Normally we know that flow velocity in the river is minimum at the bottom of a channel. As we move towards the surface of the flow in a river velocity increases. But normally maximum velocity occurs at 0.05 to 0.25 d where d is the depth of that channel. My question is why maximum velocity of flow in the river will not happen at the free surface?
In general,spiral flow and circulation in river bends, wind effects,shear stress will slow down the flow velocity at the water surface.In narrow channels, the maximum velocity does not occur at the water surface,for unsteady flow ( turbulent condition in large rivers), maximum velocity can occur in mid-depth or even near the bed. I think that the max. velocity occurs near the surface about 0.8-0.9 d from the canal bed
It happens due to the Archimedes' principle dealing with hydro-dynamics.
in surface water the max. velocity usually near the surface about 0.89 depth fron the canal bed
I would not say that maximum velocity cannot happen at the surface. The geometry of the channel and wind conditions above the water can make maximum velocities occur at the surface. Often in coastal streams that have very low gradient the maximum velocity is at the surface and is wind driven, especially if you have a set of conditions that run against gradient. i.e. storm surges, wind seiches etc.
Because of viscosity, there is a friction between water and other things. In this case, the friction between water and air is the reason.
yes, I agree with other colleagues. The reason is that the friction offered by the air slows down the flow
Though I agree that the air resistance or air friction will slow down the flow velocity at the water surface, but what if the wind direction is in the same direction of water flow ??
Depending on its direction wind does speed up or slow down the water flow at the upper free surface. Strong wind can even cause water counterflow. Then at some depth of the flow the velocity of water is zero
It can be broadly stated that the flow velocity at the upper free water surface is controlled by the wind direction. If the wind direction is against the water current, then it will slow down the flow velocity at the upper free surface, but if the wind direction coincides with the water flow direction, then the flow velocity at the upper water surface will be maximum.
Factors effecting upper layer flow velocity of water flowing in an open channel are: (overlaying air flow velocity and direction, viscosity and surface tension properties of water, water flow depth and channel width).
I've seen this effect myself, both in the lab and in small streams (not sure if it's as evident in large rivers, where some commenters mention river bends and other large mechanisms), and have wondered what causes it. In the lab, I've assumed that surface tension is the cause. In the field, I've observed this even with no wind, so I do not believe wind is the explanation. But is surface tension strong enough over large scales?
I agree with Prasad because wind direction will definitely affect the flow velocity at the water surface and at relatively deeper depths, water attains hydrostatic equilibrium and flow velocity amounts to zero.
A related question is how to interpret "particle image velocimetry" (PIV) data for estimation of streamflow. One reference is
"Flow measurement in streams using video imagery" which can be found at http://onlinelibrary.wiley.com/doi/10.1029/2002WR001317/pdf
http://onlinelibrary.wiley.com/doi/10.1029/2002WR001317/pdf
"If surface tension is irrelevant, please explain the mechanism." Dear Ted, it is because of friction between the water and the air. The friction takes place even when the air is calm, provided that the water is flowing and there is a jump of velocity at the interface.
Because of the interficial shear forces occuring between the overlying air layer and flowing water. These resisted forces are smaller than bed shear forces that predominantly diminished the water flow at river bed.
If we consider fluid resistance as a simple combination of internal resistance (turbulence/laminar condition as described by the Re-Nr) and surface resistance or also called free surface resistance (sub/super-critical condition as described by the Fr-Nr), the later Fr conditions influences your observations...so i do not know if your observations within a "river" is in the sub-critical regime...if not, eg. Fr>1, then your water surface motphology is changing just by fluid dynamics inducing a resistance to flow near the water surface....just another player in the phenomenon, just focusing on the flow dynamic ( no external additional influenced like wind, air layer, etc.)
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