Flow speed

[Tigger]

I think rivers always speed up when rising and the higher the level the faster they flow, it's pretty obvious that all the extra water will push harder downhill (gravity n'all that) and it just goes faster...no doubt in my mind.

Edited March 9, 2014 by Tigger

[Phone]

Tigger,

 

Sometimes rivers flow up stream.

 

Phone

[robd]

Simplistically, is it not like an emptying bath tub where by the restriction of the outlet pipe (in a river the outlet through weir/ gates etc) controls the flow? There would be an increase of flow on a ‘rising’ level (leave the taps running !) due to an increase in head pressure.

[Steve Walker]

I don't think the physics for flow through pipes is the same as for flow through a channel or over a flood plain.

[Tigger]

Simplistically, is it not like an emptying bath tub where by the restriction of the outlet pipe (in a river the outlet through weir/ gates etc) controls the flow? There would be an increase of flow on a ‘rising’ level (leave the taps running !) due to an increase in head pressure.

 

 

Yeaph, that's how I see it.

[robd]

You’re right, but I did say simplistically. The restrictions are similar, bath = floodplain, outlet =river channel, head pressure = depth above or below normal river level. Increase in pressure = increase in flow.

[Steve Walker]

Yeah, but the main difference is that to increase the mass flow through a pipe you have to increase the velocity, with a river not having a lid it can instead increase the size of the "pipe".

[Tigger]

Tigger,

 

Sometimes rivers flow up stream.

 

Phone

 

Phone,

my local river does often flow back upstream at speed but only in the tidal reaches but not on all tides. I reckon if it started flowing backwards beyong the tidal reaches we would be in serious trouble .

[Tigger]

Yeah, but the main difference is that to increase the mass flow through a pipe you have to increase the velocity, with a river not having a lid it can instead increase the size of the "pipe".

 

 

But there's more pressure pushing it through so the main flow/channel will still be rattling through at higher speed.

[Steve Walker]

I think it goes faster more because there is not much more "braking" from the drag of the bed and a lot more water going through - think an artic rolling down a hill with the brakes from a Mini.