Empirical Morphological Model to Evaluate Urban Wind Permeability in High-Density Cities
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In this chapter, a high-resolution frontal area density (FAD) map that evaluates urban permeability was produced using an empirical model, which takes into account the heterogeneous urban morphology and local wind availability. Using the MM5/CALMET model, the wind data of Hong Kong was simulated, the FAD map of three urban zones were calculated: podium (0–15 m), building (15–60 m), and urban canopy (0–60 m). Wind tunnel test data was used to correlate the FAD understanding of the three zones with pedestrian-level wind environment. Linear regression analysis indicated that a lower urban podium zone yielded the best correlation with the experimental data, and 200 × 200 m was the reasonable resolution for the FAD map. This study further established that the simpler two-dimensional ground coverage ratio (GCR) that is readily available in the planning circle can be used to predict the area’s average pedestrian-level urban ventilation performance of the city. Working with their in-house GIS team using available data, the GCR will provide the planners a way to understand the urban ventilation of the city for decisions related to air paths, urban permeability, and site porosity.
KeywordsUrban planning Urban ventilation Surface roughness Frontal area density
Tall and bulky high-rise building blocks with very limited open spaces in between, uniform building heights, and large podium structures have collectively led to lower permeability for urban air ventilation at the pedestrian level (Ng 2009). The mean wind speeds recorded by the urban observatory stations in urban areas over the past decade have decreased by over 40% (Hong Kong Planning Department (HKPD) 2005). Stagnant air in urban areas has caused, among other issues, outdoor urban thermal comfort problems during the hot and humid summer months in Hong Kong. Stagnant air has also worsened urban air pollution by restricting dispersion in street canyon with high building-height-to-street-width ratios. The Hong Kong Environmental Protection Department (HK EPD) has reported the frequent occurrence of high concentrations of pollutants, such as NO2 and respirable particles (RSP) in urban areas such as Mong Kok and Causeway Bay (Yim et al. 2009). These areas also have some of the highest urban population densities in Hong Kong.
The 2003 outbreak of the Severe Acute Respiratory Syndrome (SARS) epidemic in Hong Kong had brought attention to how environmental factors (i.e., air ventilation and dispersion in buildings) played an important role in the transmission of SARS and other viruses. Since the outbreak, the planning community in Hong Kong started to pay more attention to the urban design process in order to optimize the benefits of the local wind environment for urban air ventilation. As a result, the Hong Kong Government had commissioned a number of studies on this regard; the most important project among the government-commissioned studies is entitled “Feasibility Study for Establishment of Air Ventilation Assessment System” (AVA), which began in 2003 (Ng 2009). The primary purpose of this comprehensive chapter is to establish the protocol that assesses the effects of major planning and development projects on urban ventilation in Hong Kong (Ng 2007).
The importance of wind environment on the heat, mass, and momentum exchange between urban canopy layer and boundary layer has been studied by urban climate researchers (Arnfield 2003). Two modeling methods have been frequently applied to study wind environment of the city: wind tunnel tests and computational fluid dynamics (CFD) techniques. The United States Environmental Protection Agency (US EPA) conducted numerous urban-scale wind tunnel tests to understand the dispersion of particulate matters smaller than 10 μm in aerodynamic diameter (PM10) (Ranade et al. 1990). Williams and Wardlaw (1992) conducted a large-scale wind tunnel study to describe the pedestrian-level wind environment in the city of Ottawa, Canada, and identified areas of concern for planners. Plate (1999) developed the boundary-layer wind tunnel studies to analyze urban atmospheric conditions, including wind forces on buildings, pedestrian comfort, and diffusion processes from point-sources of the city. Kastner-Klein et al. (2001) analyzed the interaction between wind turbulence and the effects induced by vehicles moving inside the urban canopy. Wind velocity and turbulence scales throughout the street canyons of the city were analyzed using smoke visualization (Perry et al. 2004). In 2004, the US EPA’s Office of Research and Development (EPA-ORD) conducted a city-scale wind tunnel study to analyze the airflow and pollutant dispersion in the Manhattan area (Perry et al. 2004). Kubota et al. (2008) conducted wind tunnel tests and revealed the relationship between plan area fraction (λ p) and the mean wind velocity ratio at the pedestrian level in residential neighborhoods of major Japan cities. In Hong Kong, the Wind/Wave Tunnel Facility has conducted numerous tests at the city, district, and urban scale to understand the wind availability and flow characteristics of Hong Kong (HKPD 2008).
