Methodology for microclimatic urban canyon design case study surrounding MUST university campus in Cairo

ABSTRACT

Climate change is a major issue all over the world which has a great impact on global warming especially in countries with hot arid climates like Egypt. With rapid urbanization, streets in Greater Cairo and new cities are mostly car-oriented, which increases the Urban Heat Island(UHI) and, hence, affects the overall outdoor thermal comfort. Looking at the urban development in recent years, almost 10 new private universities are in operation in Cairo’s new cities together with a number of large recreational projects where they lack the appropriate vegetation surrounding their borders which in turn increases the UHI effect, decreasing the walkability’s thermal comfort and in turn contributes poorly to the surrounding climate. This paper focuses on the urban canyons in streets surrounding universities as they usually have high traffic rates in terms of both vehicular and pedestrian flows. This large flow of pedestrians necessitates taking into consideration the thermal comfort of the users. MUST University in the Al-Motamayez district on the 6th of October city was chosen as a case study. This case study is considered the basis for achieving an understanding and developing a methodology approach for such studies by using Envi-Met software for the simulation of different scenarios focusing on choosing trees appropriate to the climate zone of the city. Results for the simulation of suggested scenarios of intervention show a 30% improvement toward thermal comfort, which could be considered a first step for future guidelines development toward achieving thermal comfort and decreasing UHI.

KEYWORDS:

• Envi-met
• microclimate
• urban street canyon
• outdoor thermal comfort
• urban heat island
• trees and vegetation
• walkways
• walkability

Introduction

The UHI effect is a well-known phenomenon in urban areas, where the temperature of the air can be significantly higher than in rural areas. This is because urban areas include a lot of dark, heat-absorbing materials such as concrete and asphalt, which trap heat during the day and release it at night. This can lead to a significant increase in the temperature of the air, which can have a number of negative impacts on human health and well-being [1].

One of the most significant impacts of the UHI effect is on outdoor thermal comfort. When the temperature of the air is high, it can be difficult for people to stay cool, which can lead to heat exhaustion and even heat stroke. This is especially true for people who are elderly, young or have underlying health conditions [1]. While it is important to achieve thermal comfort in streets as they are considered an important part of the public space [2], walkways surrounding university campuses are most important to study in terms of achieving thermal comfort due to the regular high-density of pedestrians. This paper targets surrounding streets to universities in new cities in hot arid climates, focusing on developing a methodology for these street canyons’ design in an attempt to approach better thermal comfort and enhance walkability on a small-scale street segment as a case study while decreasing the collective UHI effect on a larger scale if applied.

Literature review

Urban canyons

Street canyons initially consist of streetscape including the horizontal and vertical surfaces within the space. Two of the main factors found to have a great impact on the street canyon are the height-to-width (H/W) ratio and street orientation [3]. Results from a previous study showed that 1.5 is the optimal aspect ratio due to the building’s increased shading; although, regardless of the street canyon aspect ratio, the streets’ orientation toward N-S tends to achieve better thermal comfort levels [2].

Pedestrian walkways

Pedestrian walkways are outdoor public spaces used by people to move on foot from one place to the other. To achieve their role efficiently with high quality, walkways must achieve an adequate microclimate with appropriate thermal comfort for users. Shading is one of the most important aspects that encourage people to use the walkways, especially in countries with hot arid climates [4].

Urban greenery

Although the urban form and geometry within the urban canyons with its streetscape consisting of horizontal and vertical surfaces could have a great impact on the microclimate of outdoor spaces [5], urban greenery has proved to increase the Thermal Comfort (TC) and cooling effect for the users of outdoor spaces [3].

Urban greenery and vegetation offer numerous benefits to the environment, such as carbon storage [6], reducing air pollution, and acting as urban biodiversity habitat [7].

Urban greenery could include different kinds of vegetation varying between trees, shrubs, vegetated facades, ground cover vegetation, and planters [3]. Among these, trees are considered one of the most suitable vegetation elements to be used in hot arid climates (such as Cairo’s climate) as they provide solar irradiance reduction as well as act as shading devices that help moisturize the air and enhance the thermal comfort and thus the pedestrian experience [8].

