Investigating Urban Geometry parameters to improve outdoor thermal performance for UK UD codes

Department of the Natural and Built Environment

MSc Technical Architecture May 2021

Investigating Urban Geometry parameters to improve outdoor thermal performance for UK Urban Design standard codes.

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Table of Contents List of Figures and Tables .................................................................................................04 Abstract ...............................................................................................................................06

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Introduction ............................................................................................................07 1.1 Background .................................................................................................................07 1.2 Problem Statement ......................................................................................................08 1.3 Research Questions ....................................................................................................09 1.4 Research Aims and Objectives ...................................................................................09 1.5 Methodology ...............................................................................................................10 1.6 Research scope and limitations ..................................................................................10 1.7 Significance of the study ............................................................................................11

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Literature Review ...................................................................................................12 2.1 Introduction .................................................................................................................12 2.2 Outdoor Thermal Comfort and the Thermal Balance Concept ...................................13 2.2.1 Thermal Comfort..............................................................................................13 2.2.2 Thermal indices ...............................................................................................13 2.2.3 Thermal Balance Concept ................................................................................14 2.2.4 Adaptive Thermal Comfort ..............................................................................16 2.3 Urban Climatology Layers and Urban Climate Zones ................................................17 2.3.1 Urban Climatology Layers ...............................................................................17 2.3.2 Urban Climate Zones and Canyons .................................................................17 2.4 Outdoor Thermal Comfort and the Thermal Balance Concept ...................................20 2.4.1 Aspect Ratio .....................................................................................................21 2.4.2 Orientation .......................................................................................................22 2.4.3 Sky View Factor...............................................................................................23 2

2.4.4 Vegetation ........................................................................................................24 2.4.5 Materiality .......................................................................................................25 2.4.6 Collaborative effects of urban geometry parameters.......................................26 2.5 Urban Heat Islands ......................................................................................................28 2.6 UK Urban Design Guidelines .....................................................................................29 2.7 Summary .....................................................................................................................32

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Methodology ..........................................................................................................33 3.1 Site Selection ..............................................................................................................33 3.1.1 Cardiff .............................................................................................................34 3.1.2 Bristol ...............................................................................................................35 3.2 Research Methods ......................................................................................................37 3.2.1 Rayman ...........................................................................................................39 3.2.2 IES VE ............................................................................................................40 3.3 Summary ....................................................................................................................44

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Results and Discussion ...........................................................................................45 4.1 Urban Design Codes ....................................................................................................45 4.2 Results ........................................................................................................................48 4.2.1 Rayman ................................................................................................................48 4.2.2 IES VE ..................................................................................................................51

4.3 Discussion ......................................................................................................................56 4.4 Summary ........................................................................................................................58

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Conclusion ...............................................................................................................59

References ...........................................................................................................................60

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List of Figures and Tables

Figure 1: Locations of Cardiff and Bristol on similar geographical latitudes.

Figure 2: Independent (Urban morphology) and dependent (Microclimate) variables of the research.

Figure 3: Schematic diagram of energy flow within an urban environment.

Figure 4: Urban Climatology Layers.

Figure 5: Local Climate Zones arranged in decreasing order of their ability to impact local climate.

Figure 6: Street Categories and description.

Figure 7: Urban canyon typologies with respect to aspect ratio (H: W).

Figure 8: Explanation of low and high Sky View Factor.

Figure 9: Urban Design and Planning Hierarchy within the UK. Figure 10: Street categorization by area.

Figure 11: Cardiff and Bristol’s city centre cores.

Figure 12: Urban fabric and geometry of Saint Mary street and The Hays in Cardiff City Centre.

Figure 13: Nature of Saint Mary street and The Hays in Cardiff City Centre.

Figure 14: Urban fabric and geometry of Broad street and Merchant street in Bristol City Centre.

Figure 15: Nature of Broad street and Merchant street in Bristol City Centre.

Figure 16: Methodology flowchart.

Figure 17: Obstacle file created in Rayman and the generated fisheye diagram.

Figure 18: Main interface in Rayman with data specified for Cardiff on June 21st and December 21st.

Figure 19: ModellT interface in IES VE. Figure 20: APlocate interface in IES VE. Figure 21: Suncast interface with the June 21st settings in IES VE. Figure 22: Microflo settings in IES VE. Figure 23: Microflo Monitor interface in IES VE. Figure 24: Checklist in Bristol’s local urban design guide.

Figure 25: Checklist in Cardiff’s local urban design guide.

Figure 26: Diagram of a well-designed place.

Figure 27: Aspect Ratio values for all four streets in two different cases.

Figure 28: SVF values for both cases in all streets.

Figure 29: Fisheye diagrams and SVF values for all four streets in the two cases.

Figure 30: Tmrt values for all four streets in the two cases.

Figure 31: PET values for all four streets in the two cases.

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Figure 32: Solar radiation simulations for St Mary street and The Hays in the two cases.

Figure 33: Solar radiation simulations for Broad street and Merchant street in the two cases.

Figure 34: Wind flow simulations for all four streets in the two cases.

Figure 35: Wind flow analysis for all four streets in the two cases.

Table 1: Definitions of net radiation elements (Lin et al, 2017).

Table 2: A thorough description of street types (Urban Design Compendium, 2000) Table 3: Literature on collaborative effects of urban geometry parameters in temperate climates Table 4: Climatic data for June 21st and December 21st at 12:00pm in Cardiff and Bristol.

Table 5: Streets widths for the four streets in Case 1 and Case 2.

Table 6: Solar Radiation values for all streets in the two cases.

Table 7: Temperature, climate conditions and thermal stress scale.

Table 8: Comparison of PET values against Matzarkis, Mayer and Iziomon’s temperature scale.

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Abstract

This research investigates the relationship between Urban Design parameters and Outdoor Thermal Comfort through attempting to answer the following questions; firstly, how do Urban geometry parameters effect thermal variables in Urban zones? secondly, What Urban zone areas have the most heat stress? and why does it affect thermal performance? And lastly, how do proposed threshold values in the National Model Design Codes effect the thermal performance of urban canyons?

The research identifies urban zones with heat stress causing low thermal performance through the use of thermal profiles, thereafter identifies urban geometry parameters’ effect on thermal performance in pre-selected urban zones through the use of simulations and thermal variables. Furthermore, the research examines the effect of mitigation techniques; in regard to urban geometry parameters, on the thermal performance of pre-selected urban zones in Cardiff and Bristol to test threshold values from the National Model Design Code (2021).

A deductive-quantitative approach is adopted for the purpose of this research that incorporates a survey of UK urban design guides and policy documents, as well as an analysis of thermal profiles and simulations carried out in Rayman and IESVE. The research deduces that Aspect Ratio and Sky View Factor significantly impacts the solar radiation exposure and wind flow in urban zones that subsequently effect the Mean Radiant Temperature (Tmrt) and the Physiological Equivalent Temperature (PET). Furthermore, the proposed threshold values by the National Model Design Codes has a positive impact on PET values during summer for the preselected sites in Cardiff and Bristol, whereas the impact on winter PET values has shown negatively.

The research concludes that an Aspect Ratio of 1:1 is not always ideal and that the proposed threshold values should be reconsidered or reformulated to incorporate a new category; high streets in old town centers.

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Introduction

1.1

Background

Urban microclimate research has become essential in the last few decades due to climatic changes that lead to temperature escalation among other factors, affecting the quality of outdoor urban environments. Outdoor urban environments are crucial to ‘shaping daily life’ as they allow for social interaction (Thorsson et al,2004). Subsequently, Nouri and Costa (2017) argue that the ‘triangulation between climate change, urban design and user based approaches’ can allow for successful outdoor urban environments.

Despite the vast amount of theoretical research available, little is translated into practicality causing a ‘gap between research and its application to urban design’ (Radfar,2012; Nouri,2015; Hirshima and Nikolopoulou, 2016; Chatzidimitriou and Yannas ,2017; Nouri and Costa, 2017). Silva and Costa (2016) state that diminishing the gap between theory and practice could inform a better design of outdoor urban environments in times of drastic climate change, and predict ‘future climatic projections provided a basis of quantifiable data’.

In the past century, various studies explored the link between outdoor thermal comfort and urban environments, through specifically investigating urban parameters such as Sky View Factor (SVF), Aspect Ratio (H:W), Vegetation and Materiality (Howard, 1833; Fanger, 1970; Steemers et al, 1992; Ahmad et al, 2005; Ali-Toudert and Mayer, 2006; Johansson, 2006; Matzarakis and Amelung, 2008; Thorsson et al, 2010; Kruger et al, 2011; Jamei et al, 2016; Chatzidimitrou and Yannas, 2017; Peng et al, 2019).

From this perspective, the research aims to investigate the effect of urban parameters on improving outdoor thermal comfort within the UK context; linking theory and practice to provide architects, urban designers and planners with threshold values for UK urban design guidelines.

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1.2

Problem Statement

Given the projected impacts of climate change, the UK government has identified the need for anticipating and planning for the effects of these changes through funding a series of projects, including the ‘Adaptation Strategies for Climate Change in the Urban Environment - ASCCUE’ in 2003 and ‘Sustainable Cities: Option for Responding Climate cHange Impacts and Outcomes – SCORCHIO) IN 2007. ASCCUE mainly focused on three key aspects; the built environment, human comfort and urban greenspace, using the input of civil engineers, geographers, urban planners and architects (Lindley et al, 2006).

