High-density Cities Research Articles

Housing Affordability in Metropolitan Areas. The Application of a Combination of the Ratio Income and Residual Income Approaches to Two Case Studies in Sicily, Italy

Housing affordability problems have become more serious over the course of the last few decades and are now also affecting the middle-class, despite the fall in prices on the housing market. This study proposes a methodology to assess threshold-income as an index for measuring housing affordability by applying a combination of the ratio income and residual income approaches. The methodology is applied to two particular areas of Sicily in Italy as case studies consisting of medium-size metropolitan areas located in a less developed European region. The areas have been chosen on the basis of their different territorial structure: a polarized area that comprises a high-density city centre and a polynuclear urban region. The results are diversified for income level, as well as for town and urban zone, and allow us to compare the housing affordability problems between towns belonging to the same metropolitan area.

Mechanical behavior of Quasi-rectangular segmental tunnel linings: First results from full-scale ring tests

The shield driven method is now a preferred choice when urban subway tunnels are constructed. In high density cities, a suitable cross-section for tunnel structures is rather important. In this study, a full-scale ring test is conducted in order to determine the mechanical behavior of Quasi-rectangular segmental tunnel linings, firstly designed and employed in Metro Line 3 in Ningbo, China. The design of experimental specimens and loading processes are introduced. Main results of tests are explored, including the failure state and measurement items, convergence deformation, strain of rebars, bending angle of joints and bolts’ strain .The ultimate bearing capacity of the structure is also reached during the test. According to the experimental results, two analysis methods are utilized to evaluate the mechanical behavior of the structure under design condition. The bearing mechanism and weakness of the lining are revealed as well. Safety factor of elastic stage and ultimate stage are also defined to evaluate safety reservation of the structure under unexpected circumstance. The test will lay the foundation for further verification and optimization of Quasi-rectangular segmental tunnel linings.

Thermal comfort and energy performance of public rental housing under typical and near-extreme weather conditions in Hong Kong

Building performance evaluation is crucial for sustainable urban developments. In high-density cities, occupants suffer from poor living conditions due to building overheating, especially during increasingly frequent near-extreme summer conditions caused by climate change. To represent this situation, the summer reference year weather data was employed for building simulations using DesignBuilder. This study aims to evaluate the thermal comfort and energy consumption of four typical public rental housing (PRH) building types in Hong Kong. For free-running flats, results show generally higher air temperatures in the oldest PRH type (Slab) with a compact linear building form and the most sensitive response to outdoor temperature changes for another older PRH type (Trident) with a Y-shaped design, possibly owing to its high wall conductivity. Occupants in all building types experience a ∼10% increase in the proportion of discomfort hours when compared to results for typical summer conditions, but overheating is the most severe in Slab type PRH. Following an initial assessment of the cooling energy usage, a simple sensitivity test was conducted to explore the potential energy savings by various passive design strategies, including shading and reducing the exposed cooled space. A cross-shaped building form also appears to be more energy efficient. These findings, complemented by further parametric analyses, may prove useful when designing buildings for climate change.

Assessing Structural Connectivity of Urban Green Spaces in Metropolitan Hong Kong

Connectivity is a vital element in landscape structure because of its importance in species–landscape interactions. Connectivity analysis of green spaces in urban landscapes, especially in high-density cities such as Hong Kong, differs from that of habitats in natural or rural landscapes. Using the human being as the target species, we formulated with GIS techniques a resistance weight, a structural connectivity index and an ecological barrier effect index to assess connectivity of green spaces. Two factors were included in the modeling, namely the resistance of different land uses related to human activities, and the distance between different urban green spaces. We analyzed the relationships between the connectivity index of green spaces and green cover, elevation, building density and population density. Our results indicate that low connectivity usually occurs in both old and new town centers with high building density and low green cover, and in areas occupied by land uses with a high resistance weight. However, urban density may not necessarily have a negative influence on the structural connectivity of green spaces. Green cover also may not necessarily have positive impact on connectivity if the green spaces have a poor spatial pattern. Adding more green stepping stones, large green spaces and green corridors to form greenways and shortening the distance between urban green spaces can offer a spatial-planning strategy to increase the green space connectivity in Hong Kong. The study provides insights to optimize connectivity of green spaces to improve the urban living environment in high-density metropolises.