Apart from wind tunnels, CFD model simulation can be applied at the initial urban planning stage in providing a “qualitative impression” of the wind environment. Mochida et al. (1997) conducted a CFD study to analyze the mesoscale climate in the Greater Tokyo area. Murakami et al. (1999) used CFD simulations to analyze the wind environment at the urban scale. Kondo et al. (2006) used CFD simulations to analyze the diffusion of NOx at the most polluted roadside areas around the Ikegami-Shinmachi crossroads in Japan. Letzel et al. (2008) conducted studies of urban turbulence characteristics using the urban version of the parallelized large eddy simulation (LES) model (PALM), which is superior to the conventional Reynolds-averaged models (RANS). Using the Earth Simulator, Ashie et al. (2009) conducted the largest urban CFD simulation of Tokyo to understand the effects of building blocks on the thermal environment of Tokyo. Ashie et al. noted that the air temperatures around Ginza and JR Shimbashi are much higher than in the surrounding areas of Hama Park and Sumida River. Ashie et al. argued that the high air temperature can be attributed to the bulky buildings at Ginza and JR Shimbashi that obstruct the incoming sea breezes (Ashie et al. 2009). Yim et al. (2009) used CFD simulation to investigate the air pollution dispersion in a typical Hong Kong urban morphology. In general, using CFD for urban-scale investigation has been gaining momentum in the scientific circle. Two important documents that provide guidelines for CFD usage have been published: Architectural Institute of Japan (AIJ) Guidebook (AIJ 2007; Tominaga et al. 2008) and COST action C14 (Frank 2006).
2.1.2 Objectives and Needs of This Study
While the application of wind tunnels and CFD model simulations to analyze the interaction between the urban area and the atmosphere has made important contribution to the understanding of urban air ventilation of the city, such applications are costly, and may not be able to keep up with the fast design process in the initial stages of the design and planning decision-making process. Instead, the outlined and district-based information based on urban morphological data parametrically understood may be more useful for planners.
This chapter employs the understandings of urban surface roughness to establish the relationship between heterogeneous urban morphologies and urban air ventilation environment. A new method with cross-section areas, which takes into account the site-specific wind information measured at 60 m height using the MM5/CALMET model simulation, was used to calculate the frontal area density (λ f). Using the site-specific wind information, the new calculation method of λ f focuses on the effects of the built environment to the wind field, which provides a spatially averaged understanding of wind permeability at the urban scale.
This study first correlates the pedestrian-level wind environment with λ f calculated at the podium layer, and then establishes an understanding of surface roughness and urban morphology based on ground coverage ratio (GCR), a term familiar to urban planners, with λ f to simplify the practical application of the understanding for professional use.
2.2 Literature Review
2.2.1 Roughness Characteristics
The roughness properties of urban areas affect surface drag, scales and intensity of turbulence, wind speed, and the wind profile in urban areas (Landsberg 1981). The total drag on a roughness surface includes both a pressure drag (τ tp) on the roughness elements and a skin drag (τ ts) on the underlying surface (Shao and Yang 2005). In this study, only the pressure drag is considered, since skin drag is relatively small and is not a factor that can be controlled at the urban scale. Oke (1987) provided the logarithmic wind profile in a thermally neutral atmosphere, which is a semiempirical relationship that acts as a function of two aerodynamic characteristics: roughness length (z 0) and the zero-plane displacement height (z d). The reliable evaluation of such aerodynamic characteristics of urban areas is significant in depicting and predicting urban wind behaviors (Grimmond and Oke 1999).
