Zoekresultaten

Cooling Urban Water Environments: Design Prototypes for Design Professionals

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.

Creating prototypes for cooling urban water bodies

When addressing urban heat problems, climate- conscious urban design has been assuming that urban water bodies such as canals, ditches or ponds cool down their surroundings. Recent research shows that this is not necessarily the case and that urban water bodies may actually have a warming e ect, particularly during late summer season nights. There are however indications that water can have a cooling potential if brought together with the right shading, evaporation and ventilation strategies. Yet, it is not clear how this should be achieved. Knowledge on such spatial configurations should thus be developed and made available to design practice. This challenge is directly addressed by the “REALCOOL” project, a research aiming to define design prototypes showing the physical processes behind the e ective cooling potential of urban water bodies, that design professionals can take as conceptual design frameworks.

To what extent are urban water bodies really cooling?

Urban water bodies like ponds or canals are commonly assumed to provide effective cooling in hot periods. Some of the evidence that feeds this assertion is based on remote sensing observations at relatively large scales. Such observations generally reveal reduced surface temperatures of water bodies during daytime, relative to their urbanized environment. This is to be expected because of the extremely large heat capacity of water in combination with its ability to transport heat away from the water surface by turbulent mixing. However, this also implies that the cooling of a water body may proceed only slowly, which may result in higher night-time surface temperatures. This can lead to water bodies contributing to night-time urban heat islands. The existence of a surface-air temperature gradient is a necessary, but insufficient condition for water bodies to influence their environment. In order to noticeably affect the atmospheric temperature, the cooler or warmer air near the water surface needs to be transported to the urban surroundings. Furthermore, for humans such effects are generally only relevant if they are present at a height of 1-2 m. This requires the fetch over the water to be sufficiently large, so that the internal boundary layer can grow to these atmospheric levels. Furthermore, since not only temperature but also wind (ventilation), humidity and radiation contribute to the heat load of humans, possible cooling or heating effects need to be considered in terms of physiologically meaningful quantities, such as the Physiological Equivalent Temperature (PET). Taking such considerations into account, it is no surprise that the effect of water bodies on their atmospheric surroundings are generally found to be small or even nearly absent when considering evidence from atmospheric measurements.Although there are indications that proper combinations of shading, evaporation and ventilation interventions around water bodies can help to keep their surroundings cooler during summer, it is virtually unknown how these strategies can be optimally combined in designs to counter urban heat effectively. The ‘Really cooling water bodies in cities’ (REALCOOL) project explores possible cooling effects of such combinations for relatively small urban water bodies (characteristic horizontal dimension up to a few tens of meters, maximum depth 3m). The goal is to create evidence-based design guidelines of cooling urban water environments — design prototypes — meant for application in urban and landscape design practice.This presentation will address the cooling effects of the design prototypes evaluated with micrometeorological simulations. Special attention will be paid to the cooling effects of the water bodies in the designs. These were assessed using ENVI_MET version 4.1.3., which allows the user to choose the intensity of turbulent mixing of the water. Comparisons with observations and results from water temperature simulations with a model that assumes perfectly mixed water (the “Cool Water Tool”, CWT) showed that enhancing the turbulent mixing in ENVI_MET strongly improves water temperature simulations. Three design experiments were implemented in ENVI_MET: Exp1) testbeds, which are spatial reference situations derived from an inventory of common urban water bodies in The Netherlands, characterized by the shape and dimensions of the water body and the type of urban environment; Exp2) testbeds in which the area occupied by the water was replaced with the paving materials or vegetation flanking the water body in the original testbed; Exp3) design options with optimal combinations of shading, evaporation and ventilation. All simulations were performed for the same set of meteorological conditions, representing a typical heatwave day in The Netherlands. The initial water temperature depends on the water depth and was determined from simulations with the CWT, run for the same heatwave day repetitively until a quasi-equilibrium state was reached.Model outcomes from ENVI_MET were evaluated for the normally warmest period during daytime (around 15:00 CET) and the coolest period during night-time (around 5:00 CET) in the summer, using water temperature just below the water surface and using air temperature and PET at a height of 1.5m. The cooling effect is defined as the difference in air temperature and PET, respectively, between the different design experiments. The differences were computed from the spatial averages over two areas: the area directly above the water surface (Exp1, Exp3) or its replacement (Exp2) and the area directly bordering the water (like quays and sidewalks, called “pedestrian area” hereafter).The simulations with ENVI_MET suggest that the cooling effect of small water bodies on the air temperature is quite small and often negligible (Exp1-Exp2). This is also true for the optimized designs (Exp3-Exp2). The presence of the water body in the testbeds reduced the daytime air temperature in the afternoon by at most 0.8°C directly over the water body and 0.6°C in the pedestrian area (Exp1-Exp2). PET was reduced by at most 1.8°C and 1.9°C, respectively. During night-time, there was a very slight warming effect in a majority of cases, of at most 0.3°C in air temperature. Warming effects in terms of PET were even smaller. The optimized designs led to a reduction of water temperature of at best 0.5°C, relative to the reference situations (Exp1-Exp3). Air temperature was reduced by at most 0.8°C, relative to the temperature in original testbeds. The Physiological Equivalent Temperature (PET) could be reduced by as much as 7°C at 15:00 CET, but this difference was mainly due to shading effects of trees, not to the presence of water.We conclude that small urban water bodies like the ones tested here may not be the most relevant adaptation measure to create cooler urban environments. Their size may simply be too small to have meaningful thermal effects in their surroundings, in accordance with micrometeorological theory on the development of internal boundary layers. Only for water bodies that are sufficiently large cooling effects may become noticeable. This is then also true for possible warming effects. However, the openness of urban water bodies and their surroundings allows ventilation and provides room for trees that provide shade. The combination of these aspects which both lead to cooling effects was found to dominate favourable changes in daytime PET in particular.

