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Magnitude and frequency of grain flows on a desert sand dune
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Geomorphology 95 (2008) 518 – 523 www.elsevier.com/locate/geomorph
Magnitude and frequency of grain flows on a desert sand dune Carrie Breton a , Nicholas Lancaster b,⁎, William G. Nickling a a
Department of Geography, University of Guelph, Guelph, Ontario, Canada N1G 2W1 b Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA Received 23 May 2005; received in revised form 5 July 2007; accepted 7 July 2007 Available online 21 July 2007
Abstract Preliminary studies of the magnitude and frequency of lee side avalanches (grain flows) on a Namib crescentic dune show that the frequency of grain flows for a given segment of the lee face is dependent on the wind speed and sand transport rate for the period preceding their initiation; and the magnitude of the flows as described by their area is inversely proportional to the interval between flows and thus wind speed and sand transport rates. These studies indicate the potential of using a simple digital video camera technique to document the magnitude, frequency and geometry of grain flows on desert sand dunes. © 2007 Elsevier B.V. All rights reserved. Keywords: Aeolian processes; Sand dunes; Grain flow; Namib Desert
1. Introduction Despite their importance to the dynamics of modern dunes and the interpretation of the rock record (Howell and Mountney, 2001), there have been few studies of the depositional processes that occur on dune lee faces. Prior studies consist of two modeling efforts (Hunter, 1985; Anderson, 1988), and three known field experiments (Hunter, 1985; McDonald and Anderson, 1995; Nickling et al., 2002). None of these studies have considered the magnitude and frequency of avalanches, also known as grain flows (Hunter, 1977). Following the terminology of Hunter (1977), grain flows occur when sediment deposited on the lee face by grain fall from the overshoot
⁎ Corresponding author. E-mail address: [email protected] (N. Lancaster). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.07.004
of saltating grains transported over the brink of the lee face builds up so that the lee slope is steepened above the angle of repose and fails, initiating a grain flow or avalanche. In this paper, we report the results of a preliminary field study of the frequency, duration and timing, and spatial extent of grain flows on the lee face of a 5 m-high crescentic (transverse) dune in the northern Namib Sand Sea. 2. Field site and methods The study dune is located on the eastern flank of a 60 m-high S–N-trending complex linear dune in the northern part of the Namib Sand Sea, approximately 200 m south of area of the dune studied by Livingstone (Livingstone, 1989, 1993, 2003). The study dune crest trends approximately WSW-ENE for a distance of 50 m and the dune is 5 m high (Fig. 1). It is one of a series of secondary dunes that migrate northward along the S–N linear dune flank in response to secondary flow generated
C. Breton et al. / Geomorphology 95 (2008) 518–523
519
Fig. 1. The study lee face, view from northeast. Note multiple grain flow tongues. Vertical lines separate lee face into 2 m-wide segments for analysis of grain flow magnitude and frequency.
by the dominant SW–SSW winds. At the time of the study (the afternoon of 11 August, 2002), primary winds were from the SSW. To determine when, where, and how often grain flows occur along the lee face of the dune we analyzed video footage taken over a period of approximately 60 min between 15:30 and 16:30 local time. A Canon ZR60 video camera was placed facing the lee side of a dune and imaged about 15 m laterally of the dune lee face. A Wind Sentry anemometer and wind vane was placed 1 m south (upwind) of the crest line and recorded wind speed and direction at a height of 0.5 m above the dune surface at 1 Hz intervals, averaged over a 1-minute period. Information on the number of grain flows, their duration (i.e. initiation to cessation), the location of individual grain flows on the dune face, the length and width of the flow, and the point where grain flow initiation occurred was obtained from the video footage. Temporal information was obtained using the clock on the InterVideoWinDVD.6 program used to play the recording. The dune outline and grain flow areas identified from the video were transferred onto acetate paper so that measurements could be made of the length and width of each grain flow. The length of grain flows measured on the overlay were geometrically corrected for slope assuming a lee slope angle of 32°. No other corrections (e.g. for distortion at the edges of the frame) were made. The area covered by each of the grain flows
Fig. 2. (A) Number of grain flows in each segment during observation period (approximately 60 min). (B) Area of grain flows in each segment during observation period.
