UHI and Its Mitigation

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CONTENT

LIST OF FIGURES

LIST OF PLATES

CHAPTER 1- INTRODUCTION

INTRODCUTION

INTENT

NEED

AIM

OBJECTIVE

METHODOLOGY

RESULTANT ISSUES

RESEARCH IDEAS AND ANALYSIS

CHAPTER 2- URBAN HEAT ISLAND BASICS

2.1 SURFACE URBAN HEAT ISLANDS

2.2 ATMOSPHERIC URBAN HEAT ISLANDS

2.3 RELATION OF SURFACE AND AIR TEMPERATURES

2.4 FORMATION OFURBAN HEAT ISLANDS

2.4.1 REDUCED VEGETATION IN URBAN AREAS


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2.4.2 PROPERTIES OF URBAN MATERIALS

2.4.3 URBAN GEOMETRY

2.5 EMERGENT ISSUES OF URBAN HEAT ISLANDS

2.5.1 ENERGY CONSUMPTION2.5.2 AIR QUALITY AND GREENHOUSE GASES2.5.3 HUMAN HEALTH AND COMFORT2.5.4 WATER QUALITY

CHAPTER 3- PRIMARY CASE STUDY- URBAN HEAT OF A RESIDENTIAL AREA

CHAPTER 4- OPTIMISING HEAT GAIN BY BUILDING MATERIALS

THROUGH LANDSCAPE ELEMENTS

4.1 INTRODUCTION

4.2 MANAGEMENT OF UHI THROUGH LANDSCAPE

4.2.1 REGIONAL LEVEL MANAGEMENT OF URBAN HEAT ISLAND

4.2.2 SITE LEVEL MANAGEMENT OF URBAN HEAT ISLAND

4.3 SUMMARY

CHAPTER 5- OPTIMISING HEAT GAIN BY BUILDING MATERIALS


5.1 ENVIRONMENTAL BENEFITS OF TREES

5.2 ECONOMIC BENEFITS OF URBAN TREES

5.3 HEALTH BENEFITS OF URBAN TREES

5.4 SOCIAL BENEFITS OF URBAN TREES

CHAPTER 6- CONCLUSION


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EFFECT OF URBANIZATION AND IMPORTANCE OF OPEN SPACES IN A CITY

DISSERTATION

FINAL ROUGH DRAFT

MBS SCHOOL OF PLANNING AND ARCHITECTURE

DISSERTATION GUIDE DISSERTATION CO-ORDINATOR NAMEOF THE STUDENT

AR. NEHA JHARIA PROF. ASHOK GROVER AYUSH AHUJA


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CHAPTER 1

1.1 Introduction

As more and more people leave villages and farms to live in cities,urban growth results. The rapid growth of cities like, Chicago in the late

19th century, Tokyo in the mid twentieth, and Mumbai in the 21stcentury, can be attributed largely to rural-urban migration. This physicalgrowth of urban areas as a result of rural migration and even sub-urban

concentration into cities is known as Urbanization . 1

The rapid urbanization of cities has led to the replacement of landscapesand vegetation with roads, buildings and other types of infrastructure.

Surfaces that were once permeable and moist become impermeable anddry. 2 This development leads to the formation of urban heat islands---thephenomenon whereby urban areas experience warmer temperatures than

their rural surroundings.

1.2 Intent

As there is rapid urbanization in different cities of the world, the openareas and vegetation is decreasing and there is rise in temperatures of

urban areas than their surrounding rural areas. Trees and other plantshelp cool the environment, making vegetation a simple and effective wayto reduce urban heat islands. Trees and vegetation lower surface and air

temperatures by providing shade and through evapotranspiration . 3 Todecrease the temperatures and also, to reduce the urban heat island

effect in an urbanized city, the need of open spaces is felt. In a rapidlyurbanizing area stress should be laid on increasing the percentage of

open areas to that off built up area, also keeping in count of the

percentage of vegetation that plays the major role in decreasing thetemperatures of that particular urbanized area.

1.3 Need

The study of the quantity of open spaces as compared to the quantityof built up of an urbanized area helps in providing the amount of heat

that the particular area generates, as compared to its rural surroundings.As a result it will provide the amount of open space required to bring

1 www.wikipedia.org2 www.epa.gov/hiri/resources/pdf/BasicsCompendium.pdf 3 www.epa.gov/hiri/mitigation/trees.htm
http://en.wikipedia.org/wiki/Chicagohttp://en.wikipedia.org/wiki/Tokyohttp://en.wikipedia.org/wiki/Mumbaihttp://www.epa.gov/hiri/resources/glossary.htm#Evapotranspirationhttp://www.wikipedia.org/http://www.epa.gov/hiri/resources/pdf/BasicsCompendium.pdfhttp://www.epa.gov/hiri/mitigation/trees.htmhttp://en.wikipedia.org/wiki/Chicagohttp://en.wikipedia.org/wiki/Tokyohttp://en.wikipedia.org/wiki/Mumbaihttp://www.epa.gov/hiri/resources/pdf/BasicsCompendium.pdfhttp://www.epa.gov/hiri/mitigation/trees.htmhttp://www.epa.gov/hiri/resources/glossary.htm#Evapotranspirationhttp://www.wikipedia.org/


