Action perspectives for rendering the city of Zwolle water-resilient

Why this pilot study?

Zwolle is surrounded by water: from the Vecht, IJssel, and Zwarte Water rivers. The Sallandse Weteringen even run right across the city centre. Flooding from the Sallandse Weteringen can have such a major impact that the current protection standards are not stringent enough. Apart from traditional dyke improvement, are there other ways to protect Zwolle from flooding? Within the STOWA Regional Flood Defences development programme, experts are exploring ways to effectively achieve the desired flood protection level of an area. The pilot study at hand examines regional measures to protect Zwolle and its vicinity.

What question does the study aim to answer?

The main question in this study is how the authorities wish to protect Zwolle. What are potential solutions? Are they opting for higher dykes and technological solutions? Or do they choose to collect more water at regulated locations outside the city, in order to contain a flood and mitigate potential damage? This would require a great deal of space. The study also addresses the risks involved in every measure. What probability of flooding does this measure entail? What are the consequences? How much damage could occur?

What solutions have been investigated?

The study is addressing four potential ways to protect the city against flooding from the Sallandse Weteringen:

• Dyke improvement. The advantage of dyke improvement is that we have extensive experience in the field. The disadvantage is that some improvement methods are costly and cause nuisance whilst in progress. In addition, larger parts of the Zwolle city centre located on the floodplains could be flooded more frequently.
• Pumping station: placing a pumping station at a location where a local river merges with a larger river. A pumping station enables water levels to be controlled, drains excess water, and can reduce water levels. The advantage of a pumping station is that measures are confined to a single location. Furthermore, a pumping station can protect the city centre, which is located on the floodplains. A disadvantage is that finding an appropriate location for a pumping station is difficult.
• Polders: constructing new flood defences and dykes in order to divert all the water around Zwolle. Water is collected outside the city, thus turning Zwolle into a type of island surrounded by a dyke. This is a drastic measure because it requires a great deal of space. One advantage is that the flood defences inside and outside the city will not need improving. It can also significantly boost the flood protection of the city.
• Water-conducting landscape: allowing areas to be flooded in a regulated manner. This is mainly a spatial measure to boost Zwolle’s climate resilience. Dykes play a part, but excess water is drained to areas in which the impact will be smaller. The advantage is that water can efficiently be collected during extreme weather. Zwolle will be less prone to flooding. And should things nonetheless go wrong, less water will flow into the urban area. This solution combines well with spatial adaptation: the construction methods used for, e.g., houses and roads. A disadvantage is that this solution is quite drastic. It requires a range of different measures and calls for the involvement of a range of parties. Who will do what? Who will bear which responsibility?

What are the conclusions?

The study shows that there is no single solution that is the most appropriate or the cheapest way to achieve the desired level of flood protection. Another option is to combine several solutions. Of note, however, is that the impact of a flood will differ from one set of measures to the next. With the spatial solution of a water-conducting landscape, a flood will have less impact compared to the technological measures involved in the dyke improvement, pumping station, and polders solutions.

What are the next steps?

The provinces, district water boards, and municipalities must collectively set down the standards in terms of flood risks. In addition, they must choose whether they wish to focus on technological solutions to stem the water or would prefer climate-adaptive and spatial solutions to contain the impact of a flood under all conditions. Or would they rather opt for having some areas flooded in exceptional situations? In any case, to garner sufficient support, it is important for the authorities to tie in with regional developments, and to involve residents and businesses in their choices.