Talk:Point bar

Updated descriptions of flow and sediment transport needed.

1. Observations consistently demonstrate that current speeds in meandering rivers are higher on the outside of the bend than the inside of the bend.
2. Observations of sediment movement in physical models of rivers generally show sediment moving downstream through bends (not rolling uphill). In general the secondary currents in a bend are just strong enough to keep sand and gravel from rolling downhill. The lateral slope of the point bar will evolve to balance the uphill force applied by secondary currents with the downhill force of gravity. — Preceding unsigned comment added by 74.240.32.69 (talk) 05:34, 22 July 2012 (UTC)

What sources are you using to support these claims? On Wikipedia, unsupported claims are regarded as no better than original research. In the article, Reference No. 3 quotes Edward J. Hickin as writing One of the important consequences of helical flow in meanders is that sediment eroded from the outside of a meander bend tends to be moved to the inner bank or point bar of the next downstream bend. Dolphin (t) 07:58, 22 July 2012 (UTC)

Quick google search turned up: http://www.iahr.org/publications/assets/jhr39-5/P2099.pdf

I suspect Fredkin (1945) may be the appropriate original reference. Also, the important phrase from Hickin is "next downstream bend." Since point bars generally alternate from one side of the river to the other, the downstream point bar is on the same side of the river as the sediment source. As work schedule permits, I will start looking for appropriate reference material. — Preceding unsigned comment added by 74.240.32.206 (talk) 04:24, 23 July 2012 (UTC)

Thanks. I assume you are aware that when a fluid follows a curved path the fluid elements will display one of two types of flow - free vortex flow or forced vortex flow. If a fluid element is constrained to follow a curved path and no forces other than gravity and pressure act on it, the fluid element will follow the flow pattern of a free vortex - velocity increasing towards the center of rotation. If some other force acts on it, such as viscous shear forces, the fluid element will follow the flow pattern of a forced vortex - velocity decreasing towards the center of rotation. Fluid elements are incapable of spontaneously displaying forced vortex motion. Whenever a fluid element displays forced vortex motion it is reasonable to inquire what shear forces are responsible for this motion.

When you write Observations consistently demonstrate that current speeds in meandering rivers are higher on the outside of the bend than the inside of the bend you are describing a forced vortex. We are entitled to say that this is not to be expected, and to inquire what shear forces are responsible for causing the entire cross-section of a river to participate in a forced vortex.

Hurricanes, tornadoes, cyclones and all manner of revolving weather patterns have high-speed winds near their centers, and progressively slowing winds as distance from the center increases. This is free vortex motion. Seeing revolving flow patterns in the atmosphere display free vortex motion, as is to be expected, I am curious as to why rivers would display forced vortex motion, which is not to be expected. Dolphin (t) 08:30, 23 July 2012 (UTC)

Bowker (1988) describes a paper by Einstein (1926) describing secondary flow (using the tea cup example) that includes the following statements:

1. "Einstein goes on to explain how helical flow develops in a meandering river, and that because the higher-velocity portions of the stream will be driven to the outside (concave) portion of the river bend, erosion will be greater there."
2. "He also noted that because the helical flow possesses inertia, the circulation (and the erosion) will be at their maximum beyond the inflection of the curve. Hence, the wave-form of the river will migrate in a down-current direction. Finally, Einstein explained that the larger the cross-sectional area of a river, the slower the helical flow will be absorbed by friction; which explains why larger rivers have meander patterns with longer wavelengths."

Bowker's paper is referenced under the "Tea leaf paradox" article and may be found at: http://www.searchanddiscovery.net/documents/Einstein/albert.htm

Bernard and Schneider (1992) state "When the depth-averaged streamlines are curved, and the vertical distribution of velocity is nonuniform, centrifugal forces create a torque that generates helicity in the flow. (This is like a screw advancing or retreating in the streamwise direction.) If no correction is added for the secondary flow, depth-averaged models will usually constrain the largest velocities to lie near the inside of a channel bend, instead of allowing the observed gradual migration of high velocity toward the outside." and describe a correction to standard, depth-averaged, free surface hydraulic models required to match observations. Their paper may be found at: http://scholar.googleusercontent.com/scholar?q=cache:cL4KZ2h8RfUJ:scholar.google.com/&hl=en&as_sdt=0,25 — Preceding unsigned comment added by 74.240.14.199 (talk) 23:15, 4 August 2012 (UTC)