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Correlation of aeolian sediment transport measured by sand traps and fluorescent tracers
Journal of Marine Systems 80 (2010) 235–242
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Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s
Correlation of aeolian sediment transport measured by sand traps and fluorescent tracers Laura L. Cabrera ⁎, Ignacio Alonso Department of Física, University of Las Palmas de Gran Canaria, Campus de Tafira s/n, 35017-Las Palmas, Spain
a r t i c l e
i n f o
Available online 20 October 2009 Keywords: Transport rates Sand traps Fluorescent tracer Wind speed Nebkha
a b s t r a c t Two different methods, fluorescent tracers and vertical sand traps, were simultaneously used to carry out an aeolian sediment transport study designed to test the goodness of fluorescent tracers in aeolian environments. Field experiments were performed in a nebkha field close to Famara beach at Lanzarote Island (Canary Islands, Spain) in a sector where the dunes were between 0.5 and 0.8 m height and 1–2 m wide and the vegetal cover was approximately 22%. In this dune field the sediment supply comes from Famara beach and is blown by trade winds toward the south, where the vegetation acts as natural sediment traps. Wind data were obtained by means of four Aanderaa wind speed sensors and one Aanderaa vane, all them distributed in a vertical array from 0.1 to 4 m height for 27 h. The average velocity at 1 m height during the experiment was 5.26 m s− 1 with the wind direction from the north. The tracer was under wind influence for 90min at midday. During this period two series of sand traps (T1 and T2) N, S, E and W oriented were used. Resultant transport rates were 0.0131 and 0.0184 kg m− 1 min− 1 respectively. Tracer collection was performed with a sticky tape to sample only surface sediments. Tagged grains were visually counted under UV light. The transport rate was computed from the centroid displacement, that moved 0.875 m southwards, and the depth of the active layer considered was the size of one single grain. Taking into account these data the transport rate was 0.0072 kg m− 1 min− 1. The discrepancy in results between both methods is related to several factors, such as the thickness of the active layer and the grain size difference between the tagged and the native material. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Aeolian sediment transport has been an important topic in earth sciences for decades, in particular in any study dealing with dunes and desert environments. The theoretical background of aeolian transport was established by Bagnold (1941), who stated that the discharge of sediments could be predicted by: 0:5
q = Cðd=DÞ
3
ρ = gu*
where C is an empirical coefficient related to the sediment sorting, d is the mean sediment diameter, D is a reference grain size of 0.25 mm, g is the gravitational acceleration, ρ is the air density and u* is the shear velocity. Bagnold (1941) assigned values of C ranging from 1.5 for nearly uniform sand, to 1.8 for typical dune sands, to 2.8 for moderately to poorly sorted sands, to a maximum of 3.5 over hard surfaces. Nevertheless Bagnold's equation was derived from laboratory observations and basic principles of physics, and it is valid when
⁎ Corresponding author. E-mail addresses: [email protected] (L.L. Cabrera), ialonso@dfis.ulpgc.es (I. Alonso). 0924-7963/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2009.10.012
applied to ideal surfaces: horizontal, unobstructed, dry, crust-free and free of vegetation. Since then, many authors have presented different formulations for ideal, quasi-ideal and non-ideal surfaces. Howard et al. (1978) and Hardisty and Whitehouse (1988) included modifications for the slope effect. Hotta et al. (1984), Sarre (1988) and Davidson-Arnott et al. (2008) made different findings to assess the influence of water content on sand movement. Nickling (1978) and Nickling and Ecclestone (1981) proposed a modified equation to account for the formation of crusts due to salts concentration. The effect of algae on crusts has been considered by Campbell (1979) and van der Ancker et al. (1985). Finally, the role of vegetation in controlling sediment transport has been widely considered by Hesp (1981), Pye (1983), Kuriyama et al. (2005), and Leenders et al. (2007) among others. Regarding the methods for measuring transport rates, Sherman and Hotta (1990) classify the different methodologies in three conceptually distinct classes: (i) detailed measurements of volumetric changes in source/sink areas; (ii) sediment trapping; and (iii) tracer experiments. We should include a fourth class consisting on the use of aerial photographs and remote sensors. Each one of these approaches has several advantages and disadvantages. The first type of measurement has been used by Illemberger and Rust (1986) in South Africa and by Valdemoro et al. (2007) in Canary
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Islands, among others. The use of traps is broad, since there are many different kinds of trapping devices. Probably the most widely used are the vertical traps described by Leatherman (1978) and Rosen (1978), although other authors have used trench collectors (Greeley et al., 1996), saltiphones (Vanhee, 2004), saltation flux impact responders (Baas, 2004) and other types. Goossens et al. (2000) gives a calibration of five different types of sand traps. Tracers are commonly used for measuring longshore drift (White and Inman, 1989; Vila-Concejo et al., 2004; McComb and Black, 2005; Silva et al., 2007), but not in aeolian environments. The only paper, that we have found, that uses fluorescent tracers in field experimentation to estimate aeolian transport rates was that of Berg (1983). Since then no one seems to have followed this approach. Finally, there are many authors who use aerial photographs to measure dunefield evolution, and to estimate transport rates (Levin and Ben-Dor, 2004; Hernández et al., 2007). Others use remote sensors (Levin et al., 2007; Stockdon et al., 2007). Here, we describe a field experiment carried out on September 2004. The experiment consisted of simultaneous measurements of aeolian transport in a nebkha dunefield using vertical sand traps and fluorescent tracers. The first aim of this study was to test the utility of fluorescent tracers in aeolian studies, and the second was to compare the sand transport measurements obtained by both methods and by Bagnold´s theoretical expression to gain insight into the aeolian transport process. 2. Study area The study area is located on Lanzarote Island (Canary Islands, Spain). This island is crossed from the northern to the southern coast by a sand strip called El Jable, where aeolian sediments cover an area of nearly 90 km2 (Fig. 1). In the northeast of El Jable there is a beach 4 km long called Famara Beach which is the input zone of marine sediments migrating inland, and an small dunefield of about 0.5 km2 mainly formed by nebkhas (Fig. 2). Since the beginning of the XX century, El Jable has evolved from a barchans dunefield to a sand sheet that in most places is only a few
centimetres thick, with a few barchan dunes on it (Cabrera and Alonso, 2006). Prevailing tradewinds blow steadily from the NNE from April until October. The rest of the year the wind is much more variable, both in direction and intensity. The nebkha dunefield cannot be properly defined as a foredune, since rather than a continuous dune ridge, it is formed by isolated dunes associated with discrete plant species. This vegetation shows gradual changes both in surface cover and dominant species depending on the distance to the shoreline. Considering its effect on sediment transport, Traganum moquinii is the dominant species closer to the shoreline, since it can reach up to 3 m height and the associated shadow dunes may reach up to 10 m length. Moving inland vegetation gradually changes to Launaea arborescens, which due to the smaller height and size forms smaller dunes ranging between 0.5 and 0.8 m height and 1– 2 m large). Both species are shrubs and in both of them nebkhas are perennial, showing a clear correlation between dune height and spacing, as stated by Tengberg (1995). The surfaces between the nebkhas are sandy and become bare during the whole year except during rainy periods at winter time. During these periods the area is more humid and it is possible to find herbaceous species. The contribution of these species to fix the sand is limited, since they die in few weeks after the rain. Sediments in the nebkha dunes have both bioclastic and terrigenous components, with a 55–60% in carbonate content derived from bioclasts (Cabrera et al, 2006).