Despite the availability of such projects and urban microclimate research on the UK context (Chowienczyk et al, 2020; Taher et al,2019; Chatzipoulka and Nikolopoulo, 2018; Kruger et al,2013; Lindley et al, 2006) it is yet considered limited in comparison to available research on Hot Arid and Humid climates. Johansson et al (2014) argues that different urban forms and parameters have been extensively investigated for Hot Arid and Humid climates, but to a lesser extent Temperate climates due to the tendency of staying indoors in such climates.

Nonetheless, with the rapid climatic changes the world is facing and rise in global temperatures; societies have evolved and adapted making outdoor thermal comfort a pressing issue in the urban design field.

In light of this perspective, the research will examine two UK cities; Cardiff and Bristol due to being located at similar geographical latitudes (51.481583° N, 51.4545° N respectively) and facing similar climatic contexts; temperate climate but different urban forms and densities.

Figure 1: Shows the locations of Cardiff and Bristol on similar geographical latitudes.

The climatic context of the UK is known as a Maritime temperate climate (Cfb), as classified

by Köppen-Geiger. Maritime temperate climate is characterized by significant precipitation during all seasons, where the average maximum temperatures range from 16 – 20 °C during summer and 2.5 – 8.5 °C during winter (UK Met office, 2020).

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1.3

Research Questions

The preceding discussion sheds light on the importance of the anticipation of climatic changes and their effect on urban climatology and outdoor thermal comfort, to enhance the usability of outdoor urban environments and the pedestrians’ experience. Accordingly, to investigate the effect of urban geometry parameters on improving outdoor thermal comfort for developing UK Urban design guidelines, the research will attempt to answer the following three overlapping questions: 1. How do Urban geometry parameters effect thermal variables in Urban zones?

2. What Urban zone areas have the most heat stress? and why does it affect thermal performance?

3. How do proposed threshold values in the National Model Design Codes effect the thermal performance of urban canyons?

1.4

Research Aims and Objectives

The research has a dual purpose; firstly, its primary aim is to examine threshold values of UK Urban design guidelines to enhance thermal performance and public realm in UK urban zones. Secondly, the research aims to bridge the gap between theory and practice and allow for the proper application of urban climatology within the process of urban planning and design. In order to achieve these aims, the current research’s objectives are as follows: 1. Identifying Urban zones with heat stress causing low thermal performance through the use of thermal profiles.

2. Identifying Urban geometry parameters’ effect on thermal performance in pre-selected Urban zones through the use of simulations and thermal variables.

3. Examining the effect of mitigation techniques in regard to Urban geometry parameters on thermal performance in pre-selected Urban zones to test threshold values from the National Model Design Codes.

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1.5

Methodology

The research intends to quantitatively investigate the effect of urban parameters; Sky View factor, Aspect ratio and Orientation on outdoor thermal comfort, through studying the nonphysical parameters; Solar radiation, Air temperature, Air speed, Humidity and Cloud cover within a specific urban context in the two preselected UK cities during summer and winter.

Figure 2: Shows the independent (Urban morphology) and dependent (Microclimate) variables of the research.

The integrated effect of urban parameters and microclimatic data, causing thermal comfort/discomfort is examined and analyzed using a range of online resources and simulation software; Rayman and IES VE. Thereafter the results and findings are evaluated using a thermal index; the Physiological Equivalent Temperature (PET).

1.6

Research scope and limitations

The purpose of this experimental study is to test the condition of heat stressed Urban zones that relates Urban geometry parameters to thermal performance for UK urban design guidelines at Cardiff and Bristol.

The independent variables of Urban geometry parameters will be defined generally as the Aspect Ratio; described as the height to width ratio, and the Sky View factor; described as the amount of sky that can be seen at any given point on the

surface

being

investigated,

Orientation,

Vegetation

and

Materiality.

The dependent variables of thermal performance will be defined generally as Solar radiation, Air temperature, Air speed, Humidity and Cloud cover and the intervening

variable

climate

will

be

statistically

controlled

in

the

study.”

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The limitations of the study would be its inability to be generalized, as it would be casespecific to the cities with similar latitudes as Cardiff and Bristol . Furthermore, the research will not take into account months of extreme weather conditions nor winter season, as well the psychological parameters and the ‘individual state of mind that expresses satisfaction’ in relation to human factors.

1.7

Significance of the study

In response to the impact of rapid climatic changes, and its effect on the outdoor thermal environments; studies have expressed a need to bridge the gap between theory and application. Thus, the research argues that there is a need for threshold values in the UK urban design guidelines to improve outdoor thermal performance and create energy saving opportunities; both indoor and outdoor to make cities more resilient towards future climate change.

Despite the research available, Chatzidimitriou and Yannas (2017) argue that there is yet a “gap between research and its application to urban design”. Similarly, Tseilou et al (2010) corroborates that there is a “missing link to urban geometry effects which are important from a designer’s point of view”.

Globally, ‘the maturing climate change adaptation agenda’ has become a prominent topic (Nouri, 2015). Despite the aforementioned, Costa (2013) states that there is ‘a lack of knowledge on how such adaptation measures can improve the experience and comfort of pedestrians in outdoor environments’.

Furthermore, Hirashima et al (2016) corroborates by expressing the need for in-depth understanding of urban climatology and its effects on enhancing outdoor urban environments to be used at different times of the year.

Nazarian et al (2019) states that ‘it appears there is limited efforts on prescribing the collective information that should be delivered to the architects, planners, and decision makers, such that the performance of outdoor space with regard to thermal comfort is fully assessed’ and is supported by Nouri (2015) assertion that ‘there is considerable theory, yet limited practical benchmarks that can directly aid local decision making and design’ 11

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Literature Review

2.1

Introduction

In the past decade, urban microclimate research has become more prominent due to climatic changes that affect the quality of outdoor urban environments. Nouri and Costa (2017) state that the ‘triangulation between climate change, urban design and user based approaches ’can allow for successful outdoor urban environments. Similarly, Thorsson et al (2004) corroborate the aforementioned by stating that outdoor thermal environments shape daily life and are hence crucial for social interaction.

Despite an inevitable level of uncertainty due to unpredictable climatological factors; Lindley et al (2006) argues that it is not enough to understand the effects of climatic changes on outdoor urban environments alone as ‘society itself will also evolve over time’. Hence, it is significant for architects, urban designers and planners to ‘become fluent in the language of climatically informed urban design’ and understand the ‘thermal behaviour of these microclimates’ to allow for a more comprehensive and balanced approach to the planning and design of outdoor urban environments. (Radfar, 2012; Nouri, 2015).

Multiple theoretical research has been published in the field of climate sensitive urban design but little is translated into practicality causing ‘a gap between research and its application to urban design’ (Radfar,2012; Nouri,2015; Hirshima and Nikolopoulou, 2016; Chatzidimitriou and Yannas ,2017; Nouri and Costa, 2017).

Therefore, the following thematic literature review is split into three sections that will discuss various aspects that constitute outdoor thermal comfort. Firstly, an explanation of the thermal comfort and thermal balance concept, thermal indices and adaptive thermal comfort.

Secondly, an analysis of the urban climatology layers, urban climate zones and urban canyons. Thirdly, a critical analysis of the urban morphology and urban geometry parameters; Aspect Ratio, Sky View Factor, Orientation, Vegetation and Materiality, followed up with a discussion of the collaborative effects of urban geometry parameters.

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2.2

Outdoor Thermal Comfort and the Thermal balance concept

2.2.1 Thermal Comfort Thermal comfort is defined as the ‘condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation’ (ASHRAE, N.d). The aforementioned was initially intended for indoor spaces; however, with the rising issues of rapid urbanization, climate change and global warming it has become transferable to outdoor spaces.

Liu et al (2012) argues that the heat transfers between a human body and its surrounding environment is a result of ‘‘comprehensive, multi-factorial effects concerning both physical and non-physical’ parameters. Physical parameters include clothing, physical activities and urban geometry; whereas non-physical parameters include Solar radiation, Air temperature, Air speed, Humidity and Cloud cover. Similarly, Fahmy et al (2008) corroborates by stating that the assessment of outdoor urban thermal comfort should incorporate the complexity of local climatology and urban mobility of climate conditions.

Due to the complexity of climatological parameters, the heat transfers between a human body and the surrounding environment is considered unstable ‘shifting from one time, place and season to the other’ (Peng et al, 2019). In Spagnolo and De Dear (2002) the study suggested that there are ‘seasonal differences in thermal neutrality and preference in outdoor areas’ such as the tendency to prefer higher temperatures in winter and lower temperatures in summer. Hence, the concept of outdoor thermal comfort is vital in climate responsive urban design to establish ‘the boundaries of ideal climatic conditions’ (Nouri and Costa, 2017).

Therefore, it is vital to study both physical parameters; urban geometry, and non-physical parameters; Solar radiation, Air temperature, Air speed, Humidity and Cloud cover and their effects on human beings using a thermal index to achieve thermal comfort.

