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Water Science and Applications



  • Paleohydrology
  • Floods

Index Terms

  • 1815 Hydrology: Erosion and sedimentation
  • 1821 Hydrology: Floods
  • 1827 Hydrology: Glaciology
  • 1824 Hydrology: Geomorphology



Initial motion of boulders in bedrock channels

P. A. Carling, M. Hoffmann, and A. S. Blatter

Natural boulders situated on flat or rugose bedrock channel surfaces may be isolated or they may occur in simple or complex groups. Grouped boulders may be imbricated with flow-normal spans (B) comparable with the channel width (W). However, before the hydraulic stability of complex boulder bedforms can be studied, an evaluation of entrainment conditions for isolated boulders and simple groupings of boulders is required. The problem reduces to a consideration of (i) isolated boulders; (ii) stacks of a few boulders of limited span (B<W;) and (iii) stacks of many spanwise boulders (BW). In this paper, preliminary results pertaining to the entrainment of isolated cubes on bedrock are reported. Latterly, data from two published studies of simple grouped blocks are re-evaluated and finally, examples of relatively simple natural boulder bedforms on bedrock are presented. Flume experiments quantify the force balance equation for initial motion of isolated smooth-faced cubes and oblongs. Flume experiments consisted of two parts: (1) Initial motion on a smooth concrete surface; (2) Initial motion on an artificially-roughened concrete surface simulating bedrock positive and negative irregular steps up to 2mm in height transverse to the flow. Three different cases of initial motion were recorded: (1) the block is emergent and pivots out of place; (2) the block is emergent and slides out of place; (3) the block is submerged and slides out of place. Thus initial motion by pivoting is induced by the flow characteristics associated with emergence. Due to the turning moment, the friction coefficient (μf) for the smooth bed was not constant, but on the simulated bedrock surface the friction coefficient was approximately constant. For the smooth concrete bed, with maximum contact between block and bed, μf was 0.80 for sandstone and 0.60 for slate. For the simulated bedrock the number of points of contact were reduced such that μf equaled 0.50 and 0.59 for the two rock types, respectively. In a natural smooth-bedded limestone channel μf was 0.81 for sandstone blocks, 0.73 for slate and 0.70 for natural limestone blocks derived from the channel bed. The variation in the drag coefficient (Cd) for conditions of initial motion was related to particle orientation (L/B) and relative water depth (h/d). For the fully submerged case Cd = 0.95, but for emergent conditions Cd increased rapidly as relative depth was reduced. For palaeohydraulic estimates of flow parameters it may be concluded that the friction coefficient is constant. Setting Cd equal to 0.95 is appropriate where blocks were deeply submerged ((h/d)(L/B) > 2) and entrained by sliding. However, where blocks were in shallow water or emergent ((h/d) (L/B) < 2), increasingly large values of Cd pertain as h/d decreases.

Citation: Carling, P. A., M. Hoffmann, and A. S. Blatter (2002), Initial motion of boulders in bedrock channels, in Ancient Floods, Modern Hazards: Principles and Applications of Paleoflood Hydrology, Water Sci. Appl., vol. 5, edited by P. K. House et al., pp. 147–160, AGU, Washington, D. C., doi:10.1029/WS005p0147.

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