3. Methods 3.1. Site selection and wind measurements The experiment site was located 500 m from the shoreline in an area where Launaea arborescens is the dominant species. The vegetation cover was about 22% estimated from field observations and very detailed aerial photographs of the area (Fig. 2).
Fig. 1. Location map of the study area.
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Fig. 2. Closer aerial view of the experiment area.
Field experiment was developed in a sector where the nebkhas are mid-size: around 2 m wide and 0.5 m high. Both traps and tracer experiments were located in sandy clear zones between the nebkhas, in order to minimize the topographic influence in the aeolian transport. Selected areas for both sand traps and fluorescent tracers were very similar in bush density and bedforms. Wind ripples in the area are about 10 cm wavelength (Figs. 3 and 4). The sand traps and the tracers were working simultaneously, so that both of them were exposed to the same conditions of wind, surrounding obstacles and sediment. During the experiments wind data were recorded by means of one anemometer mast 4 m high consisting of 4 Aanderaa three-cup anemometers located at 0.125, 0.5, 1.0 and 4.0 m and 1 Aanderaa vane at 2.0 m above the surface. The recording interval was 1 min and the recording period was from 17:00 on 6 September until 20:00 the day after, with a total record of 27 h (Fig. 5). Both the original sediment used to create the tracer and the tracer itself, were dry sieved at 0.5 Ø intervals between −1 and 5 Ø, and the sediment bulk density ρs, was obtained by picnometry (Newkirk, 1920).
Fig. 3. In situ photograph of the injection point, looking southwards.
3.2. Fluorescent tracer Several paints have been used to make tracers for measuring sediment transport in marine environments. The most common method when using fluorescent tracers consists of painting the sand previously collected from the study site. Another option is using artificial fluorescent material with the same hydrodynamic characteristics as the natural sediments (McComb and Black, 2005). In this work we painted sand from the field experiment location. Prior to the field experiment we performed several tests in laboratory in order to find the best product and procedure to make the tracer. Three different kinds of fluorescent paints were tested. Table 1 summarizes the advantages and disadvantages of each one. The best result was obtained with the fluorescent acrylic spray paint. It formed few aggregates, and these are easily disaggregated. About 0.5 kg of tagged sand was used in the experiment. It was placed in a 0.27 × 0.27 m area with a thickness of 3–5 mm (Fig. 3), and exposed to wind action from 11:10 h until 12:40 h on September 7th. For 90 min the wind moved the tracer. At 12:40 h wind action was stopped using a portable fence 2 × 1 m, so that transport was completely interrupted. Then we began to sample. Assuming the blown tracer was distributed on the uppermost layer of the sediment surface, sampling was carried out with a sticky tape 5 cm wide, which collected only surface sediments. Special care was taken to avoid disturbance of the surface during the sampling. The tape was placed in several strips normal to wind direction, being closer to the injection point where the highest tracer concentrations were found. This collection procedure simplifies the subsequent work
Fig. 4. Sediment traps location, looking northwards.
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3.3. Sediment traps
Fig. 5. Wind mast in the experiment area.
in the laboratory. One drawback of this method is that it only takes into account surface material. It does not consider any tracer grains which may be below the surface. Once in the laboratory, the sticky tape with the sediment stuck on it, was cut into 10 cm strips. Each one was conveniently marked and the tagged grains were visually counted under UV light. The greatest disadvantage of this method of counting is that it is tedious and timeconsuming. To avoid this problem Silva et al. (2007) used an automatic digital image processing system called SAND (System of Automatic Nparticle Detection). This system automatically counts the number of marked particles in collected sand samples. We tried to develop similar software based on image processing, but it was not possible because of the natural fluorescence of same carbonates. Their presence obligated to an accurate calibration of the methodology, making this approach even more time-consuming than the original method. The mass center of the wind-blown sand was computed following the method described in Madsen (1987) and Vila-Concejo et al. (2004).
Table 1 Advantages and disadvantages of different products used to make the tracer. Product
Advantages
– Do not form aggregates – Easy to make the mixture with water – Cheap prize – Easy to clean Orange fluorescent – High highlight under acrylic paint UV light – Good adherence – Medium prize Orange fluorescent – Few aggregates easy acrylic paint in spray to disaggregate – High highlight under UV light – Good adherence – Easy to use – Quick dry Red dust pigment
The sand traps used in the field experiment were vertical cylinders following the design proposed by Leatherman (1978) and Rosen (1978). These traps consisted of half-buried PVC tubes, with two longitudinal openings in the subaerial part. One of these openings was covered with a 60µm screen that retained the transported grains. This material fell into the buried part of the trap where it was deposited in a plastic bag fitted to the inner part of the trap. Traps had a trapping height of 29 cm above surface and a diameter of 4 cm. The traps were placed in a group of four, facing N, S, E and W (Fig. 4). The orientation of the traps was determined by wind action, since in this area prevailing winds were from the north. In fact, shadow dunes were facing southward during the experiment. This orientation meant that the traps were oriented parallel and normal to the shoreline, which made easier the calculations of onshore/offshore transport. In this way the effect of possible changes in wind direction could be considered by computing the vector sum of the sediments collected in each trap. During the 90 min period in which the tracer was under wind action, two series of sand traps, T1 and T2, were working for 8 and 5 min respectively. The plastic bags were marked and changed after each trapping series, and the material collected was weighted with an accuracy of 0.001 g. These relatively short sampling intervals were used in order to obtain a better correlation between sediment transport and wind data. To avoid the scour that usually takes place around the tube by wind drift, traps were slightly modified according to Alcántara-Carrió (2003). The modification consisted of an apron cloth placed around the trap and covered with sand (Jones and Willetts, 1979; Jackson and Nordstrom, 1997). This apron is very similar to the one used by Illemberger and Rust (1986) in Venturi traps and has proved to be efficient, since no scouring was observed around the traps.
4. Results 4.1. Tracer procedure Selected paint showed quite good results, since it formed less aggregates than the others, it was easy to use and it dried quickly. On the other hand, large quantities of spray were needed to paint a small amount of sediment. Grain size distribution for native sediment and the fluorescent tracer showed that both samples have similar unimodal grain size distributions, although the native one is slightly finer and better sorted than the marked one (Fig. 6). The results also indicated that the fluorescent sand formed small aggregates that slightly increased the original size of the native sample changing the medium size range from 0.208 to 0.268 mm in the tagged sample.