Planning and Construction of Sports Facilities in Urban Residential Areas in China

The function, utilization, and resource allocation of sport facilities in residential areas have attracted increasing attention in China along with the country's rapid economic development and the popularization of the health concept. To optimize the allocation of public sports resources and develop functions for sports facilities in China, this study analyzed existing policies concerning the planning and construction of sports facilities in residential areas and actual problems in the operation, management, and construction of sports facilities in the country. The analysis was conducted using document, investigation and expert consultation methods. Corresponding improvement measures were proposed by combining actual situations of urban sport participation of residents. Sports facilities in urban residential areas in China are facing various problems, such as inadequate quantity, uneven layout, poor management, and imperfect related laws and regulations as well as planning policies. Formulating sports facility planning policies and updating existing sports facilities are beneficial in relieving the sports facility shortage caused by the increased, diversified, and actual demands of residents. These measures could also guide the future construction of sports facilities in residential areas in high-density cities in China.

Spatial Variability of Geriatric Depression Risk in a High-Density City: A Data-Driven Socio-Environmental Vulnerability Mapping Approach.

Previous studies found a relationship between geriatric depression and social deprivation. However, most studies did not include environmental factors in the statistical models, introducing a bias to estimate geriatric depression risk because the urban environment was found to have significant associations with mental health. We developed a cross-sectional study with a binomial logistic regression to examine the geriatric depression risk of a high-density city based on five social vulnerability factors and four environmental measures. We constructed a socio-environmental vulnerability index by including the significant variables to map the geriatric depression risk in Hong Kong, a high-density city characterized by compact urban environment and high-rise buildings. Crude and adjusted odds ratios (ORs) of the variables were significantly different, indicating that both social and environmental variables should be included as confounding factors. For the comprehensive model controlled by all confounding factors, older adults who were of lower education had the highest geriatric depression risks (OR: 1.60 (1.21, 2.12)). Higher percentage of residential area and greater variation in building height within the neighborhood also contributed to geriatric depression risk in Hong Kong, while average building height had negative association with geriatric depression risk. In addition, the socio-environmental vulnerability index showed that higher scores were associated with higher geriatric depression risk at neighborhood scale. The results of mapping and cross-section model suggested that geriatric depression risk was associated with a compact living environment with low socio-economic conditions in historical urban areas in Hong Kong. In conclusion, our study found a significant difference in geriatric depression risk between unadjusted and adjusted models, suggesting the importance of including environmental factors in estimating geriatric depression risk. We also developed a framework to map geriatric depression risk across a city, which can be used for identifying neighborhoods with higher risk for public health surveillance and sustainable urban planning.

Detached eddy simulation of pedestrian-level wind and gust around an elevated building

Wind flow turbulence is known to have a major influence on the pedestrian-level wind (PLW) environments, particularly around a building. The elevated design of a building, as a special feature, proved to improve pedestrian-level weak wind conditions in high-density cities. The present study aims to assess three turbulence models, the detached eddy simulation (DES), the steady-state RANS (SRANS), and the unsteady-state RANS (URANS), in their simulation of the PLW flow turbulence concerning wind gust. The simulated mean wind velocities around isolated buildings with and without an elevated design were compared with those obtained from a wind tunnel experiment. The effects of mesh resolution and inflow fluctuating algorithm on the performance of the DES model were thoroughly evaluated. The DES model can better reproduce the mean flow fields than the other two models. Finally, the unsteady fluctuations of wind flow around the buildings with and without the elevated design are analyzed in terms of instantaneous wind velocity, lift coefficient, energy spectral density, and turbulence intensity. The predicted lift coefficient and Strouhal number are approximately 0.01 and 0.09, respectively, which is consistent with what are reported in the literature. Modifications of the frequency of vortex shedding, periodical wind flow pattern, and the normalized wind gust flow fields around the two types of buildings are compared in detail. The work reveals that transient turbulent flow pattern can be reasonably obtained with the DES model, indicating the potential of using the DES for PLW gust assessments in urban planning.

