Flash floods and Urban flooding The Netherlands
As the sewage systems were designed to cope with less violent downpours, heavier summer storms also will mean more flooding in urban areas.
The flooding of July 2021
Severe floods in mid-July 2021 killed at least 220 people in Germany and Belgium. The floods followed 2 days of heavy rain that caused several rivers in the region to burst their banks. The worst-hit areas were around the Ahr and Erft rivers in Germany and the Meuse in Belgium, which experienced the most rainfall in a 24–48-hour period since records began. According to a model study carried out by the ‘World Weather Attribution’ initiative, human-caused climate change had increased the rainfall intensity of such storms by 3–19%, relative to a pre-industrial climate 1.2 °C cooler than today (8). The study concludes that similar events can now be expected to hit any part of Western Europe, in a large region between the north of the Alps and the Netherlands,about once every 400 years. The authors state that the likelihood of such an event to occur today compared to a 1.2 °C cooler climate has increased by a factor between 1.2 and 9.
Strategic options for climate-proofing the Netherlands
The implementation of climate-proofing measures in urban development today may considerably reduce costs for tomorrow (3).
Structural measures, such as the construction of parks, canals, ponds, thermal energy storage and modified sewerage systems, can be implemented in new developments and urban restructuring projects. The key players for such adaptations at neighbourhood or city scales are municipal councils and real estate developers. The most appropriate measures in existing urban areas are adaptations at the scale of individual buildings or streets, such as insulation, green roofs and adapting street paving for water retention (3).
The key actors at this scale are housing corporations, companies and private property owners. Urban areas are constantly changing: new homes, offices and infrastructure are built, neighbourhoods and business estates are restructured and sewerage systems replaced. If municipal authorities, developers, housing corporations and private owners consistently incorporate climate resilience into their investments in the built environment and urban facilities, the additional costs of climate adaptations can be minimised (3).
Municipal authorities are the most suited to play a leading role in coordinating and managing these adaptations. Municipal ‘climate-proofing plans’ are a potentially interesting tool for gaining insight into the what needs to be done to make cities climate ready. These plans can clearly identify the scale of the task at hand, the options available, and the responsibilities that municipal councils take on themselves and those they delegate to businesses and local residents (3).
A list of possible adaptation measures for urban flooding due to heavy downpour has been published for measures to buildings and (parts of) cities (6):
- Building: waterproof building (for example floor above street level, high thresholds, no crawl spaces, waterproof plaster and membranes on walls, waterproof floors); green roofs; green facades; water drainage (drainage in gardens, gutters etc.); unpaved gardens (infiltration, water retention).
- Street/quarter/city level: seeping water ‘screens’; water permeable pavement instead of asphalt and other measures for better infiltration and water outlet; lower water tables; separation of rainwater and sewage water; insurances; warning systems; evacuation plans; extra green space; water storage facilities (open water such as pools); increase sewer capacity or enhanced maintenance; drainage systems; dry pumps and other provisions for water discharge and clean-up; information campaigns.
Rotterdam leading the way
In the Netherlands Rotterdam leads the way in implementing adaptation measures to deal with the projected increase of heavy downpour in the future. More ponds and canals will be dug in public spaces. The city also wants to introduce so-called ‘water plazas’, which will be dry most of the year but will, in the event of heavy rainstorms, be allowed to fill up and retain superfluous rainwater for a longer period. For the same purpose, underground reservoirs are being integrated into some new housing projects. The new car park under Museumpark, for instance, will house a collection reservoir with a capacity of 10,000 cubic metres (2).
The relatively cheapest solutions for the collection of water remain in laying out small parks and constructing green roofs: in the district of Delfshaven the largest green roof in Europe will be constructed on top of a new business centre. The green roofs and ‘pocket parks’ help to keep the city cool (2). These green measures in cities are called ‘ecoysytem-based approaches to adaptation’ (EbA) or ‘urban green infrastructure’ (UGI) (7).
Adaptation tipping points and mainstreaming adaptation
A strategy for climate proofing a certain area with respect to a certain impact is the so-called adaptation tipping points approach (4). Adaptation tipping points are points where the magnitude of change due to climate change or sea level rise is such that the current strategy will no longer be able to meet the objectives. This gives information on whether or when a water management strategy may fail and alternative strategies are needed.
In this approach, the time window of an adaptation tipping pointwill define when an alternative, adaptive strategy will be needed. This assumes that climate change is the main driver of adaptation. In urban areas, however, the ‘normal’ maintenance, modification or renewal of infrastructure, buildings and public spaces could give an opportunity for adaptation, for instance to reconsider the existing urban drainage (stormwater) system from a different standpoint (5).
Many adaptation responses can be implemented synergistically with normal maintenance, modification and renewal cycles and at next to no additional cost, such as with sewer rehabilitation, urban greening and neighbourhood regeneration projects. From this perspective, the normal dynamics of the urban area should be recognised and used as a driver of adaptation (5).
The mainstreaming method has been developed as an extension to theadaptation tipping points approach byanticipating adaptation opportunities in the normal dynamics of urban areas. Mainstreaming adaptation of urban drainage systems to climate change is expected to lead to potential cost reductions, since adaptation options can be integrated into the infrastructure and building designs at an early stage instead of being applied separately (5).
The references below are cited in full in a separate map 'References'. Please click here if you are looking for the full references for the Netherlands.
- Ministry of Housing, Spatial Planning and the Environment (2009)
- Boer (2009)
- PBL Netherlands Environmental Assessment Agency (2011)
- Kwadijk et al. (2010)
- Gersonius et al. (2012)
- Runhaar et al. (2012)
- Gaffin et al. (2012)
- Kreienkamp et al. (2021)