Sand dune classification and formation conditions
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.
Sand dune formation process
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.
What are the conditions for the formation of sand dunes?
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.
Three Elements of Sand Dune Formation
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 answer to the process of Danniang's sand dune formation
Danniang sand dunes were formed in the winter and spring season (October to March of the following year). The climate was dry and windy. The area where the strong wind passed by had little vegetation. In addition, it passed through the river surface, so it was less blocked, forming a vortex ascending airflow. These airflows carried the sand on the sandbars and river banks, which were blocked by the mountainous terrain beside the river. The wind speed slowed down, and the sand piled up on the hillside. After a long time, it formed the Danniang sand dune.
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