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Table 1 Initiation time and duration, length, area, and location of each failure occurring on the Namib dune during the video coverage Failure Start time Duration Length Area number (h:min:s) (h:min:s) (m) (m2)
Location Segment (distance from east m)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
11.45 9.60 4.64 8.40 7.36 4.85 10.47 13.47 8.51 5.51 8.78 13.85 3.71 2.07 2.07 8.07 12.38 5.73 10.96 12.22 9.98
0:04:00 0:06:10 0:08:47 0:14:20 0:19:48 0:19:50 0:26:20 0:26:30 0:31:50 0:33:19 0:42:40 0:42:40 0:46:15 0:46:59 0:47:58 0:50:30 0:51:20 0:54:25 0:56:00 1:00:30 1:00:40
0:00:37 0:00:50 0:00:46 0:00:50 0:00:35 0:00:45 0:00:50 0:00:15 0:00:40 0:00:41 0:00:37 0:00:35 0:00:57 0:00:46 0:01:02 0:00:40 0:00:55 0:00:40 0:00:45 0:00:20 0:00:27
4.22 4.19 4.34 3.43 5.00 5.00 3.95 3.00 2.87 1.72 5.00 4.24 5.00 3.99 3.36 4.28 5.00 5.00 4.42 4.06 4.30
2.00 2.81 1.13 0.69 6.95 3.13 1.09 3.15 0.36 0.77 3.62 6.25 10.50 0.88 0.45 2.91 2.53 8.80 3.00 1.77 6.00
6 5 3 5 4 3 6 7 5 3 5 7 2 2 2 5 7 3 6 7 6
was then determined by using the appropriate area formula for the given shape of the grain flow: in this case either the area of a trapezoid (A = (w1 + w2) × L/2) or a kite (A = 1/2(length)(width)). For subsequent analysis of grain flows in time and space, the lee face was divided into seven 2 m-wide segments (Fig. 1).
3. Results 3.1. Spatial distribution of grain flows On the dune of span-wise width from east to west of 15 m, the grain flows occur on all parts of the lee face, with a tendency to be more frequent in its western part. Fig. 2A shows the total number of grain flows in each 2 m-wide segment as delineated above; and Fig. 2B shows the total area of grain flows in each segment as a proxy for the volume of material moved, there being no information on grain flow thickness available. 3.2. Temporal distribution of grain flows Twentyone grain flows occurred on the dune lee face in the 60 min of the video recording, with an average time between grain flows of 2.7 min. The median duration of the grain flows was 40 s with a range from 15 to 62 s (Table 1). The grain flows appear to occur in temporal clusters (Fig. 3) during the intervals spanning elapsed times of 4–9 min, 14–20 min, 26–34 min, and 42–60 min from the start of the video. The clusters can also be grouped into two distinct periods in which the recurrence interval between flows changed: (1) from 4 to 34 min (48% of grain flows) when the average interval between flows was 202 s and (2) between 42 and 62 min (52% of flows, average interval between flows 102 s). Wind speed at the dune crest was 7.49 m/s during the first period and 7.38 m/s during the second period.
Fig. 3. Timing of grain flows compared to wind speed at dune crest.
