Dam levels

How is Lake George going, has it filled up yet, it hasn’t been a lake for at least 15 or so years.

OpenYourMind said :

Obviously it’s not the case with all this rain, but is there any truth in the rumour that Googong’s catchment has been reduced by all the hobby farm development and their associated small dams out Burra way?

It is certainly true that an increase in farm dams since googong was built has lead to a decreased total runoff from the catchment.

Along with drinking water, Googong provides flood control, and provides storage for maintaining the level of Lake Burley Griffin during dry periods.

I am lead to believe the site was selected for two main reasons. Firstly, there advantages in having two different catchments to draw from, especially if one gets contaminated. Secondly, the Googong site has been considered for a impoundment since Canberra was a pup. (I have been told that the Googong catchment was to be included within the boundaries of the ACT, however for one reason or another they chose not to.) Therefore the site was the most studied in terms of geology and hydrology.

Pork Hunt said :

The catchment cannot exceed 100% in the same way a schooner glass will not hold a pint. The overflow of the dams is not and cannot be harvested for drinking water.

It may not be harvested but the capacity can go over 100% – just look at what happened to Burrinjuck a few weeks ago and I reckon would be happening again now. It was at 110% because the inflow could not get over the wall fast enough.

Keijidosha said :

Having spoken with some ActewAGL dam engineers there is a general consensus that the choice of Googong over other locations was poor, however engineers at the time of contruction had less reliable modelling data on which to base their decision.

Also worth considering is that the Googong site is outside of the ACT, aquired by the Commonwealth and managed by ACTEW Corp. With the somewhat limited water resources within the ACT it makes sense that an opportunity would be taken to aquire land outside of the ACT where/when possible.

Is those same engineers that recommended it be built? Regarding the second point you make, go back to the reasons our founding fathers chose to site Canberra where it is now, is because of the catchment area contained within. It was not until the university trained experts came along and buggered all of that up! Local knowledge is a good thing and is never written about in text books. Even now it is proven that the Cotter corridor will provide enough water to supply well above the population that our founding fathers never dreamed of for our city.

georgesgenitals said :

D2 said :

Keijidosha said :

Despite being the largest dam in our supply system, Googong is in a rain shadow which reduces yeild.

+1

What I want to know is who’s the idiot who decided to build our largest dam where it doesn’t rain!

2 reasons. Firstly, it can be much larger than the (then) existing dams. Second, until the drought over the past 15 years or so it was used to regulate flow down the Queanbeyan River to stop the semi-regular flooding that used to occur. Googong catchment is a very large area, and forcign that water down the Queanbeyan River would produce, um, interesting results.

This: http://www.actewagl.com.au/water/catchment/watermap.aspx?facts

shows that the catchment areas are:
Cotter: 192.4 square kilometres
Bendora: 91.4 square kilometres
Corin:

196.3 square kilometres
Googong: 873 square kilometres

That’s why they built it there!

But you forgot to add – it is also shallow and the evaporation rate from that large surface area is tremendous…

Obviously it’s not the case with all this rain, but is there any truth in the rumour that Googong’s catchment has been reduced by all the hobby farm development and their associated small dams out Burra way?

I went to Googong this afternoon on the motorcycle and got home nanoseconds before the rain hit Qbn.

Looking at the two little piers from the disabled parking area, the water is just about to reach the bottom of the farther one.

The catchment cannot exceed 100% in the same way a schooner glass will not hold a pint. The overflow of the dams is not and cannot be harvested for drinking water.

The information is available, but the maths isn’t simple. Calculating remaining “days of supply” depends on many factors – current storage levels, current and predicted inflows, current and projected use, number and size of personal rainwater tanks, and more.

The simplified answer is that Canberra uses roughly 67GL of water per year. (Around 35GL of this is returned to the Murrumbidgee via stormwater and outflows from Molongolo STP, which we cannot reuse). If our total available storage is 207GL then we’d have 3-4 years worth of water if the dams were all full and inflows were zero.

What I want to know is why dam levels are always reported in percentage terms – it doesn’t matter what the percentage of the dams are its about how much water in total we have – and days of supply.
Even when the dams in Sydney were down at 30% they still had about 3 years supply in them. In some country areas dams are quite small so it doesn’t take much to get them to 100%.

Its silly if you ask me.

Having spoken with some ActewAGL dam engineers there is a general consensus that the choice of Googong over other locations was poor, however engineers at the time of contruction had less reliable modelling data on which to base their decision.

Also worth considering is that the Googong site is outside of the ACT, aquired by the Commonwealth and managed by ACTEW Corp. With the somewhat limited water resources within the ACT it makes sense that an opportunity would be taken to aquire land outside of the ACT where/when possible.

Reply

chewy14 said :

dave__ said :

Could ACTEW be keeping the level of the dam low while they finish work on the spillway?
http://www.actew.com.au/watersecurity/majorprojects/GoogongDamSpillwayUpgrade.aspx
Or has that finished already?

