"Sponge" cities
"Sponge" cities, also known as water-sensitive cities, are urban areas designed to effectively absorb and manage precipitation and runoff, thereby reducing the risk of flooding. The concept emerged in China in 2014, primarily as a response to severe flooding incidents that resulted in significant property damage and loss of life. Sponge cities employ various sustainable techniques, such as integrating vegetation, creating stormwater swales, and developing permeable surfaces that allow water to be absorbed back into the environment rather than overwhelming drainage systems.
The initiative aims to transform regions prone to flooding into safe living spaces by utilizing natural absorption methods and innovative urban designs. Funding from the Chinese government has facilitated the development of these projects, although cities are required to contribute a substantial portion of the costs. Techniques implemented in sponge cities range from constructing parks and wetlands to installing sensors in drainage systems that monitor water levels and provide early warnings during storms.
While the sponge city model began in China, it is gaining traction in other regions, including Europe and North America, where cities like Houston are exploring its benefits. Despite challenges such as high costs and the need for structural modifications, experts believe that integrating sponge city features into existing urban infrastructure can help address flooding risks effectively. As urbanization continues to challenge traditional drainage systems, the sponge city approach presents a forward-thinking solution to water management in the face of climate change.
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"Sponge" cities
A sponge city, also called a water-sensitive city, is an urban environment designed to absorb large amounts of precipitation and/or runoff to prevent flooding. Sponge cities use various techniques, such as adding vegetation and containment swales, to provide natural ways to hold excess water so it can slowly move back into the environment. Sponge cities allow areas that are susceptible to flooding to be used for housing. The concept began in China in 2014, where the increasing population, urban growth, and climate have made flooding a serious problem. However, sponge cities have been used in many other areas that are prone to flooding as well.
Background
Efforts to create sponge cities began in China in 2014 in response to increased property damage and loss due to flooding. The initiative was a response to loss of life and property over many years, especially repeated heavy rains and monsoons that resulted in months of extreme flooding in 2010. Over the course of several months, flooding and mudslides took the lives of more than 1,100 people and left more than 600 recorded as missing. More than 45,000 people were forced to flee their homes because of mudslides, and a thousand homes were either destroyed or damaged by mud. At least 10,000 farm animals were lost because of the storms and flooding, which cost billions of dollars in damage.
The 2010 disasters, which began in May and spanned several months, came after several years of increasing damage from rainstorms. Experts reported that between 2008 and 2017, the number of fatal floods affected twice as many Chinese cities as in previous years. Changing climate conditions that bring more intense storms are blamed in part for the problem.
Experts say that the problem is made worse by increased urbanization, which clears ground for construction. This removes the natural plantings that help absorb and dissipate rainwater and runoff in a slower manner. It also replaces them with hard surfaces like concrete structures and asphalt roads and parking lots that allow water to pool and/or run off into lower areas. In some cases, existing waterways are clogged with debris or degraded in some other way that prevents water from flowing freely and allows it to build to flood levels. These problems all increase the likelihood and the severity of flooding during times of heavy rain.
Overview
Under the sponge cities initiative, the Chinese government provided $12 billion to cities to develop sponge projects. Due to the number of cities affected, this money only covers about 15 to 20 percent of the cost of the projects. The cities are required to complete the projects using their own funds.
The program began with about sixteen cities. By 2024, about thirty Chinese cities were involved in some form of a sponge city project. Many different approaches are used to help minimize the damage. Some attempt to make existing surfaces more compatible with heavy rains by replacing hard surfaces with materials that allow water to go through more easily. In other cases, storm swales or basins are added to contain excess rainwater and runoff in areas away from homes, farms, and businesses.
Other projects add materials, such as various forms of greenery, to help absorb water. Some create whole new greenspaces, such as parks, or add greenery alongside roads, parking lots, and other hard surfaces. Other options include adding greenery in unconventional places, such as on top of roofs, to slow the flow of rainwater and allow the ground to absorb it without reaching flood levels. The projects also include other innovations, such as the use of sensors in stormwater drains and sewers to help officials determine when they are in danger of being overwhelmed by rainwater. This can allow them to implement alternate measures or warn people to evacuate before life-threatening flooding occurs.
Different cities are taking different approaches to the problem. For instance, Yanwiezhou Park in eastern China has paths that are raised above the green spaces. The sunken areas, which are full of grass, trees, and shrubs, help absorb more water than hard-packed dirt or paved areas would. They can also serve as basins to hold more extreme amounts of rainfall. In the meantime, the paths are raised high enough to remain passable even when the park is flooded, preventing the park from becoming a roadblock during a disaster.
Other cities are adding wetlands to areas that often flood, sacrificing a small amount of land to new plantings that will help absorb water in the hopes of protecting much larger areas. Rooftop plantings of grasses are also growing in popularity as a way to prevent the runoff from reaching the ground and adding to any flood risk. This is the approach taken in Lingang, a city near Shanghai, where the government has spent nearly $120 million on rooftop greenery and additional wetlands. The funds were also used to create permeable roads; while most roads are made of hard surfaces that allow water to run freely into nearby areas in large amounts, permeable roads catch and store water to minimize flood damage.
While the concept began in China, it has begun to spread to other areas. Some European cities are experimenting with the idea, and American cities such as Houston that are susceptible to flooding are considering the advantages of using sponge technology. For instance, many North American cities have begun restoring wetlands to help with stormwater runoff.
There are some problems with the concept. One is the cost; building swales and areas to hold water, creating parks and wetlands that fulfill the sponge concept, replacing hard surfaces with materials that catch and store water, and adding greenery to roofs and other areas is expensive. This work competes for funding with other infrastructure projects, such as replacing aging bridges and roads. One solution is to integrate the two types of projects, such as using water-friendly materials when making necessary repairs or replacements on road projects. Another problem is the need to alter other projects to accommodate the sponge city initiatives. For example, roofs often need to be shored up to be able to bear the weight of extra greenery and the water they collect. However, experts believe that there are ways to address these concerns to make sponge cities more widespread.
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