2.2.2 Thermal indices The integrated effect of urban parameters and microclimatic data, causing thermal comfort/discomfort can be evaluated using an array of thermal indices (Fahmy et al, 2008; Tseliou et al, 2010; Muller et al, 2013; Taleghani et al, 2015; Nazarian et al, 2019; Peng et 13

al, 2019). Thermal indices include the Universal Thermal Climate Index (UTCI) that defines thermal effects on the whole human body drawing on ‘multi-node dynamic thermophysiological UTCI-Fiala model’, the Standard Effective Temperature (SET) that considers effective temperature based on mean radiant temperature and is an adaption of an indoor index, the Predicted Mean Vote (PMV) ‘ an empirical index applicable for field measurements of thermal comfort or indoor analysis’ and the Physiological Equivalent Temperature (PET).

The PET index was originally introduced by Hoppe in 1999, and is defined as ‘the equivalent temperature to the air temperature at which, for a typical internal situation, the thermal balance of human does not change, considering the same core and skin temperatures in the original situation’. Tseliou et al (2010) states that for the calculation of PET a group of climatological factors are needed such as air temperature, relative humidity, mean radiant temperature and wind speed alongside individual-related factors.

Furthermore, the index originally had no reference ranges but has since been frequently used in investigating outdoor thermal comfort and mitigation techniques; in a study by Chow and Heng (2018) that incorporated a comparison of 10 major cities’ neutral PET ranges, as well as multiple studies that examined urban geometry in improving outdoor urban environments within the wide scope of research available (Kruger et al., 2011; Taleghani et al, 2014; Chatzidimitriou and Yannas, 2015; Hirashima et al, 2016; Nouri and Costa 2017; Peng et al, 2019; Taher et al, 2019).

PET index provides a clear and accurate indication of thermal comfort and assessment of the climatological effects on human health and well-being, that is also logical to people who are not experts in the field as it is in degrees (Taleghani et al, 2015). Hence, the PET index that has been used repeatedly in research, provides an indication and understanding of thermal comfort for both researchers and people.

2.2.3 Thermal Balance concept In order to fully assess the outdoor thermal comfort in urban design; spatial distributions and temporal variabilities are vital to represent (Nazarian et al, 2019). Fahmy et al (2008) states 14

outdoor heat transfer and heat exchange in urban canyons are due to an interaction between the ‘urban structure (built form+ pattern)’ and climatic conditions.

Lin et al (2017) argues that it is vital to understand the concept of energy balance to better understand ‘the energy flow within an urban environment’. As the first law of thermodynamics states that ‘energy cannot be created nor destroyed, but only converted from one form to another’; that translates into a surface-air system within an urban environment.

The surface-air system incorporates solar radiation gains or anthropogenic heat that is lost by convection, evaporation and net energy transferred or stored to other systems (Figure 3).

Similarly, Fahmy et al (2008) corroborates that heat exchange is a three-fold process; between the human body, surroundings and air due to solar radiation.

Figure 3: Schematic diagram of energy flow within an urban environment (Lin et al, 2017).

In an urban environment, net radiation is the main source of energy that is constituted by incoming solar, reflected solar, atmospheric radiation and surface radiation (Figure 2). Lin et al (2017) states that the amount of solar radiation and net radiation varies based on the season, time of day, latitude, solar position and azimuth.

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Table 1: Definitions of net radiation elements (Lin et al, 2017).

Name

Definition

Incoming solar

Direct and diffused short wave radiation emitted from the sun

Reflected solar

Reflected short wave radiation off of urban surfaces

Atmospheric Radiation

Long wave energy flux emitted by particles from the sky to urban surfaces on the ground

Surface Radiation

Long wave radiation emitted by urban surfaces

Consequently, an array of climatological parameters should be examined to comprehend the thermal balance in urban environments; most importantly solar radiation.

2.2.4 Adaptive Thermal comfort Muller et al (2013) states that the PET index allows for an ample assessment of various adaptation measures on outdoor thermal comfort. As Peng et al (2019) argues that comfort is a product of interaction between outdoor microclimatic conditions and an individual’s preferences within an urban environment, that could involve multiple processes to improve one’s adaptive ability to the thermal condition (Nikolopoulou and Steemers, 2003).

Adaptive thermal comfort has been researched widely in the past decade (Nikolopoulou and Steemers, 2003; Liu et al, 2012; Nouri and Costa, 2017) and was first introduced by ASHRAE for indoor operative temperatures followed by CIBSE for indoor office buildings (Liu et al, 2012).

The concept is defined as ‘gradual decrease of the organism’s response to repeated exposure to a stimulus, involving all the actions that make them better suited to survive in such an environment’ (Nikolopoulou and Steemers, 2003). In other words, adaptive thermal comfort is one’s ability to interact with the surrounding conditions when discomfort arises in an attempt to optimize and restore thermal comfort through a range of adaptive processes and mitigation techniques.

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2.3

Urban Climatology layers and Urban Climate Zones

2.3.1 Urban Climatology Layers To better understand thermal comfort, it is equally important to fully comprehend the urban microclimate and the urban climatology layers. Oke (1976) categorized the urban volume layers into three; Urban Air Dome layer that is the mesoscale climatic conditions, Urban Boundary Layer that incorporates the Inertial Surface Layer; an envelope for urban roofs, and lastly the Urban Canyon Layer that is the microscale climatic conditions (Figure 4).

Figure 4: Urban Climatology Layers (Fahmy et al, 2008).

2.3.2 Urban Climate Zones and Canyons Research has argued that detailed mapping of the urban environment that is covered under the Inertial Surface Layer is key to understanding the parameters that could be adapted or mitigated to enhance outdoor thermal comfort by assessing compactness and its impact (Grimond and Oke, 1999; Oke, 2006; Stewart and Oke, 2012; Raven et al,2018; Maharoof et al, 2020).

Grimond and Oke (1999) proposed a set of Local Climate Zones (LCZ) based on roughness, aspect ratio and built percentage that was later simplified by Oke in 2006 (Figure 5).

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Figure 5: Local Climate Zones arranged in decreasing order of their ability to impact local climate (Fahmy et al, 2008).

Raven et al (2018) state that cities are occupied by multiple Local Climate Zones with the most climatically troubled zones are those with high density, narrow streets and little vegetation much like downtown towers and old city cores where solar radiation is restricted, air flow is decelerated and diverted. Furthermore, the researchers argue that the Urban Canopy Layer is the layer where most influential changes happen as it is highly occupied by human beings that experience a variety of thermal exposures. Fahmy et al (2008) validates the aforementioned by arguing that conditions at the street level can be mitigated using passive and active techniques.

On the other hand, Ali-Toudert and Mayer (2006) argue that outdoor thermal comfort is almost impossible to achieve due to the connection between the Inertial Surface layer that cannot be controlled and the Urban Canopy Layer. Despite the aforesaid the researchers also argue that outdoor thermal comfort can be attempted to achieve through urban fabric design to ensure appropriate solar access, shade and air flow (Bottema, 1999; Knowles, 2003; Emmanuel et al., 2007; Chen et al., 2010).

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Various researchers have stated that well designed urban canyons play a key role in creating thermally comfortable and pleasant urban microclimates (Oke, 1988; Johansson and Emmanuel, 2006; Ali-Toudert and Mayer, 2007; Yahia et al., 2017).

Streets otherwise known as Urban canyons have been categorized by capacity and character in the Urban Design Compendium; a document published by the English Partnerships in 2000 to ‘provide guidance on good urban design’. (Figure 6)

Figure 6: Street Categories and description (Urban Design Compendium, 2000)

Table 2: A thorough description of street types (Urban Design Compendium, 2000)

Street Name

Width

Description

Mews

7-12 m

narrow streets formed by residences on either side (low movement function, high place function)

Residential Street

18-30 m

Medium movement

function,

medium to high place function High Street

18-100 m

low

to

medium

movement

function, low to medium place function.

Boulevard

27-36 m

high movement function, low place function.

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Research in the field has suggested that as urban canyons become narrower heat exchange problems arise leading to discomfort. Consequently, various studies explored the link between outdoor thermal comfort and urban environments, through specifically investigating urban parameters such as Sky View Factor (SVF), Aspect Ratio (H:W), Vegetation and Materiality (Howard, 1833; Fanger, 1970; Steemers et al, 1997; Ahmad et al, 2005; AliToudert and Mayer, 2006; Johansson, 2006; Matzarakis and Amelung, 2008; Thorsson et al, 2011; Kruger et al, 2013; Jamei et al, 2016; Chatzidimitrou and Yannas, 2017; Peng et al, 2019).

Thus, urban canyons are widely researched spaces that are used as routes and resting areas for pedestrians, that demonstrate multiple thermal issues; due to the surrounding buildings, density, enclosure, materials and lack of vegetation.

2.4

Urban Morphology and geometry parameters

Due to recent changing climatic conditions; cities have begun to ‘future proof’ its urban fabric by utilizing ‘integrated mitigation and adaptation’ techniques such as enhancing urban ventilation and solar orientation by studying the effects of urban geometry parameters (Raven et al, 2018).

Past research has shown that the most influential urban geometry parameters include Aspect Ratio (H: W), Sky View Factor (SVF), Orientation, Vegetation and Materiality. Despite the availability of such projects and urban microclimate research on the temperate climate (Ali-Toudert and Mayer, 2007; Kruger et al, 2013; Muller et al, 2013; Ketterer and Matzarakis, 2014; Taleghani et al, 2015; Chatzidimitriou and Yannas, 2015; Santos Nouri, 2015; Karakounos et al, 2018; Maharoof et al, 2020) it is yet considered limited in comparison to available research on Hot Arid and Humid climates.

