Dynamic transport capacity in gravel-bed river systems

Sediment transport capacity mediates the transfer and storage of bed material between alluvial reservoirs in a drainage system. At intermediate time scales (100-102 yr) corresponding to the evolution of sediment pulses, conditions governing bed-material transport capacity under the hydrologic regime respond to variations in storage and sediment flux as pulses extend, overlap, and attenuate through the network. Relations between transport and storage in degrading gravel-bed channels and general relations between bed-load transport rate and tractive force have similar inflection points marking the inception of armoring. This suggests that in a sediment-charged river where armoring is weak and transport rates are high, transport rates respond weakly to changes in sediment supply, and the channel adjusts primarily by erosion or deposition. At some stage in degradation, armoring commences and responds to variations in sediment input rates, causing sediment transfer rates to decline rapidly as supply and storage decline. However, it appears unlikely that the simple, linear form of transport-storage relations for degrading channels can be extrapolated to full aggradation-degradation episodes: (1) Simulations of transport rates in an initially uniform reach of a natural channel that was affected by an evolving sediment wave show longitudinal variations in relations between bed elevation and sediment flux. (2) In a flume experiment where stepped increases and decreases in feed rate forced episodes of aggradation and degradation, sediment output rates increased after feed rate was reduced and the channel began to degrade. The resulting hysteresis in relations between transport rate and storage was driven by changes in bed texture and channel morphology that increased bed mobility during early stages of degradation before armoring intensified and reduced transport.