Cooling Urban Water Environments: Design Prototypes for Design Professionals

Cooling Urban Water Environments: Design Prototypes for Design Professionals

Cortesão, João; Lenzholzer, Sanda; Klok, Lisette ; Jacobs, Cor; Kluck, Jeroen

Publication date 2018

Document Version Final published version Published in

PLEA 2018: Smart and Healthy Within the Two-Degree Limit

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Citation for published version (APA):

Cortesão, J., Lenzholzer, S., Klok, L., Jacobs, C., & Kluck, J. (2018). Cooling Urban Water Environments: Design Prototypes for Design Professionals. In E. Ng, S. Fong, & C. Ren (Eds.), PLEA 2018: Smart and Healthy Within the Two-Degree Limit: Proceedings of the 34th International Conference on Passive and Low Energy Architecture (Vol. 2, pp. 520-525).

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Cooling Urban Water Environments:

Design Prototypes for Design Professionals

JOÃO CORTESÃO ¹ , SANDA LENZHOLZER ¹ , LISETTE KLOK ² , COR JACOBS ¹ , JEROEN KLUCK ²

1

Wageningen University and Research, Wageningen, The Netherlands

2

University of Applied Sciences Amsterdam, Amsterdam, The Netherlands

ABSTRACT: This paper presents five design prototypes for cool urban water environments developed in the ‘Really cooling water bodies in cities’ (REALCOOL) project. The REALCOOL prototypes address an urgent need: urban water bodies, such as ponds or canals, are often assumed to cool down their surroundings during days with heat stress, whereas recent research shows that this is not always the case and that urban water bodies may actually have warming effects too. There are, however, indications that shading, vaporising water, and proper ventilation can keep water bodies and their surroundings cooler. Yet, it is necessary to explore how these strategies can be optimally combined and how the resulting design guidelines can be communicated to design professionals. The REALCOOL prototypes communicate the spatial layout and biometeorological effects of such combinations and assist design decisions dealing with urban water environments. The micrometeorological simulations with Envi­met showed that the prototypes led to local reductions on daytime PET from 1 °C to 7 °C, upon introducing shade. Water mist and fountains were also cooling solutions. The important role of ventilation was confirmed. The paper discusses and concludes about the use of the prototypes as tools for urban design practice.

KEYWORDS: Urban Heat, Water bodies, Thermal sensation, Prototype, Research Through Design

1. INTRODUCTION

Urban water bodies such as ponds or canals are commonly believed to solve urban heat problems and improve human outdoor thermal sensation. This assertion is often based on the claim that urban water bodies necessarily have a cooling effect [1­3]. Recent research shows that the cooling effect of most common urban water bodies during warm summer periods is quite limited over day, in particular when perceived temperature is considered [4], and often induces a night­

time warming effect [5­7].

Nevertheless, there are indications that under specific circumstances water can have a cooling effect during summer [7­9]. Shading, vaporising water (e.g.

spraying), and proper natural ventilation might help to keep urban water bodies and their surroundings cooler [10­12]. Yet, it is necessary to explore how to combine these strategies in urban design and communicate the resulting design guidelines to design professionals.

The “Really cooling water bodies in cities”

(REALCOOL) research project explores the most effective combinations of shading, water vaporisation and natural ventilation around urban water bodies, and makes them available to designers as design prototypes. The REALCOOL prototypes deal with keeping urban water environments as a whole — made up of water and other spatial features such as vegetation or ground cover — cooler during heat stress periods. The prototypes are animated 3D scenes depicting the layout and biometeorological effects of such combinations and work as evidence­based guidelines. The aim is to inform

design options targeted at improving outdoor human thermal sensation around urban water bodies.

This paper presents five of these prototypes. Light is shed on the micrometeorological processes and design principles behind the cooling effects of urban water environments.

2. METHODS

The REALCOOL team gathered experts in bioclimatic urban design, urban meteorology, water­atmosphere interaction, and 3D visualisations. The project followed a Research Through Designing (RTD) approach: an iterative process where the former iteration informs the latter [13]. In REALCOOL, quantitative and qualitative methods were combined and allocated in the different iterations.

