Project

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.

Background

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.

Health

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).