C. Breton et al. / Geomorphology 95 (2008) 518–523 Table 2 Recurrence intervals between avalanches by segment Recurrence interval (min) Segment 3 5 6 7
21.10 7.83 4.67 9.17
12.82 10.84 29.67 8.66
11.72 17.50 22.33 16.17
8.16
Mean
Standard deviation
15.21 11.08 18.89 11.33
5.13 4.49 12.85 4.20
However, for a given part of the lee face, grain flows take place much less frequently, as deposition by grain fall builds up to a point of grain flow. Four of the segments (3, 5, 6, and 7) experienced 4 or more grain flows within the study period allowing the time between grain flows within the segment to be calculated (Table 2). The average recurrence interval between grain flows within these segments ranged between 11.1 and 18.9 min (Fig. 4). 3.3. Length and area of grain flows Grain flow length ranges from 1.72 to 5 m, with 5 m indicating that the grain flow length covered the entire height of the dune lee face (Table 1). Average grain flow length was 4.11 m. The grain flows were, in general, trapezoidal in shape. As a result their planimetric area was found
521
using the standard geometric formula for a trapezoid (see Methods section). The area of the grain flows ranges from 0.45 m2 to 10.50 m2 , with a mean of 3.28 m 2 (Table 1). The majority of the grain flows (76%) have an area less than 4.0 m 2 , and 24% have areas greater than 6.0 m2 . The larger grain flows tend to occur at a position on the dune where there are relatively few grain flows and occur after a long interval between grain flows. Overall, the area (and likely the volume) of the grain flows tends to increase with the interval between flows, although there is considerable scatter in our data (Fig. 5). 3.4. Point of grain flow Initiation on the lee face Through visual observation of the video, it was determined that grain flow initiation repeatedly occurred near the top of the lee face. For many grain flows it was clearly evident that grain flow initiation began at a point approximately 0.30–0.40 m below the crest where a topographic bulge on the slope was visible. The grain flow then progressed down slope from this point while the bulge appeared to move upwards before disappearing (retrogressive grain flow). Other grain flows appeared to cascade downslope from the crest of the dune within a wide apron of sand before channeling into a narrower stream. In these cases a topographic bulge could not be distinguished by visual observation.
Fig. 4. Occurrence of grain flows in identified segments, with mean and standard deviation of recurrence interval for each segment.
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4. Discussion 4.1. Magnitude and frequency of grain flows The magnitude of grain flows (as measured by their area) shows an overall dependence on the interval between successive grain flows in the same segment of the lee face (Fig. 5). The relationship is clearest for the segments 3 and 7 of the lee face where grain flow area is strongly correlated with the interval between successive grain flows (r 2 = 0.85 and 0.95 respectively). Grain flow recurrence interval should be a function of sediment delivery to the lee face, which in turn is dependent on wind speed and sand transport rate at the dune crest (Hunter, 1977). This hypothesis is generally supported by this study. The interval between grain flows for a given segment of the lee face decreases with the wind speed and therefore sand transport rate for the period preceding their initiation (Fig. 6), although there is a considerable amount of scatter in the data for some parts of the lee face. This is likely the result of the small range of wind speeds observed and the short duration of the experiment. In addition, field observations suggest that there are two types of grain flows: (1) primary flows initiated by accumulation of grain fall in the upper parts of the lee face, and (2) secondary flows that are initiated by disturbance of the surface of the lee face by primary flows. Secondary flows are generally smaller and occur
soon after primary flows. In practice, it is often difficult to distinguish between the two types of grain flow, thus the data plotted in Fig. 5 includes both varieties of grain flow. 4.2. Initiation of grain flows The grain flows were observed to begin at a point 0.3–0.4 m from the brink of the dune where deposition over-steepens the slope, leading to grain flow. This agrees with the saltation model of Anderson (1988). Even though this model helps explain the location of the bulge, and the observed periodicity of grain flows can be explained by the rate of sediment delivery to the lee face, there is no explanation as to why some of the grain flows observed here have such great areal extent. McDonald and Anderson (1995) suggest that another depositional mode is required to induce such large flows. Nickling et al. (2002) hypothesised that sediment carried in flow cells in the lee of a dune may be the source of this additional sediment. Laboratory wind tunnel studies (Cupp et al., 2005) show that sand movement on the lee face of an experimental dune is much more complex than previously observed. Fallout of saltating grains drives sand movement by reptation down the upper part of the lee slope. A strong return cell gives rise to upslope movement of sand in the lower part of the lee slope. It appears that area of interaction between these two transport zones may be the site of
Fig. 5. Relation between area of grain flow (by segment) and the time elapsed since last grain flow in that segment.
