Characterization of wind-blown sediment transport with height in a highly mobile dune (SW Spain) (original) (raw)
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The Valdevaqueros dune is located at one of the windiest points of Europe, where the frequent occurrence of strong easterly winds has generated a highly mobile dune. Several rotating cup anemometers in vertical array and a self-designed vertical sand trap, were placed to retain the drift sands at different heights over the surface in order to determine theoretical and actual sand transport rates in the Valdevaqueros dune system. General results show that 90% of the wind-blown sand is transported within the first 20cm above the dune crest surface. Theoretical transport rates based on different empirical formulae were 0.33 to 0.78 times the in-situ sand transport rate detected, which was 2.08·10-2kgm-1s-1 under moderate wind power (mean speed ranging from 8.4 to 17.9ms-1). Analysis of different statistical grain-size parameters helped to understand sand transport distribution at different heights.
Assessment of highly active dune mobility in the medium, short and very short term
Geomorphology, 2011
Dune activity or stability has usually been studied over long time periods; however, this may not reflect changes that occur in the short term, especially for highly active dunefields. Extreme wind conditions that are generated near the Strait of Gibraltar (SW Spain) have given rise to the transgressive Valdevaqueros dunefield. The current work focuses on analyzing the sand drift potential and the evolution of the dune profile in the medium term (months), the short term (days) and the very short term (hours). Topographic data, which were collected with a differential GPS, were interpreted from reconstructed empirical orthogonal functions (EOF). The results showed that generally the dune profile presented shifting morphologies, especially around the crest and brink, and a trend towards migration to a gentler steady state. As a result, the leeward side adopted continuous slope variations during the different survey periods, whereas the windward slope did not undergo any significant change. Lateral and vertical displacements were analyzed during a severe easterly sandstorm, when the dune brink experienced an advance migration rate of 1.75 m in 24 h. Sand transport rates of 25.5-36.5 m 3 m − 1 month − 1 , 22.52 m 3 m − 1 day − 1 and 0.93 m 3 m − 1 h − 1 were measured for the medium term, short term and very short term, respectively. These values were compared to the theoretical sand transport rate for Valdevaqueros dune, based on the classic Bagnold equation as well other more recent formulae, to obtain a ratio between the real and the theoretical rates for each study period. These results together with the sand drift potential (up to 10,000 vector units) demonstrate that Valdevaqueros (Tarifa) is a dunefield with one of the highest sand transport capacities in Europe.
Sand Transport Under the Action of Wind
Coastal Engineering 1980, 1980
Field investigations on sand-transport by wind were carried out at German North-Sea coast. A special trap was constructed, which allows to measure bed load and suspended load seperated. In addition wind speed was measured up to 10 m over the bottom. The results of the field investigations were used to calibrate a transport-equation for sandtransport by wind.
Variations in wind velocity and sand transport on the windward flanks of desert sand dunes
Sedimentology, 1985
The magnitudes of increases in wind velocity, or speed-up factors, have been measured on the windward flanks of transverse and linear dunes of varying height. On transverse dunes, velocity speed-up varied with dune shape and height. For linear dunes, speed-up factors varied principally with wind direction relative to the dune, with dune shape and dune height. The main effect of velocity speed-up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas. This is greatest for large dunes, with winds of moderate velocity blowing at a large angle to the dune. Changing ratios of base to crest sand-transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase. On linear dunes, the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi-directional wind regime.