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the temperature down of the urbanized city. The vegetation required tobring the temperature down not only decreases the temperature, but

also:

Improves urban soil conditions

Improves the functioning of hydrological cycle in cities

Increases the diversity and quantity of wildlife in cities

Reduces the levels of atmospheric pollution in urban climates. 4

1.4 Aim

To study the impact of heat islands with the help of secondary case

studies and analyse measures to counter the effect.

1.5 Objective

To study the impact of urbanization over the microclimate of anurbanized space.

To study the formation and causes of urban heat islands.

To study, with the help of secondary case studies, the impacts of UHIeffect.

To provide key strategies to mitigate urban heat island effect.

4 www.wikipedia.org
http://www.wikipedia.org/http://www.wikipedia.org/


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1.6 Methodology

Need for

Study effect of urbanization

on an urbanized area.Analyzing the urban heatisland effect of a city, withhelp of existing information.

Identifying the

Providing key strategies to counter UHI


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1.7 Resultant Issues

The size or number of open spaces cannot be increased in a developedcity area.

Since trees and plants cannot grow in all climates, the specie of thetree or plant will have be kept in mind.

The maintenance of the open areas and plants or trees will also be anissue.

1.8 Research Ideas

Due the rapid urbanization of cities the built up area has increased ascompared to the open areas, as a result the microclimates of the

urbanized areas have gone up. The incorporation of more and more openareas in the citys master plan, also providing open spaces with a

suitable amount of vegetation in them will help in lowering thetemperatures of the urbanized city area.

The incorporation of vegetation in open spaces has many advantageswhich are:

Reduced energy use: Trees and vegetation that directly shade buildingsdecrease demand for air conditioning.

Improved air quality and lowering greenhouse gas emissions: By reducingenergy demand, trees and vegetation decrease the production of

associated air pollution and greenhouse gas emissions. They also removeair pollutants and store and sequester carbon dioxide.

Enhanced storm-water management and water quality: Vegetation

reduces runoff and improves water quality by absorbing and filteringrainwater.

Reduced pavement maintenance: Tree shade can slow deterioration of street pavement, decreasing the amount of maintenance needed.

Shading in parking lot medians can provide extensive shading coverage.

Improved quality of life: Trees and vegetation provide aesthetic value,habitat for many species, and can reduce noise. 5

5 www.epa.gov/hiri/mitigation/trees.htm
http://www.epa.gov/hiri/mitigation/trees.htmhttp://www.epa.gov/hiri/mitigation/trees.htm


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CHAPTER 2

Urban Heat Islands

Introduction

Many urban and suburban areas experience rise in temperaturescompared to their surrounding rural areas; this difference in temperature

is what constitutes an Urban Heat Island. The annual mean airtemperature of a city with one million or more people can be 1 to 3C

warmer than its surroundings, and on a clear, calm night, thistemperature difference can be as much as 12C. 6 All cities and towns,

irrespective of their size, produce heat islands, though the effect oftendecreases as city size decreases.

2.1 Surface Urban Heat Islands

On a hot, sunny summer day, the sun can heat up, exposed urbansurfaces, like roofs and pavement, to temperatures 27 to 50C hotter

than the air, while shaded or moist surfacesoften in more ruralsurroundingsremain close to air temperatures. 7 Surface urban heat

islands are typically present day and night, but are always strongestduring the day when the sun is shining.

On average, the difference in daytime surface temperatures betweendeveloped and rural areas is 10 to 15C whereas the difference in night

time surface temperatures is typically smaller, at 5 to 10C.

The magnitude of surface urban heat islands varies with seasons, due tochanges in the suns intensity as well as ground cover and weather. As

a result of such variation, surface urban heat islands are typically largestin the summer. 8

2.2 Atmospheric Urban Heat Islands

Warmer air in urban areas compared to cooler air in nearby ruralsurroundings is a phenomenon defined as atmospheric urban heat

islands. These heat islands are divided into two different types:

6 7 8


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Canopy Layer Urban Heat Islands are the ones which exist in the layerof air where people live, i.e., from the ground to below the tops of trees

and roofs. (Figure 1)

Boundary Layer Urban Heat Islands start from the rooftop and treetop

level and extend up to the point where urban landscapes have noinfluence on the atmosphere. (Figure 1)

Atmospheric urban heat islands are often weak during the late morningand throughout the day and become more intense after sunset due tothe slow release of heat from urban infrastructure. The timing of this

peak, however, depends on the properties of urban and rural surfaces,the season, and prevailing weather conditions.

