It is urgent to fill this gap with high standards and quality. Cities with frequent floods and waterlogging disasters | construction | flood disasters

After years of effort, China has achieved tremendous success in infrastructure construction. But the floods that occurred in some areas this summer have also exposed the shortcomings of drainage facilities, especially urban water supply and drainage infrastructure construction.

Drainage facilities refer to drainage networks, sewage treatment facilities, and sludge treatment and disposal facilities. The drainage network includes drainage pipes and their pumping stations, inspection wells, rainwater outlets, manhole covers, and rainwater grates and other ancillary facilities. As a public good, drainage facilities play an important role in preventing floods, ensuring the normal operation of the country, especially cities, and ensuring the safety of people's lives and property. Therefore, they are referred to as the "conscience of the city" by French writer Victor Hugo.

Why have floods and waterlogging disasters occurred frequently in recent years

It is undeniable that the frequent occurrence of flood disasters in recent years is caused by various reasons.

First, under the influence of global warming, El Nino and frequent extreme weather, rainstorm is becoming more and more frequent, and the scope of impact is becoming larger and larger. China is a high-risk area for floods and typhoons, with high risks of disasters such as flash floods, urban waterlogging, and agricultural waterlogging. The scope of typhoon disasters has also expanded from the coastal areas of East and South China to North and Northeast China. When the rainfall intensity is high, the area is concentrated, and the large amount of accumulated water formed over a long period of time exceeds the capacity of the drainage system, waterlogging will occur.

Secondly, the process of urbanization is accelerating, and urban infrastructure construction lags behind or has defects. The urbanization rate in China has been almost stagnant for over a decade before the reform and opening up, maintaining around 17%. After the reform and opening up, it has experienced rapid development, increasing from 17.9% in 1978 to 30.4% in 1998 and 65.2% in 2022. The consequence of the accelerated urbanization process is that the urban area continues to expand, the coverage of artificial land cover on the ground increases, and the coverage of natural land cover decreases. These have caused damage to the original natural hydrological system, resulting in rainwater being unable to naturally penetrate and discharge, and only flowing into the drainage system, thereby increasing the burden on the drainage system and causing urban waterlogging and other floods.

Thirdly, the drainage system has low standards and severe aging. In the early days of the founding of the People's Republic of China, China's economic and urban development adopted the Soviet model. The Soviet Union has a vast territory, with most areas located in the cold zone and low precipitation. Moscow, which belongs to the continental humid climate of the cold temperate zone, has a total annual precipitation of less than 600 millimeters. The Soviet Union used the average precipitation in Moscow as the maximum drainage capacity of urban sewers and established a relatively narrow "pipe network" drainage system model. Most parts of China have an unstable continental monsoon climate, and are greatly affected by typhoons, so extreme rainstorm and floods are more likely to occur. Even without rainstorm, the precipitation of most cities in China is far higher than the average annual precipitation of Moscow. This means that the design standards for the "pipe network" drainage system used in China, rather than the corridor drainage system, are low, making cities more prone to flooding. Of course, this does not mean that the sewer network type is not as good as the corridor type. As long as the design is reasonable and the scheduling is appropriate, both pipe network and corridor types can effectively play a drainage role. However, in cities where problems such as aging of urban drainage systems, pipeline blockages, and pump station failures frequently occur, the likelihood of rainwater being unable to be discharged in a timely manner leading to waterlogging is higher.

What experiences abroad are worth learning from

China has always attached great importance to water management and urban drainage infrastructure construction since ancient times, and its experience in urban flood control is longer than that of Europe and America. In ancient China, the proportion of permeable ground in courtyards was high, and all garden green spaces were sunken. By using ponds to store water and resist drought, many experiences were accumulated. Many contemporary experts put forward measures to solve urban waterlogging, such as strengthening the maintenance and management of urban drainage system, strengthening the dispatching and operation management of drainage pipe network, strengthening the emergency command of urban rainstorm waterlogging, and building a "sponge city". However, the reality of frequent rainstorm disasters shows that the drainage facilities, especially the urban water supply and drainage infrastructure construction is backward, which is still the weakness in China's infrastructure construction. It is urgent to fill this gap with high standards and high quality.

