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Well-placed mangrove forests can help protect coastal communities from rising water levels during a tropical storm, according to a modelling study.

Mangroves contain trees that grow in saltwater on coastlines, mostly in tropical and subtropical regions. In many parts of the world, they have been destroyed to make way for fish and seafood farming. However, they are important carbon sinks and their restoration is seen as a promising way to mitigate climate change.

They can also act as a first line of defence against natural disasters such as tsunamis. Previous studies have looked at the effect of mangroves in reducing wave height during a typhoon, but few have looked at their overall imact on storm surges 鈥� the rise in water level caused by wind and atmospheric pressure change during a storm.

Trees can affect water levels during a typhoon by increasing the amount of drag that water experiences, reducing its overall energy levels, says at the National Oceanography Centre in Liverpool, UK.

De Dominicis and her colleagues modelled the effects of a typhoon on water levels in the Pearl river delta in China. The region surrounding the delta, which includes the cities of Hong Kong, Shenzhen and Guangzhou, has a population of 85 million, and this is expected to rise to 120 million by 2050.

The last major typhoon to hit the region was in 2017, named Hato. Previous research suggests that warmer temperatures may be leading to

The researchers chose to study two locations in the delta: the wetlands in Shenzhen Bay and the river branches near Guangzhou. They collected pieces of mangrove from these sites and carried out experiments in the lab to estimate how much friction a cluster of those trees would produce against rising water levels.

The team then used an ocean model to estimate how mangrove placement and size could affect water levels during a storm on the scale of Typhoon Hato. They found that, in general, wider patches of mangroves had a larger effect on water levels than narrower patches.

In Shenzhen Bay, for example, the researchers found that a 600-metre-wide mangrove patch in front of a sea wall would reduce extreme water levels by up to 2.8 metres. On the other hand, a 300-metre patch would have little effect, as it doesn鈥檛 add enough friction to stop water rising against the sea wall, says De Dominicis.

The researchers also found that mangroves in river channels would reduce water levels upstream, but those on open coastlines would mainly affect levels nearby and not in river channels.

鈥淚t shows that before you do a [mangrove] restoration project, you need to do some kind of assessment of where it is best to place the trees,鈥� says at the University of East Anglia in Norwich, UK. This is also the case when restoring salt marshes to act as flood defences, he says. 鈥淚n some places, restored salt marshes can actually exacerbate the tide [and lead to a greater flood risk], so you鈥檝e got to see how the whole system works before you start such a project,鈥� he says.