The Edge of Turbulence

January 20, 2018 11:12 PM

I live near a shallow tidal lagoon. I often notice that part of the surface is turbulent, part is relatively calm, and there is a sharp boundary (as opposed to a smooth gradient) between these two areas. I don't think the boundary occurs in a consistent place from day to day. How can such a boundary form, and how is it sustained over time (i.e., why don't waves from the turbulent area spill across the boundary)?
posted by aws17576 to Science & Nature (3 answers total)

Even a slight breeze can totally change the surface texture of water. If there is a geographical feature nearby that affects the wind patterns, you would see the results in exactly this way.
posted by 5_13_23_42_69_666 at 11:38 PM on January 20, 2018

Best answer: That's called an edge and it's the edge of a current. Probably related to bathymetry- that is, its at the edge of a deep bit next to a shallow bit. You have water moving along and going places next to water that is just hanging out or back eddying. The waves are most likely standing waves and the turbulence is most likely upwelling from the bathymetry change.

Edges are where all the nightlife is as there is good opportunity for food to arrive in the fast moving water while one hangs out in the slow moving water saving energy. And detritus I(dead stuff) ends up there a lot. In a lagoon it's probably the delineation between brackish and salt water which is interesting to animals too. So watch it for fish and bird sightings and watch the eddy for animals resting to feed like seals and birds.
posted by fshgrl at 1:29 AM on January 21, 2018 [14 favorites]

Best answer: If you want to learn more than fshgrl can tell you in two paragraphs, pick up the fascinating How To Read Water: Clues and Patterns from Puddles to the Sea by Tristan Gooley, and you will learn to see even more water features than just this edge.
posted by Orlop at 7:11 AM on January 21, 2018 [9 favorites]