Dhaka, the overburdened capital city of Bangladesh, faces severe water management challenges, such as flood and waterlogging, especially during the monsoon season. The concept of a Sponge City offers a promising solution to combat these issues. This article delves into the details of the Sponge City concept, its successful implementation in other countries, and why it should be adopted in Dhaka.

The Concept of Sponge City

Kongjian Yu, a Chinese landscape architect and professor, pioneered the concept of the Sponge City. A Sponge City is designed to passively absorb, clean, and utilize rainwater in an eco-friendly manner by reducing polluted runoff. Essentially, it uses landscapes and infrastructure to preserve water and promote natural retention, infiltration, and purification of drainage.

Key techniques associated with a Sponge City include:

Rainwater Harvesting: Collecting and storing rainwater for various uses.

Permeable Roads: Roads designed to allow water to seep through and be absorbed into the ground.

Terrace Gardens: Green roofs that help absorb rainwater and reduce runoff.

Green Spaces and Waterbodies: Parks, ponds, and lakes that act as natural water retention and purification systems.

Proper implementation of these techniques can minimize the frequency and severity of floods, improve water quality, and reduce individual water usage. Green roofs, for example, enhance the quality of life by improving air quality and mitigating urban heat islands. Overall, a Sponge City can efficiently handle excess water during heavy rains and reuse it during dry periods to combat water shortages.

How China Adopted the Concept of Sponge City

In 2013, the Chinese Central Government adopted Professor Kongjian Yu's Sponge City concept, implementing it across 30 cities. Following successful trials, China aims to transform 80% of urban areas into "sponges" by 2030.

The core principle of Chinese Sponge Cities is to give water enough room and time to drain into the soil rather than quickly channeling it into large dams. Instead of fast-flowing water channels, these cities employ meandering streams without concrete walls, allowing water to spread out during heavy rains. Yu asserts that replacing concrete infrastructure with natural systems can save lives.

In Sponge City designs, natural waterways and permeable soils clean water and reduce pollution. Evaporated rainwater cools the city, while collected rainwater is used for irrigation, street cleaning, and other purposes, reducing the reliance on tap water.

The vegetation, sediments, and microorganisms in Sponge City water systems lessen the burden on energy-intensive urban water filtration facilities. As climate change brings unpredictable rainfall, Sponge City buildings are constructed to accommodate rising water levels during floods.

Compared to traditional gray infrastructure, Sponge City designs require less energy for maintenance. They reduce the load on water treatment facilities, decrease the need for air conditioning due to heat reduction effects, and use fewer resources like concrete. Additionally, vast green spaces absorb carbon dioxide and air pollutants, promote biodiversity, and provide recreational areas for residents.

China has also implemented Sponge City concepts in Ningbo, a coastal city 150 km south of Shanghai. Ningbo's sponge infrastructure includes an eco-corridor on a post-industrial site, where channeled water flows through meandering waterways surrounded by native plants. This blue space improves water quality, provides habitat for flora and fauna, and reduces the risk of waterlogging.

Application of Sponge City Model in Copenhagen

Copenhagen adopted the Sponge City concept in its Cloudburst Master Plan, developed in 2012. This plan redesigns approximately 250 public spaces to retain or redirect floodwaters, including parks, playgrounds, and the Sankt Kjelds Plads roundabout. Green spaces like trees, shrubs, and soil naturally retain water and direct it to non-destructive areas.

Several ponds around the roundabout retain excess rainwater after heavy rainfall. Wide openings on low-lying streets funnel floodwater into a network of tunnels 20 meters below the surface. During regular downpours, rainwater is directed to the harbor, but during heavy rainfall, a pumping station forces the collected water out to sea. This system prevents street flooding and is expected to be completed by 2026.

Copenhagen's Sponge City approach uses spaces and infrastructure to absorb, hold, and release excess rainwater, integrating it back into the water cycle.

Why Dhaka Should Adopt the Sponge City Concept

Dhaka faces significant water issues that cause considerable difficulties for its inhabitants. Situated on the extensive floodplains of the Ganges and Brahmaputra rivers, Dhaka experiences heavy monsoonal downpours. Seasonal tides and the city's topography contribute to waterlogging.

Unplanned spatial development, haphazard urbanization, and rapid population growth encroach on natural drainage paths and water retention areas, exacerbating the problem. The city's drainage systems struggle to handle urban runoff, leading to prolonged inundation in some areas.

Other contributing factors include low-capacity drainage systems, inadequate drainage sections, natural siltation, lack of inlets and outlets, numerous drainage outlets, poor maintenance, and the unplanned disposal of solid waste into drainage paths. These issues cause blockages and severe waterlogging.

To meet the water demands of its inhabitants, Dhaka relies heavily on groundwater extraction, with over 80% of the city's water supply coming from underground aquifers. This excessive extraction leads to a rapid decline in groundwater levels, dropping by 2 to 3 meters per year. Research indicates that if this depletion continues, the groundwater table will reach 120 meters by 2050.

The Sponge City concept offers a sustainable solution to Dhaka's water problems. Instead of relying on traditional gray infrastructure, Sponge City designs can help Dhaka absorb rainwater during monsoons and release it during dry seasons. This approach can combat waterlogging, reduce water shortages, decrease groundwater depletion rates, increase biodiversity, and expand green spaces.

Conclusion

Countries like China and Denmark have successfully implemented the Sponge City concept to combat urban surface-water flooding and manage related water issues, such as runoff purification, peak runoff attenuation, and water conservation. Dhaka can learn from these examples and apply the Sponge City concept to address its water challenges.

Implementing Sponge City designs can reduce waterlogging and rapid runoff during heavy rains, decrease groundwater depletion, and alleviate water scarcity during dry seasons. Green and blue spaces can improve air and water quality in the city. However, extensive research and investment are required to successfully implement Sponge City initiatives.

It is crucial for city authorities and the government to consider the potential of the Sponge City concept to combat flooding in Dhaka and other cities in Bangladesh. By adopting this innovative approach, Dhaka can build resilience against climate change-induced flooding and create a more sustainable and liveable urban environment.