Atmospheric heat islands vary much less in intensity than surface heatislands. On an annual mean basis, air temperatures in large cities might

be 1 to 3C warmer than those of their rural surroundings. 9

Figure 1

Source: www.epa.gov/hiri/resources/pdf/EPA_How_to_measure_a_UHI9


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Above : Factors contributing to formation of Urban Heat Islands.

Mesoscale : Shows the effect of UHI in the entire city and itssurrounding rural areas; also the formation of Urban Boundary Layer

(UBL).

Microscale : Shows the effect of UHI at micro level in a city, also theformation of Urban Canopy Layer (UCL).

2.3 Relation of Surface and Air Temperatures

Surface temperatures have no direct, but significant, influence on airtemperatures, especially that are close to the surface called the urban

canopy layer. For example, open areas like; parks and vegetated areas,

which typically have cooler surface temperatures (because of evapotranspiration occurring in such areas), contribute to cooler air

temperatures. Dense, built-up areas, on the other hand, typically lead towarmer air temperatures, because air mixes within the atmosphere,

though, the relationship between surface and air temperatures is notconstant, and air temperatures typically vary less than surface

temperatures across an area.

2.4 Formation of Urban Heat Islands

2.4.1 Reduced Vegetation in Urban Areas

In rural areas, vegetation and open land typically dominate thelandscape. Trees and vegetation provide shade, which helps lower surfacetemperatures. They also help reduce air temperatures through a process

called evapotranspiration, in which plants release water to thesurrounding air, dissipating ambient heat. Whereas in urban areas the

surface is characterized by dry, impervious surfaces, such asconventional roofs, sidewalks, roads, and parking lots. As cities develop,

more vegetation is lost, and more surfaces are paved or covered withbuildings. The change in ground cover results in less shade and

moisture to keep urban areas cool. Built up areas evaporate less water,which contributes to elevated surface and air temperatures. 10

10


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Figure 2: Impervious Surfaces and Reduced Evapotranspiration

Source: (Reducing Urban Heat Islands: Compendium of Strategies Urban HeatIsland Basics)

Highly developed urban areas (left), which have more percentage of impervious surfaces, have less surface moisture available for evapo-

transpiration than natural ground cover, which has lesser imperviouscover (right). This characteristic contributes to higher surface and air

temperatures in urban areas, resulting in the formation of Urban HeatIslands.

2.4.2 Properties of Urban Materials

Properties of urban materials, in particular solar reflectance (the ability of a material to reflect light), thermal emissivity (the ability of the surface

of a material to emit energy in the form of radiation), and heatcapacity, also have an effect on urban heat island development, as they

determine how the suns energy is reflected, emitted, and absorbed.

Solar energy is composed of ultra-violet (UV) rays, visible light, andinfrared energy, each reaching the Earth in different percentages: five

percent of solar energy is in the UV spectrum, including the type of raysresponsible for sunburn; 43 percent of solar energy is visible light, incolours ranging from violet to red; and the remaining 52 percent of

solar energy is infrared, felt as heat. Energy in all of these wavelengths

contributes to urban heat island formation. 11

11


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Solar reflectance, or albedo, is the percentage of solar energy reflectedby a surface. It has been found that much of the suns energy is foundin the visible wavelengths, so, solar reflectance has a mutual connectionwith a materials colour. Darker the surface, it will tend to have lower

solar reflectance values than that of surfaces with lighter colour.

Urban areas have surface materials for roofing and paving which have alower albedo than those in rural areas. As a result, built up areas

generally reflect less and instead absorb more of the suns energy. Thisabsorbed heat increases surface temperatures and contributes to the

formation of surface and atmospheric urban heat islands.

But, solar reflectance is the main factor that determines a materialssurface temperature, thermal emittance, also plays a role in determininga materials surface temperature. Thermal emittance is a measure of a

surfaces ability to release energy in the form of heat. But the mostimportant thing is that, surfaces with high emittance values stay cooler,

because they release heat more readily. Most construction materials, withmetal being an exception, have high thermal emittance values, as a

result, this property makes metal it of great interest to those installingcool roofs.

Another important property that affects heat island development is amaterials heat capacity, which means the ability of a material to storeheat. For eg: Many building materials, such as steel and stone, have

higher heat capacities than rural materials, such as dry soil and sand. So,it has been found that, cities are typically more effective at storing the

suns energy as heat within their infrastructure. City centres of metropolitan areas can absorb and store twice the amount of heat

compared to their surrounding rural areas during the daytime.


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Image showing varying temperatures in a city and its surroundings as a

result, making cities an Urban Heat Island.

