Designing functional urban open spaces: through user comfort & spatial analysis approach
Ruffina ThilakaratneChu Hai College of Higher Education, Hong [email protected]
Abstract: Due to peculiar topography and extreme high density, Hong Kong’s open space per capita 2 sqm is far from the WHO standards of 9 sqm per person. Open spaces are often regarded as compliance to planning requirements rather than providing desirable outdoor amenities to the community. Due to challenging Summer conditions in Hong Kong such as high temperature & humidity and stagnant wind most urban open spaces are underutilized. This research investigates the factors effecting enjoyment of open spaces and design factors that may improve their function as public amenities. Exploration is based on user comfort, user behaviour, and design attributes. Data collection was conducted on seven urban parks within Tsuen Wan district on dry days during Summer. User comfort data gathered using a WBGT monitor and simulation methods supplemented by photographic survey of users’ movement within the parks and user interviews. Findings indicate tendencies in using these parks in the afternoon between 3pm to 5pm for passive recreation and navigating in shady locations and areas with seating. Residents expressed close proximity and presence of mature trees as positive attributes in these parks, however voiced the need for rain shelters, more shady areas and better storm water drainage.
Keywords: user comfort; urban open spaces; microclimatic parameters; vegetation.
1. URBAN THERMAL COMFORT CHALLENGES IN HONG KONG
Hong Kong inherits a sub-tropical climate with hot, humid and wet Summers. Thermal comfort conditions in urban centres are influenced by its peculiar topography and high-rise high density built environment resulting in negative factors such as urban heat island effect, street canyon effect, shading from buildings, stagnant wind, air pollutant concentration, high humidity and lack of vegetation. On average wind speed in most urban areas is approximately 1 m/s. Over the past five years humidity levels from June to September ranged between 75% - 84% with mean daily maximum and minimum maintaining about 2 oC difference whilst absolute average Summer temperature reaching 34.5 oC (HKO, 2013-2017).
Due to mountainous terrain conditions, developable land area in Hong Kong is limited to less than 25% out of 1,111sqkm land area to accommodate 7.4million population. Land population density in some areas exceeds 57,250 persons per sqkm (ISD HKSAR, 2015) creating challenging liveable conditions. People live in towers as tall as 60 storeys and flats small as 15sqm where outdoor amenities play a vital role towards wellbeing in such dense living conditions.
High humidity, hot weather, stagnant wind and air pollution are some of the key challenges towards the enjoyment of outdoor life in Hong Kong. Few percentage of citizens opt to mountainous country parks during weekends to get cleaner air. Majority of citizens depend on open spaces within their vicinity for passive recreation.
Factors effecting user comfort conditions such as thermal comfort and design quality of open spaces play an important role when defining the liveability and functionality. ASHRAE 55 defines thermal comfort as a sensation that indicates the satisfaction with the thermal environment. These parks are being predominantly used by elderly on a regular basis who are more vulnerable to heat stress and safety related issues. This research investigates the factors effecting enjoyment of open spaces and design factors that may improve their function as public amenities.
Draft Outline Zoning Plan approved for Tsuen Wan district in 2017 includes land allocations for “Open Space and Green Belt” land use categories (TPB, 2017). Therefore, findings from this study may be beneficial in redesigning and improving functionality of open spaces in Tsuen Wan and similar districts.
2. URBAN PROFILE IN THE CASE STUDY AREA
Tsuen Wan district, a 6,000 hectares new town located in the South-Western part of New Territories bordered by the sea on the South and South-East sides and flanked by mountains on the North East and North West sides was selected as the case study for this research. City center is located inland away from the waterfront and the hilly terrain. Predominant Summer wind directions are from the East, South and South-East quarters (Ng, 2011) where street grids are aligned to South East and North West directions without facilitating adequate wind flow along the roads.
A new town that was developed in 1960s as an industrial hub has now been transformed in to a mixed-use residential area. Out of 302,814 population, about 70,000 people live in public and subsidized housing with the rest living in private housing. Tsuen Wan district accommodates a population density of 5,300 persons per sqkm with 103,219 households in a large number of five to six storeys low-rise high density Chinese tenement buildings. These developments often follow shop housing typology with long blocks of shops at the road level with residentials above.