Trees play a key role in microclimate regulation, especially in cities where Urban Heat Island (UHI) effects are mostly felt just like the case in the Greater Cairo Region. Tree microclimate is mainly due to radiation intercepted and evapotranspiration contributing to modifying the heat balance of the surrounding environment. Radiation interception is owed to canopy prevention of short- and long-wave radiation from the upper hemisphere, whereas evapotranspiration is owed to water content carrying capacity of the soil – tree – air system [9].

A 2014 study found that street trees can alter microclimate on pedestrian walkways, with tree species based on the Plant Area Index (PAI) determining microclimatic and PET benefits, aiding urban planners and landscape professionals in selecting suitable tree species (Index Suzana, Denise Johnstonea, Peter Maya, Stephen J. Livesley School-2016).

Previous studies have shown that the spatial arrangement of trees along the street layout and their pattern in relation to the edge buildings vary in the impact on the urban street canyon. This can be manifested in different scenarios like the spacing between each tree and the other, the placement of the tree in accordance with the street side buildings, and whether to place the trees in the middle of the building or at the edges of the building [8].

In addition, the arrangement of the trees within the urban space varies according to their size and shape as well as the properties of the trees themselves including their species, age, and canopy coverage [3].

Envi-Met software

Envi-Met is a software used for environmental analysis that one can use in order to assess an environmental situation with certain input parameters such as the location, simulation day and duration, climate, and temperature, while changing design scenarios aiming at achieving better results in terms of thermal comfort and other criteria based on choice.

Urban trees can be assessed and selected for specific locations through foliage modeling in Envi-met.

The combination of biometeorological outputs of Envi-Met gives a deep understanding of climate at the urban canyon level [9].

Predicted Mean Vote (PMV)

PMV is an index that uses a seven-point thermal sensation scale aiming at predicting the occupant’s/users’ mean value of votes. The middle of the scale (0) represents the neutral state attaining the best thermal comfort while (−5) is considered too cold and (+5) is considered too hot. (−1) to (+1) is considered the comfort thermal scale. An occupant/user could feel a thermal equilibrium when the internal heat production is equal to its heat loss. There are several factors that can affect the heat balance of people, including cloth insulation, physical activity in addition to thermal environment parameters [10].

Methodology and process

This paper aims to prove how urban canyon greenery and tree additions affect microclimatic thermal comfort, while the main goal is to find out the optimum methodology (criteria) in assessing and finding suitable proposed interventions to the street urban canyon walkways in order to achieve an improved microclimatic thermal comfort for the pedestrians which enhances the walkability in the surroundings of universities.

The process of achieving this goal is first of all, collecting data for the surrounding of five different new universities on 6^th October city in Cairo in order to be able to choose one urban street canyon as a case study based on the following aspects for data collection:

• Types of the surrounding streets

• Types of the surrounding land uses

Proposed locations to study

By surveying the existing private universities in the 6^th of October City, five universities were chosen with different circumstances to start analyzing each of their surroundings and eventually pick one that has the potential of being an example to follow.

As shown in Figure (1), the first university is a university, which is located on the edge of a highway and surrounded by commercial landmarks to its west and east as well as residential complexes to its north and south. The second one is MUST University which is located on the edge of a highway on one side and a main street on the other side, while surrounded by residential complexes to its east and south in addition to an extension to the campus to the north. The third one is the 6th of October University, which is located on the edge of a landmark square and surrounded by residential complexes to its south, north, and west in addition to commercial buildings to the east. The fourth university is MSA University, which is located on the edge of a highway and surrounded by residential complexes to its north. The fifth and last one is Al-Ahram Canadian University, which is located in the industrial zone of 6th of October City and surrounded by industrial buildings from all directions.

Figure 1. Satellite view locating the researched proposed locations.

Chosen location

Based on a quick analysis of the different university sites proposed above; MUST University shown in Figure (2) was found to be the best case with a high potential to add scenarios for intervention and achieve a comprehensive methodological study. This was due to the presence of an empty plot in front of the campus to the opposite side of the main street, which could benefit from such methodology and give more space for scenarios of intervention as well as its location in the heart of the 6th of October city in a lively area allowing more exposure to passersby.

Figure 2. The chosen location shows the existing buildings and the street canyon to be studied.

