Author A is a Lecturer in the Department of Architecture, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh. Author B is a M.Arch Student at McGill University, Montreal, Quebec, Canada.

Impact of Vegetation in Urban Open Spaces in Dhaka City; In Terms of Air Temperature

Samina Mazumder Tuli Nazmul Islam [Bangladesh University of Engineering and Technology] [McGill University] samina@arch.buet.ac.bd, samina123@gmail.com nazmul.islam@mail.mcgill.ca

ABSTRACT

Dhaka is a highly vibrant megacity with almost 15 million populations (World Bank, 2013). Due to high density in an undisciplined urban setup, the city is in real scarcity of open and green spaces for her large number of dwellers. The rapid growing population in conjunction with very immediate developing urbanization has led to unplanned and uncontrolled expansion of Dhaka which is resulted in the gradual loss of open and green spaces in the city. In the quest of meeting the great demand of urbanization, the city has hardly saved some of her natural spaces. Unfortunately these very few existing open spaces are not even well preserved with natural greeneries or vegetation; as a result this lack of green and plantation severely affects the microclimate of those spaces. Whereas the use of vegetation and plantation as an element of urban landscape, has always great environmental benefits and opportunities both in the scale of micro and global climate; like temperature control, solar radiation control, wind control, reduction of air pollution by absorption the pollutant and noise reduction etc. Urban vegetation minimizes direct solar radiation of the surface, optimizes wind velocity and its form and configuration influence temperature, air humidity and wind pattern of an urban setting. Vegetation create green barrier as visual boundary, natural screen and space buffer as a major element of landscape design.

The objective of this paper is to discuss the impact of vegetation in urban air temperature and explore the possibility of vegetation configuration to maximize the cooling effect in urban open space in Dhaka city. For researching these issues, a short field survey has done in two significant urban squares to find out the answer of the proposed research query. An evidence based microclimatic simulation software ENVI-met is also used to compare the outcome data of the field survey.

INTRODUCTION

Preface

The rapid growing population in conjunction with very immediate developing urbanization has led to unplanned and uncontrolled expansion of Dhaka which is resulted in the gradual loss of open and green spaces in the city. In the quest of meeting the great demand of urbanization, the city has been developing her infrastructure as well as built environment by continuous ignorance of nature which has hardly saved some of her natural spaces. Comparing with rural surroundings, this built environment of Dhaka city is mostly uncomfortable to her dwellers’ experience. Expansion of unplanned urbanization and built structure results cutting a large number of trees and converts the green areas in concrete surfaces. A minimum 25% of forest cover is suggested for a healthy living (Mowla, 1984) where at present in old Dhaka (old part of the city) only 5% and new Dhaka (new part of the city) 12% of land is

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green and open (Mowla, 2011). Green spaces with vegetation have great environmental benefits and opportunities. Vegetation has potential to reduce environmental temperature. Its form and configuration influence solar radiation, temperature, air humidity and wind flow of an urban setting. This paper states the impact of vegetation in urban air temperature and explores the possibility of vegetation configuration to maximize the cooling effect in urban open space in Dhaka city.

Statement of Problem

Use of excessive exposed hard surfaces like brick and concrete pavements and plaza, pitched road, buildings, air-conditioning system etc are responsible for raising urban air temperature. The urban spaces of a city are considered more vibrant when it can allow more people for outdoor activities. So it is important to create attractive and welcoming open spaces for the public, where outdoor comfort can be an important criterion. Again, air temperature is a vital aspect for this outdoor comfort. It is found that when meteorological data of average air temperature for Dhaka is 28 C (23 Feb, 2012) then the average air temperature of the paved area of Shahid Minar is 32 C (Field Study). Vegetation is an important element of nature. Being an organic element, vegetation has some impact on local microclimate as well as global climate. That is why this study is focused on the impact of vegetation in urban open spaces in Dhaka, in terms of air temperature.

Objective

The objective of this study is: To determine the impact of vegetation in open spaces in Dhaka city in terms of Air

temperature. To find out the possibility of vegetation configuration to maximize the cooling effect in

urban open space in Dhaka city.

Methodology

The methodology of this study can be described in three steps and these are: Step 1: Literature Review and Theoretical Basis: Study the relationship between vegetation and

basic four environmental components and the impact of vegetation on the open spaces from different paper, journal, books, articles etc.

