| Abstract | The main investigation of this thesis is to determine the thermal effect of different urban green systems (UGS) (as trees, living facades) and high albedo pavement at Pedestrian street level in mitigating urban heat island (UHI) and Carbon Dioxide (CO2) reductions in London, the UK. This thesis quantifies the critical contribution by changing different urban greenery-covering densities (25% and 50%) or applying high albedo pavement in 2018 and future climate scenarios for the 2050 and the 2080 based on London climate change plans. Therefore, this strategy is designed to motivate pedestrians to walk for more extended periods and further distances during the summer season following the Health Streets Initiative introduced by Transport for London and turning London to the world's biggest national park by the 2050s based on the Mayor of London Plan.
However, the influence of different UGS is based on their thermal improvement and CO2 reduction performance is mostly lacking, particularly for future climatic scenarios. The main aim of this research is to quantify the benefits and how they will influence pedestrians with streets. Different methodologies’ phases were tackled in order to investigate the performance and the benefits of different urban green systems’ interventions. This was achieved by ENVI-met software simulations, field measurements to validate the results, and a questionnaire survey. All these together have helped to achieve the objectives and to reach the required outcome of the research. First, a simulation for the current case of a typical central London neighbourhood with no vegetation (0%) is carried out, followed by simulating the same neighbourhood by applying urban green systems with 25% and 50% living facade and tree alternatives in addition to applying high albedo material on all pavements which represents 66% of the canyon area.
This simulation of different interventions was applied for 2018, 2050 and 2080 during the summer months in order to measure their influence on pedestrian thermal comfort and CO2 reduction. Subsequently, first, a real field analysis was carried out to validate ENVI-met simulation calculations that have shown a great index of agreement. Second, a questionnaire survey analysis was formatted to determine the intangible benefits of urban green systems and how these would influence pedestrians’ activities within the streets after applying them, in addition to determining the preferred UGS alternative (Living façade or Trees) with the preferred covering percentage (25% or 50%) for pedestrians.
The ENVI-met simulations results determine that the outdoor thermal comfort was measured based on physiological equivalent temperature (PET) as a reference to evaluate modification benefits towards outdoor comfort level. These results confirmed that, across the three different climatic scenarios – 2018, the 2050s and the 2080s, for best thermal comfort increase, the recommendation was 50% trees followed by 25% trees. While LF with both 25%and 50% coverage did not have a noticeable influence on pedestrians, high albedo pavement has increased thermal stress as it reflects the solar radiation to pedestrians, acting as a second source of radiation. There was not a significant reduction in CO2 levels from trees, while LF did not change the level at all. This reflects that oceans are the primary source of CO2 sequestration and oxygen production. However, to eliminate CO2, the CO2 sources should be limited and UGS cannot act solely to sequestrate it.
Different canyon orientation in London requires different UGS percentage. For instance, North-South canyons do not require high UGS percentage because buildings self-shade on the canyon and hence receive lower solar radiation, leading to decreased thermal stress. On the other hand, East-West canyons need higher UGS coverage to reach higher thermal comfort levels. Therefore, it is advised to apply UGS coverage of 50% trees for East-West street canyon and 25% trees for North-South street canyon across all years to reach the ideal thermal comfort performance.
However, the ENVI-met software determined the physiological effect; the survey was required to determine pedestrian’s perception of different UGS so as to evaluate the psychological effect. The survey reflected that pedestrians are willing to spend 30% more time walking outdoors after applying UGS. Pedestrians also preferred 50% trees as their top favourite preference. While 25% trees and 50% LF had similar preference, the UGS percentage is not the main factor determining the preferred UGS alternative.
One of the most important and crucial findings from the survey is that pedestrians’ top reasons for choosing UGS was its aesthetic value and its air pollution reduction. This was followed by relaxation, connecting to nature and biodiversity increase. Thermal comfort increase ranked fifth which reflects that it is not a priority for pedestrians. This reflects the pedestrians’ priority when they are looking at UGS. The outcomes based on the ENVI-met simulation and survey analysis including correlations and cross-tabulation propose the adequate UGS percentage and type for each canyon orientation in London. That will be able to provide the maximum thermal comfort level in order to achieve psychological satisfaction and physical heat stress relief, which motivates the city walkability through improving the overall comfort. |
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