Influence of path design cooling strategies on thermal conditions and pedestrian walkability in high-rise residential complexes

https://doi.org/10.1016/j.ufug.2023.127981Get rights and content

Highlights

  • Effects of path design strategies on outdoor thermal conditions are evaluated by micro-scale CFD model.

  • Influence of path design cooling strategies on walkability are examined by agent-based Model (ABM).

  • The cooling extent is up to 5.5 °C for Tsurf, 0.2 °C for Ta, 16.2 °C for MRT, and 5.8 °C for PET at 12.7 m away from paths.

  • Single-row planting strategy provides the best perceived travel time (PTT) reduction and can be up to 30.7 %.

  • This research proposes efficient designs to providing optimum comfortable conditions.

Abstract

Intensive urbanization exacerbates overheating in cities, leading to negative impacts on human health. Although numerous studies have investigated the improvement of pedestrian comfort through street-level treatments, few have examined the influence on pedestrian walkability, and the spatial extent of cooling effects from paths to adjacent areas remains unevaluated. This study assesses the cooling effects of different mitigation strategies on four thermal indicators—surface temperature (Tsurf), air temperature (Ta), mean radiant temperature (MRT), and physiological equivalent temperature (PET)—using ENVI-met simulations. We employ Agent-based Models (ABM) to analyze pedestrian walkability through perceived travel time (PTT). The study focuses on two high-rise residential complexes in Suwon City, South Korea, and compares reflective pavement, single-row tree planting, and clustered tree planting mitigation strategies. Results indicate that single-row planting offers more significant cooling effects across the entire site compared to other strategies, while clustered planting improves local heat conditions. Cooling effects extend from the path to the entire block, with single-row planting reducing Tsurf by up to 5.5 °C, Ta by 0.2 °C, MRT by 16.2 °C, and PET by 5.8 °C at 12.72 m away from paths during the hottest hours. ABM results suggest that single-row planting provides the best PTT reduction and can be up to 36.24 %. The proposed framework and findings provide urban designers with a data-driven approach to optimize pedestrian thermal comfort and walkability.

Introduction

Global warming is one of the serious concerns the world now faces. Global surface temperature in the first two decades of this century from 2001 to 2020 was 0.99 °C higher than that of 1850–1900, and global mean sea level increased by 0.20 ± 0.05 m from 1901 to 2018 (IPCC, 2021). Intensive urbanization has exacerbated this situation. Urbanization reduces natural green space and increases impervious surfaces, consequently diminishing evapotranspiration (Li et al., 2019). In contrast to rural areas, urban surface materials with lower albedo emit more heat (Qin, 2015). Furthermore, densely constructed buildings impede nocturnal cooling by obstructing heat dissipation (Bonan, 2015). Anthropogenic heat emissions, originating from vehicles, buildings, and industries, exacerbate urban heat conditions (Zhou et al., 2018). Although urban temperatures are generally 2–4 °C higher than those in rural areas, the difference can reach up to 10 °C, contingent upon location and local weather conditions (Bohnenstengel et al., 2011, Heaviside et al., 2015).

Severe heat waves pose considerable risks to human health. The most direct and prevalent symptoms include stroke, dehydration, and heat exhaustion, as evidenced by various extreme heat events worldwide, such as those in Chicago and other regions in the USA in 1995 (Whitman et al., 1997), the Europe-wide heat wave in 2003 (Kosatsky, 2005), and the heat wave in southeast Australia in 2009 (Nitschke et al., 2011). Indirectly, hot weather and heat extremes have been found to correlate with increased mortality rates from cardiorespiratory and other chronic conditions (Cheng et al., 2019), mental health issues (Thompson et al., 2018), and adverse pregnancy and birth outcomes (Zhang et al., 2017). Owing to the heightened severity of heat conditions in urban settings, individuals residing in urban areas face an elevated risk of mortality compared to those living in non-urban regions (Conti et al., 2005).