Spatial variability of excess mortality during prolonged dust events in a high-density city: a time-stratified spatial regression approach

BackgroundDust events have long been recognized to be associated with a higher mortality risk. However, no study has investigated how prolonged dust events affect the spatial variability of mortality across districts in a downwind city.MethodsIn this study, we applied a spatial regression approach to estimate the district-level mortality during two extreme dust events in Hong Kong. We compared spatial and non-spatial models to evaluate the ability of each regression to estimate mortality. We also compared prolonged dust events with non-dust events to determine the influences of community factors on mortality across the city.ResultsThe density of a built environment (estimated by the sky view factor) had positive association with excess mortality in each district, while socioeconomic deprivation contributed by lower income and lower education induced higher mortality impact in each territory planning unit during a prolonged dust event. Based on the model comparison, spatial error modelling with the 1st order of queen contiguity consistently outperformed other models. The high-risk areas with higher increase in mortality were located in an urban high-density environment with higher socioeconomic deprivation.ConclusionOur model design shows the ability to predict spatial variability of mortality risk during an extreme weather event that is not able to be estimated based on traditional time-series analysis or ecological studies. Our spatial protocol can be used for public health surveillance, sustainable planning and disaster preparation when relevant data are available.

Characterizing prolonged heat effects on mortality in a sub-tropical high-density city, Hong Kong

Extreme hot weather events are likely to increase under future climate change, and it is exacerbated in urban areas due to the complex urban settings. It causes excess mortality due to prolonged exposure to such extreme heat. However, there is lack of universal definition of prolonged heat or heat wave, which leads to inadequacies of associated risk preparedness. Previous studies focused on estimating temperature-mortality relationship based on temperature thresholds for assessing heat-related health risks but only several studies investigated the association between types of prolonged heat and excess mortality. However, most studies focused on one or a few isolated heat waves, which cannot demonstrate typical scenarios that population has experienced. In addition, there are limited studies on the difference between daytime and nighttime temperature, resulting in insufficiency to conclude the effect of prolonged heat. In sub-tropical high-density cities where prolonged heat is common in summer, it is important to obtain a comprehensive understanding of prolonged heat for a complete assessment of heat-related health risks. In this study, six types of prolonged heat were examined by using a time-stratified analysis. We found that more consecutive hot nights contribute to higher mortality risk while the number of consecutive hot days does not have significant association with excess mortality. For a day after five consecutive hot nights, there were 7.99% [7.64%, 8.35%], 7.74% [6.93%, 8.55%], and 8.14% [7.38%, 8.88%] increases in all-cause, cardiovascular, and respiratory mortality, respectively. Non-consecutive hot days or nights are also found to contribute to short-term mortality risk. For a 7-day-period with at least five non-consecutive hot days and nights, there was 15.61% [14.52%, 16.70%] increase in all-cause mortality at lag 0-1, but only -2.00% [-2.83%, -1.17%] at lag 2-3. Differences in the temperature-mortality relationship caused by hot days and hot nights imply the need to categorize prolonged heat for public health surveillance. Findings also contribute to potential improvement to existing heat-health warning system.

Classification of Local Climate Zones Using ASTER and Landsat Data for High-Density Cities

The local climate zone (LCZ) scheme provides a standard method to conduct urban heat island studies, in which urban landscapes are classified into different LCZs according to urban structures, land cover, and construction materials. Based on the LCZ classification scheme, the World Urban Database and Access Portal Tools (WUDAPT) is a new initiative to generate LCZ maps of cities worldwide with the use of freely available Landsat data. This paper aims to evaluate the performance of the original WUDAPT method in LCZ mapping for high-density cities. To further improve LCZ mapping accuracy for high-density cities, we investigate the usage of both freely available Landsat and advanced spaceborne thermal emission and reflection radiometer (ASTER) satellite data to generate better LCZ mapping results. Experiments on two high-density Chinese cities, Guangzhou and Wuhan, showed that combining Landsat and ASTER data can improve the overall performance of LCZ mapping results, especially for urban areas. This finding indicates that further applications of the WUDAPT method for high-density cities can include both ASTER and Landsat data.

