Communities around the world are increasingly interested in nature-based
solutions to mitigation of coastal risks like coastal forests, but it
remains unclear how much protective benefits vegetation provides,
particularly in the limit of highly energetic flows after tsunami
impact. The current study, using a three-dimensional incompressible
computational fluid dynamics model with a fluid-structure interaction
approach, aims to quantify how energy reflection and dissipation vary
with different degrees of rigidity and vegetation density of a coastal
forest. We represent tree trunks as cylinders and use the elastic
modulus of hardwood trees such as pine or oak to characterize the
rigidity of these cylinders. The numerical results show that energy
reflection increases with rigidity only for a single cylinder. In the
presence of multiple cylinders, the difference in energy reflection
created by varying rigidity diminishes as the number of cylinders
increases. Instead of rigidity, we find that the blockage area created
by the presence of multiple tree trunks dominates energy reflection. As
tree trunks are deformed by the hydrodynamic forces, they alter the flow
field around them, causing turbulent kinetic energy generation in the
wake region. As a consequence, trees dissipate flow energy, highlighting
coastal forests reducing the onshore energy flux of tsunamis by means of
both reflection and dissipation.