Wind direction and complex sediment transport response across a beach–dune system
Evidence from a field study on wind flow and sediment transport across a beach–dune system under onshore and offshore conditions (including oblique approach angles) indicates that sediment transport response on the back-beach and stoss slope of the foredune can be exceedingly complex. The upper-air flow – measured by a sonic anemometer at the top of a 35m tower located on the dune crest – is similar to regional wind records obtained from a nearby meteorological station, but quite different from the near-surface flow field measured locally across the beach–dune profile by sonic anemometers positioned 20 cm above the sand surface. Flow–form interaction at macro and micro scales leads to strong modulation of the near-surface wind vectors, including wind speed reductions (due to surface roughness drag and adverse pressure effects induced by the dune) and wind speed increases (due to flow compression toward the top of the dune) as well as pronounced topographic steering during oblique wind approach angles. A conceptual model is proposed, building on the ideas of Sweet and Kocurek (Sedimentology 37: 1023–1038, 1990), Walker and Nickling (Earth Surface Processes and Landforms 28: 111–1124, 2002), and Lynch et al. (Earth Surface Processes and Landforms 33: 991–1005, 2008, Geomorphology 105: 139–146, 2010), which shows how near-surface wind vectors are altered for four regional wind conditions: (a) onshore, detached; (b) onshore-oblique, attached and deflected; (c) offshore, detached; and (d) offshore-oblique, attached and deflected. High-frequency measurements of sediment transport intensity during these different events demonstrate that predictions of sediment flux using standard equations driven by regional wind statistics would by unreliable and misleading. It is recommended that field studies routinely implement experimental designs that treat the near-surface wind field as comprising true vector quantities (with speed and direction) in order that a more robust linkage between the regional (upper air) wind field and the sediment transport response across the beach–dune profile be established.
Sediment transport distribution along equilibrium sand dunes
The present study focuses on distribution of sediment transport along mobile dunes in equilibrium. To this end, using ACVP (Acoustic Concentration and Velocity Profiler), we have obtained simultaneous, co-located, high temporal-spatial resolution measurements of the multi-component flow velocity and suspended sediment concentration above dunes. In contrast to previous measurements of flow and sediment dynamics above dunes which are mostly carried out with more than one instrument, we are now able to address sediment fluxes directly for flow scales smaller than the separation distance between different instruments. In this paper, preliminary ACVP results are shown in terms of flow velocities, suspended sediment concentrations and suspended sediment fluxes along dune profiles .
Dune morphology and sand transport
We performed a computer simulation on patterns and dynamics of desert dunes. Dune patterns observed in deserts were reproduced. From the initial random state, barchans and linear dunes are produced, depending on the variability of the wind direction. The efficiency in sand transport is calculated through the course of development. We found that the sand transport is the most efficient in the linear transverse dune, barchans next, and the least when no pattern is formed. The efficiency in sand transport always increased through the evolution, and the way it increase was stepwise. We also found that the shadow zone, the region where the sand wastes the chance to move, shrinks through the course of evolution.
Geomorphology, 2002
Studies of the particle size and sorting characteristics of sand on the stoss slope of a 6-m high reversing dune show that the sand in transport is generally finer and better sorted than surface sand at the same position on the slope. The sand in transport becomes coarser and more poorly sorted as wind speed and rates of mass transport increase toward the dune crest. These patterns reflect changes in the competence of the wind, which is capable of transporting larger grains and a wider range of grain sizes as its speed increases in space and time. Our field observations suggest that the particle size and sorting characteristics of surface sand are highly dependent on antecedent wind conditions and are not an invariant property of the dune, as is widely assumed. The wide range of particle sizes on the surface, as well as its change through time, also has important implications for modeling sediment transport on dunes. Transport thresholds may vary by as much as 30% on the stoss slope of the study dune.
Wind Distribution Over Beach/Dune Surface: Improving Aeolian Sand Transport Estimation
2002
Aeolian sand transport estimation is extremely important to calculate for sediment budgets in beach/dune systems. However, the number and location of high frequency sampling (> 1Hz) devices deployed on the beach/dune systems still have some limitations, related basically to equipment budget and its integrity during high energy events. Field experiments took place in Portugal, mainly along the SW and S coasts, over long periods (weeks to almost 2 years) on medium energy, non-dissipative beaches with 0.3 mm to 0.5 mm and moderate to well sorted quartz sand. Data provided by all monitoring devices was used to feed a well-tested flow model and shear velocity was calculated at every node of the monitored beach surfaces. Although longshore wind events were affected little by beach morphology, cross-shore wind events on the other hand resulted in well-defined, heterogeneous shear velocity zones along the beach profile. Relative shear velocity values varied from a minimum on the berm to ...