The choice of formation between sand heart and disaster

There were many types of sand dunes. Among them, the dome-shaped sand dunes were formed by the laminar-flow effect of the airflow streamlines. The airflow separated above the dome to form an air vortex, causing the fine sand to be drawn into the air vortex and move upward, gradually accumulating into a dome-shaped sand dune. The circular dunes were formed by the combination of wind direction and surface aquatic conditions. For example, in a flat river valley, the surface water flow would wash out the depression. When the wind blew, the sand would be blown into these depression to form circular dunes. The vertical sand dunes were formed by sand that had accumulated over a long period of time on the rocks. In addition, the crescent-shaped sand dunes were formed by the turbulence caused by the interference of the sand pile on the wind field, resulting in different erosion and accumulation. The specific formation process was that the airflow from the top of the sand pile and around the two sides of the sand pile created a vortex on the leeward slope of the sand pile, and the sand was piled up on both sides of the sand pile, forming a horseshoe-shaped depression. As the wind speed and the amount of sand increased, the small depression on the leeward slope of the sand pile would further expand, and the sand would continue to accumulate under the effect of the vortex, forming a young crescent-shaped sand dune. As the sand dunes expanded and increased, the leeward slope produced a larger vortex, expanding the original horseshoe-shaped depression and forming a typical crescent-shaped sand dune. In short, the formation of sand dunes involved the interaction of many factors such as airflow streamlines, wind speed, sand volume, and surface aquatic conditions.

The formation of sand dunes was the result of a combination of natural forces and human activities. Sand dunes were mainly hills or ridges formed by the wind. They were commonly found on the sandy surface of coasts, certain river valleys, and dry areas. The formation of sand dunes could not be separated from the two factors of drought and wind. Due to the dry climate, the material became loose and could not be consolidated, allowing the sand to be transported and moved by the wind. During the formation of the sand dunes, the wind pushed the sand grains to roll. The sand rolled along the wind on the windward side, crossed the top of the sand dune, and slid down the back. The sand that slid down kept accumulating, moving, and sliding down, thus pushing the sand dune forward. In addition, the formation process of different types of sand dunes was also different. For example, vertical sand dunes were formed under the condition of a fixed wind direction, while horizontal sand dunes were formed under the condition of changing wind direction. In general, the formation of sand dunes was a complicated process, affected by many factors, including climate, terrain, wind direction, and so on.

There were several types of sand dunes. The first was the dome-shaped sand dunes, which formed an air vortex above the sand dunes by airflow. The fine sand was swept into the vortex and moved upward, gradually accumulating into tall circular or oval sand dunes. Next was the circular sand dunes, which were circular or oval sand dunes formed by the wind blowing sand into the depression. There were also vertical sand dunes, which were formed by sand that had accumulated over a long period of time. In addition, there were crescent-shaped sand dunes, which evolved from cake-shaped sand dunes to shield-shaped sand dunes, and then to embryonic crescent-shaped sand dunes. The formation of sand dunes mainly relied on the accumulation of wind power. When the wind blew, the sand grains piled up to form sand dunes of different shapes.

There were three main factors that caused sand dunes: the accumulation of wind, the dry climate, and the thin vegetation. The wind could push and move the sand to form sand dunes. The dry climate made the sand loose and unconsolidated, allowing it to be transported and moved by the wind. The sparse vegetation caused the wind to be less blocked when it passed by, forming a vortex updraft that carried sand to the hillside and accumulated over time to form sand dunes.

The classification of sand dunes was mainly based on the conditions of formation and the characteristics of their forms. According to the formation conditions, sand dunes could be divided into vertical sand dunes, horizontal sand dunes, and sand dunes formed by wind. The vertical sand dunes were formed under the condition of a certain wind direction, usually arranged in a north-south or east-west direction. Its formation depended on the interaction between the airflow streamlines and the undulating shape of the ground. Transverse sand dunes were formed under the condition of changing wind direction and were arranged diagonally to the wind direction. The process of forming a horizontal sand dune was to first form a sand dune ridge on the surface, and then gradually expand to both sides to form a sand dune slope as the wind direction changed. In addition, there were domed dunes, which were tall, round or oval sand dunes. According to the shape, sand dunes could be divided into crescent dunes, parabola dunes, grid dunes, pyramid dunes, and so on. Crescent dunes were the most common type. They were dunes that were flat like a crescent moon, with gentle windward slopes and steep leeward slopes. Crescent dunes usually appeared at the edge of the desert. The other sand dunes were named according to their appearance. The formation of sand dunes was mainly affected by wind, ground form, water, vegetation and sand supply. The wind determined the speed and direction of the sand dunes, and the undulation of the ground would affect the accumulation and shape of the sand dunes. The amount of sand supplied would also affect the size and height of the dunes. In addition, places with good water and vegetation conditions were not conducive to the development of sand dunes. Therefore, the formation of sand dunes was not necessarily limited to dry areas. Sand dunes could also be formed in river valleys, sandy lakeshore, and coasts in humid areas. In summary, the classification of sand dunes was mainly divided according to the formation conditions and morphological characteristics, including vertical sand dunes, horizontal sand dunes, domed sand dunes, etc. Sand dunes were formed under the influence of wind, ground form, moisture, vegetation, and sand supply.

The conditions for the formation of sand dunes include wind, ground form, moisture, vegetation, and sand supply. The wind determined the speed and direction of the sand dunes, and the undulating ground affected the accumulation of the sand dunes. The abundance of the sand source would also affect the size of the sand dunes. In addition, the better the water and vegetation conditions, the more unfavorable it was for the formation of sand dunes. The formation of sand dunes was not necessarily limited to dry areas. Sand dunes could also be formed in river valleys, sandy lakeshore, and coasts in humid areas.

The main reason for the formation of sand dunes was drought and wind. The dry climate made the material loose and could not be consolidated, and the wind could transport and move these unconsolidated materials, thus forming sand dunes. In addition, the formation of sand dunes also required a source of sand, which usually came from inland mountain basins with rich deposits and lowlands on the eroded plateau.

Danniang's sand dunes were formed by the accumulation of wind power. Every winter and spring, the climate in this area was dry, and the wind was strong. In addition, the vegetation was thin, and the wind was less blocked when it passed through the river. It formed a vortex updraft, which carried the sand on the sandbar and the floodplain. It was blocked by the terrain along the river, and the wind speed slowed down. The sand piled up on the hillside, forming Danniang sand dunes over time.

The sand dunes were formed by the wind. When the wind blew over the surface, it would take away some of the sand and pile it up behind the obstacles on the downwind side. As time passed, the sand dunes gradually grew larger and played a greater role in blocking the sand carried by the wind. In a hidden place downwind, the sand would be intercepted, forming a relatively steep sliding surface. When the angle of the sliding surface reached a certain degree, the sand would slide down the sliding surface, causing the sand dunes to advance forward. The speed at which the sand dunes moved depended on the height of the sand dunes, the speed of the wind, the condition of the underlying surface, and other factors. Mobile sand dunes are mainly distributed in China's Xinjiang, the Yellow Plateau, and the Inner Mongolia Plateau.

The most important reasons for the formation of moving sand dunes were water sources and strong winds. Mobile sand dunes were one of the types of sand dunes that were classified according to their mobility. There was little vegetation on their surface, and they could move along the wind under the influence of the wind. Mobile sand dunes were widely distributed in the desert. The formation of sand dunes was also related to the abundance of sand sources, the distribution of deposits, and the effect of wind. However, the current information did not provide a clear answer to the specific mechanism and process of sand dune formation.