The ability of ancient humans to transform nature and resist natural disasters is very poor, and they can only choose their residence by relying on the established natural environment. Settlements are generally required to be close to water sources, with stable and secure foundations. However, the concave bank of the river is an erosion bank, with strong water erosion and steep bank slope, which is not conducive to water intake. Rivers are easy to divert or break their banks in flood season, which is far less stable and convenient than convex banks (river accumulation banks), so concave banks are not conducive to the location of ancient settlements.
Shanghai, London and other big cities were originally developed from port cities, mainly considering traffic factors and using natural conditions to establish good ports. Most of the river depressions are natural harbors: the banks are steep, the water depth is deep, it is not easy to deposit, and the river surface is open and sheltered from the wind, which is conducive to shipping and berthing. At the same time, with the development of modern science and technology, the problem of running water scouring is weakened, the flood control ability of river banks is enhanced, and the safety is guaranteed.
A concave bank on the coastline of a curved river. The concave bank is generally washed by the main slide, and the water depth and velocity are large. Seen from the shore, the convex bank is convex and the concave bank is concave. Usually, the surface water flows from convex to concave, and the lower water flows from concave to convex. Sediment is easy to deposit on the raised bank. From the geostrophic force, the right bank of rivers in the northern hemisphere is easy to be eroded and become a concave bank, while the left bank of rivers in the southern hemisphere is easy to be eroded and become a concave bank. If the original valley has existed in the form of concave bank and convex bank in advance because of the initial topographic fluctuation and trend, then no matter which hemisphere the river is located in, the river will scour the concave bank because of its inertia, making the river bend more outward. Jingjiang reach of the Yangtze River in China is called Jiuqu Ileum, and there are concave banks on the left and right banks of the river. This is because the curvature of the river increases and the velocity slows down, and the geostrophic deviation will not play any role.