The REALCOOL RTD (Fig. 1) consisted of four iterations comprising designing and testing stages.

Figure 1: the REALCOOL RTD process.

Designing involved experimenting with different combinations of shading, vaporisation and ventilation strategies around water. The overarching design aims were to reduce the thermal load placed upon people by increasing shading, cooling the air through water vaporisation and stimulating natural ventilation.

The design experiments (hereafter referred to as

‘prototypes’) were developed over 3D spatial reference situations — representative Dutch urban water bodies — that work as ‘testbeds’. Working with these testbeds made the prototypes replicable. The testbeds were identified across nine cities on clay and peat soils (where most surface waters can be found) and with clear urban heat island effects. Eight types of water bodies were identified in four categories: canal, wide canal, ditch and pond. This number was doubled as to include east­west (EW) and north­south (NS) orientations. These resulting sixteen water body types were set as the REALCOOL testbeds.

Sketches, 2D drawings, 3D visualisations and physical models were used for producing the prototypes. A design matrix rated the efficiency of each design option in fulfilling the goal of the research. All design options addressed a typical Dutch heatwave day, with a maximum temperature higher than 30 °C. The chosen date was the 23

of June, just after the summer solstice, thus, near the annual maximum of solar elevation. Wind direction was taken to be East as this is the predominant wind direction during heatwave days in the Netherlands.

Testing included expert judgments and micrometeorological simulations on the cooling effects of the prototypes, and their assessment by external practitioners on parameters commonly encountered in practice.

The expert judgements involved a close dialogue between urban designers and meteorologists on the most influential biometeorological issues of the prototypes. The prototypes were revised accordingly and the resulting biometeorological effects quantified through simulations in Envi­met software. The Envi­met Winter1617 (V4.1.3) release was applied as it enables simulation of turbulent mixing and transport in the water column and allows adjusting the light absorption characteristics of water. The main evaluation criteria for the simulations was the Physiological Equivalent Temperature Index (PET). The simulations were performed for 3.00 p.m. (when the maximum air temperature is reached) and 5 a.m. (when the minimum air temperature occurs).

The external assessment of the prototypes was made by representatives from consultancies, a health institute, municipalities and design offices during design workshops at the end of each iteration. Aesthetical appeal, functionality, costs, maintenance requirements and public health effects were the assessment criteria.

The results derived in design principles for refining the prototypes.

As shown in Figure 1, in iteration 1 designing dealt with exploring the maximum cooling potential possible.

Testing involved expert judgements and Envi­met simulations. The bioclimatic design principles for the subsequent iterations were set and discussed during the design workshop.

In iteration 2, designing dealt with refining the cooling effects from iteration 1 and adding increase of rainwater storage as a design criterion. Testing quantified the resulting cooling effects (Envi­met simulations) and water storage increase (expert judgements), and finished with the design workshop.

In iteration 3, designing combined cooling effects and water storage with urban design criteria. Psychological cooling (enabling people to get closer to water) was added as a design criterion. Testing involved Envi­met simulations, expert judgements and design workshop. An online visual inquiry (to assess how attractive the general public would find hypothetical environments resulting from the prototypes) and a ‘reality check’ (to test the applicability of the prototypes to practice by applying them in real projects) were additionally used.

Iteration 4 dealt with setting the final prototypes and developing their 3D animated scenes, i.e. the REALCOOL final output. Envi­met simulations were used to quantify the final cooling effects.

3. RESULTS AND DISCUSSION 3.1. RTD process

The Envi­met simulations for the testbeds suggest that the cooling effect of small urban waterbodies on air temperature is quite small and often negligible — about 0.5 °C or less in air temperature and 1 °C or less in PET at 1.5m above the water surface. In pedestrian areas (e.g.

quays), cooling effects were even smaller. These findings are in line with previous studies reporting the limited cooling effect of water [4, 14]. However, the prototypes led to reductions on PET from 1 °C to 7 °C at 15.00h, at 1.5 m above the water surface, locally, upon introducing shade.