Source: www.Ecofinroofgarden.co.uk

2.4.3 Urban Geometry

An additional factor that has an effect on urban heat islanddevelopment, particularly at night, is urban geometry, which refers to

the dimensions and spacing of buildings within an urbanised area. Urbangeometry has effects on wind flow, energy absorption, and a surfacesability to return long-wave radiation back to space. In developed areas,

surfaces and structures are often at least partially obstructed by objects,such as neighbouring buildings, and become large thermal masses that

cannot release their heat very effortlessly because of these obstructions,making the air above urbanised areas typically warmer than air over

rural areas, especially at night (because, urban materials release most of


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their absorbed heat at night). As a result, the night time atmosphericheat islands can have serious health problems for urban residents during

summers.

Urban canyons are narrow streets formed by lined tall buildings. During

the day, urban canyons can have tremendous effects. On one hand, tallbuildings create shade, reduce surface and air temperatures whereas on

the other, when sunlight reaches surfaces in the canyon, the sunsenergy is reflected and absorbed by building walls, which further lowers

the citys overall albedo, i.e., the net reflectance from surface albedoplus urban geometryand can increase temperatures. At night, urban

canyons generally hinder cooling, as these buildings and structuresobstruct and trap the heat that is being released from urban

infrastructure. Urban Canyons also alter air quality , where locallystagnant air concentrates pollutants near ground level creating health

discomforts.

2.4.4 Weather

Winds and clouds have a major influence on the formation of heat

islands. Under the calm and clear sky weather conditions the intensitiesof heat islands are high. During the period of winds blowing the hot air

on the surface mixes up with the wind and gets lowered and in the

presence of clouds (when the clouds are increasing), they reduce the

radiative cooling at night, reducing the heat island magnitudes. So, as

there is variations in the patterns of season there is change in the

frequency and magnitude of heat islands.

2.4.5 Geographic Location

The factors like climate and topography of an area and its rural

surroundings affect the formation of heat islands. For example: When

there are water bodies, of cooler temperatures, close to an area they,

generate winds that help in transporting the heat away from that area,

thus affecting the intensity of heat island. Also, in case of cities closerto mountains, the wind is either blocked by the mountains in the city
http://en.wikipedia.org/wiki/Air_Quality_Indexhttp://en.wikipedia.org/wiki/Air_Quality_Index


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or wind patterns are created that pass through that city, thus, local

terrain of a city has a major influence in the formation of heat islands.

2.4.6 Time of Day or Season

Seasons play a role too in affecting the intensity of heat islands. In

case of cities located in tropical climates, the dry season favours large

heat island magnitudes. Cities with multiple seasons have higher heat

island intensities during summer and lower during the monsoon seasons.

CHAPTER 3

Impacts of Heat Islands

3.1 Impacts of Urban Heat Islands

The increase in temperatures by urban heat islands, particularly during

the summer, results in majorly affecting a citys environment and quality

of life. It has been noted that some heat island impacts are positive,

i.e., such as increasing the plant-growing season, but, most of the

impacts are negative that include the following:

Increased energy consumption (negative during summers) (positive

during winters).

Increased emissions of air pollutants.

Compromised human health and comfort.

Impaired water quality.

3.1.1 Increased Energy Consumption

Increased summertime temperatures in cities increase energy demand for

during peak periods of demand, which generally occur on hot, summer


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weekday afternoons, when offices and homes are running cooling

systems, lights, and appliances. This peak urban electric demand

increases 1.5 to 2 percent for every 0.6C increase in summertime

temperature. 12 The increasing temperatures over the last several decades,

means that the increased demand for electricity by the urban population,is used to make up for the urban heat island effect. The increase in

energy consumption also has resulted in burning of more fossil fuels,

thus resulting in more carbon emissions.

3.1.2 Air Quality and Greenhouse Gases

As discussed in Section 2.5.1, higher temperatures can increase energy

demand, which generally causes higher levels of air pollution and

greenhouse gas emissions. As, most electricity is produced from burning

fossil fuels, thus, most power plants release pollutants like sulphur

dioxide (SO2), various nitrogen oxides, etc, so these pollutants have

been proven to be harmful to human health and contribute to major air

quality problems such as acid rain. Also, fossil fuel powered plants emitgreenhouse gases, for example: carbon dioxide (CO2), which is a major

contributor towards global warming.

3.1.3 Human Health and Comfort

Increase in daytime surface temperatures, reduction in night time

cooling, and higher air pollution levels due to the formation of urban

heat islands can affect human health as they cause general discomfort,

respiratory diseases, heat cramps and exhaustion, non-fatal heat stroke,

and heat-related mortality (Chicago, 1995).

The impact of heat waves can be worsened by these urban heat

islands. Heat waves are periods of abnormally hot, and often humid,

12


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weather. Sensitive populations, such as children, older adults, and those

with existing health conditions, are at particular risk from these events.

