Laserfiche WebLink
Thresholds for motion of cobble- and gravel-sized sediment (framework grains) were <br />estimated from the relation for dimensionless shear stress: <br />ti <br />ti = BPS - P) g DSO (1) <br />where ps and p are the densities of sediment and water, respectively, g is the gravitational <br />acceleration, and D50 is the median grain size of the bed surface-(armor layer). In a simple <br />physical sense, the variable T* represents a balance between the fluid forces acting to move <br />particles on the bed versus the resistance due to their mass. Movement of a small number of, <br />framework grains begins when r* exceeds a threshold or critical value, T*,. Results from field <br />and laboratory studies suggest that values of z*c may be affected by several factors, including <br />particle shape [Gomez, 1995], sand content [Wilcock, 1998], spatial variations in bed texture and <br />bed structure [Lisle et al., 2000; Konrad et al., 2002; Church et al., 1998], and average channel <br />gradient [Mueller et al., 2005]. In addition, there is a practical problem of defining the degree of <br />bed mobilization, i.e. whether transport involves only a few coarse clasts or many clasts. Finally, <br />some gravel rivers can carry significant amounts of sand-sized sediment; this is potentially a <br />problem because sand moves at flows much lower than those required to move the coarser clasts, <br />and it can move either as bed load or suspended load, depending on the flow. Such is the case in <br />the Colorado River. Thus, it is possible to distinguish three phases of bed load transport: the first <br />phase, involving movement of sand and granules over an otherwise stable bed surface, is termed <br />overpassing (Ashworth and Ferguson, 1989); this sediment is not the same as `wash load' <br />(sediment supplied from sources other than the bed itself), and in fact may represent a significant <br />proportion (> 20 %) of the total bed load of a gravel river system (this point is pursued in detail <br />later). The second transport phase, involving sporadic motion of small to moderate percentages <br />of the framework grains, is termed partial transport [Wilcock and McArdell, 1995]. A recent <br />18