322 — Mangrove vegetation as a protective solution for coastal protections

Palmer & Withmarsh (1807.01834)

Read on 08 July 2018
#turbulence  #conservation  #ecology  #trees  #mangrove  #vortex  #water  #tide  #hurricane  #tsunami 

This paper was a very cool and pretty quick read, but it covered a lot of ground both metaphorical as well as literal: In light of the 2017 hurricane season, these researchers looked at the impact of mangrove roots on the flow of water.

Specifically, they modeled a mangrove root cluster as a series of uniformly sized ~1cm cylinders with a constant unidirectional water flow through a domain that surrounds them (imagine the tide rushing in around the roots of the trees).

They found that according to their simulation (following a time-averaged form of the fluid-dynamics-description Navier-Stokes forumlas, named Reynolds-averaged Navier–Stokes, or RANS), there were substantial differences between mangrove-like obstacles and, say, no obstacles or solid obstacles.

In particular, the Von Kármán vortex streets — something that I never knew the name of before but knew of well — were delayed in their formation when compared to the solid obstacle (the no-obstacle case obviously didn’t exhibit a vortex street).

If you’re curious and don’t know what a vortex street is: You know what it is well! Just…not by this name. A vortex street is the continuous stream of little vortices that break off of a larger vortex when a flow is occluded by a cylindrical object. The Wikipedia page has a great visualization of this, though you’d also probably recognize the shape from smoke curling around the end of a cigarette. Interestingly, before it became clear that the Tacoma Narrows Bridge collapse was a result of harmonic frequencies and aeroeslastic flutter, it was at first believed to have been caused by vortex shedding.

So what does this mean for mangroves?

In particular, it means that mangroves are probably a better barrier to turbulent flow than solid barriers (i.e. sandbags) and they’re certainly better in tsunami or hurricane areas than no boundaries. So stop cutting down trees.