L'Afrique s'urbanise rapidement et l'étalement des agglomerations urbaine s'accroît. L'une des solutions à l'expansion rapide des zones urbaines et au coût élevé des terrains est la densification car les investisseurs tirent un meilleur rendement de leurs investissements. Toutefois, la densification peut avoir des effets négatifs sur l'écologie urbaine, ce qui se traduit par une mauvaise qualité environnementale et des villes insalubres.
Deux zones de la ville de Nairobi qui présentent une forte densité de forme ont été utilisées comme études de cas. Si la densification présentedivers avantages, elle compromet l'écologie urbaine, entraînant une réductiond es services écosystémiques urbains. Ce phénomène génère des zones urbaines insalubres sujettes aux inondations, aux températures élevées, à la pollution atmosphérique et à des espaces de loisirs limités.
Africa is rapidly urbanising, and the footprint of urban settlements is rapidly growing. One of the solutions to rapidly expanding urban areas and the high cost of land, is densification allowing investors to derive greater returns from their investments. However, densification may negatively affect the urban ecology, resulting in poor environmental quality and unhealthy cities. Two areas of Nairobi City that have a high-density urban form were used as case studies. While densification has various benefits, it compromises urban ecology resulting in a reduction in urban ecosystem services. This phenomenon generates unhealthy urban areas prone to flooding, high temperatures, air pollution and limited recreation spaces. The sustainable development goals (SDGs) are instrumental in achieving healthy urban areas with the requisite ecosystem services. A healthy city has a strong natural urban environment that enhances the mental and physical health of the City's population. Finally, a collaboration between the different actors, including the government, private sector, professionals, and academia, is necessary for the successful materialisation of healthy cities regardless of their density.
According to the United Nations, globally, more people live in urban areas than in rural areas, with 55 per cent of the world's population residing in urban areas in 2018. Although Africa has more rural settlements, the rate of urbanisation is rapid, with Africa and Asia urbanising more rapidly than other regions of the world (United Nations, 2019). Nairobi City in particular, has seen densification of previously low-density zones to meet the rising demands for human settlement for both economic and residential land uses.
The densification of old Nairobi City County estates under the affordable housing project has seen the development of compact estates in areas with ample openspaces. While this project has regenerated decayed parts of the city, the question of environmental quality and its impacts on the resident's health and well-being still lingers. Environmental quality is the properties of the environment and how they affect the well being of human beings and other organisms. It refers to various characteristics such as air quality, water quality, scenic quality, noise, recreation spaces and thermal comfort, and how these impact the quality of physical and mental health (European Environmental Agency, 2004).
Many African cities are increasingly witnessing improved physical infrastructure to match their rapid urbanisation; however, such development leads to a reduction of ecological infrastructure. For example, the ongoing road construction of the Nairobi expressway between Jomo Kenyatta International Airport and James Gichuru Road has resulted in a significant loss of vegetation and amenity along the 27.1km stretch.
Densification is advanced as a tool for sustainable development due to the resultant compact urban form. Urban density increases the efficient use of natural resources as it maximises the use of space in urban areas and allows for the efficient and economic reticulation of public utilities (Pelczynski & Tomkowicz, 2019). However, dense networks of streets, buildings, industry and transportation interfere with the natural ecosystem processes, negatively affecting local hydrology, biodiversity, biogeochemical cycling, and microclimate stability (Elderbrock, Enright, Lynch, & Rempel, 2020).
From an anthropocentric perspective, urban ecology is the application of the sciences to provide well functioning urban habitats or ecosystems while improving the living conditions for the urban population (Endlicher, et al., 2007).Urban ecosystems services directly impact human health through services such as air purification, noise reduction, urban cooling, and runoff mitigation. A healthy landscape will have natural thermal cooling, air purification via vegetation, adequate spaces for physical activity and social interaction, hence improving the physical and mental health of the population. In addition, rich biodiversity provides an environment that self regulates through interaction amongst the species and with the environment, resulting in resilience.
Urban ecology comprises green and grey spaces, including open areas, road reserves, road medians, waterfronts, private gardens, and cemeteries. It provides essential ecosystem services broadly grouped as regulating, provisioning,habitat support and cultural functions(Baggethun, et al., 2013). Through proper planning of the ecological networks, these services can be enhanced and or restored.
In 2017 the Executive Committee of the International Federation of Landscape Architects (IFLA) ratified the IFLA Global Accord, which suggests that adaptation for a changing world hinges on resilience, transformation, and sustainability principles. These three pillars create a foundation for healthy landscapes, andhence landscape architects should seek to develop healthy and sustainable landscapes.