Taleghani et al (2015) argues that different urban forms and parameters have been extensively investigated for Hot Arid and Humid climates, but to a lesser extent Temperate climates due to the tendency of staying indoors in such climates. Nonetheless, with the rapid climatic changes the world is facing and rise in global temperatures; societies have evolved and adapted making outdoor thermal comfort a pressing issue in the urban design field.

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2.4.1 Aspect Ratio Aspect Ratio is described as the height to width ratio; height of the buildings (Canyon walls) to the width of the street (Canyon width). Urban canyons are categorized into three typologies: Deep, Shallow and Uniform (Ahmad et al, 2005) (Figure 7).

Deep Canyon H:W = 2

Shallow Canyon H:W < 0.5

Uniform Canyon H:W = 1

Figure 7: Urban canyon typologies with respect to aspect ratio (H: W).

Maharoof et al (2020) argues that aspect ratio is a determining factor in outdoor thermal comfort as it dictates the level of openness irrespective of orientation. On the other hand, AliToudert and Mayer (2007) confirmed that aspect ratio and orientation effects the canyons’ ability to absorb solar radiation and shading for the mitigation of heat stress based on numerical results.

In a study by Nunez and Oke (1977) that examined two urban canyons in Vancouver- Canada with different aspect ratios (0.85 and 1.15) and similar orientation; it was deduced that radiation exchanges are highly influenced and differs by canyon geometry and specifically aspect ratio. Similarly, Ketterer and Matzarakis (2014) studied the effect of aspect ratio and orientation in Stuttgart-Germany during summer and its effect on PET values.

The study suggests that shallow urban canyons with lower aspect ratios experience higher heat stress during daytime that results in discomfort, but lower PET values at night.

Furthermore, it was deduced that an ideal configuration of northwest-southeast or north-south orientation with an aspect ratio of 1.5 minimum results in enhanced thermal comfort conditions and increased solar access all year long in mid-latitude cities.

Various research in the field have examined the effect of aspect ratio on solar radiation, air flow and air temperature. A study conducted in Glasgow-Scotland established that building 21

height variations and aspect ratio have significant effects on urban wind environment (Lin et al, 2017). Likewise, a study in Athens-Greece found that by decreasing aspect ratios an increase in cooling rates is observed (Giannopoulou et al, 2010).

While researches argue that increasing building heights that lead into higher aspect ratio values and deeper canyons causes thermal discomfort, it is suggested by Lin et el (2017) that such a configuration might not necessarily lead to a worse urban microclimate. Berardi and Wong (2016) studied the effects of high-rise buildings on an open space in Toronto-Canada in which increased wind speed at the pedestrian level was observed and resulted in lower temperatures.

Despite, aspect ratio being a determining factor in outdoor thermal comfort it is often insufficient to study on its own. Thus, multiple researchers examine the correlation of orientation and aspect ratio collaboratively on outdoor thermal comfort.

2.4.2 Orientation Orientation has evident contributions on the energy balance and heat storage in urban canyons as it effects solar access due to ‘the dominant positioning of the sun in the sky’, wind exposure and surface temperatures in the urban morphology (Nouri and Costa,2017).

However, wind speed and wind direction are unpredictable; hence difficult to anticipate, unlike sun path which is known all-year round.

Erell and Williamson (2007) suggested a difference in air temperature between north-south and east-west oriented canyons. Equivalently, a study in Stuttgart-Germany found that eastwest orientated canyons suffer from longer durations of solar radiation that lead to higher levels of heat stress, whereas northwest-southeast canyons showcase lower heat stress levels (Ketterer and Matzarakis, 2014).

Taleghani et al (2015) examined five urban forms in the temperate climate of Netherlands and corroborates that East-West canyons are exposed to solar radiation for almost 12 hours a day in contrast to 4 hours a day in North-South orientated canyons that are found to be of a cooler microclimate.

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In a comparison study by Esch et al (2012) that surveyed canyons with different widths and orientations of East-West and North-South, it was concluded that East-West canyons are extensively exposed to solar radiation during summer while North-South canyons are exposed for a short duration irrespective of aspect ratio. In conclusion, researchers have corroborated that the E-W orientation is the most inadequate in any geographical context, due to the prolonged periods of solar radiation (Jamei et al, 2016).

Therefore, orientation is a key factor in addressing thermal stress as it provides in-depth knowledge of solar radiation and wind flow within an urban environment.

2.4.3 Sky View Factor SVF is a dimensionless parameter ranging from 0 to 1, and is defined as the amount of sky that can be seen at any given point on a surface that is being investigated. SVF mainly controls solar radiation and wind speed; hence very context based and optimal SVF values should be made on a case-by-case basis (Lin et al, 2017).

Low Sky View Factor - Limits long wave radiation loss - Low daytime air temperature - High night time air temperature

High Sky View Factor - Does not limit long wave radiation loss - High daytime air temperature - Low night time air temperature

Figure 8: Explanation of low and high Sky View Factor.

Lin et al (2017) argues that lower SVF values cause cooler thermal environments during daytime due to the shading effect of the urban morphology however trapped radiation is released during nighttime. The effects of SVF irrespective of aspect ratio is discussed in Nouri and Costa (2017) that suggested higher SVF values correlate with higher solar radiation exposure.

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Chatzipoulka and Nikolopoulou (2018) stated that it is vital to understand how SVF is related to urban geometry specifically density. SVF values are lower in highly dense areas that cause obstructions to the amount of sky that can be seen at the study site. Additionally, the study suggested that by ‘adjusting the horizontality and verticality’ of the urban morphology and creating more open spaces on the ground level; outdoor thermal comfort would be enhanced.

Despite its importance in the study of outdoors thermal comfort, Niachou et al (2008) argued that “SVF alone is inadequate in presenting the complex thermal phenomena occurring in Urban canyons”.

SVF is used as an indication of solar radiation, cloud cover and built obstructions. Although it is an important indicator of the urban morphology it is mainly used alone in study of Urban Heat islands and needs validation frequently as the methods used to calculate SVF values have considerable errors.

2.4.4 Vegetation Recent research has showed that vegetation is a key element to the adaptation and mitigation techniques in existing urban environments, and greatly aid in enhancing outdoor thermal conditions and overall urban microclimate. Vegetation greatly effects evapotranspiration and shade in any urban environment.

Santos and Nouri (2015) discussed vegetation’s ability to shield outdoor spaces in summer from direct solar radiation by providing shade; thus leading to reducing air temperature, while still being able to allow for much-needed solar radiation in winter. Additionally, Karakounos et al (2018) argue that vegetation and water surfaces have a significant effect on wind conditions and air temperature within an urban environment when a 0.9 C decrease was observed in Greece.

Introducing vegetation into an urban environment increases evapotranspiration causing a reduction in heat loads and providing adequate shade. Hence, achieving a higher decrease in PET values during daytime making it thermally comfortable for pedestrians but releasing trapped heat during night-time (Taleghani et al, 2015). Additionally, Muller et al (2013) corroborates that vegetation’s shading and evapotranspiration cause a decrease in PET values, while also suggesting that adequate distancing between trees allows for good ventilation.

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On the other hand, Lin et al (2017) states that vegetation’s effect ‘varies with time, season and built environment’ and that it is affected by urban geometry and the microclimate. In particular trees that are found to have the ‘largest effect on surface, globe and air temperature’ in comparison to other factors (Chatzidimitriou and Yannas, 2015). Trees change their characteristics, such as dimension and ability to shade each season, which is cause for concern when professionals utilize vegetation as an adaptation and mitigation technique.

2.4.5 Materiality In addition to vegetation being a prominent technique to enhance outdoor thermal comfort in existing urban environments; researchers have validated ‘cool materials’ to having significant effects. ‘Cool materials’ are defined as materials with high solar reflectance (albedo) and high infrared emissivity that cause a decrease in surface temperatures and enhance outdoor thermal comfort (Santamouris et al, 2012; Santos Nouri, 2015; Karakounos et al, 2018).

Albedo of cool materials is influenced by colour and roughness (Santos Nouri ,2015), where lighter colours and smoother surfaces are of a high albedo value. Raven et al (2018) states that changing the albedo of different materials present in an urban canyon by replacing impervious surfaces with permeable ones or adding an extra surface coat can enhance outdoor thermal comfort as they reduce surface temperatures, hence released heat.

Karakounos et al (2018) argues that cool materials have a positive impact on night-time surface temperature due to decreasing air temperature that leads to less heat release. On the contrary, Ali-Toudert and Mayer (2007) argue that such adaptation and mitigation techniques could lead to ‘delayed cooling of deep street canyons’ while also starting that in a similar climate and latitude as Freiburg-Germany; where summers are not dry, the use of permeable thin materials that allow for trans-evaporation is advisable.

Similarly, research has suggested that the use of ‘cool materials’ could have adverse effects affecting pedestrian comfort and building energy balance; such as increasing thermal indices, increasing surface temperatures of surrounding buildings and inducing surface glare (Chatzidimitriou and Yannas ,2015; Santos Nouri, 2015).

