Aerodynamic roughness lengths over movable beds: Comparison of wind tunnel and field data

Abstract

[1] We evaluate the increase in apparent roughness length during aeolian saltation. Using a data set comprising 291 wind profiles measured in wind tunnels or in the field, we tested the Charnock (1955), modified Charnock (Sherman, 1992), and Raupach (1991 relationships for the entire data set and for segregated wind tunnel and field data sets. We assessed the abilities of the models to predict enhanced roughness and investigated scaling differences between wind tunnels and field environments. For the entire data set, the Charnock relationship worked best, with an r 2 of 0.25. For the segregated data sets, the Charnock relationship also worked best, with an average r 2 of 0.675, as opposed to 0.66 for the modified Charnock and 0.61 for the Raupach model. For conditions with shear velocity 1.0 ms À1 , the comparable, average values of r 2 are 0.54, 0.625, and 0.645, respectively. There was an order of magnitude difference in the roughness length responses for wind tunnel and field data for each model evaluated, convincing evidence of a fundamental scaling constraint in the wind tunnel data. The scaling differences are attributed to conceptual and bias errors that include the use of the law of the wall versus the wake-defect law or differences in turbulence characteristics, especially regarding the role of large coherent structures. Raupach's (1991) A was found to be an extremely volatile constant, making the modified Charnock relationship most suitable for predicting aerodynamic roughness lengths over movable sand beds.

Field Measurement of Aeolian Sand Flux Using Ceramic Sand Flux Sensor Ud-101 at a Sand Dune

“…As shown in Fig. 7, plots at point F scattered around the logarithmic law curve of c 0 = 0.05 and those at point C scattered around the curve of c 0 = 0.29, indicating that the law with the Charnock constant could be applied to the profile at the measurement points; these values are similar to values at a range of 0.01 to 0.24 in fields (Sherman and Farrell 2008). c 0 at point C was six times as large as that at point F. This could indicate that c 0 over slope was larger than that over flat.…”

“…c 0 is Charnock constant, for which Raupach (1991) gave 0.02 for wind tunnel experiments and 0.16 for beaches. Sherman and Farrell (2008) demonstrated that the…”

Mechanisms limiting the growth of aeolian megaripples

“…First, we take into account the effect of saltation. On a flat surface, z 0 is a function of both the grain diameter and the excess shear velocity and can be approximated by [Sherman and Farrell, 2008] …”

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

“…They found that shrubs bunched into patches resulted in far greater sediment fluxes than in the case of regular grids of single shrubs, likely because of a significant increase in turbulence between the patches. However, the length scales of saltation in wind tunnels has been shown to differ from those of natural saltation [128][129][130], so findings from wind tunnel studies should be treated with some caution.…”