River flow at different depths.

[Mataji]

In various discussions about river flow I have read claims that the river can be flowing much faster below the surface than at the surface. Is there any truth in this and any real evidence? I would have thought that there would be more resistance below the surface, from the river bed and sides than from the air at the surface, and therefore the flow would be faster on the surface.

 

[Bran]

In various discussions about river flow I have read claims that the river can be flowing much faster below the surface than at the surface. Is there any truth in this and any real evidence? I would have thought that there would be more resistance below the surface, from the river bed and sides than from the air at the surface, and therefore the flow would be faster on the surface.

You are correct, surface water normally flows faster than water at depth. This is known as the velocity profile and is valid for laminar flow. River flow rates over natural river beds are calculated by measuring flow rates at various depths and relating it to river depth and velocity profile. I did study this back in the 1970’s when I did an apprenticeship in industrial instrumentation. Was told to consider it as very thin layers of water stacked on top of each other, like a stack of printer paper on a sloping table. As you tilt the table the top layers of paper start to move faster than the lower layers. Same thing in a pipe but consider it as straight rods, maximum velocity in the centre of the pipe.
Do a Google search for velocity profile in open channels for details, there is lots of data available.

 

[oldgit]

at low/ normal river levels would have thought that friction would tend to give a considerable difference in speed and rate of flow but at times of flood , perhaps less. ?


However when a weir is encountered , flows must get "mixed "going under the gates but presumably reform further down stream ?


The lock gate sluices at Faversham being used to flush the creek.
Automatically timed to open about 3 or 4 hours after high water to release tidal water in the pen above the gates.
Gates fell into disuse many years ago and could not be opened.
After some argy bargy between Peel Ports and Kent CC regards ownership ie whos going to pay for mending , gates were removed and not been seen since.
Who is going pay to mend the little bridge is also under debate.
Rumours that the actual mechanism installed in 1800 and something is in good nick but the far more recent "modern" deck is not ?

 

[Bran]

Weirs and fixed structures give better accuracy than measuring over a natural river bed. From memory weir structures for measuring flow rates were different shapes under water, broad, crump, flume in design, cannot remember the difference, was a long time ago!
Could bring up some interesting conversations with some of the lock keepers!

 

[The Q]

We river sail / race most of the year, we keep close to the banks when sailing into the tide, the tide is definitely slower close to the banks, due to the friction of water against river bank with the river bed being close.
You do have to watch for the salt water changing directing down below long before the fresh water on top.

 

[Momac]

 

[oldgit]

Some of us are known to perhaps not keep to the "correct" side of a navigation when punching any tide and may well have been admonished by a well known VTS for doing so.
When its your own boat and fuel it is probably of marginal gain in time and money.
When towing a barge or six, its not the tugs owners expences you are concerned with but purely how soon you will get home and the sooner the better.

 

[finestgreen]

In various discussions about river flow I have read claims that the river can be flowing much faster below the surface than at the surface. Is there any truth in this and any real evidence? I would have thought that there would be more resistance below the surface, from the river bed and sides than from the air at the surface, and therefore the flow would be faster on the surface.

PADI told me that when scuba diving I could expect to find the slowest currents near the sea bed.

 

[penberth3]

View attachment 166839

At last, the correct answer! In cross section it looks like an onion, maximum velocity in the centre of the flow and below the surface, the 0.2 of depth shown above.