How can we measure the benefits of urban greening?

Dar es Salaam in Tanzania is expected to double in population by 2050. This rapid growth poses significant challenges for the city, particularly relating to flood risk, urban heat, land degradation and soil erosion.

In response to this, Arup was selected to assess how urban greening could help to resolve these issues, while enabling the city to increase its climate resilience. Collaborating with the Tanzanian Government, the World Bank and specialist local partners, this research explored the benefits of implementing nature-based solutions in favour of grey infrastructure in Dar es Salaam. Insights from this assessment will allow for more climate resilient urban development and infrastructure plans that can overcome the challenges of flooding, water availability and rising temperatures. The suggested greening interventions have the potential to improve the quality of life of the city’s people, especially for those living in low-income settlements.

Project Summary:

• 730,000m³ of rainfall stored in the soil
• 17,000buildings could become less susceptible to flooding
• 4% reduced soil loss

Quantifying the impacts of urban greening and nature-based solutions

Even though nature-based solutions are becoming a more established way of supporting climate resilience and biodiversity, their future impacts are rarely quantified. Framed on numerical evidence, our study’s aim was to assess the benefits of interventions such as urban greening, open space development, erosion control, and sustainable urban drainage systems.

We analysed Dar es Salaam’s existing land use types – including the varying densities of its urban areas – to estimate the scale, extent, and types of greening interventions that could exist in the future. We discovered that, if greening interventions were incorporated into the city by 2030, approximately 300 Olympic-size swimming pools of rainfall (equating to 730,000m³) could be stored in the soil, helping to restore soil health and support urban agriculture.

Our team created geospatial data estimating Dar es Salaam’s future pattern of urbanisation – focusing on 2023, 2030, and 2040 as the project’s key timeframes. The data was presented through a series of visual maps showing the future potential land changes and enabling the team to identify where the opportunities for city-wide urban greening could be realised in new developments and infrastructure. We selected a combination of greening interventions based on how well they suited each of the city’s 17 dominant land types. This data enabled spatial calculations of the impacts of the greening interventions, for example assessing the flood reduction potential across the city, and they have provided a method of visualising the opportunities and impacts of greening that will benefit lives of local people.

Nature-based flood risk reduction

With the area’s frequent heavy rainfall and flooding, the people of Dar es Salaam face regular disruption to their livelihoods, infrastructure, and daily operations. These problems will only worsen as the city grows and the climate continues to change – unless interventions are implemented at scale throughout the city. The project considered a wide range of nature-based solutions to address these challenges, with each one having a unique role to play in helping to alleviate flood risk. We considered solutions such as rain gardens, road-side drainage, and tree planting – all of which offer ways to increase the city’s water infiltration and subsurface water storage.

When combined, these solutions can considerably lessen the extent of flooding across the city, helping to safeguard the homes of people as well as protecting critical facilities such as hospitals and schools. We found that, by implementing nature-based interventions, up to 17,000 buildings could become less susceptible to flooding by 2030. This would significantly improve the lives of people situated within unplanned areas, since they are more likely to live on land that is prone to flooding.

Reducing the urban heat island effect

Under current climate conditions, the average and maximum temperatures of Dar es Salaam are around 29°C and 37°C respectively. These temperatures are expected to rise as a result of the city’s increasing urbanisation, and this heat gain will only be compounded by the impacts of climate change. The urban heat island (UHI) effect is when an urban area is significantly warmer than its surrounding rural areas. Today, Dar es Salaam city has a maximum UHI of 4.85°C compared to air temperatures in rural areas and, with rapidly rising temperatures, the city’s UHI is projected to increase by up to 2°C by 2040.

We found that implementing greening interventions across the city – such as tree planting – could help to reduce air temperatures by between 1-5°C. By reducing grey urban areas within the city and preserving existing green areas of forest and mature trees as the city’s urban boundary extends, we would see a reduction in air pollution as well as a decreased risk of heat-related illnesses.

Green spaces that improve lives

Our research found that surface water runoff and soil erosion will worsen if Dar es Salaam continues to grow and densify without greening interventions. By implementing interventions and preserving existing natural open spaces and forests, our team estimated that the city’s total average soil loss could be decreased by approximately 4% (840,000 tons/year), reducing the negative impacts of sedimentation. The opportunity for tree planting suggests that around 1,182,000 trees could be planted by 2030, which would help to counter CO2 emissions. This would also improve Dar es Salaam’s air quality, helping to prevent health risks and illnesses associated with air pollution.

Future greening approaches could help to connect the city’s existing green spaces and water bodies, particularly its major river channels, enhancing biodiversity. A greener city would provide additional quality of life benefits for its residents, such as cooler air temperatures, improved air, water, and soil quality. Urban greening would also gradually help to replenish the city’s depleted ground water supplies. Low-income settlements are impacted more acutely by water supply and quality challenges, and some interventions such as rainwater harvesting, can even increase water availability for these communities.

Financed by the Tanzania Urban Resilience Programme, the World Bank are completing this research scope in collaboration with the Quality Infrastructure Investment Partnership – which is itself funded by the Government of Japan. The results from this will be used by the Government of Tanzania to introduce green infrastructure into World Bank-financed projects across the city, creating a healthier and more resilient urban environment for its people.