C. Breton et al. / Geomorphology 95 (2008) 518–523
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Fig. 6. Relation between recurrence interval of grain flows in a given segment of the lee face and wind speed at dune crest.
grain flow initiation. Further field studies are planned to examine these processes. 5. Conclusions The observations documented above indicate the considerable amount of information on lee face processes that can be gained using this video approach. They indicate that grain flows occur at somewhat regular intervals, determined in large part by wind speed and sand transport rates. The existence of primary and secondary grain flows has not, to our knowledge, been described in the literature. There are many lessons that can be learned from this preliminary study, which will guide future research by our group. These include: (1) the need for a much longer time series, in order to increase the sample of grain flows and improve the magnitude and frequency statistics; (2) observations that span a greater range of wind speeds in order to better determine relations between the magnitude and frequency of grain flows, winds, and mass transport rates; and (3) measurements of grain flow thickness, so that estimates can be made of the volume of material redistributed by the grain flows. Acknowledgements Field work was supported by a grant from the American Chemical Society (PRF 37754-AC8) and data
analysis was funded by NSF Grant EAR02-07893. We thank the Gobabeb Research and Training Center for logistical support; and the anonymous reviewers for their comments and suggestions. References Anderson, R.S., 1988. The pattern of grainfall deposition in the lee of aeolian dunes. Sedimentology 35 (2), 175–188. Cupp, K., Lancaster, N., Nickling, W.G., 2005. Lee slope processes on a small artificial flow-transverse dune. Eos, Transactions American Geophysical Union 86 (52, Fall Meeting Supplement) Abstract H51C-0386. Howell, J., Mountney, N., 2001. Aeolian grain flow architecture: hard data for reservoir models and implications for red bed sequence stratigraphy. Petroleum Geoscience 7, 51–56. Hunter, R.E., 1977. Basic types of stratification in small eolian dunes. Sedimentology 24, 361–388. Hunter, R.E., 1985. A kinematic model for the structure of lee-side deposits. Sedimentology 32, 409–422. Livingstone, I., 1989. Monitoring surface change on a Namib linear dune. Earth Surface Processes and Landforms 14, 317–332. Livingstone, I., 1993. A decade of surface change on a Namib linear dune. Earth Surface Processes and Landforms 18 (7), 661–664. Livingstone, I., 2003. A twenty-one-year record of surface change on a Namib linear dune. Earth Surface Processes and Landforms 1025–1032. McDonald, R.R., Anderson, R.S., 1995. Experimental verification of aeolian saltation and lee side deposition models. Sedimentology 42 (1), 39–56. Nickling, W.G., McKenna Neuman, C., Lancaster, N., 2002. Grainfall Processes in the Lee of Transverse Dunes, Silver Peak, Nevada. Sedimentology 49 (1), 191–211.
