Cities are hotter than the surrounding countryside. This so-called heat island effect can lead to heat stress during hot weather in the summer. Climate change will probably exacerbate this effect, along with the accompanying inconvenience and health problems.
The municipality of Arnhem commissioned a study, which has been going on since 2008, to assess the way in which heat affects the city. This study has revealed that the temperature difference between the countryside and the city centre is greatest in the evenings and can easily amount to more than 7 degrees. The differences are smaller during the day, but the highest temperatures are measured during the course of the afternoon. In the sun and out of the wind it can easily feel 15 degrees warmer and the risk of heat stress is then considerable.
The so-called Heat attention map shows which measures can be used in Arnhem to reduce the heating effect, as well as how cooling areas can be protected. The map and the ambitions relating to heat and the urban climate are included in the 2020-2040 Structural Vision which the municipal Council of Arnhem adopted at the end of 2012.
What is new?
Arnhem is one of the first cities in the Netherlands to have investigated the problems relating to heat stress and to have anchored the approach to that problem in policy. The guideline for all the areas which are most sensitive to heat is that construction projects must not have a worsening effect on the urban climate. More generally the municipality must take every opportunity to take account of the heat stress problem in public space and in the context of construction projects.
State of affairs
The Heat attention map of Arnhem combines, among other things, wind direction and wind speed with information about the city, such as height differences, neighbourhood composition and land use (water and greenery). The map divides the city into three categories based on sensitivity to heat. The map's greatest added value is that it shows at a glance where it easily becomes (too) hot and where not, and why that is the case.
All spatial projects are assessed by the municipality against the principles of a climate-proof city and the ambitions laid out in the 2020-2040 Structural Vision. Risky areas are subject to the condition that the urban climate is not allowed to worsen. In the most sensitive areas no more construction is permitted, or only subject to strict conditions (including compensation).
The municipality monitors the urban climate by:
- Focusing on greenery in the areas which heat up easily (particularly green façades and greenery on the ground), less paving, other use of materials, shading and more water. Also maintaining or reinforcing through-breezes wherever possible, for example by creating access for the wind.
- Ensuring that the open areas (such as the floodplains) and the parks on the periphery of the Veluwe National Park remain open so that they retain their cooling effect on the city.
Basis of support
In the past few years a broad basis of support has been created for a good urban climate, both from the perspective of municipal politics and society in Arnhem as a whole. Everyone understands that lengthy hot periods are unpleasant for everyone and even unhealthy for some. In practice it has transpired that the policy for a better urban climate links up well with Arnhem's policy on greenery and water. Although keeping the floodplains green and other green areas on the edge of the city had been policy for some time, armed with the detailed knowledge of heat in the city the municipality now has an extra and properly substantiated argument for maintaining and strengthening greenery and nature and for improving water management. Green has acquired an extra quality as it were, namely that of climate green.
Small-scale projects have since been realised to improve the urban climate locally, for example:
- Vertical greenery in Spoorhoek. There is little greenery, and little space for greenery, in the Spoorhoek district. However, there are plenty of buildings with bare end walls. Green façades have been created on two of these since 2010. The idea is for more to follow.
- In the district of Geitenkamp the combined sewer is being replaced. This opportunity is being taken to reduce the risk of flooding not only on the (raised area of) Geitenkamp, but also in the districts located further down the hill. The paved public area is to be disconnected as much as possible in the form of, for example, rain gardens, wadis and infiltration wells. The municipality and the Rhine and IJssel water board are working closely on this project. The municipality is engaged in conversations with the housing corporations to see whether the roofs can be disconnected. In the Reestraat an experiment is taking place with various types of permeable paving (including demonstrations with simulated rain showers, which is an effective way to raise awareness).
- In the city centre, the Arnhem City Centre Platform [Platform Binnenstad Arnhem], Arnhem6811 residents' platform [bewonersplatform Arnhem6811] and the municipality have joined forces. Shopkeepers, real estate owners and residents are being encouraged to make the city centre green with an inspiring Green Menu [Groene Menukaart].
The turn of Arnhem's residents
Since mid 2016 the Climate-proof Arnhem platform [platform Arnhem Klimaatbestendig] (whose members come from government bodies and social organisations) have been working in the city to make residents, entrepreneurs and institutions aware of their own role and responsibility when it comes to making their own garden or land climate-proof. After all, with specific help and support, small investments at local level can make a significant contribution to a climate-proof, and therefore heat-proof, city.
The Royal Netherlands Meteorological Institute (KNMI) predicts that, due to climate change, the centre of the country will experience between seven and fifteen days of tropical temperatures each year by 2050, as opposed to four now. The number of hot summer nights (20 degrees Celsius or more) will also increase. A heatwave, like the one in 2003 which caused many deaths in Europe, will be a more common occurrence in the future.
