New method precisely calculates how mangroves protect coasts against strong waves

Imagine a natural fortress standing strong against raging storms. That’s what mangroves and other forested wetlands do for our coastlines. But how well do they protect us, and against which storms? Researchers from Sun Yat-Sen University, China and the Royal Netherlands Institute for Sea Research (NIOZ) have uncovered a new and easy method to predict the effectiveness of these natural barriers during extreme weather events. This is an important new insight and tool for coastal managers and policymakers.

We have long known that mangroves and other forested wetlands can act as natural shields against floods. But when it comes to extreme storms, we haven’t had a clear picture of just how well they work. The available models for predicting how much these forests can reduce wave heights are often too complicated for many coastal managers to use. Plus, figuring out the right numbers to plug into these models—especially in extreme conditions—is a tough nut to crack. 

New way to measure protection
To tackle this, a team of international researchers gathered data from some of the biggest waves ever recorded in forested wetlands. They looked at waves during a typhoon in China, as well as data from labs and other field studies around the world. What they found is remarkable: mangrove forests of about 100 m width can half the storm wave height. That’s a significant lowering of the waves, giving quite some protection to the coast line behind the mangrove forest. 

But that’s not all. The team tested 20 different ways to calculate the drag that trees create against water wavy motions. Most of these methods couldn’t predict wave attenuation in storm events. So, they came up with a new, easy-to-use method they called the HU method. “This method doesn’t need complicated numbers or detailed tree measurements,” says lead researcher Zhan Hu. “It’s simple enough that coastal practitioners or volunteers with a few existing measurements can use it to figure out how well their local forested wetlands can protect against waves.”

Saving coasts as well as money
“This study is a big deal for coastal protection” says Tjeerd Bouma, researcher at the Royal Netherlands Institute for Sea Research (NIOZ) who also worked on the project. “The HU method gives coastal managers and scientists a practical tool to assess how well their local forests can reduce wave heights during storms. This can help in designing natural coastal defenses, potentially saving billions of dollars worldwide.”
“Plus, the study reminds us how important it is to protect and restore mangroves and other forested wetlands like willows,” Bouma adds. “They’re not just wave-blockers; they also capture carbon, provide habitats for wildlife, and clean our water.”

Wave non-linearity
“No, the method is not named after me”, laughs Prof. Zhan Hu. “The name rather reflects the approach that is based on the relation between wave height ‘H’ and Ursell number U, which is an index for wave nonlinearity”. In a nutshell, this method works as the wave attenuation is closely linked to the nonlinear wave effect, which is depicted by the HU relations. And the same HU relation can be applied for both calm and storm weather conditions. The novel methods use the same HU relation in calm weather (but highly nonlinear) to predict the wave height attenuation during storm events, and bypassing the need of quantifying drag. 

Other wetlands and coastal areas
While the HU method is a great start, the researchers know it’s not perfect. It works best for specific types of rigid, above-ground forests. Future work will focus on making the HU method work for different kinds of wetlands and coastal areas. They also want to explore what happens when trees start to sway in the wind and how much wave reduction is possible.

“In short, this research is a huge step forward in understanding how nature can protect our coasts,” concludes Hu. “The HU method offers a new way to predict wave reduction during extreme storms, opening the door to more sustainable coastal management.”

The full paper was published in journal PNAS, and can be read at: https://doi.org/10.1073/pnas.2410883122