The Bruneau Dune system is located in the Bruneau Dunes State Park, Idaho, USA (Fig 1). The park is 8 miles east-northeast of the town of Bruneau and about 18 miles south of Mountain Home. The park has an area of 19km², housing Sand Dune Lake. The majority of the Bruneau Dunes are located around the lake.
The dunes are unique in that unlike other American dunes such as Algodones, they form at the centre of the river basin and not at the edge. This is due to the distinctive characteristics that define their morphology. The climatic conditions and sediment supply are favourable to dune formation and maintenance. Whilst the total number of dunes is relatively small compared to some systems such as the Kelso Dunes in California, the system does provide for a range of individual dune types. This is interesting given the relatively small area that they occupy and the distances between them.
Whilst the dune system is quite remarkable and varied it is unfortunate to find that they have been relatively unstudied. To understand their formation there has to be an appreciation of general morphology.
The controlling factors of dune formation are aridity, wind velocity and sediment supply. Bagnold (1941) was instrumental in defining the relationship between the shear wind stresses to the amount of sediment flux. Bagnold’s relationship has been adapted and changed over the years to provide a more accurate interpretation of sediment flux. Kawamura (1951) first proposed a threshold limit on the minimum velocity required to entrain sediment. Lettau and Lettau (1978) and White (1979) are examples of further developments.
Sediment flux is controlled by the properties of texture, moisture content, vegetation, roughness and slope gradient effects (Lancaster, 1995). Further to this the size of sediment grains will produce different sediment fluxes when subjected to any given shear wind velocity. As particles become more spherical the sediment flux increases (Lancaster & Nickling, 1994). The presence of moisture will result in higher transport thresholds and can lead to a loss of as much as 25% of normal flux (Hotta et al., 1985). Vegetation influences the sediment availability and the surface roughness. Roughness strongly influences the sediment threshold levels (Blumberg & Greeley, 1993). Only smooth grains can move at low wind velocities (Greeley & Iversen, 1987).
These factors have affected the development of dunes at Bruneau and have culminated in the dune formations we see today. Simple and complex transverse Dunes and a Star Dune have formed and there is also the presence of a blowout (Fig 2).
Bureau’s climate is significantly arid. Rainfall reaches a maximum of 0.91 inches in November and 0.18 inches in July. Average rainfall is just 0.63 inches per month and an annual total of around 7.5inches (Lay, 2000). With precipitation so low throughout the year, sediment can easily be entrained, thus providing a basis for a high sediment transport rate.
Sediment flux requires an active wind. The dominant wind directions are the North West (NW) and South East (SE). Wind does though blow in all directions throughout the year. The strength and occurrence rates of the winds vary through the seasons (National Weather and Climate Center, 2003). In August the winds blow with almost equal intensity from the NW and SE. From August to March the winds blows for the most part from the SE (Fig 3). By April this has reversed and until August remains dominant (Fig 4). Wind therefore is mainly bi-directional and subjects the dunes to wind processes from the SE for one half of the year, and the opposite direction, the other half.
This creates a relatively stable environment and results in the dunes moving very little. They have in fact moved very little since their formation.
Another factor that fixes the dunes to their location is the presence of physical barriers in the form of the lake and wetland vegetation (Fig 5). The vegetation is a result of the saturated soil conditions and the high ground water which allows them to be sustained even in the presence of minimal precipitation. Nanninga & Wasson (1986) have indicated that sediment transport can occur even if vegetation cover is 45%. However the Bruneau dune system is subject to a small resultant wind. Sediment can be entrained but with the vegetation cover such as it is and the overall low wind velocities in any given direction the vegetation acts as a sufficient block for any dune migration.
The dunes themselves vary in type. As stated earlier a broad range exists over a small spatial area.
The transverse dunes are found predominantly to the North and North East of the lake. They form parallel to the prevailing wind directions (Fig 5). There is a great amount of sinuosity in the morphology of some of the dunes. The dunes also have sharp steep crests due to the presence of a slip face on both sides.
Even though the exact nature of seif formation is unknown (Bagnold, 1941; McKee, 1979), it is highly likely that they Seifs. The alternative choice is Reversing but this is not possible.
Reversing type dunes form via a generally persistent wind from one direction. This creates barchanoid-type dunes, while occasional reversing winds create miniature dunes on the crest. Whilst there are seasonal changes in the direction in which the wind is dominant, they both are active for similar period of time; half the year each. Thus is cannot be said that wind blowing from the NE, for example, is the dominant one whilst the SE winds are the ‘reversing’ ones.
Reversing dunes produce a slip face in the opposite direction to the dominant wind. Fig 5 shows clearly that there are two slip faces.
Bagnold (1941) and McKee (1979) put the formation of seifs down to bimodal wind directions, which would fit given known wind patterns. Although others believe that they can also form in unidirectional wind regimes (Fryberger, 1979).
We can model the formation of the dunes using a simple Discrete Ecogeomorphic Aeolian Landscape Model (DECAL). Using DECAL we are able to test the theory that at Bruneau the simple transverse dunes form by process of barchan transformation.
The Bruneau system is a product of bimodal winds. The model is only able to replicate a single wind direction. This would appear to be of concern. However given the seasonality of the wind strength the modal can still be used. The model is simulating what occurs when the wind is blowing constantly in one direction. The Bruneau winds are polar opposites and therefore the processes are repeated in exactly the opposite direction. There is a small overall result force with the NE being slightly more dominant in strength. Thus by taking the resultant force as the only direction of wind, we can still use DECAL as an effective experimental tool.
