Before the Qin and Han Dynasties, because there were few cities, the drainage system was mostly simple, and all the above-ground channels drained from high to low. During the Qin and Han dynasties, the unification of the world led to the rapid development of cities, and a large number of people flocked to cities, and the drainage systems of corresponding cities gradually developed. From the excavation of the capital at that time, people found that the city had a composite water conservancy system of water supply, drainage and water storage. Drainage also uses the height of the terrain to drain water, connecting the drainage system with rivers and lakes to achieve the effect of drainage as soon as possible. In the Sui and Tang Dynasties, there was a more perfect light and dark drainage system, and even in order to prevent floods and blockage of drainage pipes, iron fences were installed at the water inlet to prevent sundries from entering the pipes. During the Song Dynasty, most of the land was in the south, so the drainage system paid more attention to flood control. At this time, the drainage system began to connect all the drainage pipes in the city into a drainage pipe network, and connected with the surrounding low-lying rivers, lakes and ponds to achieve the purpose of emptying precipitation in time. The famous Fushou Ditch is one of them. In the Yuan, Ming and Qing Dynasties, the drainage system remained basically unchanged, but newly-built cities and places began to implement the method of urban design and drainage system design together. The representative of it is naturally the drainage system of the Forbidden City, and there is basically no water when it rains heavily.
The ancient drainage system was basically realized by using the terrain and the surrounding rivers and lakes and ponds, which can be said to be very ingenious. However, this does not mean that the current drainage system is not good enough. The main reason is the rapid development of cities and the influx of large numbers of people into cities. Many lakes and ponds have been buried to build houses, and drainage naturally becomes difficult. Moreover, at present, the urban construction basically does not consider the terrain, which leads to many reasons why bridge opening becomes a lake when it rains.
Moreover, ancient cities were not as big as big cities now, and the population was not as large as it is now, so it is impossible to develop so fast. At present, with the speed of economic development and urban development, it is good to do this.