GRAIN-SIZE PATCHINESS AS A CAUSE OF SELECTIVE DEPOSITION AND DOWNSTREAM FINING

One facet of the debate about the hypothesis of equal mobility of all grain sizes in a mixture is that equal mobility seems contrary to fiel d evidence for selective deposition as an important mechanism of downs tream fining. We argue that regardless of the ultimate outcome of the equal mobility debate, variability in local mean grain size across a r each can give rise to strong selective deposition even if locally equa l mobility is satisfied exactly. We consider a system in which the sed iment is arranged into locally well mixed zones (''patches'') whose me an grain size varies randomly across the reach. We assume that the she ar stress is randomly distributed with a mean value near the critical value for the reach-averaged median size, that the variance in stress scales with the variance in grain size, and that the development of fi ne patches in areas of high shear stress is supply limited. The main c ontrols on fining rate are then the ratio of mean stress to critical s tress for the section-averaged mean grain size and the ratio of the pa tch standard deviation (assumed constant) to the standard deviation of patch means. In addition, in any system in which fining occurs by sel ective transport and deposition, the fining rate is strongly influence d by the spatial distribution of deposition rate.