First the starting parameters must be specified. The attributes that will remain constant throughout the experiment are listed below (Table 1)
|Number of Cells Downwind||100|
|Number of Cells Lateral||100|
|Slab Height Ratio||0.1|
|Deposition Probability (With Slabs)||0.7|
|Deposition Probability (No Slabs)||0.3|
|Type of Boundary||Periodic|
|Starting Morphology||Starting Morphology|
|Max Height for Plotting||10|
Table 1: Starting Parameters for DECAL
The grid has been set at 100×100 as this is a manageable size that allows sufficient investigative depth, but is not so large that the simulation takes considerable time to run.
The depositional probabilities have been set at 0.7 and 0.3 for slabs and no slabs respectively. The probabilities have been set as such due to the fact that the Bruneau Dunes and the basin they lie in have acted as a trap for sediment since its formation. Jump length has been set to 2. This is in accordance with Bishop et al. (2002). The starting morphology has been set to flat. The maximum height for plotting is ten. This should allow a sufficient amount of detail to be mapped.
We begin by modelling the initial barchan formation. For this the iteration time is set at 100. It is set low to enable us to see if barchans do form initially. The output produced show the presence of merging barchan dunes. To see if they would eventually form a more established transverse dune. The iteration time was increased to 500.
From the outputs (Fig 6) we can see that transverse dunes have formed. They appear to have steep ridges either side. It can be seen that relatively linear dunes have formed. However some sinuosity has formed. This fits well with the current state that the Bruneau dunes appear to be at.
If we increase the iterations to 1000 we see longer and less sinuous dunes forming (Fig 7). This can be assumed to be what would happen to the Bruneau dunes, if one of the prevailing winds were to cease. If for example the wind regime were to change and for the overwhelming majority of the time wind only blew from the NW then we may get a dunes migrating and merging producing increasingly less sinuous.
The migrating and merging dunes does imply one thing however; the fact that there is space to do so. The Bruneau system is bound by vegetation to the south of the Dunes Lake and the lake itself. Depending on which hypothetical wind shall become dominant, NW or SE, we will get different dune movements. The South Easterly dunes would require a shift to a more North Westerly wind if they were to migrate, whilst the North Westerly ones would require the opposite. Complex dunes are defined by the development of smaller dunes over the top of a larger dune. A complex transverse dune forms from the south end of the lake. Superimposed barchanoid dunes can be found on the larger transverse dune (Fig 8).
One possible explanation for their formation is that wind, whilst strong enough in the quaternary period to form the larger dune, is now comparatively weak and can only form the smaller dunes that are superimposed (Lancaster, 1995). This is likely given the small resultant wind velocity that exists in the system. Wind speeds on average are 4.0ms-1 in a single direction.
Given the bimodal nature of the winds the velocity is not be great enough to counteract the effects of cohesion and gravity that exists for sediments on the transverse dune’s surface.Barchaniods are able to form as the slope of the transverse dune is sufficiently large enough for it to act as a normal planar surface.
If the barchaniods were migrating it would give an indication that the SE winds were sufficient enough to entrain sediment. However it is more likely that at some point in the dunes history it was subjected to winds for a time that resulted in barchans forming and migrating, but that wind regime is no longer present.
Given the sinuous nature of the large dune and the presence of barchaniods it can be assumed with some ease that its formation was the result of the barchan modification as suggested by Bagnold (1941). This would therefore categorise it as a Seif.
A star dune is located to the east of the Sand Dune Lake and is the tallest of the dunes at 143m (Fig 9). Star dunes are characterised by the formation of a central peak and three or more radiating arms. The star dune at Bruneau has three arms, the longest of which is 560m in length.
Star dunes are a result of large sediment availability and complex wind regime. Given the nature of the other dune formations and the wind data, it is clear that a complex regime does not currently exist. The bimodal winds that exist could not produce a star dune. This presents a problem.
It has been suggested that star dunes form from existing dunes after undergoing a change in wind regime, possibly after migrating into new locations (Lancaster, 1989). The star dune could not have formed under current conditions and therefore must have formed when conditions were suitably altered. The most likely factor to have been changed is the wind regime as opposed to the sediment supply. Sediment has always been in abundance and unlikely to be a limiting factor.
There are two possible formation scenarios. The first involves a wind regime that was able to establish a star dune and after its formation, changed to the current regime we have today. Other dunes would then have formed under the current regime but because of the self-reinforcing nature of the star dune the new wind patterns would not affect its morphology.
The second scenario hypothesises that the star dune was initially transverse and formed with the other connected dunes. After a time a freak event or sequence of events may have resulted in changes to the wind regime and thus the pattern of accumulation of sediment. This resulted in star dune formation.
The issue with the second scenario is that the events would have had to have been of great enough magnitude to alter the processes enough to form a star dune but confined to a small spatial area. If the change were not spatially limited, we have to ask why other star dunes have not formed. Given the arid conditions and abundance of sediment this is a real issue.
The Bruneau Dune system is defined by the presence of a consistent sediment supply and the seasonal bimodal wind regime. The juxtaposed nature of the winds is the main determining factor in why the sand dunes have not migrated since their initial conception, thousands of years ago. The low migration rates though suggest that at some point in the dunes history the wind regime was much different. Given the range of dune formations that require more than a bimodal or unidirectional wind regime, it is more than likely over its history the system has been subjected to different controlling processes. The dunes formations appear to have followed the Barchan merging pattern as suggested and the model helps to reaffirm this. The star dune however would have required a more complex wind regime and this would require further investigation. The whole system is bound the every presence of wetland vegetation and the Dunes Lake. Whilst dune migration can still occur in the presence of vegetation, the aerial footage seems to suggest that the dunes do not migrate. With the location of the lake blocking dune migration from the South East and North West it seems that the dune formations would be trapped. Only a significant change in the wind regime would allow the dunes to migrate and out of the river basin centre which they currently inhabit.
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