Step 2: Field Study: A short field survey (in two urban open spaces) has been done in the current situation of Dhaka. This part is very important for this research because this research is based on field study and the output provides the current microclimatic data of the study area. Two urban spaces of similar context are taken for this study. The “Shahid Minar Chattar” (approximate area 120000 sft), located in the Dhaka University area and the “Pantha Kunja” square (approximate area 260000 sft), located beside the city commercial zone, Karwan Bazar node, Dhaka. Both spaces are laid in north-south direction and consist of both paved and green area. This step will find out the answer of the query, whether is there any variation of air temperature happened for presence of green vegetation and urban pave?

For data collection, 6 Points (spot) have been taken (in almost equal distance) on an imaginary line along the centre (north-south direction) for the both cases for measure the air Temperature, Humidity and Air Velocity. Here for Shahid Minar Chattar, lump sum 500 ft distance has being taken, where after every 100 ft one data point has located in the case study 1(Figure 6 a) and for Pantha Kunja, lump sum 1000 ft distance is being taken, where after every 200 ft in park area and every 100 ft in paved area, one data point has located in the case study 2 (Figure 7 a). All data (18 set) have been taken on that imaginary line above 1m ground level with the Kestrel 3000 Pocket Weather Meter. The data was being taken on February, 2012 on morning 10.00 am, noon 1.00 pm and evening 4.00 pm.

Step 3: Simulation: Microclimatic simulation has been done by ENVI-met software to compare with the survey results. The full areas (draw in Figure 6a & 7a) of the two urban spaces are considered for ENVI-met modeling. Input data are found from field survey and weather data of Dhaka.

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Limitations

A number of limitations have observed in this study, like time, resource and some practical problems. The survey for this study is held in the month of February where June to September are the hottest (considering highest temperature database) months for Bangladesh. During the survey it was very difficult to locate accurate point for measurement. Sometimes it was difficult for the users and different activities on the spots.

LITERATURE REVIEW

For this paper, urban space means that the space which is designed or created and used for deferent public activities in Dhaka and vegetation means the plants of an area or a region. Vegetations can be classified in different categories. In broader classification: Herb, Shrub and Tree. Herbs are seed-bearing plant whose aerial parts do not persist above ground at the end of the growing season where as Shrub is a woody perennial plant, smaller than a tree. Trees are any large woody perennial plant with a distinct trunk giving rise to branches or leaves at some distance from the ground. Again In terms of density, according to John P. Caouette and Eugene J. DeGayner, they are two types, Low (SDI < 280) and High (SDI > 280) (Caouette and DeGayner, 2008). Stand density index (also known as Reineke's Stand Density Index after its founder) is a measure of the stocking of a stand of trees based on the number of trees per unit area and diameter at breast height of the tree of average basal area.

Now the relationship of vegetation and four major components of the climatic features of environment, Solar Radiation, Air Temperature, Humidity and Air Flow discuses bellow.

There is close relationship between vegetation and solar radiation. According to Wardoyo (2011), individual leaves of the vegetation allow, some radiation to be transmitted through them (20%), absorb some radiation (55%), and reflect some radiation (25%). The leaf absorbed the solar radiation and re-transmitted it by evapotranspiration which increases relative humidity and reduces air temperature (Wardoyo, 2011). Large trees with spreading canopy can also provide shade and protect surface from direct radiation. Bueno-Bartholomei and Labaki found that, the structure of the crown, dimension, shape and colour of vegetation leaves influence reduction level of solar radiation (Bueno-Bartholomei and Labaki, 2005).

Vegetation in open space has low reflectance value that helps to reduce the air temperature in an urban surrounding. Vegetation does not radiate the long wave radiation which helps to maintain lower air temperature. Evapotranspiration reduces air temperature and increases relative humidity (Wardoyo, 2011). Tree shade also helps to reduce the air temperature. Zahoor (1997) in Pakistan found that vegetation has significant influenced to local temperature and effective in reducing air temperature about 6 –7 °F.

