I can understand how this would reduce erosion (although that benefit would seem negligible ROI vs (concrete?) wall dam engineering) but how is an undersea wall going to even partially reduce flooding, exactly?
> how is an undersea wall going to even partially reduce flooding They interfere with storm surges. The same way reefs force waves to break, dissipating their energy offshore.
Surface waves are only the displacements on the surface that we see. The water column is also perturbed below the surface, and depending upon on the wavelength and amplitude, the perturbation can go down pretty far.
If you track the water particles as the waves pass by, you would see the particles traveling up and down and back and forth in orbital motions. In shallow water (relative to the wavelength), the sea bottom prevents downward motion, so the orbital motion is flattened into a back and forth motion near the bottom, and near the surface the waves increase height and break at the top.
If you have things like coral reefs or oysters (or an undersea wall), the back and forth motion is impeded, and energy from the wave is dissipated (which reduces the surface displacement).
Here's an article on experiments quantifying the energy dissipation: https://www.mdpi.com/2077-1312/9/9/1007