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Lin et al (2017) examined the albedo of cool materials against vegetation, which validated that vegetation have lower albedo values than cool materials coated in a light colour but still lead to decreased radiation and enhance thermal stress.

Despite, materiality being an important factor in addressing outdoor thermal comfort and its mitigation techniques, it is insuffient to examine the effect of ‘cool materials’ on its own but should be examined collaboratively with Sky View Factor (Karakounos et al, 2018).

2.4.6 Collaborative effects of urban geometry parameters Researchers often study the effect of two or more urban geometry parameters on urban microclimates and outdoor thermal comfort; as it provides an in-depth understanding of the issue at hand (Table 2).

Santos Nouri (2015) examined the effect of SVF and vegetation in New Zealand to mitigate heat stress and concluded that in urban environments with high SVF values; the addition of vegetation can block unwanted wind in winter and enhance evapotranspiration during summer. SVF and Cool materials were studied in Karakounos et al (2018) where a considerable decrease in mean radiant temperatures was observed. Lin et al (2017) and linked the cooling effect of vegetation to aspect ratio and their collaborative effect on wind flow.

Most commonly researchers examine the effect of Aspect ratio, Orientation and SVF collaboratively to describe the intensity of the urban environment, and its effect on the microclimate in terms of solar radiation, wind flow and thermal conditions (Ali-Toudert and Mayer, 2007; Lin et al, 2017; Chatzipoulka and Nikolopoulou, 2018).

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Table 3: Literature on collaborative effects of urban geometry parameters in temperate climates

Research

Urban Area

Climate Classification

Method

Chatzidimitriou, and Yannas (2017)

Thessaloniki - Greece

Temperate Climate

in-situ measurements

Nouri (2015)

Auckland- New Zealand

Temperate Climate

in-situ measurements

Kruger et al (2013)

Glasgow-Scotland

Temperate Climate

in-situ measurements+ Simulation

Muller et al (2013)

Oberhausen-Germany

Temperate Climate

Simulation

Ali-Toudert and Mayer (2007)

Freiburg-Germany

Temperate Climate

in-situ measurements+ Simulation

Ketterer and Matzarakis (2014)

Stuttgart-Germany

Temperate Climate

Simulation

Taleghani et al (2015)

Netherlands

Temperate Climate

Karakounos et al (2018)

Serres- Greece

Temperate Climate

Chatzipoulka and Nikolopoulou (2018)

London- UK Paris- France

Cheung et al (2015)

Maharoof et al (2020)

Geometry Parameters SVF, Materials, Albedo, Vegetation Materials, Albedo, Vegetation SVF, Materials Materials, Albedo, Vegetation H: W, SVF, Orientation H: W, SVF, Orientation

in-situ measurements+ Simulation in-situ measurements+ Simulation

Orientation, SVF, Vegetation

Temperate Climate

Simulation

SVF, H: W, Density

Manchester- UK

Temperate Climate

Simulation

SVF, Orientation

Glasgow- Scotland

Temperate Climate

Simulation

H: W, SVF, Orientation, Materials, Vegetation

SVF, Materials, Vegetation

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2.5

Urban Heat Islands

The effects of Urban Geometry parameters extend beyond thermal comfort, into a much larger phenomenon; the Urban Heat Island (UHI). Urban Heat Islands are ‘localised manmade climate changes’ due to solar energy; that is constituted by short and long wave radiation being stored within the urban environment during daytime, and then partially released back during night-time (Chowienczyk et al, 2020).

Nowadays, rapid urbanization is changing green spaces into buildings and roads affecting the local climate. The change in urban fabric and geometry causes a reduction in the Sky View Factor (SVF), meaning how much clear unobstructed sky can be seen at any given point within a site. As long wave radiation is released during night-time back to the local environment, a significant reduction in SVF values directly causes a reduction in how much long wave radiation is released back, leading to the Urban Heat Island effect.

Within the past decade, Urban Heat Islands have been a prominent topic within the field with a ‘wealth of available material’ (Cheung et al, 2015). In the UK, a collaborative research on Urban Heat Islands examined the topic in depth within multiple metropolitan cities to deduce ‘effective climate change mitigation initiatives’ (Taher et al, 2019).

Thus far, research has found that UHIs develop in areas with low wind speed and has a maximum effect on clear nights. Additionally, UHIs have stronger correlation with minimum temperatures rather than maximum temperatures during summers.

Specifically, within the UK, researchers found that there is a 7 °C difference in air temperatures between urban and rural areas (Skelhorn et al, 2014). Furthermore, in a study

by Taher at al (2019) that examined UHI effects in the city of London, it was concluded that the city of London will face an increase of 9°C in air temperature compared to its surroundings as a result of UHIs. On the other hand, in a study by Chowienczyk et al (2020) that examined multiple metropolitan cities within the UK; including Cardiff, it was found that within the Cardiff metropolitan area UHI intensity strongly correlates with maximum temperatures rather than the usual minimum.

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Overall, the research infers that Urban Geometry parameters and its effects on Urban Heat Islands have been widely researched within the UK however Outdoor Thermal Comfort has been scarcely researched.

2.6

UK Urban Design Guidelines

In the past decade, a number of policy documents and guidelines have been released by governmental authorities to guide the urban design and planning process within the UK. The Ministry of Housing, Communities and Local Government is responsible for most policy documents and guidelines that are utilized by Local Authorities to derive their own local urban design guide or planning document. In addition to the aforementioned, a number of professional bodies such as the English Partnership-Housing corporation and the Design council aid the government in supplementary material to aid the urban design and planning process (Figure 9).

UK Urban Design and Planning System

Ministry of Housing, Communities and Local government National planning policy guidance Manual for streets 1& 2 (2007,2010) National Design Guide (2019) Living with beauty (2020)

Supplementar y sources

English Partnership The Housing corporation

Design Council

Urban Design Compendium 1 &2 (2000,2013)

Councillor's guide to urban design (2003)

National Design Model Code (2021)

Local Authority (County Council, District Council , Unitary authorities, Metropolitian Districts, London Boroughs) Local Urban Design Guide ( Based on City theme) Figure 9: Urban Design and Planning Hierarchy within the UK.

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Among the numerous documents available, the National design guide, living with beauty and manual for streets 1 and 2 have been the main focus of local authorities’ own guides. The previous documents act as a guide and produce rule-of-thumb descriptions to various issues within the urban design and planning field; such as identity, built form, movement, public spaces, homes and buildings … etc. Despite the purpose of the documents, they remain descriptive and diagrammatic in nature and open to interpretation by Local Authorities; specifically, in areas that are in need of quantitative numerical values such as aspect ratio.

Furthermore, the policy documents scarcely mentioned climate change and the need for more sustainable and environmentally friendly designs when addressing public spaces.

In the National Design Guide (2019) stated that a new policy document will be published to ‘a baseline standard of quality and practice across England which local planning authorities will be expected to take into account when developing local design codes and guides and when determining planning applications’.

Furthermore, in the Building Better, Building

Beautiful Commission’s document; living with beauty (2020); ‘Policy Proposition 7: localise the National Model Design Code which will function as a template for local authorities to develop, their own codes in accordance with local needs and preferences and to support better urbanism and mixed use.’ outlines the need for codes that local authorities can utilize as a template to develop their own codes.

In 2021, the National Model Design Code accompanied by a guidance notes document was published based on the recommendation of the aforementioned policy documents, that outlined quantitative numerical values for multiple aspects in the Urban Design and Planning field and addressed the climatic aspect of urban design and planning. Within the area of public spaces, proper and reviewed categorization of streets in conjunction with movement hierarchy was suggested with numerical values for aspect ratios (Figure ure 10).

The suggested street categorization provided a deeper insight into street types, area types and functions. In addition to the suggested categorization, accompanying diagrams showcasing ideal designs of different street types in different areas was offered as a rule-of-thumb.

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Figure 10: Street categorization by area. (National Model Design Code, 2021)

Local authorities such as County Councils, District Councils, Unitary authorities, Metropolitan Districts and London Boroughs produce their own local urban design guides that are used to inform local development documents that in turn deduce core strategies and area action plans. The focus of these local urban design guides is mainly city and town centers with key themes relating to the nature of the city.

In Cardiff, a ‘Liveable Design Guide’ was published in May 2015 that highlights the main focus of the local council’s vision for the city, that being to create ‘people-friendly places and streets’, convenient and fast travel, easy access to amenities and community sharing and meeting cores. The document is descriptive in nature and merely provides guidance to developers and professionals through a masterplan checklist to ease the understanding of the process and what is required.

Similarly, Bristol’s ‘Urban Living SPD’ and ‘City Centre Framework’; published in 2018 and 2020 respectively, focus on preserving the historic nature of Bristol while advocating for improved movement, new developments and enhancing public realm within the highly dense urban fabric. The documents moderately discuss climatic aspects in relation to pedestrian level winds and sunlight/daylight, that are descriptive and diagrammatic providing guidance to current and new developments within the city.

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Despite, the current local urban design guides being well-organized and functional, they are yet considered descriptive and lack accuracy. The National Model Design Code and Guidance notes pave way to new accurate and efficient local design guides that take climate change and its effects on the urban fabric into consideration while proposing codes that would enable developers and professionals to enhance public spaces and public realm simultaneously.