Case study process

Field visits and observations were conducted in order to document the current situation forming the base case for the study. Data gathering from the urban street canyon includes the following:

• Trees types

• Trees spacing

• Asphalt percentage in the street

• Greens percentage in street

• Pavement percentage in street

• Street geometry

This step is followed by analyzing the data in order to understand the most common features of designing urban canyons in this area, then study how it affected the microclimate of the area, then simulate the designed microclimatic scenarios by using Envi-Met software for these simulations. Envi-Met was chosen as it is a software that uses a holistic approach in which all the different aspects of the microclimate are linked and simulated together in one model while simultaneously considering the number of processes between elements. That makes it extremely helpful to know the effect of each scenario and proposed solution as well as the effect of each street urban canyon element on the microclimate producing a strong real-time database and providing the opportunity to analyze the outcome data and compare it with the present situation (base case) which enables the achievement of aspired methodology to assess the microclimate targeting to improve the microclimatic thermal comfort.

For the best representation of the case study, the day chosen to apply the above-mentioned analysis and simulation on Envi-Met is the 21st of June, as it represents one of the hottest days of the year for the weather conditions in terms of temperature and solar radiation shown in Figure (3).

Figure 3. Analysis of weather data (temperature and direct solar radiation hourly average) on June 21st.

Source: Retrieved from climate.onebuilding.org

The graph shows the temperature as follows:

• 7 AM has one of the lowest temperatures during the day (24 °C).

• 12 PM has a high temperature (30°C) representing the average.

• 3 PM has the highest temperature of the day (33°C) representing the peak.

Analysis and results

Site observation and analysis

Based on the observation of the existing situation of the chosen site, Figure (4) represents the observation outputs and street canyon analysis showing that the total street width is 22.25 m with a 1 m sidewalk to the left, a 1.1 m sidewalk to the right, and a 1.9 m width island sidewalk. The street was found to be oriented toward the north which, according to the literature review, is considered a good orientation. As for the percentages of asphalt and pavement on the site, they were found as follows: 62.5% Asphalt and 37.5% pavement

Figure 4. The chosen site’s street canyon.

The existing trees on the site were observed and documented in terms of their different types and properties in addition to the distances between them. They were identified and classified based on the following aspects shown in Table (1):

• Existing height

• Maximum growth height

• Existing circumference

• Existing diameter

• The maximum circumference of the tree trunk

• Maximum crown breadth

Table 1. List of observed existing trees.

Vegetation types observed
	A	B	C	D	E	F
Taken photo on site
Name extracted	Phoenix dactylifera L. Date palm Arecaceae	Albizia julibrissin Durazz. Mimosa Fabaceae	Acacia melanoxylon R.Br. Australian Blackwood Fabaceae	Ficus benjamina L. LC Weeping fig Moraceae		Ficus microcarpa L.f. LC Chinese banyan
Existing Height	2.56 m	3 m	3.25 m	3.8 m
Max. growth height	up to 30 m	3–16 m	4–6 m and up to 20 m	12–30 m	2–6 m	up to 12 m
Existing Circumference				55 cm
Existing Diameter				17 cm
Max. circumference of tree trunk	40–60 cm	50 cm to 1 m	approximately 150 cm
Max. Crown breadth	ranges from 6–10 m	6–15 m	5–8 m	7–30 m	2–4 m	up to 12 m

Proposed scenarios

Scenarios of intervention were proposed and applied incrementally via Envi-Met simulations in an attempt to achieve better thermal comfort as follows:

1- The first scenario included adding trees to the street island and along the opposite side of the university fence, as shown in Figure (5).

Figure 5. Scenario (1).

2- The second scenario, in addition to the trees, included adding grass on the street island as well as adding more trees along the opposite side of the university fence, as shown in Figure (6).

Figure 6. Scenario (2).

Results and discussion

The chosen site was first assessed for the current situation using the Envi-met software. The current site conditions representing the base case were inserted in the software and a simulation was run to generate maps that further indicate the climatic situation which could guide us to see what needs to change. Analysis and simulation were applied at three different timings along the day as shown in Figure (7); 7 am, 12 pm, and 3 pm; on 21th of June as it represents one of the hottest days of the year in terms of weather.

Figure 7. PMV simulations of base case as well as Scenarios 1 and 2.

Scenario (1) includes adding trees to the street island and along the opposite side of the university fence. The simulation was applied following the same logic and timings as the base case simulation. Results show some improvement in the PMV and hence the thermal comfort.

Scenario (2) includes adding grass on the street island as well as adding more trees along the opposite side of the university fence. Simulation was applied following the same logic and timings of the first two simulations. Results show further improvement in the PMV and hence the thermal comfort.

