Hydrologists - how have they managed this?

[Sway]

Outside my hotel in Zurich there's a river. The Limmet or something similar.

Normally quite pedestrian (literally, walking to the tram stop I've noted I walk the same speed as a pooh stick), this week after heavy rain all around the area it's running massively faster (at least double the speed) - standing waves, whirlpools behind bridge buttresses, etc.

Yet the water level is about a foot lower.

How can this be? Never seen anything like it in the UK, wondering if the Swiss have magic rivers.

At least I do know they're not tidal, unlike my boss...

I'd upload a photo from my phone, except a) it'd probably be upside down and that would irritate me and b) I've tried and thumbsnap is telling me the photo I've selected doesn't exist.

[Octoposse]

Sluices open, hydroelectrics bypassed to get the water out of the country?

[vonuber]

Supercritical flow probably.

As in why it is running faster and lower. The increase in runoff will be either due to dam release, or the time to peak of the surrounding catchment (given it is mountainous, that is likely to be pretty rapid).

Here you go, have a video( looks like my old hydraulics lab ):
http://www.youtube.com/watch?v=Q0R5xr-BqdA

Edited by vonuber on Tuesday 4th June 19:45

[Sway]

I love this place.

So they've deliberately created supercritical flow in order to reduce the risk of flooding, and yet still flow sufficiently?

Or is it a fluke?

[john2443]

I'd go with them opening sluices downstream. Normally the sluices keep the water level up (like a shallow reservoir) and the flow is low, open sluices, level drops but providing you put enough water in upstream the flow speed goes up.

You can have a river with eddys, waves etc that gets flatter when it's in flood but it also has a fast flow, so that's different to what you can see.

[vonuber]

Sway said:

I love this place.

So they've deliberately created supercritical flow in order to reduce the risk of flooding, and yet still flow sufficiently?

Or is it a fluke?

The problem with supercritical flow is all the energy within the water has to be changed into a more stable form, and what happens is that you get a hydraulic jump (sudden increase in water level) downstream.

[Simpo Two]

I see it. You envisage the driving force logically being from above, but in fact it's from below - the water you see is not running in but running out.

[Sway]

Cheers, all seems to make sense.
My understanding is that it flows out of Lake Zurich, to wherever.

Flow is definitely faster, sticks in the middle of the river typically move at walking pace, now much faster than that. In fact, they've stopped the river ferry, not sure if that's due to flow or less depth.

[vonuber]

WeirdNeville said:

If the water level has dropped, then discharge rate must > inflow rate.
If the the river is flowing faster than usual, then the gradient across it has increased. If the increase was due to increased influx, then the river level would HAVE to rise.
If the increase in flow rate is due to increased discharge, then the river level drops.

Ergo, I should imagine they've opened all the taps downstream to maximise flow through the river and prevent flooding

Not necessarily, water is a funny bugger and all that.
Your limit will be the capacity of the channel where it is, opening 'the taps' downstream will not affect the capaity of the channel where it is (well it will alter the downstream boundary condition, but the effect on the river will be fairly limited along its reach).
Fot the defined channel, Q = VA, and if if Q increases and A decreases (i.e. level drop) then allowing for friction losses it imples V incerase. If V increases toa point where the Froude number gets high enough, then you get supercritical flow and a subsequent drop in water level.

[Mr2Mike]

WeirdNeville said:

The Limmat flows into Lake Zurich,

Other way around I think? Looking at the West end of the Limmat where it joins the Aar there are some pretty big 'waterfall' type structures that suggest the flow is from East to West.