Why are there sand dunes on the sea like the desert?
Coastal sand dunes seem to many people that sand dunes only appear in inland desert areas with dry climate, such as the Sahara Desert in Africa, the Tarim Basin in Xinjiang in China, and the Qaidam Basin in Qinghai. But did you know that sand dunes also appear in coastal areas with humid climates? They are coastal sand dunes. In fact, coastal sand dunes exist on all continents except Antarctica. They are often associated with sandy coasts and are one of the important landforms in coastal zones. So why are there sand dunes on the beach like deserts? According to scientists, the formation of coastal sand dunes is often related to conditions such as rich sand sources, suitable wind conditions and broad terrain space. Many coastal areas have good conditions for the development of coastal sand dunes mentioned above. First of all, rivers entering the sea carry a large amount of sediment, which is the main source of sediment in the coastal zone; in addition, coastal erosion and seabed erosion in the coastal zone also produce some sediment, which moves ashore under the action of coastal currents and waves, forming a wide sandy beach and also provides a rich sand source for coastal sand dunes. Secondly, most coastal areas belong to maritime monsoon climate zones. For several months of the year, onshore winds prevail and the wind force is strong, providing strong impetus for the formation of coastal sand dunes. Third, there are many coastal plains and terraces of varying sizes distributed along the coast, providing a broad space for the development of coastal sand dunes.
So, how are coastal sand dunes formed? It turns out that in the foreshore zone (intertidal zone) of sandy coasts, whenever the tide is low, the beach dries out. When the wind speed exceeds the starting wind speed of the sand grains, the sand grains on the beach are lifted up by the wind, forming a sand-sand flow. When the shoreline wind and sand flow blows to the backshore (supratal-tidal zone) above the high tide line of the spring tide, due to the increase of ground resistance, it first deposits on the beach ridge, forming a coastal foreland. During strong winds, the sand on the front mound of the coast is constantly replenished from the beach, and at the same time it is continuously transported inland as a "springboard", forming various forms of sand dunes such as horizontal sand dunes, crescent sand dunes, and parabolic sand dunes behind them., forming a tall sand dune belt parallel to the coast. On the landward side of the high sand dune belt, a broad, flat or gently undulating sand sheet is formed due to the weakening of wind and the reduction of sand volume. In addition, in some places where mountains are facing the sea, wind and sand will climb along the slopes to form climbing sand dunes.
Coastal sand dunes in different regions will differ in development scale and type due to factors such as the intersection of coastal strike and onshore wind, the amount of sediment supply, the original terrain pattern, coastal erosion and accumulation dynamics, and vegetation coverage.