Currently, three methods can be used to estimate the surface roughness: Davenport roughness classification (Davenport et al. 2000), morphologic, and micrometeorological methods (Grimmond and Oke 1999). The Davenport Classification is a surface-type classification based on the assorted surface roughness values, using high-quality observations (Davenport et al. 2000), which covers a wide range of surface types. This method is not too helpful to describe urban permeability in high-density cities, because most of the urban areas could only be described in Class 8 “Skimming: City centre (z 0 ≥ 2)”. Compared with the micrometeorological method, the morphometric method estimates the aerodynamic characteristics, such as z 0 and z d, using empirical equations (Lettau 1969; MacDonald et al. 1998; Raupach 1992; Bottema 1996; Kutzbach 1961). Grimmond and Oke (1999) validated the empirical models by Kutzbach, Lettau, Raupach, Bottema, and Macdonald. While reasonable relationships between z0 and frontal area index (λ f(θ)) for low- and medium density forms have been found, there is a tendency of overestimation of z 0 for higher density cases (Bottema 1996).
Grimmond and Oke (1999) calculated λ f(θ) in the context of the urban morphology of North America cities. Ratti et al. (2002) calculated λ f(θ) of 36 wind directions in London, Toulouse, Berlin, and Salt Lake City. By incorporating a spatially continuous database on aerodynamic and morphometric characteristics, such as λ f(θ), z 0, and z d, morphometric estimation methods can be helpful to urban planners and researchers in depicting the distribution of the roughness of the city. Using Bottema’s model equation, Gál and Unger (2009) mapped z 0 and z d to detect the ventilation paths in Szeged. Wong et al. (2010) mapped λ f(θ) to detect the air paths in the Kowloon Peninsula of Hong Kong.
2.2.2 Calculation of Frontal Area Index and Frontal Area Density
Compared with λ f(θ), which is an average value that describes the urban morphology of the entire urban canopy, λ f(z,θ) represents a density that describes the urban morphology in the interested height band. Burian et al. (2002) conducted frontal area density calculations in a height increment of 1 m in Phoenix City, and found that λ f(z,θ) is a function of land uses because the buildings in different land uses have different building morphologies.
2.3 Development of New Layer-Based λ f(z)
2.3.1 Height of the Podium and Urban Canopy Layer
2.3.2 Wind Availability in Hong Kong (MM5/CALMET System)
The terrain in Hong Kong is complex; hence, the resolution used in MM5 simulations (typically down to 1 km) cannot accurately capture the influence of topology characteristics on wind environment. Therefore, CALMET, a prognostic meteorological model capable of higher resolutions (down to 100 m), was used. Combining the data obtained from MM5 and the data obtained from an upper air sounding station, maintained by the Hong Kong Observatory, in 2004, the CALMET model adjusts the estimated meteorological fields for the kinematic effects of terrain, slope flows, and terrain blocking effects to reflect the impact of a fine-scale terrain on resultant wind fields at 100 m resolutions (Yim et al. 2007). In the CALMET model simulation, the vertical coordinates were set with 10 levels: 10, 30, 60, 120, 230, 450, 800, 1250, 1750, and 2600 m (Yim et al. 2007).
2.3.3 Calculation of λ f(z) in Grids with Uniform Size
Compared with the conventional calculation of λ f(z), the nonexisting cross-section walls in the new calculation method could result in unrealistic surface roughness. However, such cross sections may be needed to avoid underestimation of the surface roughness at the high-density urban areas covered by large and closely packed buildings. Thus, the correlations between the VR_ w,j and between the new method of λ f(z) with cross section and the traditional method of λ f(z) without cross section were compared.
Correlation between VR_ w,j and λ f(0–15 m) in different resolutions and calculation methods
R 2 (with cross sections)
R 2 (no cross sections)
Resolution: 300 × 300 m
Resolution: 200 × 200 m
Resolution: 100 × 100 m
Resolution: 50 × 50 m
Based on the regression analysis result, following understandings can be stated: the new calculation method with cross sections can correctly predict the wind velocity ratio. Furthermore, compared with the traditional method of calculating frontal area density, the new method can alleviate the underestimation of mapping urban surface roughness in high-density cities with large and closely packed buildings.
