine sediment “trapped” in the lee of obstacles during transport (because of flow separation). More likely to form during falling stage of hydrograph (i.e. after peak in discharge corresponding to significant precipitation events)

Riffle-pool sequences Very common in coarse-grained channels Large-scale bedforms (or bed undulations) that scale with channel width (i.e. the average spacing between high points along bed profile is of the order of 5-7 channel widths) Observed in relatively straight channels but better developed in meandering rivers In meander bends, deep sections (pools) associated with meander bends; riffles (shallow sections) located in-between successive pools

Pools – riffles: distinctive characteristics PoolsRiffles. Finer bed material. Coarser. Gentle water surface. Steeper slope slope. Deep sections. Shallow. Low velocity. Faster flows. Narrow cross-sections. Wider These characteristics, however, are stage dependent.

Velocity reversal hypothesis Hypothesis suggested to explain the presence of coarser sediment in riffles It is suggested that at peak flows, the velocity in pools (or the bed shear stress) exceeds that in riffles Peak flow velocities may be competent to transport coarse sediment throughout a reach. However, large particles scoured from the pool would enter a zone of slower flow in the downstream riffle, where deposition may occur.

…Velocity reversal hypothesis At low flows, flow competence may be sufficient to transport sand and fine gravel. At some point (as discharge decreases), sand or fine gravel deposited in pool where flow suddenly decelerates (after being carried over the riffle by its faster flow). Which flow variable(s) should be used to adequately test this hypothesis? –Near-bed flow velocity –Bed shear stress Conflicting evidence in the literature on the presence of velocity reversal at peak flows

Formation of pools and riffles Impossible from field measurements to determine how riffle-pool sequences are formed. Riffles and pools are stable features (do not migrate downstream); bed sedimentology and morphology may not change significantly over time. Evolve to a large extent in conjunction with meander bend migration (if any).

…formation of riffle-pool sequences What controls in the initial formation of these features? Yalin provided this interesting suggestion: –Suggested that the turbulence generated at the boundary in a straight channel would produce large scale eddies –These eddies, in turn, are associated with alternative acceleration and deceleration of the flow Once initiated, the sinuosity of the channel will also further enhance localized erosion/deposition (which, in turn, may lead to further development of pools and riffles and meander development)

Bedforms in rivers Important points: –Created by sediment transport processes –Affect flow characteristics –Are affected by flow properties –Clear distinction between sand- and gravel-bed rivers –Best example perhaps of set of interactions flow- sediment transport – bed morphology (cannot understand one without addressing the others)