Megaripples dynamics under a series of storm events

Hezi Yizhaq1*, Itzhak Katra2, Ori Isenberg1

1Institute for Dryland Environmental Research, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990, Israel.

2Department of Geography and Environmental Development, Ben-Gurion University of the Negev, Beer Sheva, 84105, Israel.

*Corresponding Author

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Phone: +972-8-6596789

Fax: +972-8-6596921

Abstract

Megaripples in Nahal Kasuy in the southern Negev desert ofIsrael are characterized by a mean wavelength of about 70 cm and by a bimodal distribution of coarse and fine particle sizes, the latter of which is necessary for megaripple formation. We show that storms can affect megaripples in different ways depended on their sizes. Larger enough megaripples which their crests protrude above the saltation layer can be flatten as the wind dislodges the cover of the coarse particles at the crest. Medium megaripples can be broken into smaller segments or change their orientation but the characteristic bimodal distribution of particles at the crestsremains.In contrast, smaller megaripples can grow under the action of storms. These different behaviorsare further depended on the wind velocity and on the size of the coarsest particles at the crests. The effects of storms are non spatially uniform but locally depended on the specific characteristics and morphometry of the ripples.Thus, megaripple height, which was believed to grow indefinitely (Bagnold, 1941) is self-limiting, and our results also explain the positive correlation between maximum grain size at the crest and ripplewavelength.

INTRODUCTION

Aeolian ripples, which form regular patterns on sandy beaches and desert floors, indicate the instability of flat sand surfaces under the wind-induced transport of sand grains. Two different kinds of sand ripple are observed in nature—normal ripples and megaripples (Bagnold 1941; Sharp 1963). Their main features are summarized in Table 1.

Megaripples have been described in many places, among them the Kelso Dunes and Coachella Valley sands in Southern California (Sharp, 1963), in the Libyan desert (Bagnold, 1941; El-Baz, 1986), the northern Sinai (Tsoar, 1990), Swakopmund, Namibia, (Fryberger et al., 1992), northeastern Iceland (Mountney and Russell, 2004), the coast of northeastern Brazil (Yizhaq, 2008) and on the Great Sand Dunes National Park and Preserve in south-central Colorado (Zimbelman et al., 2011). Ginat megaripples were documented in Carachi Pampa, Argentina, at a height of 4000 m above mean sea level (Milana, 2009). Composed of volcanic pebbles, these megaripples were formed by the action of extremely strong winds (probably the strongest winds known on Earth, ~400 km/h). Megaripple wavelengths were up to 43 m and their heights were about 2.3 m (Milana, 2009) with a crest maximum grain size of 19 mm.

Table 1: Main features of normal aeolian ripples and megaripples

The physical mechanism responsible for the formation of sand ripples is the action of the wind on loose sand. When wind strength exceeds some threshold, grains displaced by the direct action of the wind are lifted into the air. However, even strong winds cannot keep sand grains suspended indefinitely (they are too heavy),and therefore, they eventually fall to the ground. During their flight, sand grains reach velocities that are approximately equal to that of the wind.Upon impact with the ground surface, the grains impart energy and momentum into the sand and eject other grains. For sufficiently high wind velocities, the bombardment by sand grains accelerated by the wind generates a cascade process, creating an entire population of saltating grains “hopping” on the sand surface. When the saltating, high-energy grains collide with the bed, their impacts eject smaller, lower energy grains, termed reptons (Andreotti et al., 2004). The windward slope, characterized by small bumps, is subjected to more impacts than the lee slope. As such, the flux of reptons is higher uphill than downhill, which causes the bumps to increase in size. Size analyses of grains from different parts of the megaripples and from normal ripples showed that a bimodal mixture of grain sizes is needed for megaripple formation and that the coarse particles are more abundant at the crest (Isenberg et al., 2011; Yizhaq et al., 2009).

