Igone Garcia, Adrian Glodeanu / Tecnalia Research & Innovation BRTA. Climate Change Adaptation Team. Energy, Climate and Urban
Simulation models are tools used to predict and evaluate the impact of different urban designs and solutions on certain variables. These simulations are based on numerical models that take into account different variables based on a 3D model of the area. From a baseline, different design alternatives can be simulated to inform urban planning and design decisions through comparative effectiveness analysis. These simulations are a valuable tool for designers, urban planners and policy makers to make informed decisions about the implementation of NbS in urban areas.
Modelling is a well-researched tool at the micro level that can be applied to very different variables, such as air quality and noise, but also to those directly related to the potential adaptation of public spaces to climate change scenarios: Drainage and thermal comfort.
Drainage is a highly conditioned variable, but the characteristics of the basin in which the urban design is planned and the NbS can provide a complement to more integral solutions. Nevertheless, in thermal comfort, NbS can provide a direct added value for adaptation through shading and evapotranspiration, which also affect the mean radiant temperature (TMRT).
This provides an opportunity to explore the role of NbS for thermal comfort in the urban design process, since the increase in heat events is a strong public concern, as some urban configurations create areas where the rising temperatures will have greater impact, also considering future climate scenarios.
The index used to define human thermal comfort is PET (Physiological Equivalent Temperature). It assesses the impact of the thermal environment on the human body, taking into account various variables such as air temperature, humidity, air velocity and radiation, but also the physiognomy and clothing characteristics of an “average person”. It represents the equivalent temperature that an individual would experience in a comfortable indoor environment, given the specific outdoor conditions.
The steps involved in micro-scale thermal modelling are the following:
Characterisation of the study area
The climatic conditions and the land use of the study area are analysed. As input, the time distribution of the different simulation variables should be considered: air temperature, relative humidity, wind direction and intensity and TMRT. The importance of defining the climate and urban design scenarios for the thermal simulation should be emphasised:
- Climate scenarios: in addition to the current scenario, climate change should be analysed in order to assess the cost-effectiveness of NbS over time. This refers to the choice of one of the possible emission pathways (RCP2.6, 4.5 or 8.5) and a time frame (2041–2070, 2071–2100) that would affect the result and the assessment of the effectiveness of the NbS.
- Urban design scenarios: in addition to the baseline scenarios and the final solution, different urban alternatives can be modelled and compared in order to inform the design and quantify the thermal impact of the decisions taken.
Computer simulation process
The characterisation information is integrated into the ENVI-met modelling software. Advanced numerical models are used to simulate different scenarios. A resolution of 1 or 2 metres is required for the microscale and to inform urban design.
Comparison of alternatives and indices
The post-processing of the modelling results provides effectiveness indices that analyse the evolution of the variables over time or make comparisons between different scenarios and alternatives. To facilitate this comparison, the indices should relate to a specific area and the information obtained must be spatially explicit.
Understanding how different building configurations, land uses and vegetation types affect local thermal comfort allows decision-makers to make more informed choices. In this context, the relevance of CFD calculations goes beyond immediate comfort considerations and highlights the central role of informed design strategies in promoting environmentally conscious and resilient cities.
In the following paragraphs, some thermal modelling exercises in Envigado are presented. The main intention is to show how the thermal variable (understood as the comfort of the user of the public space) can become part of the decision variables in the design of new urban spaces. For this purpose, as already mentioned, the spatially explicit information on the comfort level is useful to identify, at the microscale, the specific spaces where there is more thermal stress, but it is also applicable to use an aggregate index that allows us to know, with a single value, which of the different urban scenarios designed is more efficient.
The following summarises the results obtained in Envigado:
Neighbourhood: Mesa. A central space with a mix of commercial activities and residential uses. The buildings provide little shade and natural solutions play a fundamental role. Thermal comfort varies significantly from one street to another, as trees are very unevenly distributed in this neighbourhood.
Current scenario:
- % of surface with PET compatible with a comfortable use (>41°C): 24 %
- Differences of PET in the areas with and without NbS: 19 °C
Climate future scenario:
- Increment of PET in comparison with current scenario: 1–3 °C
- Role of NbS in the area in the future: Reduce PET in 0,5 °C
Neighbourhood: Flores. A small residential area dominated by paved areas and large trees of various species. Houses are usually three storeys high with limited shading.
Current scenario:
- % of surface with PET compatible with a comfortable use (>41°C): 29 %
- Differences of PET in the areas with and without NbS: 21 °C
Climate future scenario:
- Increment of PET in comparison with current scenario: 1–2 °C
- Role of NbS in the area in the future: Reduce PET in 3 °C
Neighbourhood: Central. A commercial and residential area in square blocks dominated by asphalt and narrow pavements. Sometimes, there are trees that provide shade, but in other cases their canopy is not dense enough or absent. There are large differences between neighbouring streets.
Current scenario:
- - % of surface with PET compatible with a comfortable use (>41°C): 29 %
- - Differences of PET in the areas with and without NbS: 16 °C
Climate future scenario:
- Increment of PET in comparison with current scenario: 1 °C
- Role of NbS in the area in the future: Reduce PET in 1 °C
As can be seen from the results presented, urbanism can have a significant impact on the microscale climate. Buildings, vegetation and roads can cause changes at the microscale, and computer simulations can become an interesting tool for diagnosis.
Trees can provide shade, reduce pollutants and air temperature, and increase humidity. In places such as Alcalá and Flores, the number of large trees influences the amount of area compatible with human comfort. This is relevant in the context of urban greening. Trees alone may sometimes not be enough, which is where homes and buildings can influence thermal comfort at the city level. However, as is the case in all the neighbourhoods analysed in Envigado, they have limited shading capacity. This can increase heat stress and exposure to direct solar radiation.
In addition, nature-based solutions are often lacking in the urban areas of Envigado, due to factors such as land use pressure, lack of awareness or lack of funding. In this case, simulation tools can be an important intervention tool to look for potential areas to implement new NbS. They can be installed in relation to the rectangular block layout of neighbourhoods, particularly in the central and mesa areas. This arrangement can create wind tunnels, but can also increase wind chill. In addition, the layout of the central areas can create canyon effects that need to be studied in order to prevent NbS from creating thermal discomfort.
In general, urbanism and thermal comfort at the microscale level are linked, and in the management of urban interventions, urbanism can have a significant impact on the climate at the microscale level.
Article developped in the framework of the INTERLACE project.