Geomorphology 95 (2008) 518 – 523 www.elsevier.com/locate/geomorph
Magnitude and frequency of grain flows on a desert sand dune Carrie Breton a , Nicholas Lancaster b,⁎, William G. Nickling a a
Department of Geography, University of Guelph, Guelph, Ontario, Canada N1G 2W1 b Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA Received 23 May 2005; received in revised form 5 July 2007; accepted 7 July 2007 Available online 21 July 2007
Abstract Preliminary studies of the magnitude and frequency of lee side avalanches (grain flows) on a Namib crescentic dune show that the frequency of grain flows for a given segment of the lee face is dependent on the wind speed and sand transport rate for the period preceding their initiation; and the magnitude of the flows as described by their area is inversely proportional to the interval between flows and thus wind speed and sand transport rates. These studies indicate the potential of using a simple digital video camera technique to document the magnitude, frequency and geometry of grain flows on desert sand dunes. © 2007 Elsevier B.V. All rights reserved. Keywords: Aeolian processes; Sand dunes; Grain flow; Namib Desert
1. Introduction Despite their importance to the dynamics of modern dunes and the interpretation of the rock record (Howell and Mountney, 2001), there have been few studies of the depositional processes that occur on dune lee faces. Prior studies consist of two modeling efforts (Hunter, 1985; Anderson, 1988), and three known field experiments (Hunter, 1985; McDonald and Anderson, 1995; Nickling et al., 2002). None of these studies have considered the magnitude and frequency of avalanches, also known as grain flows (Hunter, 1977). Following the terminology of Hunter (1977), grain flows occur when sediment deposited on the lee face by grain fall from the overshoot
⁎ Corresponding author. E-mail address: [email protected] (N. Lancaster). 0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.07.004
of saltating grains transported over the brink of the lee face builds up so that the lee slope is steepened above the angle of repose and fails, initiating a grain flow or avalanche. In this paper, we report the results of a preliminary field study of the frequency, duration and timing, and spatial extent of grain flows on the lee face of a 5 m-high crescentic (transverse) dune in the northern Namib Sand Sea. 2. Field site and methods The study dune is located on the eastern flank of a 60 m-high S–N-trending complex linear dune in the northern part of the Namib Sand Sea, approximately 200 m south of area of the dune studied by Livingstone (Livingstone, 1989, 1993, 2003). The study dune crest trends approximately WSW-ENE for a distance of 50 m and the dune is 5 m high (Fig. 1). It is one of a series of secondary dunes that migrate northward along the S–N linear dune flank in response to secondary flow generated
C. Breton et al. / Geomorphology 95 (2008) 518–523
519
Fig. 1. The study lee face, view from northeast. Note multiple grain flow tongues. Vertical lines separate lee face into 2 m-wide segments for analysis of grain flow magnitude and frequency.
by the dominant SW–SSW winds. At the time of the study (the afternoon of 11 August, 2002), primary winds were from the SSW. To determine when, where, and how often grain flows occur along the lee face of the dune we analyzed video footage taken over a period of approximately 60 min between 15:30 and 16:30 local time. A Canon ZR60 video camera was placed facing the lee side of a dune and imaged about 15 m laterally of the dune lee face. A Wind Sentry anemometer and wind vane was placed 1 m south (upwind) of the crest line and recorded wind speed and direction at a height of 0.5 m above the dune surface at 1 Hz intervals, averaged over a 1-minute period. Information on the number of grain flows, their duration (i.e. initiation to cessation), the location of individual grain flows on the dune face, the length and width of the flow, and the point where grain flow initiation occurred was obtained from the video footage. Temporal information was obtained using the clock on the InterVideoWinDVD.6 program used to play the recording. The dune outline and grain flow areas identified from the video were transferred onto acetate paper so that measurements could be made of the length and width of each grain flow. The length of grain flows measured on the overlay were geometrically corrected for slope assuming a lee slope angle of 32°. No other corrections (e.g. for distortion at the edges of the frame) were made. The area covered by each of the grain flows
Fig. 2. (A) Number of grain flows in each segment during observation period (approximately 60 min). (B) Area of grain flows in each segment during observation period.
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C. Breton et al. / Geomorphology 95 (2008) 518–523
Table 1 Initiation time and duration, length, area, and location of each failure occurring on the Namib dune during the video coverage Failure Start time Duration Length Area number (h:min:s) (h:min:s) (m) (m2)
Location Segment (distance from east m)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
11.45 9.60 4.64 8.40 7.36 4.85 10.47 13.47 8.51 5.51 8.78 13.85 3.71 2.07 2.07 8.07 12.38 5.73 10.96 12.22 9.98
0:04:00 0:06:10 0:08:47 0:14:20 0:19:48 0:19:50 0:26:20 0:26:30 0:31:50 0:33:19 0:42:40 0:42:40 0:46:15 0:46:59 0:47:58 0:50:30 0:51:20 0:54:25 0:56:00 1:00:30 1:00:40
0:00:37 0:00:50 0:00:46 0:00:50 0:00:35 0:00:45 0:00:50 0:00:15 0:00:40 0:00:41 0:00:37 0:00:35 0:00:57 0:00:46 0:01:02 0:00:40 0:00:55 0:00:40 0:00:45 0:00:20 0:00:27
4.22 4.19 4.34 3.43 5.00 5.00 3.95 3.00 2.87 1.72 5.00 4.24 5.00 3.99 3.36 4.28 5.00 5.00 4.42 4.06 4.30
2.00 2.81 1.13 0.69 6.95 3.13 1.09 3.15 0.36 0.77 3.62 6.25 10.50 0.88 0.45 2.91 2.53 8.80 3.00 1.77 6.00
6 5 3 5 4 3 6 7 5 3 5 7 2 2 2 5 7 3 6 7 6
was then determined by using the appropriate area formula for the given shape of the grain flow: in this case either the area of a trapezoid (A = (w1 + w2) × L/2) or a kite (A = 1/2(length)(width)). For subsequent analysis of grain flows in time and space, the lee face was divided into seven 2 m-wide segments (Fig. 1).