Again, according to Wardoyo (2011), vegetations also influenced the pattern of air movement through guidance, filtration, obstruction and deflection. Air movement sometimes depends on vegetation characteristic and configuration. Scudo (2002) establish that geometry, height, permeability and crown of the vegetation are the structural vegetal characteristic that influenced the controlling air movement.

Evapotranspiration reduces air temperature that increases relative humidity. It is found that relative humidity is always higher in the green areas.

Again the environmental criteria in Dhaka City are also important for this study. Bangladesh is in Warm Humid Tropics. Generally she has six seasons according to natural, cultural and social activities. But climatically, the climate of Bangladesh can be divided into four seasons. According to Hossain and Nooruddin meteorologically the climate of Bangladesh is categorized into four distinct seasons Winter (cool dry), Pre-Monsoon (hot dry), Monsoon (hot and wet), Post-Monsoon (hot and wet), where Winter months (December to February) temperature 21-26 C, Pre-Monsoon (March to May) temperature max 34 C, Monsoon (June to September) avg. 31 C, Post-Monsoon (October to November) temperature bellow 30 C (Ahmed, 1996). Average Relative Humidity is 60-80%. Radiation on a horizontal surface 5.00 kWh/ m2 and Air Flow 4.1 m/s (Ahmed, 1996).

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The simulation tool, ENVI-met is a three dimensional microclimate model designed to simulate the surface, plant, air interactions in urban environment with a typical resolution of 0.5 to 10 m in space and 10 sec in time. Typical areas of application are Urban Climatology, Architecture, Building Design or Environmental Planning, just to name a few. ENVI-met is a prognostic model based on the fundamental laws of fluid dynamics and thermodynamics. (ENVI-met web)

TYPES OF VEGETATION AND CLIMATE COMPONENTS

Table 1. Impact of Vegetation and Climate Components Classify plants Solar Radiation, Air Temperature,

Air Flow and Relative Humidity Comments

In General

Herb

Figure 1: Impact of Herbs

_ Direct solar radiation and Air temperature is higher than other cases in open spaces. _ Uninterrupted wind flow. _ Relative humidity depends on density of green. _ Reduces dust and no visual barrier.

Shrub

Figure 2: Impact of Shrubs

_ Sometimes small shaded area or sometimes direct solar radiation in open spaces. _ Shrubs hinder the natural wind flow in human level, but a large or smaller shrub allows air flow in human level. _ Relative humidity is high in human level. _ Sometimes filters air and create barrier. _ Flowering shrubs are good in terms of aesthetics.

Tree

Figure 3: Impact of Trees

_ Create shaded spaces. _ Allows gentle wind movements in human level, and filters or guiding the movements in. _ Sometimes ground cover do not grow in the soil because of large shading tree and lack of solar radiation. _ Air temperature is less in the shading area. _ Relative humidity is high under the tree.

Density

Low (SDI < 280)

Figure 4: Impact of Low Density Green

_ Allows direct solar radiation. _ Allows gentle wind flow. _ Air temperature is less than a paved area. _ Relative humidity is moderate. _ Allows ground cover in the soil.

High (SDI>280)

Figure 5: Impact of High Density Green

_ Do not allow direct solar radiation. _ Hinder wind velocity sometimes allows tunnel effect. _ Air temperature is less than other situation _ Relative humidity is high. _ Sometimes do not allow good grass on ground and Create dark shade.

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FIELD STUDY

Case Study 1: Shahid Minar Chattar

“Shahid Minar Chattar” is a historically and culturally significant place for Dhaka city. This monument carries the memories of our language movements on 1952. This public space is a large hard paved area with small green and large trees. Green area is surrounded the main monument area. A number of paved stairs leading to the upper platform of the monument is an important public plaza for socio-cultural activities. There is no large structure along the monument. This area is completely exposed to the sun. 6 points are being taken for data collection, which are as follows. (Figure 6)

Point 01: Located in a Green space (soil) with tree shade behind the main monument. Point 02: Located in Red Brick pave with no shade and no green. Higher than ground level. Point 03: Located in Large Red Brick pave with no shade and no green. Point 04: Located in a Green space (soil) with tree shade. There is pave around this area. Point 05: Located in a Green space (soil) with tree shade. But beside the road pave. Point 06: Located in the road pave.

