Abstract :
[en] Green roofs (GRs) are a sustainable alternative to conventional roofs that provide
multiple ecosystem services, especially when the urban ecosystems primarily consist
of massive proportions of built infrastructures. Most existing studies analyse the
ecosystem services green roofs provide at the building or prototype levels. Some
studies examine the ecosystem services at the block or neighbourhood scale. However,
city-scale studies of ecosystem services provided by green roofs are limited. Moreover,
there are no studies using realistic proportions of potential roofs while analysing
their impact on ecosystem services at city-scale. Therefore, this dissertation aims
to assess the impact of realistic roof greening on ecosystem services of urban heat
island (UHI) mitigation and contribution to ecological connectivity at a city scale. For
this PhD, we use the city of Liège in Belgium as the case study.
Three main steps are involved to accomplish the objective of this thesis. Firstly, we
identify the realistic potential roofs for greening employing geoinformation science
(GIS) and remote sensing and using parameters such as the area and slope of the roof
along with the structure of buildings. Secondly, we analyse the role of potential green
roofs on the UHI effect using three approaches to estimate city-scale impacts. Thefirst
approach utilises a highly resolved weather research and forecasting (WRF) model
using urban physics parametrised with the BEP+BEM model using the local climate
zones (LCZ) as the land use classification. The second approach involves classifying
the urban blocks in the city of Liège into unique and realistic urban morphological
archetypes using a systematic PCA-based k-means clustering approach. With this
approach, we obtained nine unique urban morphological archetypes for Liege. Thereafter,
we analysed the microclimate of these unique morphological archetypes using
the Solene-microclimat model. The third approach employed random forest (RF)
regression to predict the impact of green roofs on the surface UHI (SUHI).
Lastly,we analysed the role of green roofs in Liege’s ecological network of dry grasslands.
Creating analogous habitats on potential green roofs can restore the urban dry
grasslands that are crucial for pollinator species in temperate Europe. Therefore, we
model the ecological connectivity of existing dry grassland patches and the potential
green roofs using graph theory with Graphab 2.8 software. The results suggest that around 20% (350 hectares) of the existing total buildings
in the city have the potential for developing green roofs in Liege. Moreover, most of
the potential area comes from large buildings in the city, which could be a suitable
target for initial implementation in the city. Regarding the approaches explored for
green roofs’ role in UHI mitigation, results show that green roofs generally reduce
air and surface temperatures. Realistic roof greening may not be sufficient at city
or regional scale. Only, it is effective in some parts of the study area. Realistic roof
greening reduces air and surface temperature in large, low-rise LCZs/archetypes. It
can be extremely beneficial in reducing the surface temperature at block scale. For a
significant reduction in air temperature, the building height must be smaller, and the
building area must be larger. Our research mainly provided insight into the interplay
between urban morphology, realistic roof greening potential, and the reduction in
temperatures, improving our understanding of UHI mitigation with green roofs at
the city scale. The remote sensing approach needs further analysis to improve the
accuracy of the results.
Apart from this, results based on the role of green roofs in ecological connectivity
indicated that urban dry grassland connectivity with analogous green roofs, particularly
benefits the high and moderate mobility pollinator species, with limited
impact on low mobility species. Introduced green roofs mainly create a compact
network of dry grasslands in the center, with limited improvement in connectivity
of the existing dry grasslands. Local connectivity analysis reveals that retrofitting
30-50% of the potential green roofs will improve connectivity for high and moderate
mobility species. For improving the connectivity for lower mobility species, however,
almost all the potential roofs need to be greened. Additionally, the study shows that
building heights and configurations can strongly influence the role of green roofs in
ecological connectivity.
This PhD effectively provides an overview of the impact of green roofs on the
UHI effect and ecological connectivity. It also broadly suggests policy recommendations
where the green roof implementation could benefit dense urban areas. Apart
from this, the study stresses the importance of urban morphology for improving the
ecosystem services provided by green roofs.
