Understanding the effect of extreme weather events on water systems is essential to curb floods and droughts successfully. Stress-tests are a useful tool to contributing to this understanding. In this article, Patrick Willems from KU Leuven and Nathalie Asselman from Deltares explain stress-tests in more detail.

Together, they form part of an international research team where they work on a joint approach for stress tests that can be applied in regional river basins between Flanders and the Netherlands. Stress tests allow one to check the resilience of water systems, which are under increased pressure due to the consequences of climate change. Climate change leads to more extreme meteorological conditions, such as droughts or heavy rainfall which leads to floods.

Patrick explains how stress tests are conducted: “You use computer models to simulate these extreme meteorological conditions. The model calculates how the system will react to them. You first validate the model by looking at historical events, and then you compare that with the measurements. If these are close to the truth, you then begin to impose even more extreme conditions on that model and to predict the impact based on that. The idea is to see with the end users, such as water managers, whether – if these predictions were to come true – we as a society find those acceptable. That way you can see how stress-resistant our water system is. If it is insufficient, then we look at possible measures to make it more stress-resistant, but that is then the next step.”

Nathalie stresses the importance of a risk approach for the Netherlands and finding the design standards of certain water systems. This can be a 1 in 25 probability of flooding per year, or a probability of 1 in 100. It is, however, important to not only look at design conditions, but also at more extreme events. “We saw in July 2021 that there can always be surprises, that rainfall can occur that is much more extreme than usual. We want to prevent these kinds of extreme situations from surprising us. A stress test helps to give us insight into how water systems react under extreme conditions, to prevent a truly big disaster.” In the Netherlands, the stress test complements the standard risk approach, and its results are meant to facilitate discussion and the creation of possible preventive measures. In combination with knowledge gathered from disaster response plans, they can help warn residents in certain areas or facilitating evacuation.

The team is currently working on a transboundary report for the catchment area, to determine what is already known through previous studies and whether this knowledge can be used for this specific purpose, as well as identifying the important water managers in the area. From the Flemish side, Patrick and his team started last year, which is now being supplemented by Nathalie and her team on the Dutch part of the river basin. This allows them to get a cross-border overview of the functioning of the water systems, their main problems, and the measures that have already been taken. Nathalie explains the follow-up: “After this, we will conduct additional analyses with available models or improved models to perform the stress tests. In the Netherlands, for example, we have a standard rainfall event of 200mm in 48 hours that is applied in all stress tests. This will most likely be calculated through, but in Flanders there may be other standard events that will be explored. We will discuss that together.”

Flooding and drought used to be viewed as two separate phenomena in the Netherlands, but in recent years it has become increasingly clear that these are two sides of the same coin. That is why stress tests are now being conducted simultaneously for both events, something that may have been common practice in Flanders for some time. Patrick adds to that: “We also examine the differences in methodologies between the Netherlands and Flanders, because we do have different traditions with regards to how we model the entire system. Not just which rainfall events are calculated, but also the way the processes in the water system are represented and possible measures are calculated. Which exceptional events will we take into account, and how? You learn a lot from each other that way.”

Patrick Willems has quite some experience dealing with floods, as he also worked on the advice of Weerbaar Waterland (resilient waterland) as part of an expert panel. This advice was written in response to the disastrous floods that hit Wallonia, Germany and the Netherlands in July 2021, and serves as a means to help prepare Flanders for extreme rainstorms and floods in the future. Patrick elaborates on the main recommendations that came from the report: “We need to arm ourselves better against these kinds of extremes. The following steps should be taken systematically for each river basin:

1. What are the current and future risks regarding droughts and floods?
2. What do we consider acceptable and unacceptable risks? Finding a consensus in discussion with stakeholders is important.
3. Define clear targets: if we want to make those risks acceptable, what is the minimum that needs to be done to achieve that? This could include reducing a minimum percentage of water runoff through more upstream water storage and infiltration and/or creating additional space in the river valley. Additional measures can also be taken, such as raising dykes, should the aforementioned measures be insufficient.”

General preparedness is also important, according to Patrick. “If, under certain circumstances, you would still have to deal with floods, you must ensure that you also have adequate forecasting, warning and evacuation systems, and crisis and intervention plans.”

Additional advice offered by the Weerbaar Waterland report goes into providing sufficient funding as well as making water more steerable in spatial policy. Whereas measures in water management were previously formulated top-down by the water manager, in the Weerbaar Waterland report, the panel recommends going for a bottom-up approach. Defining and evaluating the risks together with all stakeholders, such as the agricultural and nature sector, cities, and municipalities, and developing an action programme with solutions supported by those sectors allows for stronger societal support. It was also advised to prioritise nature-based solutions that are good for both floods and droughts, and also offer other linkage opportunities such as reducing erosion and improving water quality and biodiversity.

Both experts would be satisfied if the stress test is executed successfully and if it prompts governments to cooperate more intensively, both upstream and downstream. Nathalie: “It is strange that we limit the river basin to national borders, while rivers are not stopped by these borders. Instead, we need to tackle this across borders to be as effective as possible. With this stress test, we want to promote this cooperation.” Patrick: “I completely agree. The advice from Weerbaar Waterland is now advice on paper, but it still needs to be tested and the cross-border aspect does need to be tested along with it.”

Nathalie and Patrick consider having knowledge institutes participating as project members on both sides of the border one of JCAR ATRACE's strengths, as it allows for effective knowledge and best practice sharing, based on water management traditions and models that are used in their respective countries. Nathalie: “When we look at water management from the Netherlands, looking from a purely Dutch perspective is not enough. Cooperation between knowledge institutes and governments on both sides of the border increases the chances of success.” Patrick: I totally agree, the important thing is that we learn from this cooperation. I hope this cross-border cooperation between the Netherlands and Flanders will be an example to other regions in Europe and the world.”