Spatial planning options to mitigate the effects of urban heat Islands: a case study of Delhi

Abstract

The Urban Heat Islands are considered a critical factor contributing to the rise of temperatures in the urban areas and hence impacts heat-related mortalities and climate change. The difference in temperatures experienced in the urban areas and the surrounding rural areas can be due to many factors such as climatic factors, urban morphology, presence of permeable surfaces, lack of vegetation, etc. The more the city gets urbanized, the more it poses a threat of witnessing the formation of urban heat islands. Various researches on urban heat islands focus on parameters such as temperature, humidity, vegetation and canopy, etc. Research in terms of urban morphology assessment and simulation of microclimate is lacking. This study focuses on correlating urban morphology with climate and using software simulation as a tool for identifying suitable strategies with the degree of impact it would have for each strategy against urban heat islands at the neighbourhood level. Delhi, the capital of India has been selected as the case study area, as it has recorded rapid urbanization over the past decades. In India, the national capital territory of Delhi is the most urbanized with 97.5% population living in urban areas. Also, in the past decade, Delhi has witnessed an increase of high-density built-up by approximately 20%. There are many studies already conducted in Delhi for analysing urban heat islands, but no study has focused on suggesting mitigation measures at the neighbourhood level. In the previous studies on Delhi, the urban heat island intensity was observed to be 3 ºC to 8 ºC and also there is a presence of diurnal variations among temperatures. Through the land surface temperature analysis conducted on satellite imagery, it was observed that the regions of South-western and Western Delhi showed high-temperature ranges in the day time whereas Eastern and Central Delhi showed high-temperature ranges in the night time. On comparing the land surface temperature results with the landuse landcover (LULC) change analysis, it was further observed that the cause of temperature increase in Western and South-western regions is due to large chunks of fallow land which are displaying higher temperature whereas in the Eastern and Central regions the high temperatures were observed due to the presence of high-density built-up and industries in the Eastern region. From the literature review, Delhi was divided into 5 Local Climate Zones depending on urban morphology. For further localization of study area in Delhi, Land Surface temperature analysis, landuse landcover analysis and local climate zones were assessed to select two wards from the Eastern region for detailed study i.e., Vivek Vihar and Ram Nagar having distinct characteristics. The Vivek Vihar ward falls in LCZ 5 which has open mid-rise urban morphology and Ram Nagar ward which falls in LCZ 3 and has compact low-rise urban morphology. The analysis was carried out mainly for two categories i.e., urban morphological analysis (building height, materials, surroundings, etc.) and analysis of climate variables (temperature, wind pattern, sun-directions, etc.).For the simulation and scenario-building of different mitigation strategies, ENVI-met (a Computational Fluid Dynamics-based) software was used for climatic modelling and predicting the best-case scenario for the mitigation of selected case study areas at the neighborhood level considering respective microclimate. The scenario-building for mitigation strategies was done considering numerous parameters such as temperature, wind pattern, materials used on buildings and roads, soil type, etc. Mitigation strategies such as green roofs, vegetation based, etc. were simulated and compared against the present scenario along with their degree of cooling of each mitigation strategies for the study area. These mitigation strategies can then be implemented using Development Control Regulations and Building Byelaws for areas sharing similar climate and settlement typology.