Urban Highways and Urban Heat Island effect



Urban Highways and Urban Heat Island effect1. IntroductionEvolution of Urban HighwaysUrban Highways have evolved greatly. From the 50’s to the 70’s, planners developed Urban Highway networks to relief congested inner cities and to ease journeys between city centers and suburbs. Often elevated or in cut, urban highways relied on limited access to minimize interruptions and maximize flow. However, Urban Highways have proven to erode urban vitality, blight large sections of cities, threaten historic urban neighborhoods, and concentrate air pollution and other nuisances in highly populated areas, threatening people’s health (ITDP, 2012). Since the 90s, several city authorities worldwide have decided to dismantle Urban Highways to prioritize other usages. Concepts such as Complete Streets and Shared Space emerged to promote safe circulation for all users. Those concepts are also broadening the choice of material to be used, bringing back vegetation and promoting other paving surfaces then asphalt and concrete (Smart Growth America 2017).An iconic example of this evolution is the ongoing transformation of the Bonaventure expressway in Montreal, Canada, for which an elevated highway is being replaced by a large urban boulevard. Approximately half of the carriageway surfaces will be removed leaving space for large sidewalks; green spaces and trees, contributing significantly to the reduction of the UHI effect of the area (Projet Bonaventure, 2017).Urban Heat Island, a growing worldwide phenomenonUrban Heat Island (UHI) is described by higher surface or air temperatures in the city centers as compared to the surrounding countryside (Akbari et al., 2015). Difference in temperature between city centers and countryside increases with city size and annual-mean precipitation; humid climate experiencing stronger UHI effect then dry, desert-like climates, especially at night (Zhao L. et al., 2014). As an example, this difference is on average 2,5 °C higher in the center of Paris then in the surrounding countryside (APUR, 2012). On the other hand, in a desert city like Doha, Qatar, proximity of the sea, urban forms and material albedo can invert the UHI effect with some urban area cooler then the surrounding desert (Makido et al., 2016).UHI effects are also likely to increase in the future with global warming. Climate change leads to higher temperatures and longer, more severe, and more frequent heat waves. Urban areas already suffering from the heat island effect will bear the brunt of these harsher heat events (EPA 2017a),Contributing factorsFactors influencing UHIs are (APUR, 2012):Seasonal and weather conditions: In temperate climate, UHIs appear mainly in summer during anticyclone events in which wind strength is low and the sky is free of clouds. Water: Water bodies act as thermo regulator, freshening up the air during summer heat waves and reducing the effect of cold spells during winter.Vegetation: All plants contribute to the cooling of the air. From grass to trees, the evapotranspiration process which consumes energy from the air, acts as a passive air-conditioning system (Météo France, 2012). Furthermore, during sunny days, trees cast shade, creating comfortable space underneath their canopy (De Munck, 2013). Reduction of evaporative cooling is generally thought to be one of the dominant factor contributing to the UHI effect. (Zhao L. et al., 2014).Building and Pavement Material: Building and pavement materials are able to store a great amount of solar energy released at night. The lighter color the material the greater its capacity to reflect sun rays back to the atmosphere. Urban canopy layer: Being defined as the portion of the atmosphere in which stands buildings and urban trees, the Urban Canopy acts as a windbreaker due to higher uneven ground surfaces compare to the surrounding countryside, preventing, especially at night, to cooler wind to mix with hot air trapped between buildings (Rullier 2012).Street Canyon phenomenon and Sky View Factor (SVF): Inert surfaces which are not exposed to the sun due to narrow streets will not store energy. The deeper the street canyon, the lesser the UHI effect will be (Bakaraman et Chang, 2013). However those streets will experience less night cooling due to restrained radiating exchange with the sky. Human activities: Particularly during summer, industry, transportation, air conditioning, all activities consuming energy contribute to the UHI effect. In Paris, human activities contribute to about 20% of the UHI effect.2. Roads characteristics that contribute to UHIIn various urban areas of the United States, paved surfaces cover approximately 37% of urban surfaces (Akbari et al., 2003; Rose et al., 2003; Akbari and Rose, 2001a, b). Highway dimensions and characteristics will significantly impact how highway will contribute to the UHI effect of the urban fabric in which it will be developed. Depending of the urban context, the typology and amount of vehicles, the width of the carriageway, the material used as pavement as well as the amount and type of urban furniture and vegetation implemented in the