Disadvantages – Do not highlight much under UV light – Leaves dust pigment residues – Low adherence to the sediment grains – Forms aggregates – Forms small grains of paint
– Big quantities are necessary for a small amount of sediment – Expensive price
4.2. Wind data The wind blew onshore during the experiments with an average direction of 15° N. Wind speeds monitored on-site show that higher velocities occurred between 11:00 and 17:00, when the air was warmer. During the evening, the night and early morning, wind velocity decreased considerably (Fig. 7a). The average velocity at 1 m height was 5.26 m s− 1 during the 90 min period in which the tracer was exposed to wind action, and 5.12 and 5.21 m s− 1 during T1 and T2 respectively (Fig. 7b). Time averaged wind velocity profiles show that the wind speed both in T1 and T2 followed a logarithmic relationship (R2 associated with the velocity profiles exceeds 0.96, Fig. 8).
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Fig. 6. Grain size distribution for native sediments and fluorescent tracer.
4.3. Transport rate from traps
Fig. 8. Wind profiles for T1 and T2.
Sand roses were obtained from the vector sum of the weight of trapped sediments (Fig. 9). The transport rate obtained with the traps (Q traps) was obtained after considering the time each trap was working, and it yielded Q traps = 0.0131 and 0.0184 kg m− 1 min− 1 for T1 and T2, respectively. 4.4. Transport rate from tracer Transport rate in the fluorescent tracer experiment (Qtracer) was derived from the center of mass velocity (distance travelled by the centroid in a certain time, Vtracer). Since the centroid displacement was 0.875 m southwards (Fig. 10), and the period that the tracer was under wind action was 90 min, it yielded Vtracer = 0:00972 m min
−1
According to Komar and Inman (1970), Q tracer = Vtracer × A
ð1Þ
where A is the cross sectional area of the moving layer. The geometry of the moving layer is related in beach transport experiments to the cross-shore distribution of mixing depth (Silva
et al, 2007). This moving layer has normally the width of the swash zone and the thickness of movement of sand, which is a variable depth depending on beach slope and waves, and it is normally measured from low-tide conditions to the next low-tide (Ferreira et al, 2000). In aeolian environments the concept of a moving layer is not very common, but should be used with the same purpose: the area in the wind normal direction in which sediment transport takes place, over a certain period of time. In our case this area can be estimated assuming that the tagged particles only distribute themselves in the surface or very close to it. We observed that after the 90-min period the wind was blowing over the tracer and once the wind had been stopped with the fence, there were still many tagged grains at the injection point, though many of them were arrayed in ripple morphology (Fig. 11). Since the tracer was originally deposited in a layer 3–5 mm thick, the remains of the trace deposit indicates that not all the marked material was mobilized and, that the moving layer was less than 3–5 mm deep. This is quite reasonable considering the short period the wind was blowing over the tracer (90 min) and the low wind velocities (5.2 m s− 1). In longer experiments or under stronger winds this estimation would not be valid because the tagged grains would become buried or blown away. Since the sampling scheme consisted of collecting only the surface sediments, the thickness of the active layer should be considered the
Fig. 7. a) Running average of wind speed during the whole experiment. b) Wind record during the 90 min while the tracer experiment was running.
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Fig. 11. Tracer distribution at the injection point just before the sampling. The tracer grains were displayed following the ripple morphology. Part of the injection point was buried with original upwind sediments.
Considering the sediment bulk density (ρs = 2770 kg m− 3), the transport rate becomes: −1
Q tracer = 0:0072kg m
−1
min
5. Discussion Fig. 9. Sand roses from the sediment traps.
average grain size of tagged grains (Dtracer = 0.268 mm). Normalizing the width of the active layer to 1 m, with A = 0.000268 m2 m− 1, the transport rate from Eq. (1) becomes: −6
Q tracer = 2:6 * 10
3
m m
−1
−1
min
5.1. Transport rate from the Bagnold equation Taking into account that both series of traps demonstrated effective transport and convergence at a focal point, the shear velocity (U′) * was calculated using the logarithm law for wind velocities, and the threshold shear velocity (U*c) of Bagnold (1941): ′
Uz = 5:75U* log10 ðz = z0 Þ + Ut
ð2Þ
where Uz is the wind velocity at height z, z0 is the roughness of the substrate and Ut is the threshold velocity to move sand. The threshold shear velocity (U*c) was obtained from the expression of Bagnold for dry sand, 1=2
Uc = Ab ððρs −ρa =ρa ÞgdÞ
ð3Þ
where Ab is an empirical coefficient with a value of 0.1 (Bagnold, 1941; Sarre, 1987), ρs is the sediment density, ρa is the air density, g is the acceleration due to gravity and d is the mean grain size. Air density was determined by converting the temperature and the relative humidity obtained during the experiment using the table from Pye and Tsoar (1990). The air density for our experiment was 1.162 kg m− 3. From these data, the shear velocity was 0.29 and 0.31 m s− 1 for T1 and T2 respectively, while the threshold shear velocity was 0.221 m s− 1. The transport rate calculated from Bagnold's equation (1941) was: 1=2
Q Bagnold = Cðρa = gÞðd=DÞ
′3
U*
ð4Þ
where C is an empirical coefficient having a value of 1.8 for naturally graded sand, like samples found on dunes (Bagnold, 1941) and D is a reference grain diameter of 0.25 mm. The theoretical values obtained for QBagnold with Eq. (4) are 0.29 and 0.33 kg m− 1 min− 1 for T1 and T2 respectively. 5.2. Considerations regarding sediment transport measured with traps Fig. 10. Distribution pattern of the fluorescent tracer. Each dot indicates the center of the sticky tape where tagged grains were counted.