Really cooling water bodies in cities

Assessing the influence of floating constructions on water quality and ecology

Large floating projects have the potential to overcome the challenge of land scarcity in urban areas and offer opportunities for energy and food production, or even for creating sustainable living environments. However, they influence the physical, chemical, biological and ecological characteristics of water bodies. The interaction of the floating platforms affect multiple complex aquatic processes, and the potential (negative/positive) effects are not yet fully understood. Managing entities currently struggle with lack of data and knowledge that can support adequate legislation to regulate future projects.In the Netherlands the development of small scale floating projects is already present for some years (e.g. floating houses, restaurants, houseboats), and more recently several large scale floating photovoltaic plants (FPV) have been realized. Several floating constructions in the Netherlands were considered as case-studies for a data-collection campaign.To obtain data and images from underneath floating buildings, underwater drones were equipped with cameras and sensors. The drones were used in multiple locations to scan for differences in concentrations of basic water quality parameters (e.g. dissolved oxygen, electrical conductivity, algae, light intensity) from underneath/near the floating structures, which were then compared with data from locations far from the influence of the buildings. Continuous data was also collected over several days using multi-parameter water quality sensors permanently installed under floating structures.

Innovative water quality and ecology monitoring using underwater unmanned vehicles: field applications, challenges and feedback from water managers

With climate change and urban development, water systems are changing faster than ever. Currently, the ecological status of water systems is still judged based on single point measurements, without taking into account the spatial and temporal variability of water quality and ecology. There is a need for better and more dynamic monitoring methods and technologies. Aquatic drones are becoming accessible and intuitive tools that may have an important role in water management. This paper describes the outcomes, field experiences and feedback gathered from the use of underwater drones equipped with sensors and video cameras in various pilot applications in The Netherlands, in collaboration with local water managers. It was observed that, in many situations, the use of underwater drones allows one to obtain information that would be costly and even impossible to obtain with other methods and provides a unique combination of three-dimensional data and underwater footage/images. From data collected with drones, it was possible to map different areas with contrasting vegetation, to establish connections between fauna/flora species and local water quality conditions, or to observe variations of water quality parameters with water depth. This study identifies opportunities for the application of this technology, discusses their limitations and obstacles, and proposes recommendation guidelines for new technical designs