A vein of research has been conducted to evaluating the effects of the built environment on urban microclimates (Parsaee et al., 2019). Previous studies have revealed that some building characteristics, such as building forms, layouts, orientation, density, and average height, impact the local microclimate (Sanaieian et al., 2014). The utilization of specific construction materials, including highly reflective, evaporative, and heat-harnessing pavements, has demonstrated practical benefits (Yang et al., 2015). Moreover, the evaporation and convection processes of water bodies have been found to cool adjacent air (Rahul et al., 2020). Urban green spaces have also been recognized for their mitigation potential, as they provide shade, redirect wind flow, and cool the air through evapotranspiration processes (Morakinyo et al., 2017, Morakinyo and Lam, 2016).

Regarding detailed landscape design that directly influences human comfort, pedestrian paths have emerged as a focal element. The majority of pedestrian path analyses have been conducted at street level, primarily encompassing two components: street tree plantings and reflective pavements. Street tree plantings typically provide shade on paths, providing a favorable microclimate for pedestrians (Wang and Akbari, 2016). Reflective pavements, while mitigating overheating by reflecting solar radiation, may inadvertently worsen pedestrian thermal comfort by increasing radiant loads (Middel et al., 2020).

The interrelation between thermal comfort and walkability is crucial for optimizing pedestrian comfort levels, considering the substantial impact of thermal conditions on outdoor pedestrian activity behaviors (Huang et al., 2016, Tucker and Gilliland, 2007). Previous studies have emphasized the importance of climatic factors in shaping individual well-being in external environments (Chen and Ng, 2012). Across various seasons, occupants of outdoor spaces experience a range of thermal sensations, from heat stress and cold stress to neutral and comfortable conditions (Eliasson et al., 2007, Kántor and Unger, 2010, Li et al., 2016). Even within a single day, individuals may encounter diverse levels of thermal stress (Mahmoud, 2011). Consequently, researchers have developed comprehensive indices to accurately assess thermal comfort under both hot and cold conditions, with the Physiologically Equivalent Temperature (PET) emerging as the predominant index for evaluating thermal comfort (Johansson et al., 2014, Potchter et al., 2018). PET’s validity has been extensively corroborated across various climatic zones and urban areas, including streets, squares, and parks, through questionnaires and public feedback (Cohen et al., 2013, Lai et al., 2014, Ng and Cheng, 2012). In summary, the integration of PET with walkability considerations presents an opportunity to enhance urban walkability through strategic planning.

Previous studies have primarily focused on the cooling effects of landscape components, such as tree canopies, water features, and pavements, often examining areas of landscaped spaces that do not necessarily coincide with human activity. However, the cooling benefits of pedestrian paths in high-rise residential complexes and their potential for improving pedestrian walkability remain largely unexplored. As one of the most traveled areas in the complex, path design typically varies widely to accommodate different uses and is guided by esthetic preferences (Girling et al., 2019), with thermal comfort often not being a primary concern. First, little is known about the time-variant cooling effects of path design components. While most existing studies have investigated cooling effects during the hottest daytime hours, nighttime effects have rarely been the subject of research. Urban heat islands are also prevalent at night, albeit through different mechanisms (Azevedo et al., 2016); therefore, it is essential to determine whether daytime findings remain valid during other periods. Second, the spatial extension of such cooling effects from paths to adjacent areas has not been studied. Although some research suggests that the cooling effects of certain landscape elements could extend beyond their immediate vicinity, the specific range by distance or area has not been investigated. Lastly, prior studies have generally considered outdoor comfort for an entire site but have largely overlooked micro-scale complexities that influence pedestrian outdoor activity behaviors.

Against this backdrop, we aim to assess the improvement of thermal conditions and pedestrian walkability in high-rise residential complexes by comparing the heat conditions produced by alternative path design options that utilize varying paving materials and tree-planting schemes. Specifically, we seek to answer the following questions: (1) How do path-design strategies influence outdoor microclimates and human thermal comfort in large-scale residential complexes? (2) How do the cooling effects differ during various periods of the day, namely daytime (07:00–19:00), hottest hours (14:00–16:00), and nighttime (20:00-next day 6:00)? (3) What is the spatial extent of these cooling effects? (4) Which path design strategy offers the greatest potential for walkability improvement?