In a recent study (Isenberg et al., 2011), we used a photogrammetric technique to show that megaripples start out as normal ripples and grow due to a rapid coarsening process. Their evolution is a function of wind power and of the variability in wind direction. The final wavelength is not simply correlated to the mean saltation length (Elwood et al., 1975), but rather, it develops through the interactions between ripples of different sizes (Isenberg et al., 2011; Yizhaq et al., 2009). Larger wavelengths probably reflect longer development times and stronger winds,characteristics common to bedforms in different environments, such as ripples and dunes in rivers, oceans, and deserts (Werner, 1995).

The megaripple system exhibits self-organization, such that spatio-temporally ordered structuresemerge spontaneously (Anderson, 1990; Kocurek and Ewing, 2005; Werner, 1995). During megaripple evolution, the fraction of coarse particles at the crest increases, leading to the development of an armored layer that protects the megaripple from wind erosion and that enables its continued growth. Strong winds above the fluid threshold of the coarse particles, however, can erode the armored layer and destroy the megaripples, as we recently observed (Isenberg et al., 2011, Yizhaq et al., 2012). Therefore, a correlation exists between the large grains at the crest and megaripple wavelength and height: the coarser the grains at the crest, the larger the wavelength. Despite the knowledge gained about ripple dynamics, we still possess only a rudimentary understanding of megaripple formation and destruction processes, which dictates the need for more quantitativeresearch.

The goal of the following work is to study the spatiotemporal dynamics of megaripples under a series of storms during February and March 2009. Recently, (Isenberg et al., 2011) we showed that these storms flattened one of the plots in the study site (plot D, in Figure 2) covered with large megaripples. There are two mechanisms that can mobilize the coarse particles on the megaripple crests and bring them into saltation: direct lifting by fluid drag and indirect lifting due to the impacts of saltating particles (‘splash’).For flat sand beds, theory and numerical models show that the absorption of wind shear stress by saltating particles reduces the wind stress at the surface to a value below the fluid threshold (e.g., Ungar and Haff, 1987; Werner, 1990; Anderson and Haff, 1991; Andreotti, 2004; Kok and Renno, 2009). Particles on the surface are thus sheltered from the wind, and the wind stress at the surface actually decreases as u* increases above the fluid threshold. For a flat sand bed, coarse particles will thus not be lifted by fluid drag even for u*u*t. However, this situation may differ at the megaripple crest, which protrudes into the saltation layer, thereby reducing the sheltering effect at the soil surface due to wind stress absorption by the saltating particles above. Field measurements made by Greeley et al. (1996) and Namikas (2003) and compiled in Fig. 5 from Kok and Renno (2008) indicate that the mean saltation height for ~250-µm sand grains is ~3-4 cm. Since the height of the megaripples at Nahal Kasuy was about 5 cm, their crests likely protruded from the saltation layer, and as such, they would have experienced substantially higher wind shear stress than that felt in the troughs. As a consequence, an increase in would thus produce an increase in the surface wind stress at the megaripple crest to the point that coarse particles could be lifted. For a given megaripple height, there thus exists a threshold shear velocity below which megaripples grow but above which the megaripples are flattened due to fluid entrainment of the coarse grains. Moreover, this mechanism implies a feedback between the height of the megaripple and the wind speed at which it is flattened: the higher the megaripple, the more it protrudes into the saltation layer, the higher the shear stress at the crest surface at a given , and thus, the lower the critical at which coarse particles can be lifted by the fluid. This effect seems to explain why the high megaripples disappeared during the storms while the smaller megaripples often did not as we show in this work. Moreover, in other parts of the study site, incipient megaripples were formed by the same storms.

Our main goal here, is to show that storms impact locally depends on ripples morphology, such that it differs from one specific location to the other in the same field. The same series of storms can destroy large megaripples but build small ones.

