Crescent shaped sand dunes Causes? Wind direction in the crescent shaped dunes

Crescent-shaped sand dunes (barchan dune).

It is the most basic form of mobile sand dunes. The plane of the dune is like a crescent, and there are two wings extending downwind on both sides of the mound, and the degree of development of the two wings depends on the strength of the local dominant wind, the stronger the wind speed, the smaller the angle of the angle. The two slopes of the mound are asymmetrical, the windward slope is convex and gentle, the slope is 5-20 degrees, and the leeward slope is concave and steep, and the dip angle is 28-34 degrees.

The height of the dunes is not large, generally 1-5 meters, and the width can reach 100-300 meters. Crescent-shaped dunes are formed by the action of winds in one direction or in two opposite directions. Most of them are found on the edge of the desert.

The crescent-shaped dunes were originally just a small shield-shaped dune, which was formed by the directional wind, and the wind encountered the resistance of grass or shrubs to pile up small sand piles, and then the wind blew away from the windward slope and formed a vortex on the leeward slope to pile up. At the same time, the left and right sides of the sand pile form an inward rotating air flow, which expands the wings and gradually forms the bow shape of a crescent-shaped dune.

In the case of abundant sand, crescent-shaped dunes are connected by dense crescent-shaped dunes to form a crescent-shaped dune chain, and its height is generally 5-30 meters.

The wind blows, and the desert wind is strong.

The wind direction is , the windward slope is small (gentle) and the contour line is sparse, and the leeward side of the dune slope is large (steep) and the contour line is dense.

The crescent shape is only the appearance of the sand dunes, and the more harmful should be the mobile sand dunes.

Causes, hazards and management of mobile sand dunes:

The difference between sand dunes and mobile sand dunes is mainly based on whether there is vegetation stagnation. If the air flow containing a large number of sand particles is blocked by ground shrubs and rocks on the way, the sand particles will settle and accumulate, forming sand dunes or fixed sand dunes. If there is no vegetation to stagnate, the dunes can move under the action of the wind, forming mobile dunes.

The hazards of mobile sand dunes are considerable, often inundating villages, roads, pastures, etc. Effective measures to control quicksand are to plant trees and fix sand dunes.

The crescent-shaped dunes are small piles of sand formed by directional winds, which are blocked by grass or shrubs, after which the wind blows away from the windward slope and forms a vortex on the leeward slope for accumulation. At the same time, the left and right sides of the sand pile form an inward rotating air flow, which expands the two wings and forms gradually.

nbsp;The crescent-shaped sand dunes are one of the most basic forms of mobile sand dunes, and their flat form looks like a crescent moon. Let's talk about how crescent-shaped sand dunes are formed.

Details:

Crescent-shaped sand dunes are a type of aeolian landform, which is a sand dune with a plane like a crescent moon, and there are two wings extending downwind on both sides of the mound, like a curved moon. The crescent-shaped dunes were originally a small shield-shaped dune, which was formed by directional winds, when the sand encountered grass or shrubs to create small piles of sand, after which the wind blew away from the windward slope and formed a vortex on the leeward slope to accumulate. At the same time, the left and right sides of the sand pile form an inward rotating air current, which expands the wings and gradually forms the bow shape of a crescent-shaped dune.

Crescent-shaped dunes are connected by dense crescent-shaped dunes to form a crescent-shaped dune chain with a height of about 5-30 meters. Once the shield-shaped sand pile is formed, it can become a big obstacle to the operation of the wind and sand flow, and the air flow of the ground layer will be separated on the leeward slope of the sand pile, forming a vortex with a horizontal axis, and the speed will be weakened, so that the sand particles carried by the air flow will be deposited in the vortex area of the leeward slope after passing through the top of the hill. With the continuous deposition of sand, the size of the shield-shaped sand pile gradually increases, and the relative maximum sedimentation of the leeward slope is closer and closer to the top, so that the slope of the leeward slope continues to steepen, and when the slope reaches the maximum angle of repose of the sand of about 34°, the sand body undergoes shear movement, and some sand particles collapse, forming a small falling sand slope, which develops into a rudimentary crescent-shaped sand dune.

With the continuous accumulation and increase of sand dunes, the separation of airflow becomes more and more severe, the scale and intensity of the eddy continue to increase, and the small sand slope is further expanded. In the same development process, the air currents that bypass the sides of the sand pile carry the sand to the front of the sides and pile up, gradually forming two "horns" that stretch forward with the wind on the two wings of the dune. In this way, the typical crescent-shaped dunes are formed.

Factors influencing the movement of crescent-shaped dunes.

1) Sediment transport. The speed at which the dunes are moved is directly proportional to the cubic of the wind speed, while the amount of sediment transported is inversely proportional to the cubic of the wind speed.

2) Dune height. The speed at which the dunes move is inversely proportional to the height of the dunes.

3) The movement speed of sand dunes is also affected by vegetation, dune moisture and underlying ground factors. Vegetation can reduce wind speed, sand moisture requires higher sand speed, and rough ground can hinder sand starting.