To evaluate the heat mitigation effects, we employed ENVI-met, a Computational Fluid Dynamics (CFD) tool for the surface-plant-air interaction, to simulate surface temperature (Tsurf), air temperature (Ta), mean radiant temperature (MRT), and PET. To simulate the walkability under varying thermal conditions, we translated commuting time series and activity profiles into AnyLogic, a JAVA-based programming and simulation tool for modeling hybrid systems (AnyLogic User Manual, 2021), and performed Agent-based Modeling (ABM) simulation, an approach that models systems composed of autonomous and interacting agents (Macal and North, 2005). The study sites consisted of two residential complexes in Suwon City, South Korea, representing a typical form of large-scale residential development, wherein the site area generally spans 50,000 m2, accommodates 700 living units in high-rise towers, with ample open spaces within the grounds.

The rest of the text is organized as follows: the subsequent section provides a literature review; we then describe the study sites and methods, followed by the analytical results; and we conclude with a discussion of the implications of our findings.

Section snippets

Literature review

Numerous studies have investigated outdoor thermal comfort in residential complexes. In such environments, densely positioned tall buildings and high levels of energy usage can lead to substantially uncomfortable thermal conditions (Li et al., 2020, Zhou and Chen, 2018).

Street tree plantings contribute to microclimate cooling and thermal comfort. During the warm season, cooling effects on air temperature (Ta) provided by street trees amount to around 1 °C in studies conducted in Melbourne,

Research framework

This study comprises two primary components: a thermal comfort evaluation module, employing a microclimate CFD model, and a resident behavior simulation module, utilizing the ABM (Fig. 1). Inputs for the thermal comfort assessment module encompass plant characteristics, urban surface structures and physical properties, as well as local meteorological conditions. These parameters are incorporated into the microscale CFD model to emulate the potential impacts of heat mitigation strategies on the

Model validation result

The validation results are presented in Fig. 6 and Table 5. We calculated the correlation coefficient (R2), root means square error (RMSE), and mean absolute error (MAE) of the simulated and observed values. The correlation coefficients of Ta were 0.79 and 0.74 for Hoban and Odd, respectively, while those for RH were 0.65 and 0.68. According to recent literature (Liu et al., 2021, Tsoka et al., 2018), the recommended ranges for these metrics are R2 values higher than 0.66 for Ta and 0.63 for

Discussion

This study evaluated the improvement of thermal conditions and pedestrian walkability at high-rise residential complexes by alternative path design options in two high-rise residential complexes. We explain how these findings answer our research questions below and address the design and planning recommendations. We also address limitations and directions for future research.

Conclusion

The impact of various path design mitigation strategies on thermal conditions and pedestrian walkability is analyzed using ENVI-met and ABM simulations. We employed four thermal indicators–Tsurf, Ta, MRT, and PET–and the walkability indicator–PTT. The mitigation strategies examined include reflective pavement, single-row tree planting, and clustered tree planting. The study sites consist of two high-rise residential complexes in Suwon City, South Korea. The results indicate that, among the

CRediT authorship contribution statement

Fengdi Ma: Conceptualization, Methodology, Software, Data curation, Writing – original draft, Visualization, Investigation. Heeyeun Yoon: Conceptualization, Supervision, Writing – review & editing, Funding acquisition.

Declaration of Competing Interest

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that

Acknowledgement

This work is supported by Creative-Pioneering Researchers Program through Seoul National University, the Ministry of Education of the Republic of Korea and the National Research Foundation of Korea (NRF-2021S1A3A2A01087370), Knowledge-based environmental service Program and Climate Change R&D Project for New Climate Regime funded by Ministry of Environment (2022003570007).

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