Figure 6 (a) Top view and section of Shahid Minar (b) Point 01 (c) Point 02 (d) Point 03 (e) Point 04 (f) Point 05 (g) Point 06

Case Study 2: Pantha Kunja Square

Pantha Kunja is the only open space cum green space in Karwan Bazar area, one of the important commercial hubs of Dhaka city. It has two parts: one is Green Park with large trees and limited pave area and another consists of large pave with small green area (herbs and shrubs). This area is also completely exposed to the sun. 6 points are being taken for data collection, which are as follows. (Figure 7)

Point 01: Located in a Green space (grass) with tree shade with little pave. Point 02: Located in a Green space (grass). Point 03: Located in a Green space (grass) with tree shade. Point 04: Located in Large Yellow and Red Brick pave with no shade. Point 05: Located in Large Yellow and Red Brick pave with no shade beside small green. Point 06: Located in the footpath pave beside the road.

(a)

(b) (c)

(d) (e)

(f) (g)

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Figure 7 (a) Top view and section of Pantha Kunja Square (b) Point 01 (c) Point 02 (d) Point 03 (e) Point 04 (f) Point 05 (g) Point 06

Results and Findings from Field Survey and Software Simulation

Figure 8 (a) Average Air Temperature Diagram (b) Average Relative Humidity Diagram

ENVI-met Simulation

(a)

(b)

(c)

(d)

(e)

(f)

(g)

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Figure 9 (a) Thermal Image of Shahid Minar Chattar (b) Thermal Image of Pantha Kunja Square

Here are the charts of average value of Air Temperature and Relative Humidity which are being found from field data (Figure 8). For ensuring human comfort in any urban context, relative humidity is equivalently relevant and important with temperature. That is why, in field survey, Relative humidity has always taken in consideration for analytical observation.

VEGETATION AND OPEN SPACE CONFIGURATION

It is found from Case study 01 and Case study 02 that air temperature in a green space is remarkably less than a paved or built space in Dhaka city. In Case Study 01, Average Air Temperature in green area varies from 30.5 C to 31.8 C where in paved area varies from 32.1 C to 33.1 C. In Case Study 02, Average Air Temperature in green area varies from 30.9 C to 31.3 C where in paved area varies from 31.5 C to 32.4 C. Here some other issues can be considered as follows.

Orientation: An imaginary line is taken through Case Study 01 considering north south direction. In our country the wind flows from south east direction in summer. So point 01 is 30.1 C which is less than point 04 and point 05 (31.8 C and 31.9 C) for the wind flow through the plaza area from south. Again an imaginary line is taken through Case Study 02 in same north south direction. In this case the large green is in the south part of the square which create cool environment in pave area on north.

Shade: The shaded area of green space is much cooler than the exposed area. Shade can provide by large trees or other minimal built objects. In Shahid Minar point 01 is always shaded by large dense trees. So the air temperature in point 01 is always cooler than other green spaces. In Pantha Kunja point 02 is exposed to the sun. As a result air temperature in point 02 is higher than point 01 and 03.

Types of vegetation: It is also an important issue. In Shahid Minar, there are small numbers of trees founded in selected green area. There are a few grasses found during this survey. There is no herb, shrubs and trees in the paved area. In Pantha Kunja, there are herbs, shrubs and trees founded in the park area with grasses on ground which helps to reduce the air temperature as per survey data. In the pave area there is also small green area founded with herbs which might helps to reduce the air temperature.

Figure 10 (a) Section of Case Study 1, Shahid Minar; (b) Section of Case Study 2, Pantha Kunja Square Vegetation Density: Vegetation density also effects the cooling. More dense vegetations increase

Evapotranspiration, which reduces the air temperature. Pantha Kunja green area is denser than the Shahid Minar green area and it is also a cause of less average temperature in Pantha Kunja.

Area of the Vegetation: In Shahid Minar almost 80,000 sq ft green area and 40,000 sq ft in pave area where as in Pantha Kunja 240,000 sq ft green area and 20,000 sq ft pave area. Average temperature in Pantha Kunja is less than the Shahid Minar area (survey). Larger area of vegetation contributes in better cooling effect.