Sustainable Development Goals (SDGs) is essential to advocating for and planning healthy cities. Landscape architects can develop strategies that enhance ecological infrastructure guided by SDGs and play a critical role in creating healthy, high-density landscapes.
SDG 3 Good Health and Wellbeing advocates for health and well-being for all populationdemographics, hence planning and designing healthy cities. As outlined above,healthy cities have various landscape elements that contribute to overall environmental quality, with pollution control, vegetation, and water management the most critical dimensions for urban environmental quality evaluation (Shao,et al, 2019).
Air quality in most cities is poor due to vehicular and industrial emissions. Dense urban neighbourhoods tend to have higher levels of air pollution due to minimal vegetation coverage. Urban vegetation reduces concentrations of air pollutants,carbon monoxide, sulfur dioxide, and nitrogen oxides through carbon sequestration.
The estimated carbon content for any plant species is usually 50% of its biomass, where the biomass of a species is the sum of its above and below ground biomasses (Sharma, Pradhan, Kumari, & Bhattacharya, 2020). Planting plans for dense urban neighbourhoods should maximise pollution control, and here species selection is critical.
Edible landscapes can be incorporated in various ways in dense cities, exploring vertical gardens due to limited ground space.
This approach aligns with SDG 2 that focuses on food security and sustainable agriculture (United Nations, 2015).Further, when edible landscapes are incorporated in cities, they reduce reliance on rural landscapes, reducing food transport miles and air pollution.
As a result of densification, open areas that serve as recreational and social spaces get converted into concrete jungles. Most high-density residential area slack adequate and suitable play areas for children. In addition, many urban streets have little or no greenways to complement other ecological infrastructure elements in the city. This limited vegetation cover results in higher temperatures, reducing the thermal comfort of the urban dwellers. Plant foliage absorbs and reflects some solar radiation, reducing the heat reaching the earth's surface (Baggethun, et al., 2013). Shading of parking areas reduces the amount of heat absorbed by these surfaces with a low albedo ratio (Bouyer, Musy, Huang, & Athamena, 2011).
Green roofs can provide playgrounds and green coverage where ground coverage regulations result in minimal ground-level green spaces. Green roofs and living walls also reduce the urban heat island effect resulting in improved thermal comfort.
With landscape architectural input in the street design concept, multipurpose linear spaces emerge, which provide regulatory ecosystem services such as stormwater management and thermal cooling.
These places add to the overall ecological coverage of an urban area, with elements such as bioswales and shading providing regulatory ecosystem services. Planning of urban farms, green roofs, and living walls according to site suitability across dense neighbourhoods should be linked using ground infrastructure corridors such as road medians to create continuous ecological infrastructure corridors.
Urban riverfronts, when well managed and utilised, can contribute immensely to urban ecological infrastructure coverage. Densification may lead to canalisation of rivers, reduction of river corridors and increased water pollution. When building facades face away from the river corridor, it becomes a backyard space that more often than not suffers the fate of the tragedy of the commons. River corridors should integrate with the city's activities making the corridor a core part of the urban landscape. In addition, to contributing to ecosystem services critical for a healthy city, well managed and protected urban riverfronts lead to cleaner rivers contributing towards SDG 6, which seeks to ensure availability and sustainable management of water and sanitation for all.
Densification reduces the overall visual and landscape quality of cities. Landscape assessment considers the effects of a proposed development on the landscape. In contrast, a visual assessment considers the impact of the proposed development on specific views and the general visual amenity experienced by people (Jeffery, 2017). Hence, before development, densification should require landscape, visual impacts assessment, and prescribed suitable mitigation measures. In existing dense neighbours with limited visual quality,introducing vegetation on building facades and along transport routes should be explored.
A healthy city has a natural urban environment that enhances the mental and physical health of the city's population. Such a city will have ecological infrastructure strategies that work towards achieving SDG 11,safe, resilient and sustainable cities and SDG 13, working towards climate action. Moreover, the natural systems will provide the necessary ecosystem services for resilience and ecosystem balance.
Collaboration between the different actors,including the government, private sector, professionals, and academia, is necessary for the successful preservation and increase of ecological infrastructure. Landscape architects and allied professionals should advocate for healthy landscapes and influence policy development regarding the balance between ecological infrastructure,densification and transport infrastructure.
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Fig 1: Allan Thuku
Figs 2, 3, 4 and 5: Author