3.1.4 Water Quality

Surface urban heat islands result in degradation of water quality, mainly

by thermal pollution. Pavement and rooftop surfaces that reach tem-

peratures 27C to 50C higher than air temperatures transfer this excess

heat to storm water. When the rain comes before the pavement has a

chance to heat up, runoff temperatures from the rural and urban areas

are differed by less than 2C When this heated storm water generally

drains into storm sewers, it raises water temperatures as it is released

into big water bodies like, streams, rivers, ponds and lakes.

Change in water temperature affects all types of aquatic life, especially

the metabolism and reproduction of many aquatic species. Rapid

temperature changes in aquatic ecosystems resulting from warm storm

water runoff causes imbalance in the ecological system, which also is a

major worry to the earths inhabitants.


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CHAPTER 4

Case Study- DELHI

Source: Manju Mohan, Yukihiro Kikegawa, B. R. Gurjar, Shweta Bhati1,

Anurag Kandya, Koichi Ogawa, Delhi Experiments to Learn Heat IslandIntensity, May 2008.


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Table : Urban Heat Island (UHI) intensities during specific hours of variousplaces in Delhi:

S.no TimeDay of

May2008

Max.UHI (C)

WindSpeed

Warm pockets indecreasing order with

Name of station

1. 3:00 am 25 4.1 CalmBhikaji Cama Sitaram

Bazar Janak Puri

2. 3:00 am 26 4.2 CalmSitaram Bazar BhikajiCama Adarsh Nagar

3. 3:00 am 27 4.6 Calm- 0.6 Sitaram Bazar

Janak Puri

4. 3:00 am 28 5.6 CalmSitaram Bazar Bhikaji

Cama Adarsh Nagar Janak Puri

5. 9:00 am 25 4.6 Calm- 1.8Noida Lajpat Nagar

Rohini CP

6. 9:00 am 26 6.4 0.4- 1.3

CP Noida Badarpur

Sitaram Bazar ChiragDelhi

7. 9:00 am 27 5.1 Calm- 0.8 CP Noida

8. 9:00 am 28 5.3 Calm CP Noida

9. 3:00 pm 25 6.3 Calm- 0.4CP Bhikaji Cama Loni

Permanaud

10. 3:00 pm 26 3.8 0.5- 1.3Dwarka Majnu ka Tila

Bhikaji Cama

11. 3:00 pm 27 7.6 0.5- 1.5 CP Bhikaji Cama

12. 3:00 pm 28 6.7 Calm- 1.1 CP Loni Dwarka

13. 9:00 pm 25 2.8 0.5- 1.8Dwarka Bhikaji Cama

Sitaram Bazar

14. 9:00 pm 26 5.1 Calm

Sitaram Bazar Bhikaji

Cama Badarpur

15. 9:00 pm 27 4.2 Calm- 1.4 CP Bhikaji Cama


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16. 9:00 pm 28 8.3 CalmCP Bhikaji Cama

Badarpur

17.8:00 am

(pre rain) 26 3.7 0.3- 2.7 Dwarka Noida CP

18.10:00 am

(rain)26 2.2 0.9- 3.8

Moti Nagar SitaramBazar Sanjay Van

19.12:00 pm

(postrain)

26 2.5 0.5- 2.7Adarsh Nagar Dwarka

Rohini

Source: Urban Heat Island Assessment for a Tropical Urban Airshed inIndia, by, Manju Mohan 1 , Yukihiro Kikegawa, B.R. Gurjar, Shweta Bhati, AnuragKandya, Koichi Ogawa.


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CHAPTER 4

OPTIMISING HEAT GAIN BY BUILDING MATERIALS


4.1 Introduction

Urban areas present distinctive micro climates. The total transformationof natural landscape into houses, streets, squares, big public buildings,skyscrapers, and industrial installations has brought about changes in

climate of large cities. Temperature is one of the most importantcharacteristics of urban areas. It is known urban temperatures differ

from those of sub-urban and rural areas and the phenomenon commonlyknown as the Urban Heat Islands.

Mitigation and control of this Urban Heat Island effect is so importantfor the environmental sustainability of urban areas. The materials used inthe urban matrix play key role in causing this distinctive heat gain. The

aim of this paper is to relate the thermal conductivity of buildingmaterials to the urban heat gain and to find solutions to optimize the

heat gain by materials through landscape elements.

4.2 Management Of UHI through Landscape

Landscape comprises the visible features of an area of land, includingphysical elements such as landforms, living elements of flora and fauna,

abstract elements like lighting and weather conditions, and human

elements like human activity and the built environment.

UHI mitigation is possible by irrigated landscape treatmentsturf grassesand humid- region trees and shrubs. Evaporation from irrigated surfacescools the scorching daytime desert temperatures and thus prevents the

buildup of stored heat, a critical factor in the UHI. The challenge,however, in a desert city with limited water supplies lies in the tradeoffs

between the temperature-reduction properties of irrigated surfaces andthe water required to maintain them.