3. Results 3.1. Spatial distribution of grain flows On the dune of span-wise width from east to west of 15 m, the grain flows occur on all parts of the lee face, with a tendency to be more frequent in its western part. Fig. 2A shows the total number of grain flows in each 2 m-wide segment as delineated above; and Fig. 2B shows the total area of grain flows in each segment as a proxy for the volume of material moved, there being no information on grain flow thickness available. 3.2. Temporal distribution of grain flows Twentyone grain flows occurred on the dune lee face in the 60 min of the video recording, with an average time between grain flows of 2.7 min. The median duration of the grain flows was 40 s with a range from 15 to 62 s (Table 1). The grain flows appear to occur in temporal clusters (Fig. 3) during the intervals spanning elapsed times of 4–9 min, 14–20 min, 26–34 min, and 42–60 min from the start of the video. The clusters can also be grouped into two distinct periods in which the recurrence interval between flows changed: (1) from 4 to 34 min (48% of grain flows) when the average interval between flows was 202 s and (2) between 42 and 62 min (52% of flows, average interval between flows 102 s). Wind speed at the dune crest was 7.49 m/s during the first period and 7.38 m/s during the second period.
Fig. 3. Timing of grain flows compared to wind speed at dune crest.
C. Breton et al. / Geomorphology 95 (2008) 518–523 Table 2 Recurrence intervals between avalanches by segment Recurrence interval (min) Segment 3 5 6 7
21.10 7.83 4.67 9.17
12.82 10.84 29.67 8.66
11.72 17.50 22.33 16.17
8.16
Mean
Standard deviation
15.21 11.08 18.89 11.33
5.13 4.49 12.85 4.20
However, for a given part of the lee face, grain flows take place much less frequently, as deposition by grain fall builds up to a point of grain flow. Four of the segments (3, 5, 6, and 7) experienced 4 or more grain flows within the study period allowing the time between grain flows within the segment to be calculated (Table 2). The average recurrence interval between grain flows within these segments ranged between 11.1 and 18.9 min (Fig. 4). 3.3. Length and area of grain flows Grain flow length ranges from 1.72 to 5 m, with 5 m indicating that the grain flow length covered the entire height of the dune lee face (Table 1). Average grain flow length was 4.11 m. The grain flows were, in general, trapezoidal in shape. As a result their planimetric area was found
521
using the standard geometric formula for a trapezoid (see Methods section). The area of the grain flows ranges from 0.45 m2 to 10.50 m2 , with a mean of 3.28 m 2 (Table 1). The majority of the grain flows (76%) have an area less than 4.0 m 2 , and 24% have areas greater than 6.0 m2 . The larger grain flows tend to occur at a position on the dune where there are relatively few grain flows and occur after a long interval between grain flows. Overall, the area (and likely the volume) of the grain flows tends to increase with the interval between flows, although there is considerable scatter in our data (Fig. 5). 3.4. Point of grain flow Initiation on the lee face Through visual observation of the video, it was determined that grain flow initiation repeatedly occurred near the top of the lee face. For many grain flows it was clearly evident that grain flow initiation began at a point approximately 0.30–0.40 m below the crest where a topographic bulge on the slope was visible. The grain flow then progressed down slope from this point while the bulge appeared to move upwards before disappearing (retrogressive grain flow). Other grain flows appeared to cascade downslope from the crest of the dune within a wide apron of sand before channeling into a narrower stream. In these cases a topographic bulge could not be distinguished by visual observation.