2.3.4 Grid Sensitivity (Resolution)
As shown in Table 2.1, the values of R 2 decrease along with the reduction of the grid sizes. Choosing a larger grid size would have a positive effect on depicting the urban wind environment. However, R 2 should not be the only criterion for selecting one grid size over another. For mapping roughness, the explanatory power of the map should not be totally traded off for the sake of the correctness of λ f(z). After weighing the considerations, the resolution of 200 × 200 m was adopted in mapping urban permeability in Hong Kong.
2.4 Development of Empirical Model
2.5 Implementation in Urban Planning
2.5.1 Mapping Urban Wind Permeability Using λ f(z)
2.5.2 Ground Coverage Ratio and Frontal Area Density
Based on the above discussion, following understandings can be stated:
There is a good linear relationship between λ f(0–15 m) and GCR (R 2 = 0.77) in most of the test points. For planners, using GCR to predict the wind environment at the pedestrian level is reasonable. Compared with other traditional maps (Gál and Unger 2009; Wong et al. 2010), the proposed GCR map is more applicable to urban designers and planners due to its accessibility to the planners in the planning process.
Local values of some areas may deviate due to the extremely large building widths (large commercial podiums and industrial buildings). In this type of areas, the wind permeability cannot be predicted in GCR. However, the occurrence of this type of extreme examples is very small (approximately 2%).
2.5.3 Mapping Urban Wind Permeability Using GCR
After incorporating the respective site-specific wind roses, the areas with low wind permeability are depicted in Fig. 2.17. These areas block wind and worsen the wind environment at the pedestrian level of their leeward districts. Potential air paths in the podium layer are also marked out in this map. The potential air paths would play an important role to improve the urban ventilation and environment quality by bringing fresh airflow into the urban areas for the purpose of dissipating air pollutant and mitigating urban heat island intensity.
… it is critical to increase the permeability of the urban fabric at the street levels. Compact integrated developments and podium structures with full or large ground coverage on extensive sites typically found in Hong Kong are particularly impeding air movement and should be avoided where practicable. The following measures should be applied at the street level for large development/redevelopment sites particularly in the existing urban areas:
providing setback parallel to the prevailing wind;
designating non-building areas for sub-division of large land parcels;
creating voids in facades facing wind direction; and/or
reducing site coverage of the podia to allow more open space at grade.
This chapter shows that the qualitative understanding of the podium structure, as mentioned in the HKPSG, is valid. In Hong Kong, some areas of high podium coverage can be identified. These areas require the most significant design, planning intervention and improvement.
For better urban air ventilation in a dense, hot-humid city, breezeways along major prevailing wind directions and air paths intersecting the breezeways should be provided in order to allow effective air movements into the urban area to remove heat, gases and particulates and to improve the micro-climate of urban environment.
Breezeways should be created in forms of major open ways, such as principal roads, inter-linked open spaces, amenity areas, non-building areas, building setbacks and low-rise building corridors, through the high-density/high-rise urban form. They should be aligned primarily along the prevailing wind direction routes, and as far as possible, to also preserve and funnel other natural airflows including sea and land breezes and valley winds, to the developed area.
The disposition of amenity areas, building setbacks and non-building areas should be linked, and widening of the minor roads connecting to major roads should be planned in such a way to form ventilation corridors/air paths to further enhance wind penetration into inner parts of urbanized areas. For effective air dispersal, breezeways and air paths should be perpendicular or at an angle to each other and extend over a sufficiently long distance for continuity.
Using the urban wind permeability map of the territory, city planners can initially estimate the possible urban air ventilation environment of the urban areas with the average velocity ratios. Adjusting the pedestrian-level wind speeds and predicting the bioclimatic conditions of the city have become possible.
Overall, the chapter has demonstrated a practical and reliable way for city planners to quickly obtain district-level urban air ventilation information for their board-based design works at the early stages. Conceptually, avoiding wrong decisions that may be difficult to rectify later is, therefore, possible.
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