2.7

Summary

In response to the impact of rapid climatic changes, and its effect on the outdoor thermal environments; it is vital to understand the heat exchange between urban fabrics and the environment through examining non-physical parameters; such as Solar radiation, Air temperature, Air speed, Humidity and Cloud cover, as well as physical parameters such as Aspect Ratio, Sky View Factor, Orientation, Vegetation and Materiality.

Globally, ‘the maturing climate change adaptation agenda’ has become a prominent topic (Nouri, 2015). The previous sections have discussed and analyzed the data available on outdoor thermal comfort in a temperate climate utilizing various methods; such in situ measurements and simulations, and has concluded that urban morphology specifically aspect ratio, sky view factor, orientation, vegetation and materiality have significant effects on outdoor thermal comfort at the pedestrian level. Additionally, examining combined effects of urban geometry parameters would provide in-depth insight to the thermal situation of an urban environment (Costa, 2013). Furthermore, changes to combined urban geometry parameters is best and can enhance the outdoor thermal comfort immensely.

Hence, adaptive thermal comfort in urban environment is in the shape of mitigation or adaptive techniques constituted by informed decisions taken by architects, planners and urban designers to enhance outdoor thermal comfort.

Previous studies have expressed a need to bridge the gap between theory and application (Tseilou et al, 2010; Hirashima et al, 2016; Chatzidimitriou and Yannas, 2017). The bridging process is possible through the introduction of codes or benchmarks, i.e numerical values, in urban design guides that would greatly aid architects, planners and decision makers (Nouri, 2015; Nazarian et al, 2019).

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3|

Methodology

The purpose of the following chapter is to present the research process used in completing the research and addressing the questions, aims and objectives presented in chapter one. The chapter includes a detailed description of the site selection process, the preselected sites and methods used. Furthermore, the chapter will outline the data collection and analysis methods utilized in the research.

3.1

Site Selection

Cities are compromised by multiple Local Climate Zones (LCZ); ranging from compact high rise, industrial, suburbs and city centres (Grimond and Oke, 1999). Research in the field has established that the most climatically troubled zones are those characterised by high density, narrow streets and little vegetation much like city centres nowadays (Raven et al, 2018).

Hence, for the purpose of this research two city centre cores in the UK were examined; Cardiff and Bristol due to being located at similar geographical latitudes (51.4815°N and 51.4545°N respectively) and streets orientations (N-W) that face similar climatic conditions but different urban forms and densities.

Cardiff

Bristol

Figure 11: Shows Cardiff and Bristol’s city centre cores.

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Furthermore, two high streets that are considered both a route and resting space for pedestrians were selected within the walkable catchment areas and activity nodes of Cardiff City Centre and Bristol City Centre.

3.1.1 Cardiff Cardiff’s City Centre area consists of a highly dense urban fabric and a loose grid street layout with most streets in the N-W and S-E orientation with close proximity to the River Taff and a main road that connects the City Centre to the M4 motorway.

Climatic conditions in Cardiff vary year round, but on average the temperatures range between 2.3°C to 13.8°C during winter and 11.0°C to 21.7°C during summer with wind speed and direction ranging from 3.5 knots to 21.5 knots from the NW and SW. Cardiff has a 149 days of rainfall per year and varying cloud cover and humidity levels depending on the month.

Within Cardiff City Centre, Saint Mary Street and The Hays are considered the main high streets and have high occupancy rates.

Figure 12: Shows the urban fabric and geometry of Saint Mary street and The Hays in Cardiff City Centre.

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The street widths are 28.42m and 24.69m respectively with average building heights ranging from 9m to 18m in Saint Mary street and 9m to 21m in The Hays making the aspect ratio of both streets of a wide nature (Figure 12).

Saint Mary street consists of both pedestrian and vehicular functions with no vegetation available, whereas The Hays is a pedestrian only zone with vegetation in the middle of the

SAINT MARY ST

street (Figure 13).

(F4 map Demo, N.d)

(Google maps, N.d)

(F4 demo map, N.d)

THE HAYS

(Google maps, N.d)

Figure 13: Shows the nature of Saint Mary street and The Hays in Cardiff City Centre.

3.1.2 Bristol Bristol’s City Centre area is of a highly dense urban fabric and is comprised of an organic grid street layout with most streets in the N-W and S-E orientation with close proximity to the River Avon and an expressway that runs all the way to the North of England.

On average the climatic conditions in Bristol are similar to that of Cardiff’s, with temperatures ranging between 2.2°C to 13.3°C during winter and 11.1°C to 21.5°C during summer with wind speed and direction ranging from 3.5 knots to 21.5 knots from the WSW and W. Bristol has a 123 days of rainfall per year and varying cloud cover and humidity levels depending on the month.

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Broad Street and Merchant Street are considered the main high streets and have high occupancy rates. The street widths are 10.61m and 18.01m respectively with average building heights ranging from 6m to 25m in Broad street and 9m to 15m in Merchant street making the aspect ratio of a narrow and wide nature respectively (Figure 14).

Figure 14: Shows the urban fabric and geometry of Broad street and Merchant street in Bristol City Centre.

Broad street consists of both pedestrian and vehicular functions, whereas Merchant street is

BROAD ST

a pedestrian only zone with both streets lacking vegetation (Figure 15).

MERCHANT ST

(Google maps, N.d)

(F4 map Demo, N.d)

e.

(Google maps, N.d)

(F4 map Demo, N.d)

Figure 15: Shows the nature of Broad street and Merchant street in Bristol City Centre.

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3.2

Research Methods

The research adopts a deductive-quantitative approach to address the research questions and objectives outlined in chapter one; firstly, how do Urban geometry parameters effect thermal variables in Urban zones? secondly, what Urban zone areas have the most heat stress? and why does it affect thermal performance? And lastly how do proposed threshold values in the National Model Design Code effect the thermal performance of urban canyons?

The research is constituted of two parts; firstly, a survey of urban design guides and policy documents, and secondly, an analysis of thermal profiles and simulations. The first research question was addressed through the literature review in chapter 2, whilst the second and third research questions were explored using thermal profiles and simulations. From a positivist perspective, the researcher adopted a quantitative approach which explores the relationship between numerical variables, as the purpose of this research is to examine threshold values proposed in the National Model Design Code (2021) against PET values to assess the outdoor thermal performance of preselected sites.

Furthermore, precedent studies and research have explored the relationship between urban geometry and outdoor thermal comfort through quantitative research, specifically simulations and regression analysis (Ali-Toudert and Mayer, 2007; Kruger et al, 2013; Ketterer and Matzarakis, 2014; Taleghani et al, 2015; Karakounos et al, 2018; Chatzipoulka and Nikopoulou, 2018; Cheung et al, 2015; Maharoof et al, 2020).

A remote online survey of Cardiff and Bristol’s city centres; using google maps, and background research helped determine the selected urban canyons. Thereafter, weather data for each city were obtained through ‘Lady bug’ and viewed through ‘PD: 2D Data view’ enabling the researcher to thoroughly study the climatic conditions of both cities and determine two days in which the simulations and analysis were carried in. The chosen dates and times were June 21st (summer solstice) and December 21st (winter solstice) at 12:00 pm for the purpose of this research, as these dates mark the longest and shortest daylight periods in a year.

To examine the urban geometry of each urban canyon including building density and height, DWG files were collected from CAD mapper and validated through google earth and F4 map demo. The DWG files enabled the researcher to determine average building heights, street widths and calculate aspect ratio values for all four streets in their original state.

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Assessing Outdoor Thermal Comfort and testing National Model Design Codes

CAD Mapper

F4 map demo

Diagram Key Data Collection

Google Earth

Calculate Aspect Ratio (H: W)

Data Analysis

DWG File(s) UK National Design Model Codes

Convert to DXF

Data Validation

4 streets in 3D format (Building heights & Street widths) Case 1: Original Street width(s) Case 2 : Modified Street width(s) to comply with National Design codes H:W ratio 1:1 and width 15-20m

Results

Convert to PDF

Convert to Bitmap Ladybug Weather files (Cardiff and Bristol)

PD: 2D Data view IESVE

Rayman

Suncast

Solar radiation

Obstacle files

Microflo

Wind flow

Fisheye Diagram

-Air temperature Ta °C -Rel. Humidity RH(%) -Wind Velocity(m/s) -Cloud cover N (octas)

Compare against standard PET values

SVF values Figure 16: Shows the Methodology flowchart.

Tmrt values °C PET values °C

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Thereafter, an informed decision on the street widths to comply with the National Model Design Codes was made. The street widths were modified to attempt to achieve an aspect ratio of 1:1 and within the 15- 20m range that is presented as Case 2-Modified.

The DWG files were then converted to Bitmap and DXF formats and inserted into the analysis software, IESVE and Rayman (Figure 16).

3.2.1 Rayman Rayman is a software developed by Andreas Matzarakis that calculates PET, Tmrt and SVF values through inputting a location, date, time, air temperature, humidity, wind speed and cloud cover.

The Bitmap files for Case 1 and Case 2 in Cardiff (Saint Mary Street and The Hays) and Bristol (Broad Street and Merchant Street) were individually inserted into the software and traced over to create the buildings in 3D format with the building height data supplied through the DWG file obtained through CAD mapper. The aforementioned generated obstacle files, created the Fisheye diagrams and calculated SVF values (Figure 17).

