71 research outputs found
Limits to scale invariance in alluvial rivers
Assumptions about fluvial processes and processâform relations are made in general models and in many siteâspecific applications. Many standard assumptions about reachâscale flow resistance, bedâmaterial entrainment thresholds and transport rates, and downstream hydraulic geometry involve one or other of two types of scale invariance: a parameter (e.g. critical Shields number) has the same value in all rivers, or doubling one variable causes a fixed proportional change in another variable in all circumstances (e.g. powerâlaw hydraulic geometry). However, rivers vary greatly in size, gradient, and bed material, and many geomorphologists regard particular types of river as distinctive. This review examines the tension between universal scaling assumptions and perceived distinctions between different types of river. It identifies limits to scale invariance and departures from simple scaling, and illustrates them using large data sets spanning a wide range of conditions. Scaling considerations and data analysis support the commonly made distinction between coarseâbed and fineâbed reaches, whose different transport regimes can be traced to the different settlingâvelocity scalings for coarse and fine grains. They also help identify two endâmember subâtypes: steep shallow coarseâbed âtorrentsâ with distinctive flowâresistance scaling and increased entrainment threshold, and very large, lowâgradient âmega riversâ with predominantly suspended load, subdued secondary circulation, and extensive backwater conditions
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