Towards liveable streets urban canyon microclimate : an empirical study of Colombo

Abstract

Urban Design; Designing an Urban setting, it self can perceive as a single architectural process, not only an individual building, but also the way buildings are related each other and its three-dimensional composition represents the total image of the city. Performances of these solid-void compositions directly effected to thermal behaviour of the city. The metropolization phenomenon at Colombo has provoked deep changes in the field of "Urban design". As well, as changes in thermal comfort levels and the energy balance, leading to an altered atmosphere of the urban area, In Colombo, higher relative humidity values and moderation in wind speed, the daily (diurnal) variation in temperature and diminishing diurnal temperature variation would indicate a growing UHI problem. The phenomena of Urban Heat Island can create unpleasant microclimatic conditions at the pedestrian level. Pedestrian walkways are the key pubic domain of Equatorial towns and cities, where the greatest amount of human contact and interaction taken place. Shading can create a comfortable level, avoids the direct solar radiation. Vegetation in particular tall trees can create a cooling effect, lower temperature and increase relative humidity. The aim of this study is to investigate the effect of Orientation, built mass, tall trees with a wide foliage canopy and water bodies, on the climatic parameters, at the pedestrian level in street canyons, Colombo metropolitan. The research was conducted in either-side pedestrian walkways of two major public realms selected based on its Orientation. Each street canyon consisted with areas of deep, shallow and intermediate canyons, tall dense trees planted, street canyon relatively lacking in tall trees, directly effected water bodies etc. One pair of walkways was located parallel to and, the other perpendicular to, the coastline. Data collected in two days of December 2003: Temperature and relative humidity, by a HOBO-HT data logger, in a well cross-ventilated radiation shield at seven feet height on each pavement, Traverse walking along canyon, by which temperature and relative humidity were, measured using the mobile data logger at midday and night, in Dec 18th and 2200. The results show that during the day hours the temperature values in the canyons with vegetation were up to 2 THI values lower than those measured in the canyons without trees and it was up to 4 THI values lower in Gall face Green, where directly affected by the Ocean. Wind speed in the canyons placed perpendicular to the coastline, was generally higher than in the canyons located parallel to the coastline and blocked by rows of building from the sea breeze, which is a characteristic of Sri Lankan western shoreline. But the effect of vortex phenomena caused by perpendicular winds, can clearly experienced in N-S oriented deep canyons, Wind speed in canyons without trees was generally higher that in canyons with threes due to the windbreak and friction effects of the trees in the latter. The existence of shading devices above the pavements and tall dense trees in an urban environment could result in two opposite climatic effects. Shading devices moderate the air temperature and surface temperatures, preventing the direct solar radiation. In addition, trees act as moderators in both temperature and relative humidity levels towards comfortable levels. This research shows imperative need of interferences, of climatized architectural imlpiceations in the case of street canyons and adjoined pedestrian walkways at the Colombo Metropolitan.