The two values of sediment transport measured from traps (Q trap for T1 and T2) showed large differences (Q traps = 0.0131 and
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Table 2 Wind data at 1 m height. Series of sand traps
T1 T2
Number of data
8 5
Maximum velocity (m s− 1)
Average velocity (m s− 1)
Direction (deg N)
Max
Min
Av
Max
Min
Av
Max
Min
Av
7.56 7.26
6.07 6.39
6.77 ± 0.52 6.81 ± 0.31
5.54 5.40
4.50 5.03
5.12 ± 0.38 5.21 ± 0.16
16.17 14.77
9.14 12.31
12.54 ± 2.66 13.48 ± 0.82
0.0184 kg m− 1 min− 1 for T1 and T2 respectively). This difference cannot be attributed to wind velocity changes, since average wind velocities at 1 m height were 5.1 and 5.2 m s− 1 respectively. When considering the maximum wind velocities (6.77 and 6.82 m s− 1 for T1 and T2 respectively) instead of the averages, the differences are still too small to explain the transport measurements. The other variable to consider is wind direction, which is similar between T1 and T2. Nevertheless, the wind direction variability is much higher at T1 compared to T2 (Table 2). This variability could explain the measured difference in Q trap T1 and Q trap T2. Even though the wind instruments only record 1 reading per minute, it seems clear from Table 1 that the wind during T1 was more variable (both in average speed, maximum speed and direction) than at T2, when it was more constant. This small change in uniformity and steadiness of the flow field seem to be the determinant factor in sediment transport (Bauer et al, 1990), so that the more uniform the flow field, the greater the transport rates. 5.3. Relationship of measured transport rates Measured transport rates from traps and tracer make possible to compare both values, which gives Q trap T1 = 1:82 Q tracer
On the other hand, comparison of field transport rates with those derived from Bagnold´s equation demonstrates that the theoretical ones are much higher than those measured in the field. Other authors have already considered this discrepancy (e.g. Bauer et al, 1990), and attribute it to the difference between ideal and non-ideal conditions. 6. Conclusions Fluorescent tracers are valid for sediment transport studies, although the values we have obtained are approximately half the value of those obtained when using vertical traps. This difference is mainly related to two aspects: (i) the differences in grain size between the tagged and the original material, and (ii) the way in which Q tracer is acquired. Since Q tracer derives from the displacement of the centroid and the thickness of the active layer (Eq. (1)), it is important to measure this layer, especially because it may change depending on the sampling scheme. According to the methodology followed in this study, the active layer is reliably the average size of the tagged grains, even though the movement of the native sediment may eventually cover the tagged material. Subsequent work in this topic should include more field experiments on the thickness of the active layer. Acknowledgements
Q trap T2 = 2:56 Q tracer Even though we have not found any equivalent expression in the literature, this relationship seems to be quite reasonable. Thus the difference between both methods can be explained by three factors. (1) Q tracer only takes into account the distribution of marked particles, while Q traps accounts for any particle in movement. (2) The native sediment is also transported, and may partially cover the tagged material rendering it uncollectible by our sampling technique. This would mean that the thickness of the active layer should be thicker than a single grain. (3) The difference in grain size between the tracer and the native material. These factors contribute to reduce the mobility of the marked grains compared to the native ones. This would explain the lower transport rates measured with the tracer as compared to those from traps. Comparison of transport rates measured with traps in this study and results from other authors reveal that our values are normally lower (Table 3). Apart from changes in environmental factors, such as vegetation cover and moisture, the difference in average grain size is likely the main reason for discrepancies in transport rates.
This work is a contribution from research project PI 2002/008, funded by the Canarian Government, and from research contract “Sedimentología, Geomorfología y Dinámica Sedimentaria de Famara-El Jable (Lanzarote)”, funded by Cabildo de Lanzarote. Thanks are given to Alondra Díaz for her help during the field survey and the laboratory work, to two anonymous reviewers whose comments helped to improve the original manuscript and to Tedd Packard for his comments with the English translation. References Alcántara-Carrió, J., 2003. Dinámica sedimentaria eólica en el istmo de Jandía (Fuerteventura). Modelización y cuantificación del transporte. Cabildo de Gran Canaria, Gran Canaria. 288 pp. Baas, A.C.W., 2004. Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport intensity. Geomorphology 59, 99–118. Bagnold, R.A., 1941. The physics of blown sand and desert dunes. Methuen, London. 265 pp. Bauer, B.O., Sherman, D.J., Nordstrom, K.F., Gares, P.A., 1990. Aeolian transport measurements and direction across a beach and dune at Castroville, California. In: Nordstrom, K.F., Psuty, N.P., Carter, R.W.G. (Eds.), Coastal Dunes, Form and Process. Wiley, New York, pp. 39–55.
Table 3 Comparison of transport rates measured from traps and the more relevant variables concerning sediment transport. Author
Q (kg m− 1 min− 1)
V (Vmax) (m s− 1)
U* (m s− 1)
D50 (mm)
Vegetation
Weather conditions
Jackson and Nordstrom (1997) Jackson and Nordstrom (1998)
0.32–0.41 0.235 0.293 2.337 0.00033–1.333 0.013 0.018
5–7 7.8 (9.2) 8.1 (9.6) 10.4 (12.3) – 5.1 (6.8) 5.2 (6.8)
0.20–0.32 –
Fine sand 0.16
Unvegetated backbeach Unvegetated
Rainy and humid Rainy and humid
0.23–0.65 0.29 0.31
0.17 0.21
Foredune Small nebkhas
Rainy and humid Dry weather
Sherman et al. (1998) Present study
V is average wind speed al 1 m height; Vmax is the maximum wind speed at same height; and D50 is the average grain size.
