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Storm water retention and infiltration at city-scale

This solution caters to local governments that have the mandate to manage stormwater in a city. It addresses the management of rainfall and runoff from public open spaces and transport infrastructure. The management of runoff from residential, commercial and institutional buildings are covered in a separate Solution as a different approach is required for areas under private ownership.

The solution promotes the creation and enhancement of permeable surfaces, waterscapes and networks of open drainage channels and watercourses which slow down runoff and maximize infiltration and treatment by natural processes. Aiming to collect, treat and infiltrate as much water as possible from public open spaces and transport infrastructure by the use of Sustainable Drainage Systems (SUDS), namely a combination of drains, swales, vegetated trenches, retention basins, wetland systems, multiple use green spaces, etc., this solution has benefits at multiple levels. [1]

To implement this solution, the local government undertakes an integrated strategic storm water planning process, policy enactment, standardisation of SUDS design, land use planning, stakeholder involvement for community sensitisation and mobilisation, project design and implementation, operations and maintenance, as well as monitoring and evaluation of project(s).

Motivation / Relevance

Rapid urbanisation has increased impervious surface areas in cities resulting in increased surface run-off, causing water logging and floods. Around 70% of all land use in a city consists of residential, commercial, institutional areas of which, 30-35% constitute transport infrastructure. Various studies show that, highly impervious area (75 to 100%) can increase urban runoff by approximately 50% compared to a 10% from low impervious areas (10 to 20%) [2].

Traditional urban drainage systems (combined sewer networks) primarily focus on removal of stormwater and wastewater away from urban settlements as quickly as possible. To reduce costs of conveyance infrastructure, stormwater and domestic wastewater are transported together, and receive the same treatment. However, such systems are often expensive to build, operate and maintain, and also consume large amounts of energy. Additionally, combining stormwater with wastewater increases the frequency with which the storage and treatment capacity are exceeded during high rainfall events. This results in the discharge of untreated stormwater and raw sewage directly into receiving streams, or other water bodies, causing pollution.

Moreover, changes in precipitation patterns as a result of climate change can be very disruptive and have devastating consequences on cities due to high-intensity precipitation. Conversely, cities are also susceptible to water shortages and droughts. With increasing population, the demand for water in cities will also increase.

Furthermore, stormwater is much less polluted than raw domestic sewage and if collected, treated and infiltrated separately, it can be utilized as an additional water resource for the city. In doing so cities can increase water security, reduce costs for expensive infrastructure and reduce the potential risk of flooding in cities. Additionally, creating a wetland system within the city limits can improve carbon sequestration potential in the city by 1 ton per acre per year [2].

Main impacts

  • Decrease the flood risk for low-lying urban areas
  • Decrease costs of sewer conveyance infrastructure
  • Protect and / or enhance water quality
  • Increase local water resources
  • Decrease the pressure on natural fresh water resources and ecosystems
  • Contribute to water security of urban water uses
  • Increase evapotranspiration and reduce the heat island effect, thus decreasing energy consumption for cooling in buildings
  • Provide additional recreational space in the city
  • Improved urban resilience to climate change events (floods, droughts and heat waves)
  • Attract investments into the city due to improved quality of life

Benefits and Co-Benefits

  • Multi-use urban spaces
  • Increase reliability of wastewater treatment system and reduce risk of pollution from untreated sewage discharge
  • Decrease costs of wastewater treatment, in some cases by 50% [6]
  • Reduction in greenhouse emissions from energy consumption in wastewater treatment plants
  • Improve urban biodiversity and aesthetic value of settlements

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