Experimental study of sand streaks formed in turbulent boundary layers

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Abstract
Fine‐ to medium‐grained sand transported as bedload moves in lanes parallel to the flow that are thought to be preserved as parting lineation. A series of six flume experiments was designed to discover the morphology and spacing of these lanes, here called sand streaks, as functions of local shear velocity, U* (9 × 10‐3 to 4.8 × 10‐2 m s‐1), depth (5 × 10‐2 and 9.5 × 10‐2 m), mean grain diameter (150, 200, 290, 1380 μm), and sediment bedload concentration (0.0–0.39). Low U* flows produce predominantly straight, non‐intersecting sand streaks, moderate U* flows produce sub‐parallel and en échelon sand streaks, and moderate to high U* flows produce wavy sand streaks and secondary streaks with a spacing an order of magnitude larger. The wavy sand streaks are thought to be composed of sand grains in suspension close to the bed. An upper grain‐size limit for the sand streak structure occurs at a grain size between 290 and 1380μm. The spacings of the fine‐and medium‐grained sand streaks, at low to moderate U* (0.9 × 10‐2 to 3 × 10‐2m s‐1), are similar to those predicted for low‐speed fluid streaks, although the fine‐grained sand forms more closely‐spaced streaks than the medium‐grained sand. The spacings of sand streaks formed at moderate to high U* and at bedload concentrations greater than 0.15, are wider than those predicted for the low‐speed fluid streaks. The wider spacing is thought to reflect a new type of flow immediately above the moving bed layer in which the formation of low‐speed streaks is inhibited. This results from an increase in either grain concentration or grain size. The spacing of parting lineation, also wider than that predicted for low‐speed streaks, may reflect this.