These results suggest that there is little that can be done through design to cool down water and, thereby, make it cool its surroundings. What urban design can do is to create urban water environments — an ensemble of water, greenery, paving materials, etc. — offering improved conditions for human thermal sensation during heat stress periods by combining shading, vaporisation and ventilation strategies around water. Hardly any PET reduction in the pedestrian areas was due to cooling effects from water itself. The computed PET reductions were mainly due to the shading of trees. Water fountains and sprays also had significant local cooling effects.

Furthermore, the simulations confirmed the important

role of ventilation: openness enhances cooling by wind during hot periods.

Trees are vital cooling elements but may also hamper ventilation and, thus, increase PET. During heat stress periods, air should be able to flow over the water body as unobstructed as possible as to reduce PET. Finding the right balance between shading, vaporisation and ventilation appeared to be a major design challenge.

Often, slight spatial changes can result in significant PET differences. The synergetic balance between the three strategies requires a holistic view on the biometeorological effects of design options.

The results from the simulations suggest that vaporising water from sprays and fountains can be a cooling solution. Locally, the cooling can be, typically, 3­

5 °C in PET, with a spatial extent depending on wind speed and direction. Water mists are more effective than fountains. These results are also consistent with earlier reports in the scientific literature [11, 15, 16].

The outcomes of the design workshops indicated positive aesthetical qualities and functionalities to the prototypes. The former relates mainly to the diversity and excitement brought by vegetation and the latter to increased recreational activities near/at the water.

However, the prototypes were considered more costly and requiring more maintenance than the testbeds. The prototypes were revised for reducing costs while maximising cooling effects. Regarding public health, the prototypes were considered to increase the chances for physical exercise and social cohesion, and to foster human encounters. Finally, the psychological ‘cooling’

from being close to water was seen as a relevant strategy.

These results show that creating cool water environments does not conflict with common urban design criteria and may even enhance them. Designers should articulate biometeorological effects with these criteria according to assignment, context and their own design ‘signature’.

The online visual inquiry counted on the responses of 1210 people. The statistical analysis showed that 12 environments (Fig. 2) were perceived as more attractive than current situations (testbeds) and 4 as attractive as current situations. Beauty, harmony, excitement and height­to­width ratio were the main reasons. The results indicated, however, the need for design refinements on excitement and chances for interacting with water.

Figure 2: Example of environments depicted in the inquiry.

Regarding the reality check, the Envi­met simulations and the assessment matrices suggest that, overall, the prototypes lead to site­specific cooling effects and are suitable to practice.

3.2. The REALCOOL prototypes

Identifying testbeds and, afterwards, undertaking multiple designing and testing stages resulted in the REALCOOL prototypes. From the sixteen prototypes, five representative examples of each water body type will now be briefly described, focusing on the relationship between design options and microclimate.

Canal

Figure 3: REALCOOL prototype CANAL1 EW.

CANAL1 EW (Fig. 3) is an averaged 40 m wide canyon

with a central 20 m wide water body bordered by two

symmetrical and high quays. The southern quay consists

of steps towards the water whereas the northern is a

grassed slope. This brings people closer to water and

increases rainwater storage capacity by 275 m

(per

50 m).

The 12 m wide tree crowns shade the intended sojourn locations, i.e. the areas near the water. On the southern quay, shade is provided throughout the day over the steps; on the northern quay, there is more exposure to sun since some people might prefer it.

Water vaporisation is provided as water mist along the northern edge and (well­kept and well­watered) grass provides evapotranspiration and a cool surface.

This is aimed at offsetting the higher exposure to direct solar radiation by increasing evaporative cooling.

The shape and pace of trees (15 m) as well as the absence of further built obstacles allow wind to flow unobstructed along the canyon. Ventilation and nocturnal radiative cooling are, thus, enabled.

Figure 4: REALCOOL prototype CANAL3 NS.

CANAL3 NS (Fig. 4) is an averaged 10 m wide canyon where the water body is bordered directly by buildings.

Pedestrian platforms (e.g. wooden deck) were installed on both sides to allow people to be closer to water.

Rainwater storage was not a criterion suitable for this canal type due to its confined layout.

