The top 10 wettest months since 1960 were therefore all after 1997. The wettest month was August 2006. Almost 5,400 mm of rain fell then. That is 67 times more than the average rainfall for August! The wettest day was 26 August 2010. On that day, 895 mm of rain fell in a single day. That is more than the average annual rainfall. Heavy rainfall can cause all sorts of problems in a town like Apeldoorn. Fortunately, there are a number of measures you can take to prevent flooding.
Here are some measures you can take to prevent flooding
Water roof
A water-retention roof is a green roof with an additional layer beneath the substrate layer to store rainwater. On a retention roof, rainwater is stored during a downpour. The water remains beneath the planted layer. In this way, a retention roof helps to capture rainwater in the city.
A retention roof has a drainage system and usually empties within 24 hours. This reduces the peak flow of roof water into the sewers. A retention roof is fitted with an overflow for heavier downpours. It is also possible to install retention roofs and polder roofs without vegetation. In that case, we refer to them as ‘water roofs’. Retention roofs are generally heavy. When installing a retention roof in existing or new-build situations, it is therefore necessary to assess on a case-by-case basis whether additional structural measures are required.
The polder roof
The polder roof is a retention roof fitted with a control system linked to the weather forecast. It drains rainwater just before a shower is forecast, so that the retention roof remains full for as long as possible. A prototype of the polder roof has been built in Amsterdam. This roof features a dynamic control system, which has the advantage of allowing water to remain on the roof for longer, enabling it to be used for irrigation, infiltration and cooling. Thanks to the dynamic control system, the roof can also be emptied when frost is forecast.
Rainwater pond
Rainwater ponds temporarily collect rainwater and release it slowly. By creating a rainwater pond in your garden or neighbourhood, you can prevent flooding.
Pond in the garden
Depending on the intended purpose of the pond and the type of soil, various designs are possible. Natural ponds can cope with greater variations in water level. At a depth of 1.5 metres, the water warms up only slightly, which helps to maintain good water quality. If desired, this is also deep enough for fish to survive, as the bottom part of the pond does not freeze over quickly in winter. By making the banks of the pond as gently sloping as possible, it is easier for animals (such as frogs) to climb out of the pond. It is also safer for children. It also allows a greater variety of plant species to grow in and around the pond. Not all pond plants thrive in rainwater. It may therefore be a good idea to first see which plants grow naturally in and around the pond. This helps to create a natural balance.
Groundwater-related problems
Find out what you can do if there is groundwater in your crawl space or cellar.
Read morePlant a tree
In urban areas with a high proportion of hard surfaces, rainwater struggles to infiltrate the soil. An increase in the number of downpours means the sewer system is overloaded more frequently. It is therefore important – particularly in urban areas – to incorporate plenty of greenery. Trees, in particular, absorb a great deal of water thanks to their extensive root systems. A red beech, for example, absorbs as much as 27,000 litres of water a year! This significantly reduces the risk of flooded streets and an overloaded sewer system.
Soften and green up
If 10% of a municipality’s garden surface area is de-tiled, 85 million litres less water will flow through the sewers each year! Ideally, limit the area of hardstanding to a maximum of 20%. If you wish to replace paving with grass or plants, it is important to ensure the soil is well prepared. There is often sand beneath paving, which does not provide a nutrient-rich growing medium for plants. It is therefore important to improve the soil using, for example, fertilised garden compost. In areas where you do not want plants, you can opt for gravel, wood chips or shells. These are easy to walk on, but allow water to drain away more effectively than tiles. For car parks, for example, there are an increasing number of attractive alternatives such as the Rain(a)way tiles.
At the front and sides of houses, it is often possible to remove tiles and create a façade garden. We can help you with the installation. Submit an application on our website. In it, you describe what you would like to do and where you would like to do it. We will get in touch with you and discuss the options with you.
Disconnect the drainpipe
By disconnecting your downpipe, you reduce the risk of the sewer system becoming overwhelmed. This also means that not all the water has to pass through the water treatment plant, helping to maintain groundwater levels. You can choose to collect rainwater in a rain barrel or rainwater tank to use for your plants or, for example, to wash your windows. In addition, you can channel the rainwater via a ditch or gutter to a rainwater pond, infiltration field or wadi. You can even create a wadi under a trampoline!
What do we do?
We believe it is important that you play your part in preventing rainwater from entering the sewer system. That is why we are happy to help you with this. You can apply for a grant to disconnect your property from the sewer system. You can apply for the grant via the page Grant for rainwater diversion. You can also find out what the conditions are there. For example, which areas of Apeldoorn the grant applies to.
Lay a green roof
Green roofs capture a lot of rainwater. Depending on the type of green roof, between 25 and 160 litres per m² are captured. There are various types of green roofs: sedum roofs, grass/herb roofs, walkable roof gardens and planted pitched roofs. Most homeowners opt for a sedum roof. These plants store a lot of water, enabling them to withstand long periods of drought. A sedum roof is lightweight and can almost always be installed on existing flat roofs. They are also fairly straightforward to install yourself.
Collect water using a rainwater harvesting system
Collecting rainwater in a rainwater harvesting system can reduce your drinking water consumption by between 40 and 75%. The rainwater flows from the roof down the drainpipe and through a filter to remove leaves from the water. The rainwater is stored in a tank or barrel. It can also be stored in a tank in the cellar or garage, or in an underground tank. In the tank, dust particles settle to the bottom and insects float via the overflow into the sewer. The remaining water is suitable for use in, for example, the washing machine, flushing the toilet and watering the garden. There are even systems where the water is purified to such a high standard that you can use it for showering.