Surrounding Vegetation and Presence of Built Objects: Surrounding area has also an impact in the air temperature. Here surrounding area is not considered enough. For Shahid Minar, if a 500 ft diameter circle is considered, more green area with few built area are found which has a cooling impact on local air temperature. In Pantha Kunja if a 1000 ft Diameter circle is considered, more exposed pave and glaze area with more built structure are found.

Vegetation and Relative Humidity: Relative Humidity is much related component to air temperature. As per graph (Figure 8(a) & 8(b)) air temperature and humidity varies in vice versa. Temperature rise is responsible for reducing the relative humidity. In this study the relative humidity data has also taken for observation.

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OBSERVATION

Vegetation has tremendous impact on local microclimate. From the observation of this study some points can be illustrated as follows:

Presence of vegetation minimizes direct solar radiation and reduces microclimatic air temperature (3C - 4C) in an urban space.

As wind flows from south east in Bangladesh (generally), vegetation locating in south can help to reduce the air temperature on the pave locating in the north.

Large and dense trees create shade. Air temperature is less in the shaded space than an exposed area (1C – 2C). Even air temperature in the shaded pave is less than the pave exposed to the sun.

Air temperature also depends on types of vegetation used in a space. Types and configuration of herbs, shrubs and trees are important for local air temperature.

Density (trees) is also an important component for reducing air temperature. Dense vegetation helps to reduce air temperature by screening solar radiation.

Area of vegetation affects the local air temperature. Large area of vegetation can keep lower the air temperature where as presence of hard exposed material or structure in the green area is responsible for increasing the air temperature.

CONCLUSION

The urban open spaces are very vital part of a city and considered more successful when these spaces can allow more people for different outdoor activities. So it is always very essential to design comfortable urban spaces for city dwellers. Dhaka is a city of hot and humid climate with high average temperature. Direct solar radiation and scorching heat of sun not allow city people to have maximum use of city open spaces specially in day time. Vegetation has great potential to reduce this air temperature (3C - 4C) of urban microclimate and ensure sustainable breathing space. Again vegetation creates shaded cool area with lower air temperature for the users. Designing an urban space with large area of vegetations on south side may create comfortable environment. Vegetation configuration can be one of major determining factors to get benefit of urban open spaces in Dhaka city with the best utilization of it, in her every aspect of urban landscape design.

ACKNOWLEDFEMENT

The research was started for a term paper of a M. Arch course, Arch 6106: Eco System and Built Environment. We want to thank the course teacher Prof. Dr. Qazi Azizul Mowla. The further research simulation was inspired by another M.Arch course Arch 6105: Design in the Tropics by Prof. Dr. Khandaker Shabbir Ahmed.

REFERENCES

Ahmed K. S. (1996) Approaches to Bioclimatic Urban Design for the Tropics with Special Reference to Dhaka Bangladesh, PhD Research

Bueno-Bartholomei, C.L., & Labaki, L.C. (2005) How Much Does the Change of Species of Trees Affect Their Solar Radiation Attenuation?, www.geo.uni.lodz.pl/~icuc5/text/O_1_4.pdf - 25 feb, 2012

Caouette J. P. and DeGayner E. J. (2008), Broad-Scale Classification and Mapping of Tree,Size and Density Attributes in Productive,Old-Growth Forests in Southeast Alaska's Tongass National Forest, Society of American Foresters

Mowla, Q. A. (2011) Crisis in the Environment of Dhaka: An Overview, Conference on Engineereng Research, Innovation and Education 2011, CERIE 2011, 11-13 January, Sylhet, Bangladesh

Mowla, Q. A.(2005) Eco Design and Planning for Sustainnability in Urban Dhaka, International Conference of the Bangladesh Geographical Society, Oriental Geographer, December 9-11, Dhaka Bangladesh

Scudo, G. (2002) Thermal Comfort in Greenspace, Proceeding COST C 11 "Green structures and urban planning" - Milan Oct 2002

Shelton D. P. (2008) Air Properties: Temperature and Relative Humidity, Institute of Agriculture and Natural Resources, University of Nebraska–Lincoln Extension Publications

Wardoyo J, (2011) “Vegetation Configuration as Microclimate Control Strategy in Hot Humid Tropic Urban Open Space” Zahoor, A. (1997) Effect of Trees in Ameliorating Air Temperature in Urban Setting in Pakistan, Dissertation

unpublished, University of Idaho

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