GIS-based mapping of Local Climate Zone in the high-density city of Hong Kong

This study aims to develop a Local Climate Zone (LCZ) classification map and establish the associated urban morphology database as a cross-discipline information platform for climate research and planning decision making in Hong Kong. Geographic information system (GIS) was applied to synergize various kinds of planning data, and classify land surface properties following the LCZ framework.Sensitivity tests were performed to identify the appropriate raster resolution and geolocation for LCZ classification. Next, a set of urban morphology analysis maps were developed. Based on the spatial information supported by the urban morphology analysis maps, land surface properties were classified according to the LCZ classification criteria, and an LCZ classification map was formulated. Finally, spatial distribution pattern of LCZ classes was analyzed, and LCZ datasets were established following the standardized procedure suggested by the LCZ framework.The LCZ classification system not only provides site metadata for UHI observation in Hong Kong, but also expands the global LCZ database under the high-density urban scenario. This study proposes an integrative GIS-based method to process various kinds of planning data for urban morphology analysis and LCZ-based land surface classification, which is also applicable to other cities with a comprehensive set of planning information.

Effects of urban street spatial parameters on sound propagation

Many studies have demonstrated that the effect of urban street spatial shape on sound propagation cannot be ignored. Most previous studies are based on idealised spatial models and but not systematically and comprehensively examine the real and complex street space. This paper takes the actual streets of a high-density city as research objects, select reliable spatial parameters, obtain the acoustic propagation data using computer simulation, identify the sound propagation characteristics and establish sound propagation models of urban streets. In total, 144 samples have been tested, 13 spatial parameters, including the width information, height information, section information and plan information of streets, have been selected, and three acoustic indices, which include the sound attenuation, reverberation time and early decay time, have been analysed in this paper. The sound propagation in the urban street is consistent with the propagation characteristics of the semi-free sound field, i.e. the sound attenuation is linearly correlated with the logarithm of the sound propagation distance. This linear correlation becomes more pronounced for the greater Plan enclosure degree and more even distribution of façades. The trend of sound attenuations decreases with the increasing Cross-sectional enclosure degree, increasing Plan enclosure degree in the Near Zone or decreasing Vehicle lane width. Reverberation time is primarily distributed between 1.0 s and 3.0 s and tends to be stable when the propagation distance increases. The mean reverberation time increases with the increasing Mean façade height, Sidewalk width, Cross-sectional enclosure degree in the Near Zone or Standard deviation of Plan enclosure degree in the Near Zone. The typical early decay time distribution curve is clearly divided into two areas along the propagation direction. In the first area, the early decay time value is notably small and nearly equals zero. With a sudden increase, the early decay time maintains at a relatively stable value in the second area (stable area) of 0 to 3.0 s. The mean early decay time in the stable area increases with the increasing Vehicle lane width, increasing Cross-sectional enclosure degree or decreasing Standard deviation of Plan enclosure degree in the Near Zone.

Towards energy efficient skyscrapers

As of 2007 more than half of the world’s population is living in urban areas (a figure expected to rise to 60% by 2030). Thus, the liveability of the high-density city is gradually becoming a central point of focus and concern. A successful skyscraper model of urban planning could provide the possibility to increase city-space vertically as opposed to the current continuous expansion outward, which has obvious environmental consequences. However, skyscraper development, as well as all other new construction and gradually the older building stock, has to comply with current strict regulations on building energy efficiency. Contemporary high-rise examples do not present a sustainable solution to an increasing population or as models of prosperity, as they are linked to high-energy demand, environmental and social imbalances.This paper looks at design strategies towards promoting skyscraper energy efficiency by considering a climatically responsive design, where orientation, the thermal properties of the building envelope and the effect of altitude, become the main design tools. Initial simulations were performed for a residential and an office reference structure 100m high. Different scenarios were implemented for gradually upgrading the building envelope and studying its relationship with the changing microclimate with altitude (wind speed increase and dry bulb temperature drop) between ground and top level. The advanced envelope was then simulated to up to 400m high (120 stories high), and heating and cooling loads were compared in relation to different building heights and uses.EnergyPlus is used as the main simulation tool as it accounts for wind speed increase and dry bulb temperature drop with height. The location chosen is Tel Aviv, Israel, a city already growing upwards and expected to have a significant increase in skyscraper construction in the coming years. The results of the simulations performed present the base upon which further design strategies can be implemented towards reducing the environmental impact of this challenging building type.

