Storm water flooding Amsterdam, from a quick Scan analyses to an action plan

Dealing with the issue of urban storm water flooding is becoming increasingly urgent. In the Netherlands there are no clear guidelines on the level of acceptance of urban flooding. Based on an accurate DEM, a detailed quick scan tool has been used to assess an extreme storm event in Amsterdam. The resulting flood maps for the whole of the city show where flooding is likely to occur after 60 or 100 mm of rain in one hour, as well as which buildings are at risk.Based on the results from this mapping study, Amsterdam decided to start a program to make the city rainproof (Amsterdam Rainproof). Part of the program was the validation of the model based on field research. An example of this is the use of data from the extreme storm event that occurred in Amsterdam on 28 July 2014. In this paper several pilots from Amsterdam will address the relevance and effectiveness of the quick scan tool.

Extreme showers cause more and more frequent flooding in Dutch urban areas (Spekkers, 2015). This is due to a combination of climate change, an increase of the paved area and a lack of focus on possible flooding. In the Netherlands, where the land surface is predominantly flat and where surface water levels often reach ground level, drainage systems are built to prevent water spilling onto the streets at design storms of once every year or once every two years. Flooding of streets has been accepted for more extreme rainfall events. In the past this did not lead directly to the flooding of houses and buildings, as at street level, there was normally additional space available to store water (e.g. between the street kerbs). General design standards for kerb heights, street profiles and floor levels (25 cm above street level) resulted in only rare occurrences of premises being flooded. Unfortunately those design principles have either been neglected or changed over the last three decades (in favor of easier access), resulting in a higher risk of flooding.

It is becoming more apparent that changes in the global climate are having a significant impact on precipitation patterns. From the ten most extreme storm events in the Netherlands between 1951 and 2013, seven were from after 1995 (Kluck, 2013-a, table A1). Because of the changes in rainfall patterns and the experienced more frequent floodings, municipalities have started to enquire whether they need to put more effort into anticipating extreme storms.

The national agency for sewer related issues (Stichting Rioned) expresses that street flooding is something that needs to be accepted and prepared for by designing the streets in such a way that water in the streets does not result in the flooding of premises. And even national policy has stated that cities have to become climate resilient by 2050 and that they should use the upcoming maintenance works on urban space to implement this (Delta Programme, 2015). Also Kluck et al (2013-b) suggest that municipalities should anticipate extreme showers and explains that this should be the work of both water managers and other urban managers.

There is currently no clear standard on how often flooding is acceptable and therefore Dutch municipalities have started investigating how best to deal with this issue. A logical way to approach this problem is, at a minimum, to evaluate what happens following a real extreme storm event and then decide what to do. Blanksby (2011) describes this as way of triage. By analyzing the floodings due to a really extreme event, it is possible to identify the possible problem areas, and to gain more clarity of what actions are needed and in which locations further investigation is needed.

After research and discussions with focus groups that included municipalities, Amsterdam University of Applied Science and Hanze University of Applied Sciences a consensus was agreed upon that a storm event of 60 mm/hour could be used as a first standard level for an extreme event. This was reviewed by different stakeholders, municipalities, provinces, and the local water authority for Amsterdam. This figure is in (Kluck, 2013-a) roughly based on rainfall statistics and arbitrary a return period of once per 100 year has been chosen.