Weather data viewed through PD: 2D Data view enabled the researcher to collect the climatic data; Air temperature, Relative Humidity, Wind Velocity and Cloud Cover for June 21st and December 21st at 12:00pm for both Cardiff and Bristol (Table 4).

Figure 17: Shows an obstacle file created in Rayman and the generated fisheye diagram.

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Table 4: Climatic data for June 21st and December 21st at 12:00pm in Cardiff and Bristol.

Cardiff June 21st December 21st

Air Temperature (°C) 20.0 5.0

Relative Humidity (%) 48.0 80.0

Wind Velocity (m/s) 4.0 5.0

Cloud Cover (octas) 2 7

Wind Velocity (m/s) 7.2 3.5

Cloud Cover (octas) 8 1

Bristol June 21st December 21st

Air Temperature (°C) 16.0 5.5

Relative Humidity (%) 67.0 65

Afterward the climatic data was inserted into Rayman along with specifying the geographical longitude and latitude, date and time. Furthermore, Rayman has a pre-set of personal data, clothing and activity values that are set according to standards to measure PET values. For the purpose of this research the clothing values (clo) were specified as 0.50 for June 21st and 1.00 for December 21st (Figure 18).

Figure 18: Shows the main interface in Rayman with data specified for Cardiff on June 21st and December 21st.

Finally, Rayman allowed for the analysis of the provided data and the calculations of SVF, Tmrt and PET values in addition to the fisheye diagrams of each street in the two different case scenarios.

3.2.2 IES VE IES VE is a building performance analysis software that enables designers to examine passive solutions, solar radiation and wind flow among other features. The generated DXF files were

40

imported into IES VE and traced over in the ModellT application to create 3D buildings with the building height data supplied through the DWG files by CAD Mapper (Figure 19).

Figure 19: Shows the ModellT interface in IES VE.

Thereafter using the Apache application, the location data and weather data were specified from a pre-set Site location list in IES VE to Cardiff or Bristol depending on the street that is being examined (Figure 20).

Figure 20: Shows the APlocate interface in IES VE.

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The Suncast application was used after specifying the location and weather data to assess the solar radiation impact on the buildings and most importantly the urban canyon through a simulation as shown below in Figure 21.

Figure 21: Shows the Suncast interface with the June 21st settings in IES VE.

The Suncast simulations were run for all four streets in two different cases and in the selected days; June 21st and December 21st at 12:00pm, as a result visual simulation images were produced that were used for comparison and analysis. Similarly, MicroFlo; a CFD simulation application in IESVE, was used to examine the average wind flow within the pre-selected urban canyons in Cardiff and Bristol, in two different cases; the original and modified.

Within MicroFlo a group of settings were inserted to generate the resulting simulations. The settings include wind direction, exposure and the grid settings, whereas IES VE deduces the wind velocity from the Apache weather file and generically sets the turbulence model and discretisation scheme as shown in Figure 22.

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Figure 22: Shows the Microflo settings in IES VE.

Thereafter, the grid was automatically generated and the simulation was run through the MicroFlo monitor with the outer iteration value set to 500 for accuracy and viewed through the MicroFlo Viewer (Figure 23).

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Figure 23: Shows the Microflo Monitor interface in IES VE.

3.3

Summary

The research examined 2 high streets in Cardiff City Centre and 2 high streets in Bristol City Centre that are within the walkable catchment areas and activity nodes of both cities.

Additionally, Cardiff and Bristol are within the same geographical latitudes and share similar climatic conditions. The pre-selected urban canyons are of different aspect ratios but similar canyon orientations, lack vegetation and are part of highly dense urban fabrics. The preselected urban canyons were surveyed through Google maps, Google Earth and F4 demo map and chosen according to density, orientation and street functions.

The research utilized a number of online resources and software to collect, simulate and analyse data. CAD mapper and Ladybug were used to collect DWG files and weather files respectively that were converted into DXF files and Bitmap images and inputted into two analysis software for simulations; Rayman and IES. The aforementioned software generated Fisheye diagrams, SVF values, PET values, Tmrt values, solar radiation diagrams and wind flow diagrams.

The research examined the pre-selected canyons in their original state and street widths, then modified cases that comply to codes published in the UK National Model Design Codes for Aspect ratios and streets widths for High streets within Town centres.

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4|

Results and Discussion

The following chapter will discuss firstly, the findings and results of the urban design guidelines survey, then the results of the analysis software used; Rayman and IESVE, lastly, a discussion of the results.

4.1

Urban Design Codes

The results of the survey in Chapter 2.6 found a clear support for the need of Urban design codes, as most guidelines reviewed for the purpose of this research were mainly descriptive and vague in providing guidance. The National Model Design Codes (2021) has set codes for Aspect Ratio and street widths for different types of streets and local zones and has stated that these are merely a guidance and that the Local Councils should deduce their own values derived from the general codes.

Furthermore, a survey of the Cardiff and Bristol’s local urban design guides incorporates a checklist that covers multiple aspects of Urban Design, such as transport, development density, housing...etc as shown in Figures 24 and 25.

Figure 24: Shows the Checklist in Bristol’s local urban design guide.

(Urban Living SPD,2018)

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Figure 25: Shows the Checklist in Cardiff’s local urban design guide.

(Liveable Design

Guide,2015)

Planned comparisons between the two guides revealed that in Bristol’s local guide there is no mention of Aspect Ratio or Building heights, whereas in Cardiff’s local guide it vaguely mentions building heights.

Furthermore, the survey found that both guides lack the climatic aspect of Urban Design that has recently been integrated into the National Model Design Codes’ features for a well-designed place as shown in Figure 27.

Figure 26: Shows the diagram of a well-designed place. (National Model Design Code, 2021)

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Hence, there is a need for each council to derive their own codes and local guides and incorporate sustainable or climatic sensitive attributes into their guides and checklists.

After surveying the National Model Design Codes’ guidance on high streets in town centres and carefully reviewing the two conditions; Aspect Ratio of 1:1 and street width between 1520 m, Aspect Ratios for Case 1 were calculated thereafter the street widths for Case 2 were modified accordingly, to best attempt to comply to the two conditions, the results are shown in Table 5 and Figure 28.

Table 5: Streets widths for the four streets in Case 1 and Case 2.

Street Width(s) Street Name

Cardiff Bristol

Average Building Heights (m)

Case 1- Original (m)

Case 2- Modified (m)

St Mary Street

12

28

15

The Hays

15

25

15

Broad Street

13

11

15

Merchant Street

11

18

15

Aspect Ratio 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 St Mary Street

The Hays Original

Broad Street

Merchant Street

Modified

Figure 27: Shows the Aspect Ratio values for all four streets in two different cases.

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It is important to highlight that in St Mary street, Broad street and Merchant street a 1:1 Aspect ratio could not be achieved as the street width values would fall below the recommended street width of 15-20m by the National Model Design Code. Hence, the research attempted to increase or decrease the Aspect Ratio valued to a close proximity of the 1:1 recommendation within the 15-20m range.

4.2

Analysis

In the following section, the results of the analysis software will be presented through two subsections; Rayman and IES VE. The section will firstly present the effects of Aspect Ratio on Sky View Factor, Tmrt values and PET values. Secondly, the effect of Aspect Ratio and Sky View Factor on Solar Radiation and Wind Flow within the urban canyons.

4.2.1 Rayman The results in Figure 29 and 30 indicate that SVF values increased in The Hays, Broad street and Merchant street while decreasing in Saint Mary street affecting the amount of sky visible at the central point of all four streets.

Sky View Factor 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 St Mary Street

The Hays Original

Broad Street

Merchant Street

Modified

Figure 28: Shows the SVF values for both cases in all streets.

48

Figure 29: Shows the Fisheye diagrams and SVF values for all four streets in the two cases.

49

Tmrt values were automatically calculated in Rayman and are presented in Figure 31, where it is evident that the Tmrt values have minimally decreased in The Hays and Merchant street.

Whereas St Mary street and Broad street present a unique case, where the Tmrt values for St Mary street minimally decreased on June 21st but drastically decreased on December 21st. On the other hand, the Tmrt values for Broad street increased during June 21st but drastically decreased on December 21st.

Tmrt Values (°C) 70 61.60

61.30

60 52.00

50.60

51.80

50 38.80

40 30

36.20

35.40

30.10 28.40

29.60 27.40

23.90

29.90 28.50

20 10.80 10 0 St Mary Street St Mary Street Original Modified

The Hays Original

The Hays Modified 21st June

Broad Street Original

Broad Street Merchant Modified Street Original

Merchant Street Modified

21st December

Figure 30: Shows the Tmrt values for all four streets in the two cases.

Consequently, Physiological Equivalent Temperature values were calculated using the Mean Radiant temperature (Tmrt) values, Air temperature, Relative Humidity, Wind Velocity and Cloud cover. The PET values are an important finding in the understanding of Outdoor Thermal Comfort. As shown in Figure 32, the PET values for St Mary street and The Hays have decreased, with the PET value for December 21st drastically decreasing in St Mary street.

As for Broad street, the PET values have increased on June 21st however decreased on December 21st. Contrariwise, PET values for Merchant street have increased in both June 21st and December 21st.