Cities are always several degrees hotter than the surrounding countryside. In the summer this can lead to the so-called heat island effect. Stone and asphalt easily absorb the sun's rays and then emit them as heat. If there is poor ventilation, the heat will not dispel.
Whether a real heat island is formed depends on many factors, but a large number of buildings, a large paved surface area and a small amount of greenery will certainly make the situation worse. The hottest areas are roads and streets, industrial estates and harbours. Flat-roofed buildings with asphalt roofs also get extremely hot (for example shopping centres). Research in the Bergpolder-Zuid district of Rotterdam revealed that streets with an east-west orientation stay much cooler because, on hot days, the easterly wind provides ventilation.
Wind chill factor
The wind chill factor is particularly important for heat stress. Apart from the measured temperature, the wind chill is also dependent on factors such as the wind, shadow and heat radiation from paving or buildings. Land use, the materials used (for example asphalt or bitumen), use of colour and height differences also play a role. Trees and bushes also create shade. In addition, greenery and open (unpaved) soil are important for the urban climate because they affect water management. A lot of water evaporates from plants and that too has a cooling effect. A lawn also emits little heat.
Open water is not a source of cold (which is paradoxical because it does cool you down if you jump into it). The water absorbs a lot of heat but also emits it again, just like buildings and asphalt.
Heat stress can result in health problems, ranging from mild problems such as reduced concentration and tiredness to serious problems as a result of, for example, dehydration and breathing difficulties. The mortality rate is higher than normal during hot summers. In the Netherlands the mortality rate during the heatwaves over a period of almost 20 years (1979-1997) increased by 12% to approximately 40 deaths each day. According to Statistics Netherlands (CBS) between 1,400 and 2,200 more deaths were reported in the Netherlands than normal in the summer of 2003 as a consequence of the higher temperatures. An increase can be observed throughout Europe and more people have died from heat than from flooding since 2000. (By contrast fewer people die in the winter because the winters are now milder).
The human body has two cooling mechanisms, namely dilating blood vessels and sweating. In times of extremely high temperature or wind chill these mechanisms may not be able to cope.
Renal disorders may occur (due to dehydration) and people may experience respiratory problems (caused, for example, by increased smog with ozone and particulate matter).
During heatwaves, disturbed sleep can result not only in general malaise and tiredness, but sometimes also in illness and/or the flaring up of existing chronic disorders, such as heart failure or COPD.
Groups at risk are the elderly, the obese and people with heart and blood pressure problems. In addition to that, people who have physically very demanding jobs or use alcohol, medicine or drugs also run a higher risk.
'Values of green and blue in the city' city deal
Urban planning with greenery and water is a powerful way of making cities climate-proof. Without water and greenery there will be no ecosystem services such as cooling, water storage, natural heritage and outdoor recreation. The importance of greenery and water for the locational quality for residents and companies is therefore considerable. That importance is expressed in monetary terms in the TEEB city tool , which has been in use for some time. TEEB stands for The Economics of Ecosystems and Biodiversity. In the 'Values of green and blue in the city' city deal seven municipalities, private parties and knowledge institutes share user experience and know-how so that the TEEB city tool links up even better with implementation in practice. The participating municipalities are Apeldoorn, Amersfoort, The Hague, Dordrecht, Haarlem, Rotterdam and Utrecht
Reducing heat stress as part of sustainable spatial planning
Similarly, to reducing flooding and desiccation, reducing heat stress is seen as an element of climate adaptation and is therefore a factor which has a bearing on sustainable spatial planning. Smart measures to reduce heat include:
- More trees, vertical greenery and different plants, including in private locations (gardens) and trying to increase the amount of greenery when maintaining roads and streets.
- Allowing more water to infiltrate into the soil (disconnecting, infiltration, wadis etc.).
- Thinking carefully about scope, location and design of buildings and paving. Taking account of the ventilation coming in from the countryside when (re)developing districts.
- Green and heat-reflecting roofs instead of asphalt roofs. Sunblinds for windows and doors and improved insulation in houses.
Many of these kinds of measures are the task of municipalities who are responsible for managing public space. However, private individuals can also do a lot. See, for example, the green menu. In some municipalities private individuals can obtain a subsidy, for example for the construction of green roofs or green walls (see also the practical example, Improving rainwater discharge in the city.
Green measures are attractive because greenery can fulfil various functions and provide access to numerous ecosystem services. Although planning at urban level is important in this respect, the effectiveness of such measures depends very much on local circumstances. When redesigning a street, the focus is on, for example, the building (the type of bricks, insulation), properties of the roof and the paving, the orientation (sun ingress, ventilation), the presence of sunblinds and greenery and water in or near the street. Often, a combination of measures is required.
This is also reflected in the climate app, developed by Deltares, the KNMI and three consultancies). One of the related adaptation goals is to reduce heat (in addition to reducing flooding and drought).