Out of total population of 302,814 residents, 36.5% represent home makers, retirees, self-employed and others who are not employed who spend more hours of the day within the district. About 21.5% population are above 60 years of age (CSD, 2016). With increasing percentage of elderly in Hong Kong, provision of active and passive recreational areas has become an agenda towards facilitating healthy communities. Being an old district with a large number of old housing estates, these parks are often being used by senior citizens, home makers, parents and domestic helpers who are waiting to pick up their children from schools.
2.1 Open space planning and design guidelines
Out of total 6,000 hectares 131.84 hectares of land is zoned as “Open Space” to provide outdoor areas for active and passive recreational uses for the community (PlanD, 2017). Majority of these open spaces are pocket open spaces that are located among commercial and residential land uses orientated parallel to the street grid.
Hong Kong Planning Standards and Guidelines (HKPSG) (PlanD, 2017) have stipulated for key principles for planning open spaces: quantity, quality, good practices and vision. Quality integrates environmental standards whilst good practices promoting easily accessible network of open spaces linked with pedestrian or cycling paths. Guidelines recommend that open spaces should be functional, accessible and usable spaces for the community; not merely fulfilling regulations. The Planning Standards also acknowledges that it is challenging in old districts to provide quality open spaces and to meet the minimum required area. To fulfil environmental standards, where possible parks are required to have a nature buffer to reduce pollutants entering from sources. In urban areas, the standard provision of open spaces is a minimum of 2 sqm per person which is far below from WHO International Standards which require 9 sqm per capita within 15 minutes walking distance (WHO, 2012 in Pafi et al., 2016).
Further the guidelines draw attention to surface finishes and spatial layout considering the elderly warranting for adequate outdoor furniture under shading and away from the pedestrian paths with slip resistant hardscape finishes.
In Tsuen Wan predominant Summer wind directions are from the East, South and South-East quarters (Ng, 2011). Longitudinal direction of 90 percent of the parks are aligned to North East and South West direction allowing parks that are abutting streets or major roads to receive prevailing wind from their shorter span.
3. METHODOLOGY
Seven urban pocket parks / open spaces were selected for this study which represent different geometries, contextual attributes and land use compositions to investigate their success in functioning as public amenities providing user comfort.
The research engaged a multi-dimensional approach for data collection and analysis; field data on microclimatic conditions during Summer months based on Wet Bulb Globe Temperature readings, contextual analysis of the parks, photographic survey of user behaviour and interviews with open space users. Behavioral indicators contributing heat stress such metabolic rate, clothing insulation and moisture permeability (Fanger, 1970 in Epstein 2006) were excluded as the WBGT index is an empirical index of three variables not counting physiological information. Field data were collected on these seven parks spanning over weekdays and weekends at 1130hrs and 1530hrs. Based on initial observations, 1130hrs and 1530hrs are the times that these parks are being most used by non-working residents in the vicinity. Random users were interviewed to gather their perception on the factors that affect usability of these parks during summer and their preferences for redevelopment. Interviews were supplemented by a structured questionnaire. Number of respondents were limited when the responses started becoming similar. Extech HT200 Heat Stress WGBT Meter and Kestrel 5000 Environmental Meter were used for gathering data related to microclimatic conditions. Field data were cross checked with the nearest Observatory weather station data. Photographic survey was conducted for recording user behaviour, park layout, landscape and other features. Shadow analysis of these parks were conducted using Grasshopper Ladybug plugin; results were correlated with field measurements and the photographic survey.
4. FINDINGS
4.1 Spatial quality of the open spaces
As illustrated in Figure 1, all these parks located amidst of high density areas within 15 minutes walking distance to residential estates and nearby schools creating a network of open spaces.
