CGRG Bibliography of Canadian Geomorphology
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Author : Church, M.; and Zimmermann, A.
Date : 2006.
Title : Streambed Hardening: the production of self-organized conditional stability.
Publication : Eos Transactions. AGU,
Issue : 87(52), Fall Meeting Supplement,
Page(s) : Abstract H51G-0581.
Abstract
Gravel, cobble and boulder stream beds are strengthened by the interlocking arrangement of adjacent grains to have greater reluctance to be entrained than is afforded by the inertia of individual clasts. Imbrication is the most common example. Experiments demonstrate that structures develop by local movements of mostly larger grains from less stable positions to more stable ones in conditions of low net transport. The process promotes the "clumpy" release and transport of mainly smaller bed grains. Individual entraining events signify failure and "repair" of weak spots in the bed analogous to strain hardening in metals by sequential dislocations in the crystalline structure. The result is to "harden" the bed against stresses near threshold so that the critical Shields number rises to values in the range 0.06 to 0.08. This outcome identifies the Shields number as a bed state parameter. It is reduced to a grain stability parameter (its traditional interpretation) only in the absence of structural effects. Stability remains conditional on the flow history: a state of self-organized conditional stability. Sufficiently high flows surpass a critical limit for general mobilization and the bed becomes absolutely unstable. Sand beds characteristically are absolutely unstable.
Bibliography of Canadian Geomorphology