Measurement and Analysis of the Spatial Organization of Bed Particles In Boulder-Bed Streams

Reaches of boulder-bed streams that do not exhibit step-pool morphology are not well characterized, but they can extend several kilometers in length. These reaches often appear in rivers as a transition state from boulder-bed step-pool morphology to gravel-bed riffle-pool morphology. These reaches do not have clearly-defined morphological structures such as a steps or riffles, but the spatial organization of bed particles may still affect the hydraulics of the streams. We measured a set of 6 reaches where we examined the placement of particles within the reach. These data indicated that bed particles are not randomly arranged on the streambed and that bed particle arrangement significantly enhanced bed stability. Detailed bed particle arrangements, however, are not a practical measure for comparison among many stream channels. We examined, therefore, whether aspects of the grain size distribution could be used for bed characterization. In boulder-bed streams, the distribution of bed particles in the upper tail size (> 84%) is more heterogeneous than in lower percentiles. In some cases, the largest particles are not mobile and are often protrude from the flow at discharges less than bankfull. For these reasons, the sum of the large (> 84%) particles sampled across the stream emerges as a good choice to represent a horizontal roughness length, which affects flow in the channel cross-section. This sum of particles greater than 84% is referred to as topsum in this research. The topsum metric is intended as a horizontal length scale to represent the potential size of a step structure and constrictions in the channel width. We examined 50 field sites on boulder and cobble bed streams with varying particle size and sorting, we found topsum to be linked to both particle size and sorting. It also provides a horizontal roughness length scale that can be used to explain some of the variations in flow resistance among reaches.