The management strategies can be categorized under:


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Management at regional level

Management at site level

4.2.1 Regional level management of urban heat island is possiblethrough the following methods:

Increase in open spaceregulation, as this would provide the followingbenefits : -

To meet positive Human needs recreation (physically andpsychologically).

To enhance and protect resource base air, water, soil, plants and fauna.

To meet social and cultural needs social gathering, cultural exchange.

Urban green pockets to be developed & existing greenery to be preserved.Encouraging urban forestry.

Road landscaping can be encouraged as this would enable the reduction inheat reflectance by the road surface.

Regulations to be framed on paved areas at the regional level (advising on thenature of material).

More community parks to be developed.

Water bodies in the regional scale to be preserved and landscaping canbe done to preserve their existence.

Industrial Zones of a city can be buffered with vegetation as this wouldreduce the UHI effect.

Possible reduction of concrete usage. Encouraging communityparticipation in reduction of UHI effect.

4.2.2 Site level management of urban heat island

Ground level

Vertical Surface


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Roof level

4.2.2.1 Ground Level

PAVERS(materials and

constructiontechniques that are

used in roads,driveways, parking

lots,sidewalks,

pedestrian waysand other hardsurfaces):

CoolPavements which

act to reduce the absorption, retention and emittance of solar heat.Landscape shading of paved and hardscape surfaces and use of high

reflective and porous materials, can significantly reduce the heat gain.Pervious and open grid materials such as pavers, stone, blocks andinterlocking concrete pavements with high- Albedo reflective materials

reduce heat absorption.


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Figure : Standard Pavement reflect sunlight and absorb and emit moreheat than cool pavements Source: (Brochure on the Use of CoolPavements to Reduce the Urban Heat Island Effect)

Cool Pavements mean materials and construction techniques that areused in roads, driveways, parking lots, sidewalks, pedestrian ways andother hard surfaces, which act to reduce the absorption, retention andemittance of solar heat, a factor contributing to the urban heat island.

Cool pavements utilize coloration, materials, porosity and processes thatincrease solar reflectivity to reduce surface heating and also to promote

cooling through increased air filtration and evaporation.

The landscape shading of paved and hardscape surfaces and use of highreflective and porous materials can significantly reduce the heat gain of

pavements by the sun. Light colored surfaces are more reflective thandarker. Pervious and open grid materials such as pavers, stone, blocksand interlocking concrete pavements with high-albedo reflective materialreduce heat absorption from the sun and result in lower emitted heat.

The use of cool paving materials helps to reduce the heating of roads,sidewalks and parking lots as a result of reduced heat absorption. Coolpavement can help to limit the impact of the heat island by reducingabsorption, retention and emitting of heat.

The combination of high albedo and pervious pavements are especiallywell suited for relatively light traffic flow areas such as driveways andparking lots while helping to mitigating the heat island effect andallowing storm water to pass through and permeate into the sub-baseand ground.

Research has found that cool pavements can help to reduce thesummertime heat island effect sufficiently to result in lower airtemperatures, improved air quality and improved quality of life of

residents. Large parking areas, terminals, air fields, urban roadways andlarge paved areas are especially suitable for cool pavement. Solarreflecants and permeable materials result in cooler pavement surfaces inthe summer. Cool pavements containing porous/permeable pavingpromotes cooling and evaporation and due to infiltration of rainfall,reducing runoff and the need for storm water retention.


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Figure : Cool Pavements reflect sunlight and absorb and emit less heatthan standard pavements Source: (Brochure on the Use of CoolPavements to Reduce the Urban Heat Island Effect)

Use of Cool Pavements and White Roofs to reduce the Urban Heat

Island Effect

An effort to decrease the urban heat island effect can help to producethe benefits of improving air quality, reducing air-conditioning costs andpromoting the health and comfort of city residents. Pavements, roads,driveways, parking lots, sidewalks and other hard surfaces may comprise33-35% of the land area of large urban centers. Efforts to reduce the

urban heat island effect primarily involve:

Use of urban landscape and vegetation to reduce direct sunlight onbuildings and hard-scape surfaces to provide shading and reduce heating,

Use of white roofs for commercial, industrial, institutional and someresidential buildings, where the use of a white surface on primarily flator low-sloped roofs reflects light and reduces heat gain, and

Use of cool pavements which also act to increase surface reflectantsand promote porosity to reduce absorption and retention of heat.