50

PET Values (°C) 24.40

25.00

20.00

22.30 18.60

17.90

15.00 11.30

11.70

12.10

12.30

10.00 7.00

6.70

6.90 4.10

5.00

3.50

3.90

Broad Street Broad Street Merchant Original Modified Street Original

Merchant Street Modified

1.30

0.30

0.00 St Mary Street Original

St Mary Street Modified

The Hays Original

The Hays Modified

21st June

21st December

Figure 31: Shows the PET values for all four streets in the two cases.

4.2.2 IES VE The results of the simulations run in IES VE found evidence of decreasing solar radiation exposure in St Mary street, The Hays and Merchant street during June 21st and December 21st. On the other hand, solar radiation exposure increased in Broad street as can be seen in Table 6, Figures 33 and 34.

Table 6: Solar Radiation values for all streets in the two cases.

Solar Radiation (kWh/m2) June 21st

December 21st

St Mary Street- Case 1

1.37 - 2.38

0.372 - 0.582

St Mary Street- Case 2

1.17 - 1.58

0.302 – 0.512

The Hays - Case 1

1.17 - 2.18

0.162 - 0.578

The Hays - Case 2

0.97 - 1.98

0.092 - 0.549

Broad Street - Case 1

0.97 - 1.37

0.092 - 0.440

Broad Street- Case 2

1.17 - 1.78

0.301 - 0.510

Merchant Street Case 1

1.17 – 2.38

0.301 – 0.580

Merchant Street- Case 2

0.77 – 2.18

0.162 – 0.510

51

Figure 32: Shows the solar radiation simulations for St Mary street and The Hays in the two cases.

52

Figure 33: Shows the solar radiation simulations for Broad street and Merchant street in the two cases.

53

Additionally, the wind flow simulations found a significant decrease in the wind speed in all three streets; St Mary street, The Hays and Merchant street. Whereas Broad street has shown a slight decrease in wind speed as can be seen in Figures 35 and 36.

Figure 34: Shows the wind flow simulations for all four streets in the two cases.

54

Wind Speed (m/s)

St Mary Street Wind Flow

The Hays Wind Flow

2.50

5.00

2.00

4.00

1.50

3.00

1.00

2.00

0.50

1.00

0.00

0.00 Original

Original

Modified

Broad Street Wind Flow 8.00

Modified

Merchant Street Wind Flow 7.00 6.00

6.00

5.00 4.00

4.00

3.00 2.00

2.00 1.00

0.00

0.00 Original

Modified

Original

Modified

Figure 35: Shows the wind flow analysis for all four streets in the two cases.

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4.3

Discussion

The research has examined how urban geometry parameters effect thermal variables in urban zones; specifically, Aspect Ratio and SVF’s effect on urban canyons in Cardiff and Bristol.

Moreover, the study has examined the effect of the recommended codes in the National Model Design Codes document on the thermal performance of urban canyons.

The results provided evidence that the recommended codes have had a significant effect on the thermal performance of the pre-selected urban canyons in Cardiff and Bristol. The results have demonstrated that by modifying the width of the street, whether increasing or decreasing, and affecting the Aspect Ratio and Sky View Factor has significant effects on the solar radiation exposure and wind flow.

As by minimizing the street width, the amount of visible sky is reduced causing a decrease in solar radiation exposure on the canyon and neighboring building facades, consequently effecting Tmrt values. Similarly decreasing the street width, reduces Aspect Ratio values and forms a narrower canyon that causes the wind speed to become lower. Therefore, PET values are reduced and the level of thermal stress is mitigated.

Similarly, by increasing the street width, the amount of visible sky is expanded causing an increase in solar radiation exposure on the canyon and neighboring facades, therefore effecting Tmrt values. An increase in street width, increases Aspect Ratio and forms a wider canyon allowing the wind speed to amplify. Consequently, causing an increase in PET values and thermal stress levels.

Drawing on Matzarakis, Mayer and Iziomon (1999) categorization of different temperature scales to climate conditions and thermal stress (Table 7), the results adhere to the proposed categorization and corroborates that the mitigation of the street widths and thus the Aspect Ratios of the pre-selected urban canyons on PET values for June 21st has enhanced thermal stress levels for all four urban canyons (Table 8).

Table 7: Temperature, climate conditions and thermal stress scale (Matzarakis, Mayer and Iziomon,1999)

Temperature Scale Below 4°C 4°C - 18°C 18°C - 23°C 23°C - 41°C Above 41°C

Climate condition Very cold conditions Slightly cool to cold Comfortable conditions Slightly warm to hot Very hot conditions

Thermal Stress Extreme cold stress Slight to strong cold stress Absence of thermal stress Slight to strong heat stress Extreme heat stress

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Table 8: Comparison of PET values against Matzarkis, Mayer and Iziomon’s temperature scale.

Street Name – Case X

June 21st

PET Temperature

December 21st

PET

Scale

PET

0.68

18.6 °C

Comfortable conditions

6.7 °C

Slightly cool to cold

0.80

0.41

17.9°C

Comfortable conditions

1.3°C

Very cold conditions

25 m

0.60

0.16

24.4°C

Slightly warm to hot

7.0°C

Slightly cool to cold

The Hays - Case 2

15 m

1.00

0.23

22.3°C

Comfortable conditions

6.9°C

Slightly cool to cold

Broad Street - Case 1

11 m

1.19

0.30

11.3°C

Slightly cool to cold

4.1°C

Slightly cool to cold

Broad Street - Case 2

15 m

0.80

0.83

11.7°C

Slightly cool to cold

0.3°C

Very cold conditions

Merchant Street - Case 1

18 m

0.61

0.52

12.1°C

Slightly cool to cold

3.5°C

Very cold conditions

Merchant Street - Case 2

15 m

0.73

0.65

12.3°C

Slightly cool to cold

3.9°C

Very cold conditions

Street

Aspect

Width

Ratio

Saint Mary Street - Case 1

28 m

0.43

Saint Mary Street - Case 2

15 m

The Hays - Case 1

SVF

PET Temperature Scale

57

On the other hand, the PET values for December 21st remained the same in The Hays, whereas it slightly enhanced in Merchant street. On the contrary, in Saint Mary street and Broad street the PET values deteriorated causing extreme cold stress.

The results corroborate with Lin et al (2017) and Nouri and Costa (2017) findings that low SVF values create cooler environments whereas high SVF values cause higher solar radiation exposure, hence more heat stress.

Conversely, the results contradict Ketterer and Matzarakis (2014) finding that shallow canyons with low Aspect Ratio values causes higher heat stress and thermal discomfort. As in Saint Mary street case 1 the Aspect Ratio was of a 0.43 value that is considered a shallow canyon, but had a PET values of 18.6°C that is considered thermally comfortable and is characterized by the absence of thermal stress.

Overall, the results and findings reveal that proposed threshold values recommended by the National Model Design Codes have had a positive effect on the Outdoor Thermal Comfort of the Urban canyons during June 21st, but a deteriorating effect during December 21st.

4.4

Summary

This chapter highlighted the following results; Aspect Ratio and SVF values have a significant impact on the solar radiation exposure and wind flow in urban canyons that directly impacts Tmrt values and PET Values. Furthermore, the findings and results corroborate with previous literature on the effects of SVF on Outdoor Thermal Comfort.

The proposed threshold values recommended in the National Model Design Codes; that are Aspect ratio 1:1 and street width 15 – 20 m for high streets in town centers, are difficult to achieve in some cases, specifically old town cores as they are characterized by low building heights.

Moreover, the proposed threshold values positively impact PET values during summer but negatively impacts the thermal comfort during winter as it causes a decrease in PET values.

58

5|

Conclusion

Urban geometry parameters, specifically Aspect Ratio, Sky View Factor and Orientation effect the solar radiation exposure and wind flow within Urban zones, that directly effect Air Temperature, Wind Velocity and Humidity. For example, Lower Aspect Ratios lead to higher SVF values that increases the amount of visible sky at any given point within an urban zone, hence increasing solar radiation exposure. Moreover, lower Aspect Ratio values characterize shallow canyons that increases the wind flow within the urban zone and consequently the wind velocity.

Highly dense urban zones, much like city centers are the most heat stressed as they are characterized by narrow streets; higher Aspect Ratios, and little vegetation. High Aspect Ratios cause extreme cold stress in urban zones making them thermally uncomfortable for pedestrians, as they limit solar radiation exposure that effects Mean Radiant Temperature values, thus Physiological Equivalent Temperature values (PET).

The proposed threshold values in the National Model Design Codes are found to have a significantly positive impact during summer (June 21st) on the pre-selected urban canyons in Cardiff and Bristol, as can be deduced through the generated PET values. Whereas, during winter (December 21st) they are found to cause extreme cold stress.

The research was limited to walkable streets, streets of N-W orientation and cities with similar geographical latitudes, making generalization difficult as Outdoor Thermal Comfort is a case-by-case assessment.

The research recommends that further studies should be carried during the winter season to further validate the effect of the proposed threshold values on urban canyons. Additionally, in old city center cores, building heights are lower than new build, hence compliance to the two proposed threshold values; an Aspect Ratio of 1:1 and a street width of 15-20 m is difficult to achieve. It is recommended to either create a new category; High streets in Old Town cores or alter the recommended widths to have a lower minimum value.

59

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