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Levin, N., Ben-Dor, E., 2004. Monitoring sand dune stabilization along the coastal dunes of Ashdod-Nizanim, Israel, 1945–1999. Journal of Arid Environment 58, 335–355. Levin, N., Kidron, G.J., Ben-Dor, E., 2007. Surface properties of stabilizing coastal dunes: combining spectral and field analyses. Sedimentology 54, 771–788. Madsen, O.S., 1987. Use of tracers in sediment transport studies. Proceedings of the International Conference on Coastal Sediments'87. ASCE, pp. 424–435. McComb, P.J., Black, K.P., 2005. Detailed observations of littoral transport using artificial sediment tracer, in a high-energy rocky reef and iron sand environment. Journal of Coastal Research 21, 358–373. Newkirk, W.B., 1920. Picnometer for determination of density of molasses. Journal of the Franklin Institute 189 (6) (781 pp.). Nickling, W.G., 1978. Eolian sediment transport during dust storms. Canadian Journal of Earth Sciences 15, 1069–1084. Nickling, W.G., Ecclestone, M., 1981. The effects of soluble salts on the threshold shear velocity of fine sand. Sedimentology 28, 505–510. Pye, K., 1983. Coastal dunes. Progress in Physical Geography 7, 531–557. Pye, K., Tsoar, H., 1990. Aeolian sand and sand dunes. Unwin Hyman, London. (396 pp.). Rosen, P.S., 1978. An efficient, low-cost eolian sampling system. Current Research, Part A. Geological Survey of Canada 78-1A, 531–532. Sarre, R.D., 1987. Aeolian sand transport. Progress in Physical Geography 11, 155–182. Sarre, R.D., 1988. An evaluation of aeolian sand transport equations using intertidal zone measurements. Sedimentology 35, 671–679. Sherman, D.J., Hotta, S., 1990. Aeolian sediment transport: theory and measurement. In: Nordstrom, K.F., Psuty, N.P., Carter, R.W.G. (Eds.), Coastal Dunes: Form and Process. John Wiley, Chichester, pp. 17–37. Sherman, D.J., Jackson, D., Namikas, S.L., Wang, J., 1998. Wind-blown sand on beaches: an evaluation of models. Geomorphology 22, 135–149. Silva, A., Taborda, R., Rodrigues, A., Duarte, J., Cascalho, J., 2007. Longshore drift estimation using fluorescent tracers: new insights from an experiment at Comporta Beach. Portugal Marine Geology 240, 137–150. Stockdon, H.F., Sallenger, A.H., Holman, R.A., Howd, P.A., 2007. A simple model for the spatially-variable coastal response to hurricanes. Marine Geology 238 (1–4), 1–20. Tengberg, A., 1995. Nebkha dunes as indicators of wind erosion and land degradation in the Sahel Zone of Burkina Faso. Journal of Arid Environments 30, 265–282. Valdemoro, H.I., Jiménez, J.A., Alonso, I., Lorente, P., Rodríguez-Herrerías, M., 2007. Coastal dunefield evolution in conditions of limited sediments availability. Natural and anthropogenic controls on Corralejo dunes. Proceedings Coastal Sediments 2007, vol. I. American Society of Civil Engineers, New Orleans, USA, pp. 1283–1296. Vanhee, S., 2004. Aeolian transport and fetch on intertidal barred beaches: Leffrinckoucke example, cote d'opale. Bulletin De l'Association De Geographes Francais 3, 427–437. van der Ancker, J.A.M., Jungerius, P.D., Mur, L.R., 1985. The role of algae in the stabilization of coastal dune blowouts. Earth Surface Processes and Landforms 10, 189–192. Vila-Concejo, A., Ferreira, Ó., Ciavola, P., Matias, A., Dias, J.M.A., 2004. Tracer studies on the updrift margin of a complex inlet system. Marine Geology 208, 43–72. White, T.E., Inman, D.L., 1989. Transport determination by tracers. In: Seymour, R.J. (Ed.), Nearshore Sediment Transport. New York, Plenum, pp. 115–128.
Contents lists available at ScienceDirect
Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s
Correlation of aeolian sediment transport measured by sand traps and fluorescent tracers Laura L. Cabrera ⁎, Ignacio Alonso Department of Física, University of Las Palmas de Gran Canaria, Campus de Tafira s/n, 35017-Las Palmas, Spain
a r t i c l e
i n f o
Available online 20 October 2009 Keywords: Transport rates Sand traps Fluorescent tracer Wind speed Nebkha
a b s t r a c t Two different methods, fluorescent tracers and vertical sand traps, were simultaneously used to carry out an aeolian sediment transport study designed to test the goodness of fluorescent tracers in aeolian environments. Field experiments were performed in a nebkha field close to Famara beach at Lanzarote Island (Canary Islands, Spain) in a sector where the dunes were between 0.5 and 0.8 m height and 1–2 m wide and the vegetal cover was approximately 22%. In this dune field the sediment supply comes from Famara beach and is blown by trade winds toward the south, where the vegetation acts as natural sediment traps. Wind data were obtained by means of four Aanderaa wind speed sensors and one Aanderaa vane, all them distributed in a vertical array from 0.1 to 4 m height for 27 h. The average velocity at 1 m height during the experiment was 5.26 m s− 1 with the wind direction from the north. The tracer was under wind influence for 90min at midday. During this period two series of sand traps (T1 and T2) N, S, E and W oriented were used. Resultant transport rates were 0.0131 and 0.0184 kg m− 1 min− 1 respectively. Tracer collection was performed with a sticky tape to sample only surface sediments. Tagged grains were visually counted under UV light. The transport rate was computed from the centroid displacement, that moved 0.875 m southwards, and the depth of the active layer considered was the size of one single grain. Taking into account these data the transport rate was 0.0072 kg m− 1 min− 1. The discrepancy in results between both methods is related to several factors, such as the thickness of the active layer and the grain size difference between the tagged and the native material. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Aeolian sediment transport has been an important topic in earth sciences for decades, in particular in any study dealing with dunes and desert environments. The theoretical background of aeolian transport was established by Bagnold (1941), who stated that the discharge of sediments could be predicted by: 0:5
q = Cðd=DÞ
3
ρ = gu*
where C is an empirical coefficient related to the sediment sorting, d is the mean sediment diameter, D is a reference grain size of 0.25 mm, g is the gravitational acceleration, ρ is the air density and u* is the shear velocity. Bagnold (1941) assigned values of C ranging from 1.5 for nearly uniform sand, to 1.8 for typical dune sands, to 2.8 for moderately to poorly sorted sands, to a maximum of 3.5 over hard surfaces. Nevertheless Bagnold's equation was derived from laboratory observations and basic principles of physics, and it is valid when
⁎ Corresponding author. E-mail addresses: [email protected] (L.L. Cabrera), ialonso@dfis.ulpgc.es (I. Alonso). 0924-7963/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2009.10.012
applied to ideal surfaces: horizontal, unobstructed, dry, crust-free and free of vegetation. Since then, many authors have presented different formulations for ideal, quasi-ideal and non-ideal surfaces. Howard et al. (1978) and Hardisty and Whitehouse (1988) included modifications for the slope effect. Hotta et al. (1984), Sarre (1988) and Davidson-Arnott et al. (2008) made different findings to assess the influence of water content on sand movement. Nickling (1978) and Nickling and Ecclestone (1981) proposed a modified equation to account for the formation of crusts due to salts concentration. The effect of algae on crusts has been considered by Campbell (1979) and van der Ancker et al. (1985). Finally, the role of vegetation in controlling sediment transport has been widely considered by Hesp (1981), Pye (1983), Kuriyama et al. (2005), and Leenders et al. (2007) among others. Regarding the methods for measuring transport rates, Sherman and Hotta (1990) classify the different methodologies in three conceptually distinct classes: (i) detailed measurements of volumetric changes in source/sink areas; (ii) sediment trapping; and (iii) tracer experiments. We should include a fourth class consisting on the use of aerial photographs and remote sensors. Each one of these approaches has several advantages and disadvantages. The first type of measurement has been used by Illemberger and Rust (1986) in South Africa and by Valdemoro et al. (2007) in Canary
