Potential inertial effects in aeolian sand transport: preliminary results (original) (raw)
1996, Sedimentary Geology
Most models to predict aeolian sand transport rates incorporate a threshold shear velocity term to specify the condition when sand flux begins. This term usually takes the form derived by the equation of Bagnold (1936). When shear velocity falls below the threshold value, the transport equations predict an immediate cessation of sediment movement. However, under some conditions this is not so and transport may continue for a period. This paper describes field observations of an inertia-like process that be considered when wind velocity fluctuates at near-threshold values. This study found that, although wind conditions had decreased to below threshold levels, transport was still taking place. Therefore using standard models that incorporate a threshold term, an underestimation of sediment transport would occur.
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Modelling aeolian sand transport using a dynamic mass balancing approach
Knowledge of the changing rate of sediment flux in space and time is essential for quantifying surface erosion and deposition in desert landscapes. Whilst many aeolian studies have relied on time-averaged parameters such as wind velocity (U) and wind shear velocity (u ⁎) to determine sediment flux, there is increasing field evidence that high-frequency turbulence is an important driving force behind the entrainment and transport of sand. At this scale of analysis, inertia in the saltation system causes changes in sediment transport to lag behind de/accel-erations in flow. However, saltation inertia has yet to be incorporated into a functional sand transport model that can be used for predictive purposes. In this study, we present a new transport model that dynamically balances the sand mass being transported in the wind flow. The 'dynamic mass balance' (DMB) model we present accounts for high-frequency variations in the horizontal (u) component of wind flow, as saltation is most strongly associated with the positive u component of the wind. The performance of the DMB model is tested by fitting it to two field-derived (Namibia's Skeleton Coast) datasets of wind velocity and sediment transport: (i) a 10-min (10 Hz measurement resolution) dataset; (ii) a 2-h (1 Hz measurement resolution) dataset. The DMB model is shown to outperform two existing models that rely on time-averaged wind velocity data (e.g. Radok, 1977; Dong et al., 2003), when predicting sand transport over the two experiments. For all measurement averaging intervals presented in this study (10 Hz–10 min), the DMB model predicted total saltation count to within at least 0.48%, whereas the Radok and Dong models over-or underestimated total count by up to 5.50% and 20.53% respectively. The DMB model also produced more realistic (less 'peaky') time series of sand flux than the other two models, and a more accurate distribution of sand flux data. The best predictions of total sand transport are achieved using our DMB model at a temporal resolution of 4 s in cases where the temporal scale of investigation is relatively short (on the order of minutes), and at a resolution of 1 min for longer wind and transport datasets (on the order of hours). The proposed new sand transport model could prove to be significant for integrating turbulence scale transport processes into longer-term, macro-scale landscape modelling of drylands.
SPATIAL AND TEMPORAL VARIABILITY IN INTENSITY OF AEOLIAN TRANSPORT ON A BEACH AND FOREDUNE
2003
This paper presents results from measurements of the intensity of sand transport by wind on the beach and stoss slope of a vegetated foredune over one day, at Greenwich Dunes, Prince Edward Island, Canada. Measurements of wind speed and direction were made with arrays of cup anemometers and 2-D sonic anemometers. Sediment transport intensity was measured at a height of
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