remaining public space, the road projects can either improve or worsen a pre-existing UHI effect.Urban contextAs seen above, the impact of factors such as the urban canopy layer, the sky view factor and the street canyon phenomenon, the location of the road either in peri-urban, industrial or urban contexts, all those factors will contribute to the UHI effect.In peri-urban and industrial areas located in the outskirt of the city, impervious surfaces are often exposed to direct sun with no adjacent tall buildings that cast shade, unless trees’ canopy are overhanging above them. Typically, strong surface UHI effect will be observed during the day. However because of the high Sky View Factor and the Low Urban Canopy Layer, heat released by this pavement is likely to be easily dissipated at night. On the other hand, streets located in denser central urban districts, depending of the width of the transportation corridor and its orientation can be partly shaded by adjacent buildings reducing the UHI effect.The orientation of the road plays also another role. When the direction of the prevailing winds follows the road’s direction, wind can evacuate and dissipate more effectively heat released by impervious surfaces and play a positive role in reducing the UHI effect (Du et al. 2008).Alternative design conceptsUrban highways are designed to accommodate traffic, provide safety at the speed to which they are intended to be used and meet national standards. Greater number of lanes and greater carriageway width result in more pavement in the transportation corridor which in turn translates to greater UHI effect. However, new trends in Urban Highway Design are considering other requirements then meeting traffic demand and complying to standards for the sole benefits of motor vehicles. Urban highways design is influenced by concepts such as Complete Streets which are designed and operated to enable safe access for all users, including pedestrians, bicyclists, motorists and transit riders (Smart Growth America, 2017) or Shared Space, an urban design approach which seeks to minimize the segregation of pedestrians and vehicles (Wikipedia, 2017). Those concepts tend to reduce the carriageway width, introduce a larger variety of pavement material and provide more space for vegetation.Albedo and pavement Properties of urban materials, in particular solar reflectance (albedo), thermal emissivity, and heat capacity, influence UHI development, as they determine how the sun’s energy is reflected, emitted, and absorbed (EPA 2017b).Urban highways surface material are typically made of asphalt and concrete which approximately only reflect less than 10% of the solar energy (Kushari B. et al, 2011).Vehicle colors and quantityThe amount and the color of vehicles that will use the road has also an impact on the contribution of Urban Roads to UHI effect as heat emitted by vehicles engines added to air conditioning units of adjacent buildings are considered the main source of anthropogenic heat emissions in urban areas (Li et al. 2014). 3. Tools for defining the existing situationSatellite imageryAt a regional level, to cover a whole city, thermal imagery is widely used to quantify land surface temperatures and to monitor the spatial extent and thermal intensity of the UHI effect. Early researches have applied Landsat images, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) images, Moderate Resolution Imaging Spectroradiometer (MODIS) images and Advanced very-high-resolution radiometer (AVHRR). However, because of the coarse- to medium-resolution of the thermal imagery (around 1km), researchers are unable to correlate these temperature data with the more generally available high-resolution land cover classification, which are derived from high-resolution multispectral imagery. Recently, the development of advanced thermal sensors with very high-resolution thermal imagery such as the MODIS/ASTER airborne simulator (MASTER) allows quantifying the relationship between detailed land cover and land surface temperature. (Zhao Q. et al, 2016).Thermographic cameraHand held thermographic cameras are often used to illustrate surface temperatures of any given urban space where UHI effects are suspected. Thermographic cameras can also be used airborne, in plane or helicopter, to provide images of the land surface temperature at a much higher resolution than satellite imagery which can allow investigating for example the contribution of Urban Highways to UHI (Parlow E. et al, 2014).Albedo (solar reflectance) measurementThe solar reflectance of material is now often provided directly by manufacturers thanks to Green Buildings Certification schemes such as LEED or GSAS. Solar reflectance of any material can be measured by using an albedometer. Meterological micro-sensingIn large cities such as Paris, during summer nights a simple transect walk through several neighborhoods, taking regular temperature measurements will illustrate the change in temperature due to the presence or not of vegetation (APUR, 2012). Readings taken from fixed meteorological stations located in the center of a city, the suburbs and the surrounding countryside can also illustrate air temperature variations. 