MATERIALS AND METHODS

Field experiment

Our field experiment was carried out in the southern Negev at Nahal (wadi) Kasuy sand dunes (Figure 2), which cover an area of 15 km2 (Ginat, 1991). Seif, falling and climbing dunes developed in the area.

Sand in this area is comprised 60% calcite and 35% quartz. It drifts into Nahal Kasuy from the UvdaValley on southwestern storm winds and piles up in the wadi bed (Fig.2). The sand particles were transported from the valley margins and from the extensive Pliocen Conglomerate, west of UvdaValley. The particles were deposited about 3 km westward in the Hiyyon stream in an area that contains 70 m of fine alluvial sediments. Western winds transported the sand into valleys of Nahal Kasuy and Nahal Yitro, and the northern winds accumulate it into dunes.

The annual precipitation here is about 37 mm concentrated in the rainy season (November to April), and shrubs of Haloxylon persicum cover the wadi bed sparsely.

The megaripple field is located in the middle of the wadi, where coarse grains abound. The mean megaripple wavelength is about 70 cm, with a mean height of about 7 cm (; ripple index defined as the ratio between the wavelength to height). Smaller ripples superimposed on the megaripples reflect the most recent wind direction. Because the Kasuy megaripples are small compared to those in other parts of the world, they are expected to be more sensitive to the storms that form and modify them and that can even destroy them (Isenberg et al., 2011).

To study mega-ripple evolution we flattened four plots (plots A, B, C and F) and hand mixed the grains to achieve a uniform distribution of coarse and fine grains. The plot sizes and the dates of flattening are given in Table 2 (see also the map in Figure 2). The fourth (D) and the fifth (E) plots were not flattened but were marked to track changes of the large and medium size mega-ripples. In this work we show results from plots B, D (see Isenberg et al., 2011), F (see Yizhaq et al, 2012) and area N.

Table 2: Plot descriptions at Nahal Kasuy field study.

Wind measurements

Wind speed and direction measurements were taken at 10-min intervals at a height of 3.3 m using an anemometer recorder placed at the eastern edge of the megaripple field. To complement our data, we compared it with the wind data (averaged hourly) from a nearby meteorological station at Uvda airport (30N, 34.883E; 3 km west to Nahal Kasuy). There was good agreement between the wind measurements from the two locations (Yizhaq et al., 2009). The wind speed was used to calculate the drift potential (DP) and the resultant drift potential (RDP) (Fryberger, 1979). Theoretical and empirical studies show that the potential sand volume transported by the wind through a 1-m-wide cross section per unit time is proportional to DP (Bullard, 1997; Fryberger, 1979). DP is calculated from

, (1)

where is the wind speed (in knots; 1 knot=0.514 m/s) measured at a height of 10 m and averaged over time, and is the minimal threshold velocity (=12 knots) necessary for sand transport for a typical sand grain (with an average diameter of 0.25mm) (Fryberger, 1979). RDP is the vector summation of DP from different directions and over the measurements. Mathematically, it can be written as:

(2)

where, , and is related to the wind direction. The direction of RDP is referred to as the resultant drift direction RDD, which is defined as . RDD expresses the net trend of sand drift, namely, the direction in which sand would drift under the influence of winds blowing from various directions. The ratio of RDP to DP (RDP/DP) is an index of the directional variability of the wind (e.g., RDP/DP=1 stands for unidirectional wind, and RDP/DP=0 characterizes multidirectional winds that vectorially cancel each other out). DP is the potential sand drift, but the actual sand drift potential further depends still on the mean grain diameter, the degree of surface roughness, the amount of vegetation cover, and sand moisture.

In order to use our wind speed measurements to calculate DP we extrapolated it to the height of 10 assuming a logarithmic profile and calibrated surface roughness (see Isenberg et al. 2011). Validation tests of the method vs. direct measurements confirmed that it was very good (see Yizhaq et al., 2009; Isenberg et al., 2011).

