Description: Watching water flow on the dock is a great way to pass the time. On a calm day with a strong current flowing against the dock, there is often what appears to be a stable standing wave a few inches to a couple of feet upstream of the dock. It is very thin and short (so it's frustratingly difficult to capture on camera), but can stretch a long ways perpendicular to the current.
If anyone knows about this phenomenon, I'd be curious to find out if there is a better explanation for it than mine (described below).
For a long time I thought this "standing wave" (I'm not sure it's quite correct to call it a standing wave, but I don't know a better term) was a hydraulic jump, a place where a rapid change in flow velocity causes a change in water surface height (like when the faucet is running and the water striking the sink moves rapidly away from the stream as a thin sheet until it hits slower moving water). However, the other day I saw that the speed of ripples and subsurface particles do not seem to change at the "standing wave". So it seems like the subsurface water does not change speed at all. However, particles on the water surface flow with the current upstream of the "standing wave", but are still downstream of the wave.
So I thought there might be a layer of molecules (or other material) adsorbed on the water surface that would prevent the area of the water surface from shrinking. They might pile up at the dock, forming a layer which couldn't flow with the rest of the current. The "standing wave" might be the boundary between this region where this adsorbed material is trapped, keeping the surface still with respect to the dock, and the region where the water surface moves with the current.
To try to test this, I added a tiny bit of soap upstream or down stream of the "standing wave" to see if the drop in surface tension would make it shift (as I would expect for a phenomenon that depends on the surface rather than just water height and currents). The "standing wave" rapidly sprang __towards__ the soap, whether the soap was upstream or downstream (I should double check my memory of this). I think this makes sense given my hypothesis: the soap weakens the surface tension, so it should allow the region of surface on the other side of the wave (opposite the soap) to penetrate deeper into the region where the soap was added.
Unfortunately, I have not been able to get decent video yet.
Page creator's name: Mickey von Dassow
Page creator's contact info: With IGoR contact link
Created: 29 Mar 2014 19:45
Updated: 06 Jan 2015 21:41