4. Evaluating the impact of Urban Highways on Heat Island Effecta) Microclimate Modeling SystemsThere are various Microclimate Modeling Systems available on the market. For Urban Highway Projects, Microclimate Modeling Systems such as ENVI-MET? can simulate Heat Exchange processes, impact of vegetation and Bioclimatology.As for ENVI-MET? modeling calculation are based on:Shortwave and longwave radiation fluxes with respect to shading, reflection and re-radiation from the building environment and the vegetation;Transpiration, Evaporation and sensible heat flux from the vegetation;Water- and heat exchange inside the soil system including plant water uptake.b) Green buildings rating systemsThe impact of an Urban Highway project can be evaluated following principles developed by gfreen building rating systems such as LEED? (Leadership in Energy and Environmental Design) (North America) or GSAS? (Global Sustainability Assessment System) (Gulf Region)LEED? provides accreditation for reducing the UHI effect through simple calculation using percentages of surfaces. LEED? Neighborhood Development (ND) system, which could be applied to Urban Highway projects, evaluates the following aspects:Shade casted by trees and shading structures;Solar reflectance of material;Percentage of pervious paving.GSAS? uses similar calculation but parameters are differing from LEED? due to the hot and arid climate prevailing in the Gulf countries. Shade casted by trees and structures, as well as solar reflectance of material are also taken into consideration, but pervious paving is not considered, instead Sky View Factor is considered.5. Engineering and nature-based solutions Solutions to avoid, reduce and mitigate UHI effects are now well known and can be adapted to Urban Highways projects. They can be grouped in 3 families of solutions:Road designDivert traffic from areas of cities already prone to UHIs in order to reduce carriageway width and heat emission;Take into consideration prevailing wind directions to canalize wind for evacuating heat;Adjust height/width ratio of the highway corridor to optimize both the Canyon Effect and the SVF factor;Reduce carriageway dimension to strict minimum;Maximize trees and vegetation to cast shade and contribute to the vegetation evapotranspiration effect; Incorporate rain gardens and swales.Material selectionSelect paving material with high albedo;Use pervious paving.Vehicles and usersPromote pedestrians, cyclists and transit riders use;Promote pale color vehicles.ReferencesAkbari H., Rose L.S. (2001a) - Characterizing the fabric of the urban environment: a case study of metropolitan Chicago, Illinois. Akbari H., Rose L.S. (2001b) - Characterizing the fabric of the urban environment: a case study of Salt Lake City, Utah. Akbari H., Rose L.S., Taha H. (2003) - Analyzing the land cover of an urban environment using high resolution orthophotos. Akbari H. (2005) - Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation. Akbari H. et al. (2015) - Local climate change and urban heat island mitigation techniques - The state of the art. APUR (2012) - Les Ilots de Chaleur Urbains à Paris.Bakaraman M.A., Chang J.D. (2015) - The influence of height/width ratio on urban heat island in hot-arid climates.De Munck C. (2013) - Modélisation de la végétation urbaine et stratégies d'adaptation pour l'amélioration du confort climatique et de la demande énergétique en ville.Du M., Sun W., Chen Y. (2008) - Impact of corridor Structure on Urban Heat Island in Beijing, China.EPA (2017a), Climate Change and Heat Islands, accessed 20 January 2017, EPA (2017b), Reducing Urban Heat Islands: Compendium of Strategies, Urban Heat Island Basics, accessed 20 January 2017, ? accessed 28 January 2017, ? - Neighborhood Development v.4, accessed 28 January 2017, B., Kanitpong K., (2011) - Surface Albedo of Bangkok Roads. Li C., Cao Y., Zhang M., Wang J. Liu J., Shi H., Geng Y., (2014) - Hidden Benefits of Electrical Vehicles for Adressing Climate Change. Institut for Transportation and Development Policy (ITDP) (2012) - The life and death of urban highways.Météo France, CSTB, Apur (2012) - ?tude Pluridisciplinaire des Impacts du Changement climatique à l’?chelle de l’Agglomération Parisienne (EPICEA) - volet 1, 2 et 3.Parlow E., Vogt E., Feigenwinter C., (2014) - The urban heat island of Basel seen from different perspectives.Projet Bonaventure, accessed 28 January 2017, F. (2012) - La place du paysagiste en ville dans un contexte de réchauffement climatique global. La question des ilots de chaleur urbains, application à Montréal. Rose L.S., Akbari H., Taha H. (2003) - Characterizing the fabric of the urban environment: a case tudy of Greater Houston, Texas. Shared space, Wikipedia, accessed 28 January 2017, Growth America (2017), What are Complete Streets ?, accessed 20 January 2017, L. et al. (2014) - Strong contributions of local background climate to urban heat islands.Zhao Q., Wentz E. A. (2016) - A MODIS/ASTER Airborne Simulator (MASTER) Imagery for Urban Heat Island Research. ................
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