Climate Resilient Neighbourhood Typologies in Hot-Humid Climate - Case Study of Ha Tinh, Viet Nam

Severe heat stress has been already chronic in many Vietnamese cities, especially the ones located in Central region where extreme hot summer is present annually. As one of the most vulnerable part of this region, Ha Tinh city,  the provincial capital of Ha Tinh, experiences frequent disasters including heat waves that will be intensified and prolonged as a result of climate change, causing more discomfort, sickness and even mortality.

Intervention from urban design is one of the key potential approach to reduce heat, advocate the sustainable cooling and thus protect citizens from extreme heat. Despite several efforts from local authorities of Ha Tinh into climate-adaptive housing development as well as increasing street vegetations and urban parks, comprehensive plans to mitigate urban extreme heat is still missing, and cooling-responsive interventions are never reported in all new construction or redevelopment plan of local neighbourhoods due to the shortage of scientific basis and / or practical guidelines. This is a big gap that requires immediate studies on heat mitigation at neighbourhood scale which should later be showcased as effective sustainable cooling potentials for governors, urban planners and designers.

The neighbourhood resilience is, on one hand, affected by spatial (physical) form, but, on the other hand, should be considered in the trade-off relation with social and demographical aspects. The new development or re-plan of a given neighbourhood provides thus better climate resilience assessed by the outdoor thermal comfort (OTC), while at the same time satisfies the populational demands. How a neighbourhood is physically configured and internally self-regulated forms its particular typology. The global objective of this thesis is, therefore, to develop climate resilient neighbourhood typologies with the focus on the peri-urban region of emerging cities. The objective is the first step toward sustainable cooling at urban level, and can be replicated or tailored to other specific contexts of tropical cities. To achieve the ultimate goal, this thesis targets the following specific objectives: (i) identify current neighourhood typologies, (ii) analyze the thermal performance observed over the neighbourhoods of various typologies, (iii) predict the impact of urban densification on OTC, and finally (iv) propose improved scenarios for better climate resilient typology of a neighbourhood of interest.

For objective (i), physical and non-physical parameters were considered to identify five neighbourhood typologies in the city of Ha Tinh. Types (1) and (2) represent historical urban quarters of very high density, and type (3) is formed by the new development of less compact urban neighbourhood. The two last ones (4) and (5) are located in peri-urban and rural districts highlighted by sparsely-built low-rise buildings and great amount of blue and green factors. Despite the fact that the current spatial form and inner residential organization of these neighbourhoods are homogenous, profound transformation onto neighbourhood typology (4) is foreseen when proposed municipal and master plans of Ha Tinh are realized.

To gain deep insights into the impact of neighbourhood typologies on OTC as stated in objective (ii), measurement campaigns in the years 2019, 2020 and 2021 were carried out in the most critical neighbourhoods of typologies (1), (3) and (4). The results show that the neighbourhood typology has a real impact on meteorological variables namely air temperature, relative humidity, wind speed as well as globe temperature, and consequently results in different OTC level, calculated in Thoms’ DI, WBGT, PET and UTCI. The peri-urban neighbourhood typology (4) experiences considerable better comfort level than the two other typologies (1) and (3). The most determining factor is the green coverage that is observed by far higher in the peri-urban and rural areas if comparing to urbanized and compact quarters. Building shadow cast is also very important to reducing the heat pollution thanks to the potential of blocking solar radiation, especially on the sunny and cloudless days. Strong effects on OTC from the ground materials were not observed although it influences the mean radiant temperature. Grassed surfaces and water results in slightly better comfort than hard and paved surfaces, but in the absences of shades these green/blue elements do not lead to a considerable OTC shift from extreme heat to almost neutral heat ranges. As an important part of neighbourhood typology scheme, the impact of street geometries on OTC, i.e. aspect ratio and orientation, was studied by field measurement and ENVI-Met simulation. The experimental work shows the clear positive correlation between aspect ratio and daytime OTC (higher aspect ratio leads to higher OTC level), but no clear evidence on strong impact of street orientation on OTC could be found. In addition, the influence of street aspect ratio and orientation on nocturnal OTC was not clearly observed. However, the effect of both aspect ratio and orientation can be well discerned from simulations which indicate the NE-SW street direction as the preferable direction for obtaining lower PET. In addition to the experimental and computational work, a survey on the outdoor thermal perception of local people was planned but postponed after only two initial days due to Covid-19 pandemic. Though the gathered interview data is insufficient for a statistical analysis, a general uncomfortable outdoor thermal comfort sensation during  the summer  was observed, revealing the necessity of OTC improvement across the city.

For the prediction of OTC changes due to urban development, a most typical neighbourhood typology was selected. It is the typology (4) that is prone to a radical re-landscape with densification and removals of existing blue and green elements. The as-is and business-as-usual (planned) cases were simulated for analysis of OTC change, calculated in PET. The replacement of large water body (the river) and agricultural field with low-rise detached houses cause an increase in both temperature and PET due to the altering of the evaporation potential. However, the resilient proposal that introduces higher amount of tree coverages in combination with the preservation of river parts results in better PET and a shift of  the OTC level closer to the comfortable sensation. The placement of high-rise apartment buildings may be also a potential strategy for triggering the reduction of diurnal PET values by increasing building shadows and leaving more room at ground level for high-canopy trees and grassed surfaces. Although the thermal benefits of expanded shades by increased building height was proven, the high-rise buildings may come with  potential risks related to higher cooling demand and to higher anthropogenic heat released to the environment, to higher embodied and operational energy, as well as to higher social segregation. Therefore, these issues should be considered in further studies. In addition, parametric studies of street geometry and vegetation were performed, leading to the conclusion on the provision of higher OTC level due to increased aspect ratio by altering shallow (aspect ratio of 1) with deeper urban canyon (aspect ratios of 2 and 3). The PET is even more improved with additional vegetation and further intervention of damped road and pavement surfaces. A comprehensive improvement plan was introduced to the considered, case-study neighbourhood at a larger scale to couple the enhancement of OTC with aspects of mobility, ecology and society. Finally, the thesis develops a workflow that integrates the consideration of heat mitigation into urban plan and design scheme which is highly recommended to local governors and players in urban development practice.