Wave flume tests reveal artificial salt marshes tame extreme crest statistics only at densities above natural levels

The arXiv preprint details controlled wave flume runs that systematically varied vegetation density immediately upstream of a depth transition. Excess kurtosis, a proxy for extreme crest probability, dropped with increasing density in intermediate water, but the benefit saturated quickly at low densities and only became pronounced at the highest experimental values. These findings indicate that second-order nonlinear effects and shallow-water conditions largely override vegetation damping once waves shoal further.

Natural salt marshes therefore offer only partial statistical mitigation rather than reliable extreme-wave protection. Coastal managers relying on green infrastructure for flood reduction must account for this density threshold, because typical field stem counts fall short of the levels that produced meaningful kurtosis attenuation in the lab. The result aligns with earlier field observations showing modest wave-height reductions but limited crest clipping during storms.

Related work on real marsh canopies, including a 2022 Journal of Geophysical Research study of UK sites, similarly found that nonlinear wave statistics persist landward of vegetation patches. A 2021 Coastal Engineering paper on hybrid structures reached parallel conclusions: vegetation alone rarely suffices for extreme-event design criteria without supplementary hard engineering.

Next steps include scaled field deployments that measure both kurtosis and overtopping at natural densities to test whether the laboratory saturation regime holds under irregular, directionally spread waves.