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Promoting greater reflectance of pavement can be achieved by:

a. Roller compacted concrete

b. Concrete over asphalt (white topping and ultra thin white topping)

c. Use of light colored aggregate in asphalt

d. Asphalt, concrete and pavers with modified colors

e. Porous and open grade pavements permitting water and air to passthrough to permit cooling by the air movement and evaporation.

f. Use of set in place material such as brick, stone, pavers, interlockingconcrete pavement, compacted decomposed granite and grass-crete and

gravel-crete or alternate high density high-polyethylene base grid forgrass and gravel.

Cool pavements have lower heating levels and emit their stored heatmore quickly during evening and early nighttime hours than standardpavement. The rapid cooling helps to reduce the nighttime heat islandeffect. Cool pavements can be achieved with placement of a layer of rubberized asphalt over the concrete road surface. Rubberized pavements

have been found to be cooler at night than adjacent concrete surfaces.Rubberized pavement is both cooler in the day and at night thanstandard asphalt.

This type of cool pavement and water retention system utilizes porousconcrete or asphalt pavements with a stone recharge bed. During storms,water drains through the open graded asphalt or concrete mix surfaceinto the sub-grade stone infiltration bed, providing stormwater storagevolumes similar to retention basins. These paved surfaces are designed

to have infiltration rates of about inch of water per hour and canreduce total peak volume of runoff and stormwater retention needs.Water passes through the porous pavement surface, and is temporarilystored in the stormwater recharge bed that acts as a retention basinuntil it is absorbed into the sub-soil. Puddles, surface ponding and runoff from storms on paved surfaces are also reduced or eliminated.

A porous asphalt pavement consists of 2-4 inch open graded (uniform

size of aggregate) surface and an underlying deep stone recharge bedwith a bottom filter fabric placed over natural un-compacted soil. Therecharged bed contains 1 to 2 uniform graded, clean washed,


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crushed stone, which acts like an underground retention basin during astorm event. The 40 percent void spaces between stones provides areafor the stormwater storage volume with the stormwater storage designedto drain away within 24-72 hours allowing underlying soils to drycompletely between storms. The natural underlying soil base must belevel or terraced and provides for water infiltration while helping toeliminate pollution by removing suspended solids from stormwater. Thenon-woven geo-textile fabric liner maintains a separation between thestone recharge bed and natural soils base.

Pervious Pavement. Source: (Town of Gilbert, Cool Pavements Brochure).


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Pervious Pavement showing evaporation. Source: (Town of Gilbert, CoolPavements Brochure).

Permeable Interlocking Concrete PaversPermeable Interlocking Concrete Pavers (PICP) are a type of coolpavement that reduces runoff, promotes cooling through reflection of sunlight and the movement of air permitting evaporation to occur aroundpaver materials. This occurs through the use individual 3 1/8th" thickconcrete paver blocks with openings and joints between individual paversthat are wide enough to allow air and water infiltration. These openingsare maintained by filling with sand or small sized, open graded crushed

stone and built on an open graded stone base and sub-base. PICP canachieve infiltration rates of up to 3 of rainfall per hour and whencombined with stone recharge beds, can also be used for stormwaterretention. These are well suited for walkways and parking lots.

4.2.2.2 Vertical Surface Green Walls

Green wall is wall, either free-standing or part of a building that ispartially or completely covered with vegetation and, in some cases, soil

or an inorganic growing medium.

There are two main categories of green walls: green faades and livingwalls. Green faades are made up of climbing plants either growing

directly on a wall or, more recently, specially designed supportingstructures. The plant shoot system grows up the side of the building

while being rooted to the ground. In a living wall the modular panels areoften comprised of polypropylene plastic containers, geo textiles,

irrigation systems, a growing medium and vegetation.

Living walls can be further broken down into passive and active systems. Active living walls' are a new, concept in which the living wall is

integrated into a building's air circulation system.


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Figure: Section showing the details of the Green Walls, Source:(www.GSky.com)

4.2.2.3 Roof Level

The roofs gain direct heat. Surface area and thermal property of theroofing material play key role in heat retention. There are strategies like

green roofing help in reducing the heat retention and reduce thecontribution of roofs to the urban heat island effect. Green roofs have

been around for thousands of years. One of the first notable appearancesof green roofs occurred in the Hanging Gardens of Babylon around 500

BC.

There are two types of green roofs, namely, Intensive and xtensive.

Intensive green roofs can accommodate large trees, shrubs, and wellmaintained gardens.


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The extensive green roof accommodates many kinds of vegetative groundcover and grasses.

Benefits of Green Roof:

Reduce sewage system loads by assimilating large amounts of rainwater.

Reduce urban heat island effects Absorb air pollution, airborneparticulates, and store carbon.

Protect underlying roof material by eliminating exposure to the suns

ultraviolet (UV) radiation and extreme daily temperature fluctuations.

Serve as living environments that provide habitats for birds and othersmall animals.

Offer an attractive alternative to traditional roofs, addressing growingconcerns about urban quality of life.

Reduce noise transfer from the outdoors.


