"Seeing the sea in cities" is not a "Chinese characteristic". According to Zhang Linwei, deputy director of Department of Urban Construction of Ministry of Housing and Urban-Rural Development, developed countries also had similar situations in development of urbanization. Urban development and construction destroys the natural "spongy body", leading to "waterlogging whenever it rains and drought after the rain", and causing water environment pollution, shortage of water resource, lack of guarantee of water safety, disappearance of water culture and other problems. They timely adjusted concept of urban planning, construction and management, and effectively solved the above problems by controlling rainwater runoff.
How did rain-flood control develop at abroad and what successful experience and mode can be drawn on? As introduced by Wang Hao, academician of Chinese Academy of Engineering and director of Department of Water Resources, China Institute of Water Resources and Hydropower Research (IWHR), exploration of construction of sponge cities can be traced back to the 19th century, and large-scale construction started in the 1970s at abroad.
Urban drainage system of Paris was incorporated into construction planning as early as 1852; underground drainage system of London was commenced in 1859, and completed 6 years later, extending for 2,000km. There was no waterlogging prevention system in the United States before 1972, and large drainage system was planned and constructed later for pollution of combined system and urban waterlogging. Australia started to plan and construct urban waterlogging system in 1975 due to the flood in previous year; construction of "underground temple" was commenced in Tokyo, Japan in 1992, and the most advanced sewer drainage system in the world was completed at a cost of $3 billion after 15 years.
The following are relatively mature in concept of rain-flood control: low impact development (LID) of the United States, which realizes permeation, filtration, storage and retention by source reduction, process control and end treatment to prevent waterlogging; sustainable urban drainage system (SUDS) of UK; water sensitive urban design (WSUD) of Australia; urban flood discharge system and underground rainwater storage system of Japan.
Low impact development (LID) of the United States
LID concept was first put forward by Maryland Department of Environmental Resources in 1990 for best management practice in urban rainstorm. It mainly controls the source of rainwater runoff by decentralized small-scale measures, simulates natural hydrological conditions with reasonable site development mode and takes comprehensive measures to reduce significant change in hydrological conditions caused by development and influence of rainwater runoff on ecological environment from the source. Integrating economy, environment, development and other elements, it is a design strategy based on sustainable development of economy and ecological environment.
Sustainable urban drainage system (SUDS) of UK
SUDS system also requires treatment of runoff and potential pollution source from the source to protect water resources against point source and non-point source pollution. UK updated national sustainable development strategy and agenda in the 21st century, incorporated long-term environmental and social factors into drainage system, and established SUDS system to solve frequent flood, serious pollution and damage to environment of traditional drainage system in May 1999. It mainly gives comprehensive consideration to the comfortable and pleasant leisure value of water quality and quantity and surface water in urban environment.
Apparently, SUDS system is a sustainable drainage system that maintains virtuous water cycle different from traditional drainage system centered on drainage, and considers water quality and quantity, landscape potential and ecological value of runoff in design. It changes into optimization of water system of the whole region from optimization of urban drainage facilities, considers rainwater as well as urban sewage and reclaimed water, and takes comprehensive measures to improve the overall water cycle of cities.
Water sensitive urban design (WSUD) of Australia
As improvement of traditional development measures by Australia,WSUD reduces negative influence on natural water cycle and protects health of water ecosystem through urban planning and overall analysis of design, regards urban water cycle as a whole and integrates rain-flood management, water supply and sewage management. Regarding rainwater, water supply and sewage (reclaimed water) as separate links that link to and influence each other, and taking them into full account is different from the traditional single mode and gives consideration to both landscape and ecological environment.
Urban flood discharge system and underground rainwater storage system of Japan
Urban flood discharge system and underground rainwater storage system of Japan collect rainwater around buildings and store it in underground canals and ditches for irrigation. Japan amended its building law, requiring large buildings and building groups to be equipped with underground rainwater storage and recycling system in early 1990s.
Wang Hao said, construction concepts and measures of other countries are basically consistent with LID, but emphasis is different. SUDS of UK emphasizes "storage, retention and permeation", putting forward 4 methods for "digesting" rainwater (storage water tank, soakaway, reservoir and artificial wetland) to reduce pressure of urban drainage system. WSUD of Australia emphasizes "cleanness and use" and "quasi-natural design" in urban water cycle. Urban flood discharge system and underground rainwater storage system of Japan emphasize "retention" and "drainage".
Which foreign experience can be used for construction of sponge cities in China? Analysis of Wang Hao shows that with a small population, low land development intensity and high greening rate, there is sufficient space at the source of site to absorb increment of runoff after site development in developed countries.
However, with land development intensity high in most cities of China, decentralized source reduction on the site alone can not keep the total runoff and peak capacitybasically unchanged before and after development, so comprehensive measures must be taken in the midway and at the end from planar interception to vertical interception, "changing flat ground into uneven ground", and room is left for flood that happens once in 50 or 100 years in light of characteristics of cities.
Foreign experience can not be copied, but can provide inspiration for construction of sponge cities in China to formulate technical routes and measures with "Chinese characteristics" in line with national conditions. Zhang Linwei said, technical route of construction of sponge cities in China is "source reduction, process control and system governance", and main engineering technical measures are simpler, "permeation, retention, storage, cleanness, use and drainage", but it is difficult to realize them.
Source reduction means effective "runoff control" where buildings, roads, squares, etc. in cities tend to form hard underlying surface (area of rainwater runoff yield and concentration), namely reducing runoff from the source of rainwater runoff yield and concentration.
This needs to combine green buildings and LID to implement requirements for rainwater runoff control in line with regional rainwater drainage control system during plot development and construction. Source reduction resolves responsibilities and funds (similar to implementation of requirements for greening rate by developers in plot development and construction) as well as pressure of municipal pipe network and other drainage facilities from the source.
Process control is a technical means that uses green buildings, LID and green infrastructure. Runoff peak is postponed or reduced through process control and regulation of rainwater runoff to prevent "synchronous" rainwater runoffs. System governance is to control water from integrity of ecosystem, make full use of functions of landform, natural vegetation, green space, wetland and other natural "spongy bodies" and give full play to force of nature. Moreover, relationship between "upstream and downstream, and left and right banks" of water body should be considered without causing waterlogging pressure or intercepting normal runoff to affect ecology of the water body.
The "six words of maxim" as main engineering technical measures for construction of sponge cities have their own methods. Permeation: reduce hard paving and make full use of permeation of natural underlying surface to reduce runoffs, cultivate ecology and environment and accumulate water resources. Retention: replace space with time through retention of rainwater to improve permeation, and slow down convergence of rainwater, postpone peak and reduce drainage intensity and risk of disaster. Storage: reduce peak capacityand regulate spatial and temporal distribution to create conditions for utilization of rainwater. Cleanness: reduce non-point source pollution, and degrade chemical oxygen demand, suspended matters, total nitrogen, total phosphorus and other main pollutants to improve urban water environment. Use: make full use of rainwater resources and reclaimed water to improve efficiency of water use and relieve shortage of water resources. Drainage: build green-gray storage and drainage system to avoid waterlogging and other disasters and ensure safety of cities.
With guidance of advanced concept and support of definite implementation path, sound and fast development will be achieved for construction of sponge cities in China as long as measures are taken according to local conditions. Zhang Linwei was confident in this, "conservatively estimated, these concepts will be comprehensively realized in cities and towns with a population of 50,000 ～100,000 after 10～20 years and bring the best livable towns in China if implemented well."