Good news for deltas: increase in sediment since 2000

When you build a dam on a river, less sediment can get to the sea, which makes deltas more susceptible to floods. This idea’s been believed for a long time, however new research from Utrecht University shows that the amount of sediment in deltas has actually increased since 2000. This is good news for preserving deltas – like the Netherlands – from sea level rise.

Coastal protection

Sediment is important for the coast. Against flooding, or for coastal protection projects where nature plays a central role – nature-based solutions. “For example, planting new mangrove forests,” says Jaap Nienhuis, research leader of the new paper. “Planting mangrove seedlings makes no sense if there is not enough sediment, as the seedlings will wash away in no time. Many of those projects fail as a result.”

The fact that the amount of sediment in deltas has increased over the last 20 years is good for those kinds of projects. But more sediment and an increased water turbidity can also bring disadvantages. For instance, it can make it harder for some fish to hunt, and corals may not get enough sunlight.

We do know that the sediment is not coming from rivers.

Surprising results

The increasing sediment concentrations worldwide was a surprising result for the researchers. “We don’t really know what is causing this increase. What we do know is that the sediment is not coming from rivers, as they are mostly dammed.” However, the researchers do think that tides and waves play a role. “And because river and delta land are nowadays separated by levees in many places, sediment may not be able to ‘escape’ onto land but remain in the water.”

Global delta data

“We see our research findings as good news for deltas,” says Nienhuis. The researchers collected satellite photos from around the world and developed an algorithm that allowed them to calculate sediment concentrations using those photos. So they can see exactly where extra focus is needed. “There are also places where there is too little sediment. It is important to focus new coastal protection projects there,” he concludes.

Publication

Hou, X., Xie, D., Feng, L. et al. Sustained increase in suspended sediments near global river deltas over the past two decades. Nat Commun 15, 3319 (2024). https://doi.org/10.1038/s41467-024-47598-6