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Figure 1. Park network in the case study area (Author, 2018)
Table 1. Analysis of spatial attributes (Author, 2018)
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Table 1 presents an analysis of spatial attributes indicating hardscape to greenery ratio and orientation of the parks in relation to wind direction. Park sizes varies from 200 sqm to 10,000 sqm predominantly comprising of hard surfaces and large canopy trees with limited green cover. Five out of nine parks including extensions consisted of less than 40 percent greenery whilst four parks maintaining over 75 percent greenery. These parks did not meet the HKPSG requirement for 70 percent soft landscape inclusive of 60 percent of trees for passive recreational areas.
Hong Kong is notorious for Urban Heat Island effect reporting an average UHI intensity of about 2o-3.5oC (Fung et al., 2009). Shahmohamadi et al. (2010) opined that replacement of soil and vegetation by concrete and asphalt increase ambient temperature. A study conducted in Taipei with a similar climate suggests how solar radiation absorption within unshaded hardscape areas effected the thermal environment within a park during daytime resulting in elevated temperature in the surrounding areas. This study recommends reducing unshaded paved areas to less than 50 percent (Chang and Li 2014) to alleviate negative effects. Supporting the above argument, Wong and Chen (2006) report positive contributions from greenery in parks on microclimate within the parks and the surrounding areas. These studies support the role of urban greening on thermal comfort.
Five out of seven parks used cement blocks and two parks used orange color eco blocks as the paving material; two parks also have used dark color rubber paving in the activity areas. Although light color high albedo materials have been used on hardscape areas, compared to greenery, hard surfaces contribute to high emissivity and therefore elevated mean radiant temperature and Urban Heat Island Effect. Santillán-Soto, N. et al. (2015) in a study in a desert climate that compared thermal properties of urban material concluded that unlike concrete and asphalt, grass and poly material contribute to the least amount of thermal energy during a 24-hour average, thereby reducing UHI. EPA US (n.d.) reports that evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 1–5°C.
4.2 Microclimatic data analysis
The WBGT meter recorded relative humidity (RH), ambient temperature (TA), globe temperature (TG), wet bulb globe temperature (WBGT), and wet bulb temperature (TW) in nine parks. WBGT results as a projection of combined effects was considered as the indicator of thermal comfort these parks. Figure 2 presents a comparison of TA, WBGT and RH in these parks for discussing their effect on thermal comfort. Based on CBE Thermal Comfort Tool analysis none of parks meet the thermal comfort standards required in ASHRAE 55 standards (CBE).
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Figure 2. Microclimatic conditions in the seven parks collected in Summer (Author, 2018)
As reported in Figure 2, Ambient Temperature varied from 29.9 to 32.1oC in park 2 and park 5 respectively with a median of 30.9oC. Whereas WBGT results varied from 26.7 to 27.5 oC within a median of 27.1 oC maintaining a narrow range. Humidity varied across the parks between 60.5% to 65.5% with a median of 62.4. In order to meet ASHRAE 55 standards, at 60% RH, the TA should be between 24-29 oC. CUHK (2008) reports that an air temperature of 27.9 oC, relative humidity of 80%, and wind speed of 0.53-1.30 m/s would provide thermal comfort for a person standing or walking under shade in urban spaces, in the summer months. Although all parks reported air temperature above 27.9 oC, WBGT levels were between 26.7-27.5 oC, with lower humidity levels and higher wind speed providing users with comfortable thermal environments. Parks (3B, 4, 5, 6 & 7) that are abutting to major traffic roads or surrounded by streets or located amidst of buildings reported elevated Ambient Temperature around noon. Parks 5, 6 and 7 cooled off around 4pm perhaps because of high wind speed reported in parks 6 and 7 and high ratio of greenery in park 5. Hong Kong Observatory station located on a higher plane surrounded by vegetation and about 1.5km direct distance away from the study area reported, 26.8 to 32.2oC range of Ambient Temperature alluding to the effect of greenery on outdoor temperature.