I’ve heard that this is correct, they are letting more water go down the river from the dam.
Because of the spillway works, they have been trying to keep Googong a bit low just in case we have floods or massive rains (which we have been).

I say let it back up all the way to the top, teach some people the stupidity of their decision to build housing estates on flood plains all based on the theory of a few years ago that we are in a never ending drought and all about to die and surely it won’t ever rain again… I personally can’t wait to see Harrison go under. I watched it getting built and was amazed to see great swathes of water across the site even after moderate rain. There would be a number of other areas all unprepared for serios flooding and I’ll enjoy watching all the poor floating downstream from my castle on the hill.

dave__ said :

Could ACTEW be keeping the level of the dam low while they finish work on the spillway?
http://www.actew.com.au/watersecurity/majorprojects/GoogongDamSpillwayUpgrade.aspx
Or has that finished already?

I’ve heard that this is correct, they are letting more water go down the river from the dam.
Because of the spillway works, they have been trying to keep Googong a bit low just in case we have floods or massive rains (which we have been).

D2 said :

Keijidosha said :

Despite being the largest dam in our supply system, Googong is in a rain shadow which reduces yeild.

+1

What I want to know is who’s the idiot who decided to build our largest dam where it doesn’t rain!

2 reasons. Firstly, it can be much larger than the (then) existing dams. Second, until the drought over the past 15 years or so it was used to regulate flow down the Queanbeyan River to stop the semi-regular flooding that used to occur. Googong catchment is a very large area, and forcign that water down the Queanbeyan River would produce, um, interesting results.

This: http://www.actewagl.com.au/water/catchment/watermap.aspx?facts

shows that the catchment areas are:
Cotter: 192.4 square kilometres
Bendora: 91.4 square kilometres
Corin: 196.3 square kilometres
Googong: 873 square kilometres

That’s why they built it there!

Googong has the LARGEST catchment area of all the dams, but is HEAPS bigger than all the others, meaning it takes longer to fill (and empty). If we keep getting weather like this for the next month or so I would expect used storage capacity to hit 100%.

Keijidosha said :

Despite being the largest dam in our supply system, Googong is in a rain shadow which reduces yeild.

+1

What I want to know is who’s the idiot who decided to build our largest dam where it doesn’t rain!

Mark Sullivan tells us there has been more than 2.5 gigalitres flow into Googong in the last 24 hours. Meanwhile over 1 gigalitre per day is spilling over at the Cotter.

And he also thinks we have had enough rain already:
http://twitter.com/#!/Sullivan_ACTEW/status/9765589912395776

Captain RAAF said :

I think someone needs to head out to Googong and get a photo of the dam! Are we agreed that ‘full’ means that water is lapping at the top of the spillway? This will likely be a lot lower than the level of the road over the wall, btw.

I might head out there over the weekend, it would make a great photo opportunity in the beast!

I have it on good authority that it is 2.5m from capacity at the wall. Spread that 2.5m in height by the surface area of the dam & there is quite a bit of water to go before it goes over.

A week ago Googong looked like this.

If you’re interested you can get daily updates of all Australian water storages and systems via the BOM’s free Water Storage iphone app.

Certainly warrants, given the comment of Googong being in a rain shadow and the other dams overflowing, having the ability to pump excess to the dams not currently full.

i was wondering, with my own poor maths, how dams could be much more than 100% – surely they would then overflow and hover at round 100%? (heard a comment on the talking box yesterday about dams at 110%…)

Could ACTEW be keeping the level of the dam low while they finish work on the spillway?
http://www.actew.com.au/watersecurity/majorprojects/GoogongDamSpillwayUpgrade.aspx
Or has that finished already?

I think someone needs to head out to Googong and get a photo of the dam! Are we agreed that ‘full’ means that water is lapping at the top of the spillway? This will likely be a lot lower than the level of the road over the wall, btw.

I might head out there over the weekend, it would make a great photo opportunity in the beast!

Wasn’t excess water from the Cotter supposed to flow into Googong via Stromlo?

I’ve heard (and feel free to correct this) that Googong has the largest capacity, but has the smallest catchment area of all the dams.

ACTEW seem to be pretty slow to update dam levels on their website these days too. The roadside signs look to be more up to date than what’s online – the sign I passed yesterday was saying levels were at 92%, so they are going up slowly.

Just have a look at the graph on the linkyou posted.
Googong has by far the largest capacity of all the dams and earlier this year it had about the same amount of water in it as Corin. The others look like they’ve been basically overflowing since September.

Cotter, Bendora and Corin dams (on Cotter River) are all at 100%, but Googong Dam is hovering around 80% – hence the total catchment is around 90%

This shows it – http://www.actew.com.au/water/damlevels.aspx

I have wondered this too – flying into canberra over googong dam last week I could not see any of the foreshore exposed – unless “100%” is now at the top of the trees around the dam?

A quick visit to the ACTEW Corp website shows that Googong is not full:
http://www.actew.com.au/water/damlevels.aspx

Despite being the largest dam in our supply system, Googong is in a rain shadow which reduces yeild.