Shade is provided throughout the day by the buildings and, at heat peak hours, by green shading devices (e.g.

pergolas) on both sides of the canal.

Water vaporisation is provided as water mist distributed on sunlit areas between the shading devices.

When made up of greenery, the shading devices also enable evapotranspiration.

Due to the high height­to­width ratio of the canyon, the design elements are not likely to have a large effect on wind flow. Ventilation is anyway allowed by interspersing the shading devices.

Wide canal

Figure 5: REALCOOL prototype WIDE CANAL1 NS.

WIDE CANAL1 NS (Fig. 5) is an averaged 62 m wide canyon with a central 12 m wide water body bordered by symmetrical, slightly sloped and easily accessible green areas. The green slopes along the water are lowered and, thereby, water storage capacity is increased by 250 m

(per 50 m).

The different crown shapes (8 m and 6 m wide) shade the sojourn locations throughout the day.

Water vaporisation was not included since from aesthetical and functionality viewpoints water features may not be desirable in this waterbody type.

Ventilation is enabled by the absence of further built

obstacles and by the shape, pace (15 m) and distribution

of trees. Smaller and larger trees are interspersed to

avoid a continuous green cover. Moreover, trees are

grouped to create wind flow channels.

Ditch

Figure 6: REALCOOL prototype DITCH1 EW.

DITCH1 EW (Fig. 6) is an averaged 22 m wide profile with a 3 m wide water body bordered, on one side by a sloped green area (public domain) and, on the other side, by private backyards (private domain). The green public area (southern side) has a formal character (alignment of trees and steps between the green area and the roads) and grants easy access to water. Together, green slopes and steps on the Southern side and slope on the Northern side increase water storage capacity by 350 m

(per 50 m).

The different crown shapes (5 m and 4 m wide) shade the sojourn locations on the southern side. Due to the low width of the water body, the northern side benefits from this shade.

Water vaporisation was not included for the same reason as for the two previous prototypes.

Ventilation is enabled by the absence of further built obstacles and by the shape and interspersed distribution of trees (spaced by 5 m) that allows wind to flow unobstructed along the canyon.

Pond

Figure 7: REALCOOL prototype POND EW.

POND EW (Fig. 7) is a shallow, averaged 30 m wide and 40 m long water body bordered by outdoor public spaces. Rainwater storage was not a criterion suitable for this water body due to its confined layout. The interaction people­water is enhanced by pedestrian platforms (e.g. wooden decks) following hypothetical desire lines (i.e. preferential crossing routes) over the water body.

Shade is casted over the sojourn locations by trees (8 m wide tree crowns) placed next to the platforms.

Water vaporisation is provided by water mist nozzles along the platforms and by fountains in sunlit areas.

Additionally, trees provide evapotranspiration.

The spacing between trees and the absence of further built obstacles enable proper ventilation.

4. CONCLUDING REMARKS

The REALCOOL prototypes can assist design professionals in creating urban water environments with improved thermal sensation during heat stress periods.

Resulting from an RTD, the prototypes can help legitimising design decisions without the need to embark on further investigations on the topic.

Two remarks should be made about the

micrometeorological data presented. Firstly, the

assumptions on the cooling effects of water do not

comprise large water bodies like rivers or lakes. Secondly,

the prototypes are generic layouts and the results from

the simulations indicate generic cooling effects that cannot be directly applied in specific situations — the magnitude and spatial pattern of cooling effects may be modified in specific situations (e.g. the ultimate cooling from the shading of trees depends on the exact location of the trees).

The prototypes work as ‘half­products’ between general design guidelines and site­specific solutions and should be regarded as conceptual frameworks rather than prescriptive tools. For example, the proper articulation of cooling principles with aesthetics, functionality, costs or maintenance can only be made for site­specific cases. It is up to designers to creatively translate the conveyed design principles into end­

designs.

ACKNOWLEDGEMENTS

This study is part of the research programme Research through Design with project number 14589, which is financed by the Netherlands Organisation for Scientific Research (NWO), the Taskforce for Applied Research SIA and the AMS Institute.

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