Bank Holiday Weekend

Cuchilo said:
Its making my head hurt .
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Don't hurt your head:) its a river so its going downhill, the riverbed at one end of a reach is higher than at the downstream end, locks and weirs were put in to try and level it out but it still slopes downhill. Pour a load in at the top of the hill too fast and it sits there until it can escape downhill, its trapped by the banks with only a small channel to go down.
 
Chris_d said:
its trapped by the banks with only a small channel to go down.
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well technically the hydraulic gradient depends on the depth of flow, the frictional coefficient of the substrate, and the level turbulence within the flow.

It is, as has been mentioned, obeying the Bernoulli theorem, due to the constraints of flow the head constant H1 just downstream from the weir is greater than H2, just above the next weir. To maintain equilibrium assuming constant pressure and density of the fluid, the river is in fact flowing slightly faster at the weirhead :cool:

The same equation can be used to calculate the velocity of the water as it passes over the weir simply by measuring its depth, and therefore used to calculate the volume of water going down

Is that any clearer... or shall I get the aspirin :)
 
Ive got it now . All you had to say was
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rr_123 said:
The same equation can be used to calculate the velocity of the water as it passes over the weir simply by measuring its depth, and therefore used to calculate the volume of water going down

Is that any clearer... or shall I get the aspirin :)
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But are the channels leading to the weirs rectangular??
 
jecuk said:
But are the channels leading to the weirs rectangular??
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This is a good and valid point.

The channel is certainly rectangular at the weir, but I think that a degree of approximation would need to be applied to the immediate upstream section.
 
Teddington Weir this morning ( 0545 ) click to view video

Doesn't look that fast , but the volume of water passing under the bridge is incredible.

 
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I've seen a couple of boats out this week, a professional looking geezer on a sports cruiser, a stoic 60' narrow boat (Lord knows what would have happened if he tried to turn that round!!) but the best was a Salter's party boat today - cruised very slowly upstream then came back down still pointing upstream but ferry gliding at half the speed of the current so going forward slowly through the water and backwards over the ground - lovely bit of boat handling in a wacking great thing with a single prop and a rudder :)
 
Question about average levels and bridge heights:

My boat is out of the water at Reading and arrived by truck. The river map says Henley bridge is 4.1m above the water and I have calculated 40cm clearance so no problem? Obviously it would not get under today. I saw real waves at Reading yesterday!

I need some good summer weather for painting and by then the level should be down.

If I only have 40cm between the 4.1m (on the map) and want the freedom to go up and down the river at any time of year I suppose I will have to modify the boat.

What does the 4.1 mean anyway?
 
4.1m is measured from a height in the water known as 'headwater' or zero.

So , if 'headwater' is zero , the bridge has 4.1m of clearance , you need to phone the lock before the bridge and ask them how much above 'headwater' they are running at to get the true bridge clearance.

If they run at 4 inches above headwater , then the bridge height is reduced by 4 inches or 10 cm , and would instead be around 4.0m
 
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