Planning strategies for roadside tree planting and outdoor comfort enhancement in subtropical high-density urban areas

Hot summers in the subtropics cause thermal discomfort, which is intensified by the urban heat island effect in heavily built-up areas. Use of urban greenery has been proposed as a mechanism for microclimate regulation. In this study, the microclimatic effect of urban trees was investigated in the context of high-density cities in subtropical hot and humid climates. Measurements were conducted in urban areas with sky view factors (SVF) ranging from 0.2 to 0.8, and mean radiant temperatures were calculated. The measurements show that the effects of urban trees are related to SVF and that the impact of building morphology is more evident under cloudy conditions than under sunny conditions. In heavily built-up areas, the mean radiant temperature (Tmrt) was reduced to a comfortable level (33 °C) by roadside trees in the early afternoon. Simulation results indicate that roadside trees reduce the physiological equivalent temperature (PET) to 29 °C in urban areas with SVFs of 0.2 under cloudy conditions. The SVF-oriented planning method was tested using the existing building geometry of two high-density districts of Hong Kong, Mong Kok and Sham Shui Po, and site-specific design strategies for tree planting were developed. The study results show that a comfortable microclimate can be provided by roadside trees in heavily built-up urban environments in subtropical cities for nearly 70% of the summer. Design suggestions for refining the Green Master Plans of Hong Kong in future planning are provided.

A study of plot ratio/building height restrictions in high density cities using 3D spatial analysis technology: A case in Hong Kong

Hong Kong is an international metropolis with a highly dense population. As a result, it faces enormous challenges in terms of land supply. As part of the Hong Kong Government's initiative to increase land supply, the Civil Engineering and Development Department (CEDD) proposed minor relaxation of the maximum plot ratio/building height restrictions for 21 target sites in Kai Tak Development Area (KTDA). Although CEDD has explored the feasibility of increasing development intensity by assessing environmental impacts, infrastructure capacity and public consultation, these reviews and assessments were conducted based on the 2D GIS. Since the spatial distribution of land unit in the real world is three-dimensional, 3D GIS can help us look into the world in true perspective and make informed decisions. This study aims to investigate the viability of minor relaxation of maximum plot ratio/building height restrictions of 21 sites in KTDA through 3D modeling and 3D spatial analyses, including skyline, visual impact, shadow and solar exposure. Regarding to the 21 target sites, four scenarios with different plot ratios and building heights were built and compared. The results indicate that minor relaxation of maximum plot ratio and building height leads to (i) minor effect on skyline (ii) minor effect on visual impact and (iii) slight changes in shadow and solar exposure both in winter and summer. Therefore, in light of the findings from this study, scenario 4 is the recommended reasonable scale to relax the maximum plot ratio/building height restriction for the target sites in KTDA. Besides, this study can also be applied in the urban renewal studies and other new development areas in Hong Kong, or even in other densely populated cities.

Minimizing the gender difference in perceived safety: Comparing the effects of urban back alley interventions

Urban alleys are perceived as unsafe, especially by women. We conducted a photograph-questionnaire survey to examine gender difference in perceived safety of alley scenes. Photograph simulation technology was used to create three categories of intervention scenes: Cleaning, Vegetation, and Urban Function & Vegetation. For the existing (Baseline) and Cleaning scenes, perceived safety remained low for both genders, though men's perceived safety was significantly higher than women's. Vegetation scenes were perceived as moderately safe for both genders, but men's ratings were still significantly higher. For Urban Function & Vegetation scenes, perceived safety was high for both genders, and the gender difference largely disappeared. Geometric vegetation yielded higher perceived safety than naturalistic vegetation for both genders. These findings provide clear evidence to support the efforts of policy makers, environmental designers, and community associations seeking to create safe and vital back alley environments for men and women in high-density cities across the world.

Defining the Risk of Zika and Chikungunya Virus Transmission in Human Population Centers of the Eastern United States.

