h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

So what's the question?

Do the dams contribute to the flooding?

The town is on a lake. The level of water in the lake is controlled at the dam. If the controller of the dam decides to put an extra 10 metres of water in the lake, then the town is under 10 metres of water. Yes?

Hi kea

Tell me if I have this straight:
-Queenstown is low-lying and at the outlet of Lake Wakatipu.
-The river leaving the lake flows for some distance to the river outflow from Lake Dunstan.
-The first Hydro dam is at Clyde.
-There is another dam further down this river at Lake Roxburgh.
-There is a 100m contour at Roxburgh, a 200m at Clyde and a 400m at Queenstown.

As far as I can see the dams would not affect the flooding in Queenstown unless the river between there and Clyde backed right up and rose almost 200m.

Or have I missed something?

t.

You've missed something. The impression I'm now getting from Kea's latest description, is that the town is at one end of the lake, and the dam is at the other (the actual distance is irrelevant, as they're on the same body of water).

In this circumstance, the dam does affect the water level at the town, as there will be a negligable height differential over as small a distance as several miles. (Even a spring tide, where the moon and the sun's gravities combine to raise the tidal differential are in the order of metres over a quarter radius of the Earth).

Sorry Kea, but the impression I'd formed about this is that the dam was on a lake, and the town was on a feeder body leading into it.

From what I gathered the Queenstown Lake is a natural lake, not damned [sic], the (two) dams are further down the valley.

Yes. That's my impression too.

Take a look at her link but change the view to terrain and you can work out where things are. I also had a look on Google Earth as the zoom is better.

t.

>>
The town is on a lake. The level of water in the lake is controlled at the dam. If the controller of the dam decides to put an extra 10 metres of water in the lake, then the town is under 10 metres of water. Yes?
<< Gnomon

Yes, but of course that's not what happens. What happens is that the power company tries to keep the lake high at this time of year to generate power over the winter, but can't control big rains. So it's not that they're keeping the lake high and the result is a flood, it's that the water from the big rain can't get out quick enough. My questions is how much the dam affects this along with other issues like the narrowness of the gorge.

>>As far as I can see the dams would not affect the flooding in Queenstown unless the river between there and Clyde backed right up and rose almost 200m.<<

Turvy your synopisis of the geography is good. And I understand the physics in that sentence above. What about the bath analogy with the pipe coming out that's dam-ed or not. Does that affect the rate at which the bath empties or not?

>>
Sorry Kea, but the impression I'd formed about this is that the dam was on a lake, and the town was on a feeder body leading into it.
<<

Deadangel, have a read of the OP again and look at the map. The first dam is over an hour's drive from the town which is on the lake.

Speaking of tides, I just remembered that the Queenstown lake does have this strange tide thing where it rises and falls 10 cm every half hour.

You can take it that the water can flow over the dam as fast as it can flow into the lake. That's the way dams are designed.

Oh, I've had a look at the photo and I can't understand it all now.

Queenstown is on a lake. Is that a natural lake or is it a lake formed by damming the river?

It's a natural lake. The dam is many many miles away.

Sorry, just looked at the OP, and it's not entirely clear. Look for the word 'Clyde' on the map. That's where the first dam is.

>>
You can take it that the water can flow over the dam as fast as it can flow into the lake. That's the way dams are designed.
<<

Through the dam?

I'm not sure that's true, because in a really big weather event they open the flood gates (or whatever they're called) and spill the dam. They don't like doing that of course, for a number of reasons.

Most of the time they are limiting the flow of water to generate power, that's the point isn't it?

In normal generation, all the water that flows into the lake goes through the turbines (the things that turn the generators). If there's too much water flowing in, there are various spillways that can take the excess. If they didn't do this, the water would eventually build up behind the dam so high that it would flow over the top, which doesn't happen. So the dam is always capable of letting through all the water that flows into the lake.

Since the dam is far downstream of the natural lake, it can not affect the level in the natural lake in any way.

Summary::
The lie of the land is such that those dams cannot affect the lake at Queenstown (unless Clyde could hold back the 200m required, which it can't - it's a few metres at most, from memory).

Therefore:
The outflow, or lack thereof, from Lake Wakatipu is the cause of Queenstown's flooding.

. Unless there's a tight enough restriction, like a submerged bar of rock, at the mouth of Queenstown's bay, which seems unlikely because tourist steamers ply their way about the whole lake.

QED

No?

So you don't think how much water is going through the dam affects the lake when it's in flood?

In you OP you say that the lake drains East to the confluence at Cromwell, whence to Clyde.

There'll be hour-by-hour adjustments to the flow through the Clyde dam - pretty minor stuff compared with floodmaking flow.
When such a flood flow occurs, the dam can hold back some but surely not all - otherwise it'd overflow (not advisable even if it's capable of taking that punishment) - so the excess will be deliberately released before that time - and, away from Queenstown.

We've been (reliably) informed that the dam would need to hold back some 200m depth of water before it could reach the level of the lake. It can't. Anyway, if it did, Cromwell would be affected well before it reached Queenstown.

Just conceivably there may be some effect upstream, from back pressure waves, but that seems pretty unlikely - and would anyway surely be insignificant (if measurable) compared with the river turbulence and a whole lakeful of water needing to be affected.

So no, I don't think however much water is going through the dam will affect Queenstown at all.

... barring the electrickery supply!

- but, having said all that there're rather more than a few mountains (& watersheds) around there.

I don't recall (without a re-read) anyone saying anything about subterranean waterways -?
No doubt the lake has been fairly well surveyed, but still, lots & lots of little inflows or large areas of permeable rock may not have shown up?