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How to read sand dunes formation and texture?


Frederic

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On 7/6/2020 at 9:22 AM, Frederic said:

While offroading, we cross dunes by using different methods, based on how the dune is formed and what the drive level is. Once you have spent some time in the desert, you will slowly start to pay more attention to the dune formations and how we cross them. Once you reach Intermediate and Advance level, you'll need to start learning how to read these dunes in order to navigate and create your planned routes in the desert.

As a rule of thumb, the prevailing wind in UAE is blowing from a North-Western direction to South-East direction: 

image.png.b8cb1796b06e2dd8898556c801a14dc5.png

This is something you should remember and you will notice when you zoom into certain areas with your navigation app (Google Maps, Gaia, MotionX, Mytrails,...) you will notice that the wind from that side has caused the dunes to take on their shape. This is a general rule and will not always apply, as you will see that in certain areas where wind is coming from different directions, you will see for example "star" dunes appearing.

There are different types of dunes which are separately described below.  As you can see in below picture, a dune is formed when the prevailing wind blows on a mound of sand, where the grain will move into the wind direction and create a ridge. The sand which falls on the other side of the ridge, creates a slipface, and this sand is not compacted by the wind. This is the main reason why we generally drive on the prevailing wind side or the convex side of the dunes on the Newbie level.

 

sand-dunes-2.jpg.8b4501e6d676bcee63c2e3faa91e177f.jpg

 "A dune is a curiously dynamic creature," wrote Farouk El-Baz in National Geographic. “Once formed, a dune can grow. It can change shape and move with the wind. It can even breed new dunes. Some of these offspring may be carried on the back of the mother dune. Others are born and race downwind, outpacing their parents. [Source: Farouk El-Baz, National Geographic, February 1982]

The shape of dunes is affected by things like the strength, direction and consistency of the winds; the consistency and amount of sand; the hardness of the terrain and the amount of vegetation. 

 

1552294325_WhatsAppImage2020-07-06at10_05.08(1).jpeg.80792f6058fde357fa23fb5dbe36dcbf.jpeg

Below we will describe the most common dunes found in the UAE.

 

Different Dunes

Crescent or Barchan Dune

image.png.1aeedbbba4e49de1bd5f20809ee9a6ad.png

This is the most common dune. It forms the shape of a crescent moon when the wind blows from one direction.

We generally use the words concave and convex to describe both sides of the dune:

* Concave is a shape that curves inwards.

* Convex is a shape that curves outwards.

Crescent-shaped mounds generally are wider than long. On the concave side you will have the slipface. These dunes form under winds that blow from one direction, and they also are known as barchans. 

Capture.JPG.d31b11715d6e4a6b5395f6a67131d0b8.JPG

 

Parabolic Dune

image.png.9a4e18cb44058835b1d8f22d7a19e3d0.png

A parabolic dune is similar in shape to a barchan, but it is just the opposite. The tips of this dune point into the wind, and its main body migrates with the wind, forming a depression between the tips. Because of this formation, parabolic dunes are also known as blowout dunes. These dunes often occur when vegetation stabilizes sediments and a U-shaped blowout forms between clumps of plants.

Example of Parabolic dunes near Faya:

https://goo.gl/maps/ob8xu3y8VJJvF6hk8

 

Longitudinal or "Seif" Dunes

image.png.249196869b1769ffebff7c869781179f.png

They are also often referred to to as "sand ridges" or "seifs". Their length can range from a few meters to many kilometers and their height from a couple meters high to a couple hundred meters high. Wind pushes the sand and forms ridges parallel to the prevailing wind direction with slip faces either side of the crest. Linear dunes are straight or slightly sinuous sand ridges typically much longer than they are wide. They may be more than 160 kilometers long. Linear dunes may occur as isolated ridges, but they generally form sets of parallel ridges separated by miles of sand, gravel, or rocky interdune corridors. Some linear dunes merge to form Y-shaped compound dunes. Many form in bidirectional wind regimes. The long axes of these dunes extend in the resultant direction of sand movement. 

Example in Al Qudra area:

https://goo.gl/maps/RXHFzR3GW6iFr16C7

 

Barchanoid Dunes

image.png.55f49b0e7a2214cde9886bd93ac435fe.png

These ridges are similar to Barchan dunes. They are connected in long crescentic waves as the sand supply is much greater than that which forms a Barchan Dune.

 

Star Dunes

 

image.png.8a5a9db3a40e594197d9152e23d7c462.png

These dunes are formed when there are several prevailing wind directions - there is a complex wind regime. Typically there are three or more slip faces, whilst the dunes do not migrate they grow in height.

Star dunes are radially-symmetrical, spyramidal sand mounds with slipfaces on three or more arms that radiate from the high center of the mound. They are created when the winds blow equally from every direction, causing the sand dune to grow many extensions and resemble a star. They are rare and are fairly stable and remain in the same place. Some are landmarks with names.

Example of a Star dune:

https://goo.gl/maps/jRnYghQc8ACzVz6T7

218566767_WhatsAppImage2020-07-06at10_01_35.jpeg.dead1c9131ab3055b37f0c5a10d174f4.jpeg

 

With this knowledge, the next time you step into the desert, you can start looking at the different dune formations, and this will give you lots of knowledge that you'll be able to apply in the future when leading and navigating off-road drives.

@Frederic very informative and clear description . Thank you for the info indeed which is basic for us at entry level 👍

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  • 10 months later...

Truly when ever I read this article i realize something new .. one of the check list to do before almost any drive I do . 
 

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2 hours ago, M.Seidam said:

Truly when ever I read this article i realize something new .. one of the check list to do before almost any drive I do . 
 

Good you comeback to read and even made a checklist of it. 👍

Most dont read anything at all, as they think offroading is same as driving on szr.

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Let's root for each other & watch each other grow.

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Sure the resources are wide . However the one here gives really a theoretical platform for a better and safe practice . 
even reading own past reports or comments when encountering similar or about doing similar route is useful . 
 

realy I recommend every one especially newbies / fewbies and any one to read and re read  such relevant topics to our weekly practice . 
 

cheers 

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