Fig. 4. Occurrence of grain flows in identified segments, with mean and standard deviation of recurrence interval for each segment.
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C. Breton et al. / Geomorphology 95 (2008) 518–523
4. Discussion 4.1. Magnitude and frequency of grain flows The magnitude of grain flows (as measured by their area) shows an overall dependence on the interval between successive grain flows in the same segment of the lee face (Fig. 5). The relationship is clearest for the segments 3 and 7 of the lee face where grain flow area is strongly correlated with the interval between successive grain flows (r 2 = 0.85 and 0.95 respectively). Grain flow recurrence interval should be a function of sediment delivery to the lee face, which in turn is dependent on wind speed and sand transport rate at the dune crest (Hunter, 1977). This hypothesis is generally supported by this study. The interval between grain flows for a given segment of the lee face decreases with the wind speed and therefore sand transport rate for the period preceding their initiation (Fig. 6), although there is a considerable amount of scatter in the data for some parts of the lee face. This is likely the result of the small range of wind speeds observed and the short duration of the experiment. In addition, field observations suggest that there are two types of grain flows: (1) primary flows initiated by accumulation of grain fall in the upper parts of the lee face, and (2) secondary flows that are initiated by disturbance of the surface of the lee face by primary flows. Secondary flows are generally smaller and occur
soon after primary flows. In practice, it is often difficult to distinguish between the two types of grain flow, thus the data plotted in Fig. 5 includes both varieties of grain flow. 4.2. Initiation of grain flows The grain flows were observed to begin at a point 0.3–0.4 m from the brink of the dune where deposition over-steepens the slope, leading to grain flow. This agrees with the saltation model of Anderson (1988). Even though this model helps explain the location of the bulge, and the observed periodicity of grain flows can be explained by the rate of sediment delivery to the lee face, there is no explanation as to why some of the grain flows observed here have such great areal extent. McDonald and Anderson (1995) suggest that another depositional mode is required to induce such large flows. Nickling et al. (2002) hypothesised that sediment carried in flow cells in the lee of a dune may be the source of this additional sediment. Laboratory wind tunnel studies (Cupp et al., 2005) show that sand movement on the lee face of an experimental dune is much more complex than previously observed. Fallout of saltating grains drives sand movement by reptation down the upper part of the lee slope. A strong return cell gives rise to upslope movement of sand in the lower part of the lee slope. It appears that area of interaction between these two transport zones may be the site of
Fig. 5. Relation between area of grain flow (by segment) and the time elapsed since last grain flow in that segment.
C. Breton et al. / Geomorphology 95 (2008) 518–523
523
Fig. 6. Relation between recurrence interval of grain flows in a given segment of the lee face and wind speed at dune crest.
grain flow initiation. Further field studies are planned to examine these processes. 5. Conclusions The observations documented above indicate the considerable amount of information on lee face processes that can be gained using this video approach. They indicate that grain flows occur at somewhat regular intervals, determined in large part by wind speed and sand transport rates. The existence of primary and secondary grain flows has not, to our knowledge, been described in the literature. There are many lessons that can be learned from this preliminary study, which will guide future research by our group. These include: (1) the need for a much longer time series, in order to increase the sample of grain flows and improve the magnitude and frequency statistics; (2) observations that span a greater range of wind speeds in order to better determine relations between the magnitude and frequency of grain flows, winds, and mass transport rates; and (3) measurements of grain flow thickness, so that estimates can be made of the volume of material redistributed by the grain flows. Acknowledgements Field work was supported by a grant from the American Chemical Society (PRF 37754-AC8) and data
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