From the experience of some countries and cities, water governance and urban drainage infrastructure construction should adhere to the principle of adapting to local conditions and focusing on the future, implement high standards, high quality, and comprehensive policies, and coordinate their implementation. For example, the French have been thinking about urban water management issues for a long time, rather than repairing them after the city has been built to a certain scale and problems have arisen. Starting from 1850, it took Paris over a century to complete the sewage engineering that could be opened to traffic and ships. Nowadays, the total length of Paris sewer exceeds 2000 kilometers and has become a tourist attraction for tourists to visit by car. Based on the experience of Paris sewer construction, London began the underground drainage system renovation project in 1859, and the total length of the constructed sewer system also exceeded 2000 kilometers. Influenced by the concept of sewer construction in Paris and London, Western European cities generally determine the drainage volume of their sewers based on peak rainfall and adopt a corridor style drainage pipeline model. More than 100 years ago, Germans brought this concept to Qingdao and had a significant impact on its high-level urban planning and construction, especially the construction of sewage facilities.

After the proposal and successful implementation and promotion of the "Sponge City" as a new generation of rainwater and flood management model in Europe, China also started the pilot work of "Sponge City" construction in 2014. This "sponge city of natural accumulation, natural penetration and natural purification" has been successful in Copenhagen, Stockholm, Amsterdam and other cities with uniform precipitation, few rainstorm, small population and small size, but it is worth considering whether it is suitable for China's natural conditions. Some experts pointed out that sponge cities may be able to cope with moderate and light rain, but they cannot cope with extremely heavy rainstorm. And China happens to be the place where extremely heavy rainstorm frequently occurs. Of course, this is not to negate the concept of "sponge city" planning and construction and its functions in flood control, ecological protection, but to emphasize the need to plan and construct drainage infrastructure at higher standards and quality based on the scale and climate characteristics of Chinese cities.

In this regard, the experiences of Japan and Singapore are worth learning from. After being hit by Typhoon Mireir in September 1991 and causing a flood disaster, Tokyo began constructing high standard underground drainage systems in 1992. After 14 years of effort, a globally advanced underground drainage system known as the "underground temple" has finally been built, which has an astonishing effect on preventing flooding in the Tokyo area. This sewage system can withstand at most 50 mm of precipitation per hour, that is, the precipitation of a "extremely heavy rainstorm" can be emptied within one hour, which is much higher than the Beijing rainproof standard specified in the latest Urban Drainage and Waterlogging Prevention Standard and Corresponding Rainfall in 2022. In addition, Tokyo has also adopted measures to suppress rainwater outflow, including using permeable bricks, green roofs, water storage modules, infiltration wells, permeable asphalt, permeable concrete, etc., to minimize the amount and time of rainwater collection and prevent rainwater from forming floods in the city. On the basis of drawing on the experiences of Europe and Japan, Singapore has creatively proposed the "Active, Beautiful and Clean Water Project" and explored a comprehensive rainwater intelligent management model. By setting up water level sensors for real-time monitoring, a water network system has been constructed that serves as a landscape in normal times and a flood discharge channel in emergency situations. Tokyo did not mention "sponge city", but its measure of zero rainfall and flooding is precisely the core essence of "sponge city". Singapore has not hyped up "sponge cities", but its urban water management concept is similar to "sponge cities".

It should be pointed out that constructing drainage facilities, especially urban water supply and drainage infrastructure, with high standards and quality is a project with huge investment, long construction period, and long-term benefits. In the current state of economic recovery, with the basic completion of large-scale traditional infrastructure construction, relatively low funding for new infrastructure construction, and insufficient domestic demand, launching water governance and urban drainage infrastructure construction can also become new fields and engines for expanding government investment, increasing employment, and promoting economic growth. It can be said that it is timely.

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