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Islands, among others. The use of traps is broad, since there are many different kinds of trapping devices. Probably the most widely used are the vertical traps described by Leatherman (1978) and Rosen (1978), although other authors have used trench collectors (Greeley et al., 1996), saltiphones (Vanhee, 2004), saltation flux impact responders (Baas, 2004) and other types. Goossens et al. (2000) gives a calibration of five different types of sand traps. Tracers are commonly used for measuring longshore drift (White and Inman, 1989; Vila-Concejo et al., 2004; McComb and Black, 2005; Silva et al., 2007), but not in aeolian environments. The only paper, that we have found, that uses fluorescent tracers in field experimentation to estimate aeolian transport rates was that of Berg (1983). Since then no one seems to have followed this approach. Finally, there are many authors who use aerial photographs to measure dunefield evolution, and to estimate transport rates (Levin and Ben-Dor, 2004; Hernández et al., 2007). Others use remote sensors (Levin et al., 2007; Stockdon et al., 2007). Here, we describe a field experiment carried out on September 2004. The experiment consisted of simultaneous measurements of aeolian transport in a nebkha dunefield using vertical sand traps and fluorescent tracers. The first aim of this study was to test the utility of fluorescent tracers in aeolian studies, and the second was to compare the sand transport measurements obtained by both methods and by Bagnold´s theoretical expression to gain insight into the aeolian transport process. 2. Study area The study area is located on Lanzarote Island (Canary Islands, Spain). This island is crossed from the northern to the southern coast by a sand strip called El Jable, where aeolian sediments cover an area of nearly 90 km2 (Fig. 1). In the northeast of El Jable there is a beach 4 km long called Famara Beach which is the input zone of marine sediments migrating inland, and an small dunefield of about 0.5 km2 mainly formed by nebkhas (Fig. 2). Since the beginning of the XX century, El Jable has evolved from a barchans dunefield to a sand sheet that in most places is only a few
centimetres thick, with a few barchan dunes on it (Cabrera and Alonso, 2006). Prevailing tradewinds blow steadily from the NNE from April until October. The rest of the year the wind is much more variable, both in direction and intensity. The nebkha dunefield cannot be properly defined as a foredune, since rather than a continuous dune ridge, it is formed by isolated dunes associated with discrete plant species. This vegetation shows gradual changes both in surface cover and dominant species depending on the distance to the shoreline. Considering its effect on sediment transport, Traganum moquinii is the dominant species closer to the shoreline, since it can reach up to 3 m height and the associated shadow dunes may reach up to 10 m length. Moving inland vegetation gradually changes to Launaea arborescens, which due to the smaller height and size forms smaller dunes ranging between 0.5 and 0.8 m height and 1– 2 m large). Both species are shrubs and in both of them nebkhas are perennial, showing a clear correlation between dune height and spacing, as stated by Tengberg (1995). The surfaces between the nebkhas are sandy and become bare during the whole year except during rainy periods at winter time. During these periods the area is more humid and it is possible to find herbaceous species. The contribution of these species to fix the sand is limited, since they die in few weeks after the rain. Sediments in the nebkha dunes have both bioclastic and terrigenous components, with a 55–60% in carbonate content derived from bioclasts (Cabrera et al, 2006).
3. Methods 3.1. Site selection and wind measurements The experiment site was located 500 m from the shoreline in an area where Launaea arborescens is the dominant species. The vegetation cover was about 22% estimated from field observations and very detailed aerial photographs of the area (Fig. 2).
Fig. 1. Location map of the study area.
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Fig. 2. Closer aerial view of the experiment area.
Field experiment was developed in a sector where the nebkhas are mid-size: around 2 m wide and 0.5 m high. Both traps and tracer experiments were located in sandy clear zones between the nebkhas, in order to minimize the topographic influence in the aeolian transport. Selected areas for both sand traps and fluorescent tracers were very similar in bush density and bedforms. Wind ripples in the area are about 10 cm wavelength (Figs. 3 and 4). The sand traps and the tracers were working simultaneously, so that both of them were exposed to the same conditions of wind, surrounding obstacles and sediment. During the experiments wind data were recorded by means of one anemometer mast 4 m high consisting of 4 Aanderaa three-cup anemometers located at 0.125, 0.5, 1.0 and 4.0 m and 1 Aanderaa vane at 2.0 m above the surface. The recording interval was 1 min and the recording period was from 17:00 on 6 September until 20:00 the day after, with a total record of 27 h (Fig. 5). Both the original sediment used to create the tracer and the tracer itself, were dry sieved at 0.5 Ø intervals between −1 and 5 Ø, and the sediment bulk density ρs, was obtained by picnometry (Newkirk, 1920).
Fig. 3. In situ photograph of the injection point, looking southwards.
3.2. Fluorescent tracer Several paints have been used to make tracers for measuring sediment transport in marine environments. The most common method when using fluorescent tracers consists of painting the sand previously collected from the study site. Another option is using artificial fluorescent material with the same hydrodynamic characteristics as the natural sediments (McComb and Black, 2005). In this work we painted sand from the field experiment location. Prior to the field experiment we performed several tests in laboratory in order to find the best product and procedure to make the tracer. Three different kinds of fluorescent paints were tested. Table 1 summarizes the advantages and disadvantages of each one. The best result was obtained with the fluorescent acrylic spray paint. It formed few aggregates, and these are easily disaggregated. About 0.5 kg of tagged sand was used in the experiment. It was placed in a 0.27 × 0.27 m area with a thickness of 3–5 mm (Fig. 3), and exposed to wind action from 11:10 h until 12:40 h on September 7th. For 90 min the wind moved the tracer. At 12:40 h wind action was stopped using a portable fence 2 × 1 m, so that transport was completely interrupted. Then we began to sample. Assuming the blown tracer was distributed on the uppermost layer of the sediment surface, sampling was carried out with a sticky tape 5 cm wide, which collected only surface sediments. Special care was taken to avoid disturbance of the surface during the sampling. The tape was placed in several strips normal to wind direction, being closer to the injection point where the highest tracer concentrations were found. This collection procedure simplifies the subsequent work
Fig. 4. Sediment traps location, looking northwards.
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3.3. Sediment traps
Fig. 5. Wind mast in the experiment area.
in the laboratory. One drawback of this method is that it only takes into account surface material. It does not consider any tracer grains which may be below the surface. Once in the laboratory, the sticky tape with the sediment stuck on it, was cut into 10 cm strips. Each one was conveniently marked and the tagged grains were visually counted under UV light. The greatest disadvantage of this method of counting is that it is tedious and timeconsuming. To avoid this problem Silva et al. (2007) used an automatic digital image processing system called SAND (System of Automatic Nparticle Detection). This system automatically counts the number of marked particles in collected sand samples. We tried to develop similar software based on image processing, but it was not possible because of the natural fluorescence of same carbonates. Their presence obligated to an accurate calibration of the methodology, making this approach even more time-consuming than the original method. The mass center of the wind-blown sand was computed following the method described in Madsen (1987) and Vila-Concejo et al. (2004).
