Reducing Risk of Urban Thermal Discomfort by Integrating Sustainability in Urban City Planning
1. Introduction
More than 50% of the world’s population is living in cities [1]. Thus, cities are the most significant energy users in the world. At least 60 to 80 percent of the world's energy consumption is in the urban environment [2]. Therefore, energy-consuming such as fuel energy and electricity consumption can produce heat, and this effect would enhance the temperature of cities, which causes local or even global warming. In addition, using huge scale glazing of building surfaces, wide surfaces of asphalt all over the cities, and very low areas of vegetation would create a warmer area than suburbs. That is why bringing sustainability to the cities is one of the most important actions that need to be considered by urbanism.
2. Literature Review
In 1810, Urban Heat Island (UHI) concept was introduced by Luke Howard, a British scientist, as a phenomenon in metropolises, such as London [3]. Rapid city development, which is caused by industrialization, has a substantial negative effect on the urban environment. Cities’ new conditions caused urban planners to consider the root of problems, such as high density and pollution. After defining the outlines of these problems in developed cities, researchers focused on finding the solutions [4]. Ebenezer Howard was one of the pioneers in Garden Cities Theory. He suggested vegetation and planting not only as a place for people scaping, but also as a means to create a clean and healthy environment [5]. It was the first step to reducing UHI temperature, especially in dense neighborhoods. Then, many countries worldwide established their rules and strategies to improve the decision-making process to design sustainable and well-developed cities.
The United States Environmental Protection Agency (EPA) defined UHI and proposed strategies to reduce UHI temperature. The EPA defines UHI as an area in a city that is warmer than its neighboring countryside (Figure 1). 1.8-5.4°F would be the appropriate differences for the annual average air temperature between an urban area and its surroundings [6]. The EPA has individual and urban strategies for mitigating the UHI effect. Individual strategies are related to houses and residential complexes; while urban strategies are more about comprehensive urban planning. Individual or household activities include using green roofs, vegetating courtyards, reducing waste, using energy-efficient applications, sustainable building materials, increasing shades around homes, and installing cool roofs. Similar strategies are helpful for municipalities or governments to apply in cities. In addition, they can apply cool street pavements to prevent the UHI effect. In 2009, Rizwan presented UHI mitigation tools, strategies, methodologies, and measures [7, 8, & 9].
In 2005, Solecki tried to analyze the role of urban vegetation and reflective roofs on reducing UHI utilizing the CITYgreen application. He found that urban vegetation has a significant effect on mitigating UHI; however, he believed high-density neighborhoods have limited open spaces for vegetation, and those neighborhoods have low open spaces for a wind flow [15]. In 2015, O'Malley used ENVI-met to analyze various mitigation strategies, such as vegetation and water surface on mitigation UHI. He used the resiliency term in his research and considered it as a measurement tool for measuring the effectiveness of those strategies. He provided some practical suggestions to enhance the resiliency of those strategies to make them more effective for cities and people [11].
Other scholars looked at, Air Ventilation Assessment (AVA ), which is the rule for sustainable urban planning that was established in 2005 by the Office of the Chief Secretary for Administration. Some researchers reported the scientific process of the AVA system in his research to clarify this technical method for urban planners [17]. Another effective way for mitigating the UHI effect in metropolitan areas is Urban Ventilation Corridor (UVC). This technique uses natural ventilation in an urban context by pre-designing paths to reduce the temperature of UHI. However, applying UVC in cities is not effective without considering the Cold Island effect around the cities [10], knowing that a Cold Island is defined as an area with low temperature because of a near water body, vegetation area, or both.
3. Discussing UVC techniques
Most studies in UHI mitigation and UVC simulation focus on high-density neighborhood or parallel street urban configuration with equal street width. Ng argues that connecting low-density and low-roughness areas in a city would create a path to direct wind through the city. He believed that the UVC phenomenon has a significant effect on reducing urban temperature [16]. Recently, researchers compared various urban configurations and densities to find the most accurate result related to each methodology and the best urban configuration for applying UVC. For instance, Ramponi chose parallel street configurations with various street widths to simulate airflow at the pedestrian level. The results show that main streets are efficient for applying UVC when wind directions are perpendicular to the main roads. In other words, parallel wind direction to the main streets in higher-level reduces the airflow rate at the pedestrian level in narrow streets. That study was a comparison of different wind directions in different urban configurations [12]. Another study compared various urban densities to find the most reliable Computational Fluid Dynamic (CFD) simulation result. Shirzadi used simulations to analyze cross-ventilation in a city, in 2018. The study demonstrates that CFD simulation and the RANS model are accurate for urban areas with densities less than 0.2 (CA=0.2) (Figure 2). To simulate an urban area with a higher density, it is necessary to develop CFD software [13]. The result is important for finding the suitable neighborhood with appropriate density to be simulated with CFD.
In 2019, He studied the effect of road angle and pattern on the air velocity and airflow direction in a high-density neighborhood. He realized that road pattern in a city like New York has a massive effect on directing air to the urban context. He used numerical and experimental methods for his study and compared the results to show the accuracy of the numerical analysis. His models considered four types of four-way roads with various patterns from 45 to 90 angles (Figure 3). The study results show that the road direction should be close to the natural air direction to simplify the airflow in the context. He used CFD simulation for this study, and it is one of the most recent CFD simulations for UVC.
4. Summary
The literature shows the importance of UHI mitigation in controlling the climate change effect, especially for metropolitans [14]. Nevertheless, there are many gaps in the field that need to go under consideration and research. For example, UVC and the effect of road pattern and angle is a relatively new field of study that has a great potential to expand, especially when considering different urban contexts in a different culture.
References
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