Urban adaptation measures for climate change: study of urban wetlands in view of potential urban cooling

Abstract

Urbanization in the developing world took place at an unprecedented rate which outpaced that of the developed world except for few exceptions. Much of the urban areas of the developing world are in the tropical zone, which has gone through an inadvertent environmental modification due to increased built density. This environmental modification due to high built density resulted in an adverse thermal environment in the urban area which is further exacerbated by global warming due to climate change. This adverse thermal environment in the city resulted in a muchreported phenomenon known as Urban Heat Island. An adaptation measure against Urban Heat Island in the tropical cities of the developing world can be use of the natural cycle to cool down the overheated urban fabric. Dhaka is one of the most densely populated cities in the world, where the juxtaposition of wetlands and built up area coupled together may have an opportunity for creating a viable and distinct character of its own while addressing the challenges of inadvertent environmental modification. Increased built density and reduction of wetlands are making Dhaka’s climate uncomfortable in comparison to its rural surroundings, resulting increasing number of air-conditioned buildings, which in turn elevate urban air temperature and consequent energy demand. Thus, urban land use in Dhaka is directly contributing to Urban Heat Island effect. Coolth produced by urban wetland can be an adaptive measure against urban heat island in Dhaka. This study approaches the problem by identifying wetland as existing and potential cool spot and analyzing their characteristics for cooling efficiency. Remote sensing technique was used to identify the existing and potential cool spot at surface level for analysis. This technique applied to analyze the morphology of the wetland as a producer of coolth with potential to counterbalance for the urban heat. As a part of the study field campaigns were conducted and environmental variables were recorded on the buffer area of selected wetlands indicated in the remote sensing study. Evaluations were undertaken with respect to the design factors such as the orientation of the wetlands, riparian shading characteristics, urban permeability for the cooling effect, as those factors affect the cooling intensity produced by the wetland. Through simulation studies, some aspects are identified which could not be derived from the fieldwork. They are the effect of differential riparian shading on the wetlands, effects of variable temperature and relative humidity on the coolth produced by the wetland. Based on the simulations some urban design recommendations was derived. The research finding that indicated the relationship between some important factors like the interdependency between fetch and inversion height to control inversion layer over the water surface that regulate the urban cooling effect of the wetland at an urban scale. This relationship informs as to what extent urban wetland design might impact the thermal environment leading to possible urban microclimatic cooling. These findings, in turn, might help in the development of certain policy guidelines for the urban designers and planners to more positively impact the urban thermal environment.