More than 200 urban waterlogging disasters occur in China every year.

　　In recent years, serious urban waterlogging disasters have occurred frequently in many large and medium-sized cities in China. Since 2000, there have been more than 200 urban waterlogging disasters in different degrees every year on average, which not only seriously affected the normal life order of cities, but also caused serious loss of life and property, which has aroused widespread concern from all walks of life.

　　The problem of frequent waterlogging in cities in China is the rapid expansion of city scale.

　　The frequent and serious causes of urban waterlogging disasters in China come from both natural and man-made aspects, but unreasonable social and economic activities are the main reasons for the frequent occurrence of urban waterlogging disasters in China, and the problems reflected by them are mainly the rapid expansion of urban scale.

　　First, the development mode of "above ground first, then underground" leads to the mismatch between the urban waterlogging prevention system and the modern urban development needs. China’s urbanization process is extremely rapid. In the process of the rapid rise of modern cities, due to the development mode of "above ground first, then underground", the drainage system is not up to standard, the city’s flood control and drainage capacity can’t keep up with the rapid development of cities, and the infrastructure, drainage pipe network and flood discharge facilities are not perfect. At the same time, due to poor management, aging and disrepair, pipe network blockage and other problems, the whole waterlogging prevention and control system is extremely incompatible with the development needs of modern cities.

　　Second, with the rapid urbanization process, the area of urban and rural construction land and the density of impervious surface have increased rapidly. The sharp increase of urban hardened area and the decrease of green area directly lead to the increase of storm runoff and the acceleration of confluence speed, thus increasing the risk of urban waterlogging disaster.

　　Third, "land finance" accelerates the reduction of land area with natural flood discharge and drainage function in some cities. Local development depends on "land finance" to a certain extent. In the process of transferring urban construction land, the more low-lying and waterlogged land developers are scrambling to get it, because the demolition is difficult and the investment recovery is fast. Some of the original natural lakes and ponds in the urban area were filled and rebuilt, ditches were blocked and occupied, and the original smooth natural drainage system was destroyed, resulting in the lack of its own drainage capacity within the city.

　　Therefore, the problem of urban waterlogging we are facing is much more difficult than that of developed countries. The prevention and control system of waterlogging in modern cities includes a whole set of systems such as source control, runoff process control and drainage terminal control, not just drainage. The lack of modern waterlogging prevention and control system in cities is not only the lack of pipe network construction, but also comprehensive means such as storage, detention, separation, infiltration and regulation, as well as a series of regulations and policy adjustments.

　　Foreign cities have also experienced a similar process, and gradually formed a "underground first, then above ground" urban construction and expansion model.

　　In fact, the urbanization process in developed countries in Europe and America has gone through a long period, and urban development and construction is also a process of gradual exploration and improvement. The government should plan and build a new urban area, give priority to infrastructure such as underground backbone pipe network, and complete the construction first, so that developers can only pay a good price when they buy land, and it is easy to use loans and financing to raise the initial investment in infrastructure construction, which makes it form the basic dynamic mechanism of the development model of "underground first, then on the ground" in the process of design and planning to urban construction in foreign cities.

　　Japan, South Korea and other countries have also experienced a very rapid stage of urbanization. At that time, it was difficult to avoid the development model of "above ground first, then underground", and they also suffered from frequent urban floods. Later, they all paid great efforts and costs for preventing urban floods.

　　To eradicate urban waterlogging, it is necessary to develop urban land in the mode of "planning first, then construction" and "underground first, then above ground", and at the same time actively promote the construction of sponge city.

　　First, attention must be paid to flood control and drainage in the urban planning and construction stage. To build a complete waterlogging prevention and control system, we should plan from three levels, one is river basin planning, the other is urban planning, and the third is community planning. Different levels of planning should solve different problems and explore their own reasonable and feasible solutions according to local conditions. Systematic planning of drainage facilities, combining water conservancy with municipal planning and design, planning of rainwater control and utilization of storage facilities from the source, rational planning of construction land, not arbitrarily burying and occupying areas with flood storage and drainage capacity such as existing internal rivers and ditches and ponds, minimizing the hardened area of the ground in road design, and increasing the green area by using infiltration pavement, which not only beautifies the environment, but also increases the infiltration and implication of rainwater.

　　The second is to develop urban land in strict accordance with the "underground first, then above ground" model. For the new urban development area, the way of building common ditches (water supply, gas supply, heating, power supply, oil supply, communication, network and drainage) is adopted to form the backbone drainage pipe network, and the development mode of "underground first, then above ground" should be promoted by legislation, and it is stipulated that the inherent discharge flow cannot be increased due to development.

　　The third is to accelerate the construction of sponge cities. Building a sponge city can improve the living environment, alleviate the contradiction between supply and demand of water resources, reduce the urban heat island effect, reduce the losses caused by urban floods, and make the land effectively used in value. Realize the maximization of its economic, social and environmental benefits. The construction of sponge city should follow the principles of ecological priority, combine natural ways with artificial measures, realize the accumulation, infiltration and purification of rainwater in urban areas to the maximum extent, and promote the utilization of rainwater resources and ecological environment protection under the premise of ensuring the safety of urban drainage and waterlogging prevention. In the process of sponge city construction, we should coordinate the systematicness of natural precipitation, surface water and groundwater, coordinate all aspects of water supply and drainage, and consider its complexity and long-term nature.

　　International experience shows that the urbanization rate of population is 30-60%, which is a stage of rapid development of urbanization in a country or region, and the process in China is even more rapid. Under the background that there are too many debts in urban infrastructure construction in the early stage, the process of urbanization aggravating the flood risk is not over yet, and the pressure caused by the rapid economic development in the early stage continues to increase and the challenges continue to increase. It is of great significance to curb the frequent and serious situation of urban waterlogging disasters in China as soon as possible and control them well for the steady development and promotion of urbanization in China.

　　(Author: Industrial Layout Research Office of China Academy of Social Sciences)