The recent spread of mosquito-transmitted viruses and associated disease to the Americas motivates a new, data-driven evaluation of risk in temperate population centers. Temperate regions are generally expected to pose low risk for significant mosquito-borne disease; however, the spread of the Asian tiger mosquito (Aedes albopictus) across densely populated urban areas has established a new landscape of risk. We use a model informed by field data to assess the conditions likely to facilitate local transmission of chikungunya and Zika viruses from an infected traveler to Ae. albopictus and then to other humans in USA cities with variable human densities and seasonality. Mosquito-borne disease occurs when specific combinations of conditions maximize virus-to-mosquito and mosquito-to-human contact rates. We develop a mathematical model that captures the epidemiology and is informed by current data on vector ecology from urban sites. The model demonstrates that under specific but realistic conditions, fifty-percent of introductions by infectious travelers to a high human, high mosquito density city could initiate local transmission and 10% of the introductions could result in 100 or more people infected. Despite the propensity for Ae. albopictus to bite non-human vertebrates, we also demonstrate that local virus transmission and human outbreaks may occur when vectors feed from humans even just 40% of the time. Inclusion of human behavioral changes and mitigations were not incorporated into the models and would likely reduce predicted infections. This work demonstrates how a conditional series of non-average events can result in local arbovirus transmission and outbreaks of human disease, even in temperate cities.

A study on the impact of shadow-cast and tree species on in-canyon and neighborhood's thermal comfort

Tree-planting is one of the veritable tools for combating urban heat island and improving thermal comfort in the wake of global warming and urbanization. However, trees of different species and morphological properties have variable solar attenuation capacity and consequently, thermal comfort regulation potential. Besides, the shadow-cast effect by buildings helps in reducing pedestrian radiant load and consequently improves thermal comfort especially in high-density cities even though ventilation is reduced. Therefore, a holistic and contextual understanding of tree-planting and shadow-casting can help in designing climate-proof cities. In this study, we employed the ENVI-met model to better understand the interaction between these two forms of shading (trees and buildings) on the pedestrians' thermal comfort in Hong Kong and the influence of one over the other. The impact of different urban densities on the thermal comfort improvement potential by eight (8) common tree-species in Hong Kong was specifically studied. Results show that shallow canyons are susceptible to worse thermal condition when compared to their deeper counterparts with similar aspect ratio value. Of all tree configuration parameters, leaf area index, tree height and trunk height are most influential in improving and aggravating daytime and nighttime thermal comfort, respectively. We also found that trees' effectiveness in improving daytime thermal comfort reduces with increasing urban density and vice-versa for nighttime. For the reference of planners and landscape architects, this study recommends tall trees of low canopy density with high trunk in deeper canyons and vice-versa for shallow canyons and open-areas.

Explore Children's Outdoor Play Spaces of Community Areas in High-density Cities in China: Wuhan as an Example

China has witnessed accelerating urbanization in many cities bringing both economic benefits and hardships for citizens. Often children bear the heaviest burden in the high-density cities. The value of outdoor play has been emphasized in many research fields with its physical, psychological and social benefits for children's development. However, by a range of measures, there is growing evidence that children in China spend less time enjoying outdoor play currently. This study research aims to explore the children's experience of outdoor play in their community areas by taking Wuhan as an example. Questionnaires designed for both children and their parents had been collected and analyzed to find out the relationship between play value and outdoor play spaces.

An Evaluation Index for Cross Ventilation Based on CFD Simulations and Ventilation Prediction Model Using Machine Learning algorithms

High-density cities usually surfer deterioration of the urban built environment due to slow or stagnant air movement. This deterioration could be mitigated by improved urban natural ventilation through good urban planning and building design. However, indoor natural ventilation requirements in current building standards only define absolute indoor air speed without considering outdoor conditions. To fill this gap, this paper developed generic urban-scale coupled indoor and outdoor CFD models and proposes a new integrated index (CIOI index) to evaluate indoor natural ventilation potential, which couples both indoor and outdoor wind environments. CIOIv regression models are developed by applying machine learning algorithms based on 3,840 CFD simulation results. The Gradient Boosting (GB) model shows an improved performance in terms of higher correlation (R2=0.8) and lower errors (MAPE=16%) compared with the multivariate linear regression model. Building designers and urban planners can use this CIOI prediction model to quickly check, in the early design stage, the influence of their design variations on indoor ventilation potential.