Due to high rise high density built environment, most urban areas in Hong Kong experience stagnant wind. Average Summer prevailing wind speed at 200m elevation for Tsuen Wan is approximately 3.3 m/s (PlanD). Attributed by microclimatic conditions such as orientation to wind direction, vegetation ratio and surrounding density etc., parks demonstrated varying wind speeds; parks that are orientated towards the prevailing wind direction demonstrated higher wind speeds. As reported in Figure 3 satisfactory wind speed could be observed in Parks 1, 4, 6 and 7 perhaps because of the wind tunnel effect created by the surrounding built up areas where wind escapes through low-pressure areas. Although having a high greenery ratio amongst all the parks, Park 5 reported stagnant wind levels perhaps because of the friction created by the tree crowns. Parks with high percentage of hardscape areas reported better wind performance indicating the possibility of high emissivity material creating low pressure pockets drawing winds in particularly in the afternoons.
Figure 3. Difference in wind speed in m/s in the parks at 11:30 and 15:30 in Summer (Author, 2018)
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4.3 Shadow analysis by the surrounding built environment
Shadow analysis indicates a positive effect by the surrounding built environment in creating partial shading on the exposed areas at different times of the day contributing to lower solar radiation thereby elevated temperature. Most parks seem to benefit from shadows in the afternoon mitigating radiant heat in the parks. Shadow patterns could inform positioning of shelters and seating for better integration of amenities. Shadow analysis also correlated with people’s preferred locations within the parks.
Figure 4. Shadow analysis that compare 1130 & 1530 hrs on a sunny day (Author, 2018)
4.4 User behaviour and perception analysis
Users who predominantly occupy these parks can be categorised as sedentary or slightly active. Around 80% of the users are senior citizens from the vicinity who use these parks 2-7 days per week for socializing with friends and for relaxation. Young respondents mentioned they bring their children to these parks. Majority of the users occupy the parks between 3-5pm. Field visits and photographic survey confirmed users’ preference on shaded areas and where benches are provided. Young respondents didn’t really mention about the need for shade, but the need for benches. Majority of respondent reported the benefit of large mature trees and nature on these parks providing them breeze and comfort; however, none of them make any remarks about the high temperature or high humidity.
Some were attracted to the parks because of the close proximity to their homes and spaciousness. As measures for improvement, the need for more amenities such as more seating and toilets and proper drainage system to deal with storm water were suggested. They also suggested the preference for large parks with big squares, more trees and seats and more natural elements such as water and rocks if there are future parks developments in the district.
5. CONCLUSIONS & RECOMMENDATIONS
Open spaces are vital in Hong Kong given the nature of high density high-rise living and compact size of apartments. Parks in Tsuen Wan district have been planned prior to the release of the Draft Outline Zoning Plan in 2016 which has provided a number of recommendations for creating sustainable and liveable open spaces. Although the current open spaces are within close proximity to neighborhoods, creating a network of open spaces, they have only been partially successful in terms of providing functional and thermally comfortable open spaces and promoting greening within the district. These parks are often been used in the afternoon around 4pm when the parks are cooled off. Since majority of these parks are not aligned with prevailing wind directions, measures such as wind deflectors may improve wind penetration through the parks.
Regardless of Urban Heat Island effect in Hong Kong urban areas, these parks consist of high percentage of hardscape also leading to storm water stagnation. Considering the increasing amount of rainfall in the recent years, it would also be a worthy consideration to replace impervious paving with vegetation.
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User profile should be taken into consideration when designing urban parks; considering the high percentage of elderly users, safer material choices and more seating and shaded areas are important elements to integrate. Although most areas in parks are shaded by the surrounding built environment, seating are located in exposed areas. These parks should adopt an integrated approach informed by the findings in comfort analysis, user analysis and shadow analysis in promoting greening, improving microclimatic conditions of the parks and the surrounding and improving amenities.
ACKNOWLEDGEMENTS
The author is grateful to Chu Hai College of Higher Education for research funding. Author would like to thank research assistants, Yana Xiao, Lam Ho and Yiksun Lai for their contribution in data collection and simulation studies and Prof. Paul Chu and Mr. Roger Tan for advice and thought-provoking ideas.
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