Modelling blue space effects in the urban microclimate

Abstract

Extreme heat and high temperatures have become more frequent and more intense and are projected to continue to worsen as a result of climate change, causing major impacts on human health and urban infrastructure. This increased risk of heat stress is and will continue to be even more pronounced in cities, which are known to exhibit their own climate, typically being warmer than their rural surroundings, a phenomenon known as the Urban Heat Island (UHI).

There is a growing awareness that Nature-Based Solutions (NBS)—actions inspired by, supported by, or copied from nature—can contribute to mitigating the adverse effects of climate change in cities, while promoting biodiversity and enhancing citizens' well-being. The incorporation of green or blue NBS (blue and green spaces, for simplicity) into urban areas is often touted as a way to alleviate heat stress through cooling from evapotranspiration and the increased vertical transport of heat and air. Nevertheless, the mechanisms that underpin blue space performance are not fully understood, as the focus of recent advances in NBS has been almost exclusively directed towards green spaces.

This thesis aims to develop numerical model capabilities to systematically appraise blue space effects within an urban context to aid solutions to the challenges of mitigating climate change, in particular the UHI. A comprehensive numerical approach was developed by adapting and extending an in-house numerical code that is capable of reproducing evaporation and heat transfer in an urban context. An idealised urban neighbourhood is conceived and employed as the case study. It comprises a 7×3 uniform building array of equidistant cubic buildings of the same height. The central building is removed forming an open-square configuration, providing the space for the incorporation of waterbodies of different sizes and shapes. The focus of the analysis is on exclusively addressing five key elements that influence blue space performance, namely air-water temperature difference, local convection regime, geometry, atmospheric stability and pollutant dispersion.

Results have shown that under increased wind speeds, indicative of forced convection regimes, blue space effects mostly concern local temperature and humidity changes. On the other hand, under calm conditions, indicative of mixed convection regimes, blue spaces can have a significant impact on the flow structure, while cool and warm waterbodies exhibit distinct behaviours. A unique feature of warm waterbodies under mixed convection, which can be considered representative of nocturnal conditions, is the break-up of the canopy layer in the open square and the destruction of the skimming flow in the downwind canyons. This is an indication that during late-summer calm nights, warm waterbodies can promote the dispersion and vertical mixing of pollutants from within the urban canopy layer to the atmosphere above by enhancing lateral and vertical ventilation.

This study has taken a systematic approach to appraise the influence of the size and shape of blue spaces on their interaction with the urban surroundings. Results suggest that during late-summer nights inadequately sized warm waterbodies are not capable of breaking up the canopy layer and, consequently, the flow is re-directed downwards and within the open square leading to increased temperature and humidity levels at the pedestrian level, thereby worsening environmental conditions and increasing the risk of heat-related illness and mortality. This observation implies a potential change in the use of waterbodies of different sizes: larger waterbodies, which create a vertical plume and disrupt the skimming flow, are better suited to nocturnal transport of pollutants and accumulated warm air away from the urban surface, while smaller waterbodies, which do not create a strong vertical plume, are better suited to providing localised evaporative cooling due to enhanced horizontal advection. In addition, it has been demonstrated that irregular waterbodies may lead to both greater cooling effects and larger influencing areas.

Understanding the processes and the effects of atmospheric stability on urban flows is key to developing blue spaces as a climate change resilience strategy. Results suggest that increased stability suppresses the height up to which temperature effects are felt. At the horizontal plane, the cooler the waterbody the greater its cooling effectiveness, i.e. the distance up to which its effects are felt, whereas the warmer the waterbody the less the area of influence. Interestingly, under strong stable conditions, the introduction of cool (warm) waterbodies resulted not only in a temperature decrease (increase) above and around the water surface, but also in a warming (cooling) effect elsewhere in the domain. A similar phenomenon was observed when the dispersion of pollutants was assessed, whereby a cool waterbody can disperse pollutants away from the open square only to deposit them in the neighbouring street canyon. Lastly, the results corroborated the hypothesis that the presence of warm waterbodies that are not able to generate a thermal plume strong enough to break up the canopy layer leads to the accumulation of warm air and pollutants at the pedestrian level. These observations have significant implications for the design and implementation of urban blue spaces. Urban planners and scientists should ensure not only to maximise local beneficial effects, but also to protect neighbouring areas, particularly residential ones, from receiving indirect unwanted effects.

Date of Award	15 Nov 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Tristan Kershaw (Supervisor), Jun Zang (Supervisor), Silvana Di Sabatino (Supervisor) & Carlo Cintolesi (Supervisor)