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Insulate a building from extreme temperatures.

Figure: Greenroof layers.

Source: (American Wick Drain Corp.)


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Framework for identifying information needed to assess the benefits of materials used to

mitigate urban heat island effect:


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Source: (Optimising heat gain by building materials through landscapeelements, Vol. 2 Issue 5, ISSN: 22490558)

4.3 Summary:

The severity of the UHI threat to human health is considerable, and islikely to intensify as the climate continues to change. This issue deserves

serious attention. While international cooperation on climate change isstagnating, local levels of government should take the lead in reducing

anthropogenic contributions to climate change and take steps to adapt tonew climatic conditions. In addition to global warming, physical factors of urban settlements, such as the mineralization of surfaces, low vegetation

cover, and the production of waste heat contribute to UHI.

Since anthropometric contribution needs a serious concern, as architectsone must employ the correct usage of material in areas so that thematerial does contribute less to the urban heat gain. Along with the

proper usage of building materials the impact of heat generated by themcan also be reduced by involving landscape elements like water and

vegetation in the design.


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CHAPTER 5

BENEFITS OF TREES

5.1 Introduction

More than half of the worlds population live in urbanized environmentsand the urbanization process is increasing through time. With increasing

urbanization, several environmental problems arise as a result of differenthuman activities. Among several human induced environmental problems,

urban thermal problems are reported to be negatively affecting urbanresidents in many ways. The built up structures in urbanized areas

considerably alter land cover thereby affecting thermal energy flow whichleads to development of elevated surface and air temperature, the

phenomenon termed as Urban Heat Island (UHI) which implies island of high temperature in cities, surrounded by relatively lower temperature in

rural areas. Vegetations are reported to be among the cheapest andsustainable alternatives of moderating UHI effects.

Trees and other plants help cool the environment, making vegetation asimple and effective way to reduce urban heat islands.

Trees and vegetation lower surface and air temperatures by providingshade and through evapotranspiration. Shaded surfaces, for example, maybe 1125C cooler than the peak temperatures of unshaded materials.Evapotranspiration alone or in combination with shading, can help reducepeak summer temperatures by 15C.

Trees and vegetation are most useful as a mitigation strategy whenplanted in strategic locations around buildings or to shade pavement inparking lots and on streets. Researchers have found that plantingdeciduous trees or vines to the west is typically most effective forcooling a building, especially if they shade windows and part of thebuildings roof.

5.2 Environmental Benefits of Trees

Half a hectare of trees produce enough oxygen for 18 people every dayand keeps pollutants in check.


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Trees absorb and deflect sunlight which cools the air and alters rainfallpatterns.

Trees act as natural water filters and help reduce runoff, soil erosionand flooding.

Trees absorb sound waves, reducing noise pollution.

Trees create natural environments which attract and provide habitats forbirds and other wildlife.

5.3 Economic Benefits of Urban Trees

Trees planted around a home reduce cooling and heating costs by asmuch as 30 percent.

Healthy trees can add up to 15 percent to residential property values.

Greenspace encourages outdoor activity.

Community trees affect storm water control, transportation and airquality.

Streets with little or no shade need to be repaved twice as often as

those with tree cover.

5.4 Health Benefits of Urban Trees

Greener surroundings encourage outdoor physical activity.

Children who spend more time outside pay better attention inside.

Trees filter airborne pollutants and reduce the conditions causing asthma.

Asthma incidents increase in urban communities where trees areeliminated in favour of new roads, homes or commercial developments.

Post-operative hospital stays are shortened when patients have a view of trees and open spaces.

5.4 Social Benefits of Urban Trees


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Common community spaces foster social interaction between neighbours.

Urban forests, parks, and open spaces offer places to walk, run, bike,and hike.

Less violence occurs in shaded urban public housing. Trees provide privacy, reduce glare and direct pedestrian traffic. 13

Figure: Shows the benefits of trees. Source: (Akbari, 1992)

The presence of plants can reduce not only the ambient air

temperature, but also temperatures inside buildings, thus cooling energyload. Plants can cool a building, by shading windows from direct solargain, by providing additional thermal resistance to the fabric and throughtheir latent heat. The relative importance of those methods depends onthe vegetation being used, the climate, the building structure andorientation. The effectiveness of vegetation depends on its intensity,shape, dimensions and placement. But generally, as he concludes, anytree, even one bereft of leaves, can have a noticeable impact on energy

use.

13 www.saveourplanet.org Accessed on
http://www.saveourplanet.org/http://www.saveourplanet.org/


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Figure: Showing application of trees in the neighborhood and theirrespective benefits. Source: (www.treesaregood.com).


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Table: Factors that Create Urban Heat Islands

Source: (Reducing Urban Heat Islands: Compendium of Strategies Urban HeatIsland Basics)

Documents

UHI and Its Mitigation