Table 1 Advantages and disadvantages of different products used to make the tracer. Product
Advantages
– Do not form aggregates – Easy to make the mixture with water – Cheap prize – Easy to clean Orange fluorescent – High highlight under acrylic paint UV light – Good adherence – Medium prize Orange fluorescent – Few aggregates easy acrylic paint in spray to disaggregate – High highlight under UV light – Good adherence – Easy to use – Quick dry Red dust pigment
The sand traps used in the field experiment were vertical cylinders following the design proposed by Leatherman (1978) and Rosen (1978). These traps consisted of half-buried PVC tubes, with two longitudinal openings in the subaerial part. One of these openings was covered with a 60µm screen that retained the transported grains. This material fell into the buried part of the trap where it was deposited in a plastic bag fitted to the inner part of the trap. Traps had a trapping height of 29 cm above surface and a diameter of 4 cm. The traps were placed in a group of four, facing N, S, E and W (Fig. 4). The orientation of the traps was determined by wind action, since in this area prevailing winds were from the north. In fact, shadow dunes were facing southward during the experiment. This orientation meant that the traps were oriented parallel and normal to the shoreline, which made easier the calculations of onshore/offshore transport. In this way the effect of possible changes in wind direction could be considered by computing the vector sum of the sediments collected in each trap. During the 90 min period in which the tracer was under wind action, two series of sand traps, T1 and T2, were working for 8 and 5 min respectively. The plastic bags were marked and changed after each trapping series, and the material collected was weighted with an accuracy of 0.001 g. These relatively short sampling intervals were used in order to obtain a better correlation between sediment transport and wind data. To avoid the scour that usually takes place around the tube by wind drift, traps were slightly modified according to Alcántara-Carrió (2003). The modification consisted of an apron cloth placed around the trap and covered with sand (Jones and Willetts, 1979; Jackson and Nordstrom, 1997). This apron is very similar to the one used by Illemberger and Rust (1986) in Venturi traps and has proved to be efficient, since no scouring was observed around the traps.
4. Results 4.1. Tracer procedure Selected paint showed quite good results, since it formed less aggregates than the others, it was easy to use and it dried quickly. On the other hand, large quantities of spray were needed to paint a small amount of sediment. Grain size distribution for native sediment and the fluorescent tracer showed that both samples have similar unimodal grain size distributions, although the native one is slightly finer and better sorted than the marked one (Fig. 6). The results also indicated that the fluorescent sand formed small aggregates that slightly increased the original size of the native sample changing the medium size range from 0.208 to 0.268 mm in the tagged sample.
Disadvantages – Do not highlight much under UV light – Leaves dust pigment residues – Low adherence to the sediment grains – Forms aggregates – Forms small grains of paint
– Big quantities are necessary for a small amount of sediment – Expensive price
4.2. Wind data The wind blew onshore during the experiments with an average direction of 15° N. Wind speeds monitored on-site show that higher velocities occurred between 11:00 and 17:00, when the air was warmer. During the evening, the night and early morning, wind velocity decreased considerably (Fig. 7a). The average velocity at 1 m height was 5.26 m s− 1 during the 90 min period in which the tracer was exposed to wind action, and 5.12 and 5.21 m s− 1 during T1 and T2 respectively (Fig. 7b). Time averaged wind velocity profiles show that the wind speed both in T1 and T2 followed a logarithmic relationship (R2 associated with the velocity profiles exceeds 0.96, Fig. 8).
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239
Fig. 6. Grain size distribution for native sediments and fluorescent tracer.
4.3. Transport rate from traps
Fig. 8. Wind profiles for T1 and T2.
Sand roses were obtained from the vector sum of the weight of trapped sediments (Fig. 9). The transport rate obtained with the traps (Q traps) was obtained after considering the time each trap was working, and it yielded Q traps = 0.0131 and 0.0184 kg m− 1 min− 1 for T1 and T2, respectively. 4.4. Transport rate from tracer Transport rate in the fluorescent tracer experiment (Qtracer) was derived from the center of mass velocity (distance travelled by the centroid in a certain time, Vtracer). Since the centroid displacement was 0.875 m southwards (Fig. 10), and the period that the tracer was under wind action was 90 min, it yielded Vtracer = 0:00972 m min
−1
According to Komar and Inman (1970), Q tracer = Vtracer × A
ð1Þ
where A is the cross sectional area of the moving layer. The geometry of the moving layer is related in beach transport experiments to the cross-shore distribution of mixing depth (Silva
et al, 2007). This moving layer has normally the width of the swash zone and the thickness of movement of sand, which is a variable depth depending on beach slope and waves, and it is normally measured from low-tide conditions to the next low-tide (Ferreira et al, 2000). In aeolian environments the concept of a moving layer is not very common, but should be used with the same purpose: the area in the wind normal direction in which sediment transport takes place, over a certain period of time. In our case this area can be estimated assuming that the tagged particles only distribute themselves in the surface or very close to it. We observed that after the 90-min period the wind was blowing over the tracer and once the wind had been stopped with the fence, there were still many tagged grains at the injection point, though many of them were arrayed in ripple morphology (Fig. 11). Since the tracer was originally deposited in a layer 3–5 mm thick, the remains of the trace deposit indicates that not all the marked material was mobilized and, that the moving layer was less than 3–5 mm deep. This is quite reasonable considering the short period the wind was blowing over the tracer (90 min) and the low wind velocities (5.2 m s− 1). In longer experiments or under stronger winds this estimation would not be valid because the tagged grains would become buried or blown away. Since the sampling scheme consisted of collecting only the surface sediments, the thickness of the active layer should be considered the
Fig. 7. a) Running average of wind speed during the whole experiment. b) Wind record during the 90 min while the tracer experiment was running.
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Fig. 11. Tracer distribution at the injection point just before the sampling. The tracer grains were displayed following the ripple morphology. Part of the injection point was buried with original upwind sediments.
Considering the sediment bulk density (ρs = 2770 kg m− 3), the transport rate becomes: −1
Q tracer = 0:0072kg m
−1
min
5. Discussion Fig. 9. Sand roses from the sediment traps.
average grain size of tagged grains (Dtracer = 0.268 mm). Normalizing the width of the active layer to 1 m, with A = 0.000268 m2 m− 1, the transport rate from Eq. (1) becomes: −6
Q tracer = 2:6 * 10
3
m m
−1
−1
min
5.1. Transport rate from the Bagnold equation Taking into account that both series of traps demonstrated effective transport and convergence at a focal point, the shear velocity (U′) * was calculated using the logarithm law for wind velocities, and the threshold shear velocity (U*c) of Bagnold (1941): ′
Uz = 5:75U* log10 ðz = z0 Þ + Ut
ð2Þ
where Uz is the wind velocity at height z, z0 is the roughness of the substrate and Ut is the threshold velocity to move sand. The threshold shear velocity (U*c) was obtained from the expression of Bagnold for dry sand, 1=2
Uc = Ab ððρs −ρa =ρa ÞgdÞ
ð3Þ
where Ab is an empirical coefficient with a value of 0.1 (Bagnold, 1941; Sarre, 1987), ρs is the sediment density, ρa is the air density, g is the acceleration due to gravity and d is the mean grain size. Air density was determined by converting the temperature and the relative humidity obtained during the experiment using the table from Pye and Tsoar (1990). The air density for our experiment was 1.162 kg m− 3. From these data, the shear velocity was 0.29 and 0.31 m s− 1 for T1 and T2 respectively, while the threshold shear velocity was 0.221 m s− 1. The transport rate calculated from Bagnold's equation (1941) was: 1=2
Q Bagnold = Cðρa = gÞðd=DÞ
′3
U*
ð4Þ
where C is an empirical coefficient having a value of 1.8 for naturally graded sand, like samples found on dunes (Bagnold, 1941) and D is a reference grain diameter of 0.25 mm. The theoretical values obtained for QBagnold with Eq. (4) are 0.29 and 0.33 kg m− 1 min− 1 for T1 and T2 respectively. 5.2. Considerations regarding sediment transport measured with traps Fig. 10. Distribution pattern of the fluorescent tracer. Each dot indicates the center of the sticky tape where tagged grains were counted.