Figure (8) represents a comparison of applied simulations between the base case and the two proposed scenarios during the day showing that, at 7 am, while applying the first scenario for intervention (adding trees) improved the PMV value and hence the thermal comfort, the second scenario of intervention (adding more trees and grass) did not result in further improvement as the temperature is considered to be one of the lowest during the day (24 °C) with low solar radiation which caused a stabilization in the level of thermal comfort improvement.

Figure 8. PMV comparison for simulations applied on 21st June.

As for the second assessed timing, results show that, at 12 pm, while applying the first scenario for intervention (adding trees) improved the PMV value, a further remarkable improvement resulted from applying the second scenario of intervention (adding extra trees and grass). This is due to the temperature at this time of the day that just starts to rise (30°C) while representing the peak of the solar radiation. Hence, the results show that the simulation for assessment at 12 pm has the highest impact on improved PMV value and thermal comfort as it represents the average temperature with the highest solar radiation, so the asphalt just started to absorb heat but did nt yet release it back.

The results for the third and last assessed timing show that, at 3 pm, while applying the first scenario for intervention (adding trees) improved the PMV value, only little improvement resulted from applying the second scenario of intervention (adding extra trees and grass). This is due to the high temperature at this time of the day (33°C), while the solar radiation starts to decrease again. This causes the asphalt to release a lot of heat which makes any improvement in thermal comfort harder to achieve.

Conclusion

This study focused on the urban street canyon adjacent to the MUST University fence with the aim to improve the microclimate in the urban space surrounding the campus. Field visits were conducted in order to observe the urban space components with their actual measurements and properties. Assessment of the current situation using Envi-Met software was the first step to having a base case to build. The next step was then to simulate the proposed interventions using Envi-Met software as well in order to be able to assess their impact on the microclimate at different times of the day.

Findings reveal that the largest impact on the microclimate is at 12 pm as it is considered the average temperature with the start of the increase in solar radiation, so the asphalt just started to absorb heat but did not yet release it back. While at 7 am, the temperature and solar radiation are still a bit low which makes the impact of the interventions minimal and at 3 pm, the very high temperature and start of the social radiation decrease, and the impact of the interventions is not high because of the large amount of released heat from the asphalt during this time.

Based on the above results, scenarios of intervention proved a 30 % improvement toward thermal comfort which means that these interventions could be considered as a first step toward developing an applicable guideline for achieving thermal comfort in urban street canyons and decreasing UHI.

This case study along with its results forms the basis for the development of a methodology that is to be adopted for the assessment and analysis of urban spaces including pedestrian walkways in an attempt to reach an improved Predicted Mean Vote (PMV) value toward thermal comfort.

The following Table (2) represents the framework for this methodology designed with a checklist to facilitate and enhance its usability.

Table 2. Methodological framework checklist.

Steps done to reach an improved PMV value toward thermal comfort
	Phase 1: Site Visit	✔
1	Measuring the street elements: Asphalt width + length (vehicular area) Pavement width + length (pedestrian walkability area) Island pavement width + length (pedestrian walkability area)	✔
2	Acquiring the building heights on both sides of the street.	✔
3	Measuring the heights of poles and the distance between each one.	✔
4	Plotting vegetation elements and types.	✔
5	Measuring the distance between trees.	✔
6	Measuring the trees’ height, trunk diameter, and crown diameter.	✔
	Phase 2: Plotting and Simulating	✔
1	Plotting the street using any adequate software. (Revit was used in this study)	✔
2	Determining the common tree types observed to validate the adequacy of use (from a shading and maximum growth point of view.) See attached table next.	✔
3	Inserting the plotted street into an environmental analysis software to analyze the current climatic state. (AutoCAD form or Google Earth, Google Earth and Envi-met were used in this study)	✔
4	Extracting and analyzing the current values of PMV, MRT, and Wind Speed to discern an accurate current climatic condition. (The hottest day of the year was chosen, plotting three times of the day as control points)	✔
5	Suggesting scenarios to be added in order to improve the observed discomfort. (Envi-met was used)	✔
6	Analyzing the results, calculating the average for all the values acquired for the times and simulations done.	✔
	Phase 3: Concluding the outcome.	✔

Disclosure statement

No potential conflict of interest was reported by the author(s).