About ‘sponge city’, you know what?

I. What is a sponge city？

Sponge city is a new generation of urban stormwater management concept, refers to the city can be like a sponge, in the adaptation to environmental change and response to natural disasters brought about by rainfall and other aspects of good elasticity, can also be called ‘water resilience city’.

II. Sponge city six-word policy

The six-word policy of ‘seepage, stagnation, storage, purification, use and discharge’ is the main content of the construction of sponge city, and each link is crucial. The next step is to explain in detail, the specific content of the six-word policy.

Step1: Seepage, that is, the source to reduce runoff, purify the first rain pollution.

Due to the traditional way of construction of the city's subsurface is too hard,everywhere is cement, change the original natural ecological background and hydrological characteristics, cut off the natural water cycle, rain comes, canonly be used as sewage discharge, therefore, to strengthen the natural infiltration. The benefit is that it can reduce surface runoff, reduce from the concrete ground, pavement pooled into the pipe network, while purifying waterquality through the soil, improve the urban microclimate. There are various ways to infiltrate rainwater, mainly changing various pavement and ground cover materials, transforming green roofs, adjusting the vertical orientation of green spaces, and leaving rainwater at the source and then ‘seeping’ it down.There are mainly three technical ways to achieve the role of ‘seepage’ -permeable landscape paving, permeable road paving and green roofs.

Step2: Stagnation, which is delaying the peak time of rainfall.

The main role is to delay the formation of rainwater runoff in a short period of time. Urban rainfall is usually measured in minutes or hours, the city short calendar time rainfall, the impact on the subsurface, the formation of rapid runoff, the water saved up to lead to flooding. Therefore, ‘hysteresis’ is very important to delay the formation of runoff peaks. Commonly used ways to achieve‘stagnation’ include rain gardens, grass-planted swales, rainwater ponds,stormwater wetlands, etc.

Step3: Storage, that is, to reduce the peak flow, to create conditions for the use of rainwater.

‘Storage’is to keep the rainwater, respecting the natural topography and geomorphology,so that rainfall can be naturally dispersed. The main projects include theprotection, restoration and renovation of urban built-up areas within the riverand lake waters, wetlands and to use, according to local conditions, theconstruction of rainwater collection and storage facilities, the main purposeis to reduce the peak flow of runoff, to create conditions for the use ofrainwater.

Step4: Net, that is, to reduce pollution and improve the urban water environment.

Throughthe adsorption of soil, the absorption of vegetation roots, green spacesystems, water bodies and other filtration, can have a purifying effect on waterquality. Through the previous step of water storage, purification treatment,and then back to the city, therefore, ‘seepage, stagnation, storage’ of theessence of the service in the ‘clean’. After the rainwater purification, it canbe used as ecological supplementary water, and can also be used for greenwatering under the condition.

Inthe process of building a sponge city, it is necessary to systematically layoutwater pollution treatment facilities to effectively isolate rainwater and sewage. The project mainly includes the construction of sewage treatment facilities and pipe networks, comprehensive improvement of rivers, the construction of ecological slopes along the coast to reduce surface pollution and improve the urban water environment.

Step5: Use, that is rational use of rainwater to improve the quality of the urban environment.

After purification of rainwater should be used as much as possible, regardless ofwater-abundant or water-scarce areas, should strengthen the use of rainwater resources. Not only can alleviate the disaster of waterlogging, the collected water resources can also be used, such as the collection of rainwater above thecar park after purification can be used for car washing, through the previousstep of purification to put the water to practical use, the project includes green watering, road washing, cooling water and landscape water.

Step6: Drainage, i.e., to reduce waterlogging and enhance the city's resilience.

Some cities are flooded because of excessive rainfall, infiltration and utilisation.This necessitates artificial measures to drain it away. The combination of urban vertical and engineering facilities, drainage and flood prevention facilities and natural watercourses and rivers, and underground stormwater drains and surface drainage are used to achieve general discharge and excess stormwater discharge to avoid disasters such as flooding.

The main works include the renovation of village rainwater and sewage diversion pipe networks and the renovation of drainage facilities in low-lying waterlogged spots. The main purpose is to combine drainage and flood prevention facilities with natural waterways and rivers as well as surface drainage with underground rainwater irrigation channels.

III. The construction of sponge city, they are doing so

1.Germany

In recent years, Germany has begun to widely promote the ‘depression - soakawaysystem’, so that the various on-site installation of the depression, soakaway and other facilities and drainage pipes with holes connected to the formationof a decentralised rainwater treatment system. Low-lying grassland can store infiltrated rainwater for a short period of time, while soakaways can store rainwater for a long period of time, thus reducing the burden on urban drainage pipes.

Germany's rainwater use technology has matured after years of development, in addition,they are also exploring other methods: the construction of a huge area of urban parks in the city centre; to encourage citizens to participate in the construction of ‘green roofs’, experts assessed that the green roofs can reach a certain density of green roofs, the future can be retained at least 60% of the rainfall.

2.United Kingdom

In order to solve the increasingly serious water shortage problem and enhance the municipal drainage capacity of major cities such as London, the British government actively encourages the establishment of rainwater harvesting and utilisation systems in residents' homes, communities and commercial buildings,so as to solve these two major problems at source.

Currently,rainwater harvesting systems for domestic use in the UK are mostly used to meetthe needs of non-potable water such as home irrigation and laundry. Household rainwater harvesting systems are mostly set up in the home 1000 litres to 7500 litres of water storage tanks, rainwater is collected directly from the roof,and through the guide pipe simple filtration or more complex self-cleaning filtration system into the underground storage tanks for storage.

3.Singapore

As a tropical island country with abundant rainfall, Singapore's maximum annual rainfall has been on a continuous upward trend over the past 30 years, but there are few cases of urban flooding.

Firstly,the urban drainage system is planned in advance. Singapore usually plans and designs the underground and surface drainage system of a building before the construction of a ground-level building, so that every building, including pavements and roads, has a certain number of drains around the perimeter. Secondly, rainwater diversion is strengthened and large-scale cisterns are built. Rainwater collected by the city's rainwater harvesting system will eventually be fed into 17 large cisterns around the city, and these largecisterns are also the key to solving the problem of rainwater diversion and urban flooding in Singapore. Thirdly, strict drainage standards for surface buildings have been established. The Public Utilities Board (PUB) of Singapore has revised and raised the standards for surface building drainage several times,requiring all new buildings to raise the height of their waterproofing thresholds.