The two values of sediment transport measured from traps (Q trap for T1 and T2) showed large differences (Q traps = 0.0131 and
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Table 2 Wind data at 1 m height. Series of sand traps
T1 T2
Number of data
8 5
Maximum velocity (m s− 1)
Average velocity (m s− 1)
Direction (deg N)
Max
Min
Av
Max
Min
Av
Max
Min
Av
7.56 7.26
6.07 6.39
6.77 ± 0.52 6.81 ± 0.31
5.54 5.40
4.50 5.03
5.12 ± 0.38 5.21 ± 0.16
16.17 14.77
9.14 12.31
12.54 ± 2.66 13.48 ± 0.82
0.0184 kg m− 1 min− 1 for T1 and T2 respectively). This difference cannot be attributed to wind velocity changes, since average wind velocities at 1 m height were 5.1 and 5.2 m s− 1 respectively. When considering the maximum wind velocities (6.77 and 6.82 m s− 1 for T1 and T2 respectively) instead of the averages, the differences are still too small to explain the transport measurements. The other variable to consider is wind direction, which is similar between T1 and T2. Nevertheless, the wind direction variability is much higher at T1 compared to T2 (Table 2). This variability could explain the measured difference in Q trap T1 and Q trap T2. Even though the wind instruments only record 1 reading per minute, it seems clear from Table 1 that the wind during T1 was more variable (both in average speed, maximum speed and direction) than at T2, when it was more constant. This small change in uniformity and steadiness of the flow field seem to be the determinant factor in sediment transport (Bauer et al, 1990), so that the more uniform the flow field, the greater the transport rates. 5.3. Relationship of measured transport rates Measured transport rates from traps and tracer make possible to compare both values, which gives Q trap T1 = 1:82 Q tracer
On the other hand, comparison of field transport rates with those derived from Bagnold´s equation demonstrates that the theoretical ones are much higher than those measured in the field. Other authors have already considered this discrepancy (e.g. Bauer et al, 1990), and attribute it to the difference between ideal and non-ideal conditions. 6. Conclusions Fluorescent tracers are valid for sediment transport studies, although the values we have obtained are approximately half the value of those obtained when using vertical traps. This difference is mainly related to two aspects: (i) the differences in grain size between the tagged and the original material, and (ii) the way in which Q tracer is acquired. Since Q tracer derives from the displacement of the centroid and the thickness of the active layer (Eq. (1)), it is important to measure this layer, especially because it may change depending on the sampling scheme. According to the methodology followed in this study, the active layer is reliably the average size of the tagged grains, even though the movement of the native sediment may eventually cover the tagged material. Subsequent work in this topic should include more field experiments on the thickness of the active layer. Acknowledgements
Q trap T2 = 2:56 Q tracer Even though we have not found any equivalent expression in the literature, this relationship seems to be quite reasonable. Thus the difference between both methods can be explained by three factors. (1) Q tracer only takes into account the distribution of marked particles, while Q traps accounts for any particle in movement. (2) The native sediment is also transported, and may partially cover the tagged material rendering it uncollectible by our sampling technique. This would mean that the thickness of the active layer should be thicker than a single grain. (3) The difference in grain size between the tracer and the native material. These factors contribute to reduce the mobility of the marked grains compared to the native ones. This would explain the lower transport rates measured with the tracer as compared to those from traps. Comparison of transport rates measured with traps in this study and results from other authors reveal that our values are normally lower (Table 3). Apart from changes in environmental factors, such as vegetation cover and moisture, the difference in average grain size is likely the main reason for discrepancies in transport rates.
This work is a contribution from research project PI 2002/008, funded by the Canarian Government, and from research contract “Sedimentología, Geomorfología y Dinámica Sedimentaria de Famara-El Jable (Lanzarote)”, funded by Cabildo de Lanzarote. Thanks are given to Alondra Díaz for her help during the field survey and the laboratory work, to two anonymous reviewers whose comments helped to improve the original manuscript and to Tedd Packard for his comments with the English translation. References Alcántara-Carrió, J., 2003. Dinámica sedimentaria eólica en el istmo de Jandía (Fuerteventura). Modelización y cuantificación del transporte. Cabildo de Gran Canaria, Gran Canaria. 288 pp. Baas, A.C.W., 2004. Evaluation of saltation flux impact responders (Safires) for measuring instantaneous aeolian sand transport intensity. Geomorphology 59, 99–118. Bagnold, R.A., 1941. The physics of blown sand and desert dunes. Methuen, London. 265 pp. Bauer, B.O., Sherman, D.J., Nordstrom, K.F., Gares, P.A., 1990. Aeolian transport measurements and direction across a beach and dune at Castroville, California. In: Nordstrom, K.F., Psuty, N.P., Carter, R.W.G. (Eds.), Coastal Dunes, Form and Process. Wiley, New York, pp. 39–55.
Table 3 Comparison of transport rates measured from traps and the more relevant variables concerning sediment transport. Author
Q (kg m− 1 min− 1)
V (Vmax) (m s− 1)
U* (m s− 1)
D50 (mm)
Vegetation
Weather conditions
Jackson and Nordstrom (1997) Jackson and Nordstrom (1998)
0.32–0.41 0.235 0.293 2.337 0.00033–1.333 0.013 0.018
5–7 7.8 (9.2) 8.1 (9.6) 10.4 (12.3) – 5.1 (6.8) 5.2 (6.8)
0.20–0.32 –
Fine sand 0.16
Unvegetated backbeach Unvegetated
Rainy and humid Rainy and humid
0.23–0.65 0.29 0.31
0.17 0.21
Foredune Small nebkhas
Rainy and humid Dry weather
Sherman et al. (1998) Present study
V is average wind speed al 1 m height; Vmax is the maximum wind speed at same height; and D50 is the average grain size.
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