HR: 16:15h
AN: H32I-02    [PDF]
TI: Spatial Variability in Surface and Subsurface Grain Sizes in a Meandering Gravel-Bed Stream
AU: * Clayton, J A
EM: claytonj@colorado.edu
AF: University of Colorado, Boulder, Geography Department 260 UCB, Boulder, CO 80309-0260 United States
AU: Pitlick, J
EM: pitlick@colorado.edu
AF: University of Colorado, Boulder, Geography Department 260 UCB, Boulder, CO 80309-0260 United States
AB: Predictions of bed load transport in gravel streams with natural topography and curvature are complicated by spatial variations in the flow field and heterogeneities in the bed surface texture. Locally, differences in the ratio of surface to subsurface grain size, or degree of armoring, should reflect differences in the local transport rate for a given discharge and sediment supply, but in any reach these differences should balance in order to maintain equilibrium transport. Detailed measurements of bed topography, surface texture, and substrate sediment size were made in two gravel-bed reaches of the Colorado River in Rocky Mountain National Park in the summer of 2002. The data set for the upper site includes 95 surface pebble counts and 55 bulk samples of the substrate. The data set for the downstream site includes 73 surface pebble counts and 40 bulk samples of the substrate. Overall, the median grain size, D$_{50}$, of the bed surface at the upper site averages 23 mm, and the average D$_{50}$ of the substrate is 12 mm. At the lower site the bed surface D$_{50}$ averages 18 mm, and the average D$_{50}$ of the substrate is 11 mm. Maps of the substrate illustrate that the subsurface grain sizes varied substantially through both bends, and local values appear to be related to the local surface grain sizes (r$^{2}$=0.44, p$<$0.001 and r$^{2}$=0.66, p$<$0.001 for upstream and downstream bends respectively). Ratios of surface to subsurface grain size were compared for regions greater than and less than the mean bankfull depth using a t-test. At the upstream site there were no significant differences in the degree of armoring between these two regions (p=0.68). However, at the downstream site the region with depths greater than bankfull was significantly less armored than the rest of the reach (p=0.01). Mapped values of the ratio of surface to substrate D$_{50}$ for the upstream bend are irregular but nonetheless show that bed armoring is rather spatially uniform at the upstream crossing and bend apex. Downstream from the apex the region of low armoring is confined to a narrow band along the outside bank. Mapped values of bed armoring were much more systematic at the downstream site, implying that the spatial partitioning of bed load size fractions increases with reach radius of curvature. To test this hypothesis, nearly 190 coupled measurements of flow and sediment transport were made at the upstream site during the summer of 2003 at numerous locations within the bend. The majority of these measurements were made at flows greater than bankfull and several captured mobile gravel as large as 64mm. Preliminary results suggest that bed load sample bulk weights mirror the aforementioned pattern of armoring.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting