Due to climate change, periods of heavy rainfall increasingly coincide with storms, causing rivers to discharge more water than they can handle. This increases the risk of flooding, especially in areas where people have modified the landscape, for example with dams and straightened channels. 'Dams block the natural supply of sand and silt,' explains delta researcher Ton Hoitink. 'This sediment is precisely what is needed to keep delta levels up. Without this sediment, the delta slowly sinks, causing the land behind the dikes to sink lower and lower. This creates a kind of "bathtub" - a low-lying area that is vulnerable. You don't want to live in a bathtub when the water outside is rising.’

When everything comes together at the "wrong place"—a storm, heavy rainfall, and the bathtub effect—it can permanently change the entire system. This is an example of a tipping point. In a new study, the researchers have looked into the processes behind such tipping points in deltas: what precedes them, when are they expected, and what are the consequences. This helps to better assess how to manage rivers, or whether we should let them take their own course in a changing climate.

‘In addition to the formation of deep erosion pits, we examined three other tipping points,’ explains Roeland van de Vijsel, physical oceanographer and lead author of the study. ‘Rivers can change course, water can quickly become very murky, and salt marshes can drown due to land subsidence and eroding banks. A tipping point has already occurred in the Ems River, making it permanently turbid and changing the entire ecosystem. Potentially, the Western Scheldt could also become murkier, but has not yet reached a tipping point. We want to prevent that, and this kind of research helps with that.’

Global risks for deltas

The team has created a map that visualizes the vulnerability of deltas worldwide. The map shows how much sediment has been blocked by dams and other interventions over the past 30 years, whether river peaks and storm surges have occurred more frequently at the same time, and the expected sea level rise combined with land subsidence. This provides a clear overview of the greatest risks for each delta around the world.

Van de Vijsel: "For example, the map shows that in the Rhine-Meuse delta, 70% less sediment has been supplied by the river in the last 30 years, mainly due to dam construction. We also see that sea levels are expected to rise while the inner land continues to sink. These are two factors we know can push the entire river system towards a tipping point. It is crucial to recognize these risks in time and take action to prevent further damage."

Back tipping

The good news is, there is also a solution: allowing and restoring natural processes. ‘This requires a clear vision and the willingness to make choices,’ emphasizes Hoitink. ‘For example, by temporarily taking agricultural land out of use and opening it up for the supply of sand and silt from the sea, or by transitioning from agriculture to aquaculture. The story of the Netherlands in 2120 is an inspiring example of a long-term vision that can also help us design healthier and safer river systems.’

‘Sometimes, a little push is needed to kickstart natural restoration,’ adds Van de Vijsel. ‘We now better understand how the negative spiral towards a tipping point works. We can use this knowledge to push the delta in the right direction at the right time. We call this 'back tipping'. Tipping points are not only negative; we can also use them positively. The new study also provides more insight into how we can achieve this."

Tipping points
In climate science, the concept of tipping points is increasingly discussed, and there is growing international attention to this topic. In 2023, more than 200 researchers worldwide released the Global Tipping Points Report, the most comprehensive study of global system tipping points to date. Climate tipping points occur in various areas: the melting of polar ice, the loss of tropical forests, and the acidification of oceans. The consequences can be both positive and negative. It is difficult to predict when a tipping point will occur. ‘You can compare it to a burnout,’ says Ton Hoitink. ‘All the warning signs are there, but the moment you receive the final push that tips you over the edge is impossible to predict.’