Abstract
Boulders and cobbles are often used in stream restoration projects to increase flow resistance and enhance channel stability and habitat diversity. Particle size metrics determined from the particle distribution are often used as a proxy for shear stress in field equations. Clustering of large particles has been thought to contribute to shear stress, but the effect of clustering is not accounted for in equations that use a representative particle size, such as the D84. In this paper, clustering is defined using the upper tail (≥84%) in a variable called Topsum. The number of clusters, average size of clusters, and shear stress are evaluated using the proposed definition of cluster. Findings suggest that the upper tail represents the roughness height better than the commonly used proxy of D84 for boulder bed streams (streams which have a D84 particle 0.05 - 0.15 meters).
Highlights
Boulders and cobbles are often used in stream restoration projects as a way to increase flow resistance and enhance channel stability and habitat diversity [1] [2] [3] [4] [5]
The size of the particles applied to the project is often determined by the desired flow resistance; flow resistance equations do not account for particle clustering or what is known as small-scale particle organization [6] [7] [8] [9]
To evaluate how well the ratio compared to the field-based equation that relates shear velocity to (Equation (1)) to the flume derived equation (Equation (5)) data were graphically compared to the following ratios: d/D84 and the d/Topsum, top channel width/D84, and width channel/Topsum
Summary
Boulders and cobbles are often used in stream restoration projects as a way to increase flow resistance and enhance channel stability and habitat diversity [1] [2] [3] [4] [5]. The size of the particles applied to the project is often determined by the desired flow resistance; flow resistance equations do not account for particle clustering or what is known as small-scale particle organization [6] [7] [8] [9]. Large-scale particle organization or bed forms such as dunes, riffle-pool sequences, and step-pool sequences are well characterized and recognized as a significant factor in flow resistance [1] [10] [11] [12] [13]. Particle flow resistance in natural channels is often determined using the Wolman pebble count method [16]. The Wolman pebble count requires sampling 100 particles along a stream cross section and measuring the intermediate axis. The distribution is linked to the flow hydraulics using the following equation [16]
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