Grain size analysis

Samples of sand, all of which were collected using the same method, were retrieved from the field with a tin can (diameter 84 mm, height 35 mm) by pressing the can into the cross-section of the ripple under study. The samples were scooped out of the can with a flat scraper. Here, we concentrated on samples taken from the crests as the crest GSD is a good indication of the ripples development (see Yizhaq et al., 2012).

Average sample weight was 310 g (with values ranging from 282 to 336 g). Grain size analysis was performed by ANALYSETTE 22 MicroTec Plus laser diffraction, which measures particlesin the range of 0.08 to 2000μm. The preparation of each soil sample included sample splitting for replicate samples by a micro-splitter device and the removal of distinct organic matter. For the analysis, three replicates (4 to 5 g) of each sand sample were dispersed in a Nahexametaphosphate solution (at 0.5%) and by sonication (38 kHz). Due to the negligible number of clay-sized particles in the sandy samples,GSD data were calculated using the Fraunhofer diffraction model with an error < 5.0%.

Photogrammetry

We produced digital photographs using red-green-blue (RGB) images from a digital Nikon D80 camerawith a Sigma 10-20 mm lens. Processing took place with Erdas Imagine ver. 9.1 and its Leica Photogrammetry Software (LPS) extension. The small focal-length of the lens (10 mm), which corresponds to a 94.5° field of view, reduced the number of photographs needed to cover the plots. To avoid interfering with plot dynamics, the imaging and the ground control point (GCP) markings had to be made from outside each plot. The camera was mounted on a special rail (5 m long and secured at each end to a tripod), along which it could be moved using two cords attached to the camera for this purpose. We used a remote control cable to operate the camera (see Figure 3). Image analyses were carried out with the LPS Project Manager. To reduce the need for a large number of GCPs for each plot, the LPS Project Manager uses the self-calibrating bundle block adjustment method. With this approach, the internal geometry of each image and the relationship between overlapping images is determined with a small number of GCPs.

The only manual process needed to implement this approach is geometric rectification, after which the program automatically extracts all the data needed for the “Automatic Terrain Extraction” feature embedded in the LPS. The main photogrammetry output we used in this study was the DEM that provided the two most important parameters in ripple measurement, i.e. wavelength and ripple height. Measuring these parameters together with continuous wind measurements enabled us to track temporal topographic changes.

DEM quality depended on many factors. We obtained the best results when images where taken in the late afternoon when contrast was maximized. By selecting the camera’s Auto Mode option, aperture and shutter speed were chosen automatically; no significant deviations in color or hue were noticed among the pictures (for more details about the method see Isenberg et al., 2011).

RESULTS

Wind speed measurements

Two strong storms were recorded during our study (Figure 4). In each of them the wind speed reached 15 m/s (at a height of 3.3 m) and their prevailing direction was south westerly typical to the winters storms. Table 3 summarizes the wind statistics of the period 20/2-31/3/09. The windiest time interval was between 20/2-8/3/12 where DP was 12.99.

Table 3: Wind data from Nahal Kasuy for the period 20/2-31/3/09. DP is the potential drift potential; RDP is resultant drift potential; RDD is RDP direction; RDP/DP is the wind directionality and is the time (in percent) that the wind is above the fluid threshold for sand transport (taken as 6 m/s).

Grain-size analysis

During these storms an intense saltation occurred in the plots as shown in Figure 5. Figure 6 shows the GSD of the saltation traps (near plot C). The medians (D50) w during the first two periods were 217.4 and 188.4 m respectively whereas during the calm period (24/3-31/3) the median was 177.7 m, which is still above the median of typical normal ripples in Nahal Kasuy which is 158 m (Yizhaq et al., 2009). The coarse mode during the period with the storm of 27/2 was 338.5 m. These coarse grains have large momentum and kinetic energy which upon impact with the surface can dislodge the grains at the crest into reptation and even to saltation. Figure 7 shows the GSDof plots B, D F and N and the full statistics of the GSD is in Table 4.