2 December 2010

Dam levels

| miz
Join the conversation
42

OK, I admit I wasn’t good at maths, graphs and stuff at school – but does anyone know why, after all the rain we have had in the last month, dam levels are still hovering around the 90% mark?

I would have thought it would be over 100% by now, and that we would be hearing about ACTEW having to let the overflow down river.

According to BOM, in November 2010,

Mount Ginini received 210mm rain;
Tuggeranong (and presumably Googong) received 148mm;
Tidbinbilla received 155mm; and
‘Canberra’ (Airport) received 121mm.

So, what’s going on? Is it a conspiracy to prevent us from thinking that water is plentiful, so we can continue to be charged the proverbial ‘arm and a leg’ for this essential commodity?

Join the conversation

42
All Comments
  • All Comments
  • Website Comments
LatestOldest
ARightCharlie11:56 pm 07 Dec 10

There are two reasons why a water storage system might be showing a capacity greater than 100%:

1. Inflow volumes are greater than total outflow volumes ie usage + spill capacity. (Yes Virginia, land around lakes can flood too); or
2. The designated storage capacity is less than the actual carrying capacity of a storage. This occurs where dams are designed with a dual role of water storage and flood mitigation. e.g. Wivenhoe Dam in SE QLD was designed and built after the disastrous 1974 floods. It is deemed to be at 100% when the water is actually 30 feet or so below the spillway. When it goes over 100% of nominal, as it has recently, water is released in a controlled way after the rain event to bring the dam back to 100% without causing downstream flooding. However, no ACTEW dam is treated this way.

Captain RAAF12:44 pm 03 Dec 10

C’mon mate – you and I both know that until the graph on the website says 100%, that there’s spare dam capacity!

I’ll believe it only when those mystical illuminated signs on the side of the Parkway tell me so because they are solar powered, therefore powered by God….and he never lies!

georgesgenitals12:34 pm 03 Dec 10

Captain RAAF said :

georgesgenitals said :

Interestingly the graph of dam combined capacity provided on the actewagl website says that at 2 Dec stored amount was a a bit under 94%, but the actual graph image shows it today and only a fraction under the 100% mark.

I’ll be honest here – I didn’t expect to ever see the dam combined capacity at 100%. And yet that is what will likely happen later today.

Well George, I hate to say I told you so….but I told you so, a few months ago (maybe not you directly, but the RA in general)

I’ll say it again, slowly, this

is

just

the

start

!

The dams are all now at 100%, the ground is saturated, there is now nowhere for the water to go except up!

I live on a hill, I don’t care.

C’mon mate – you and I both know that until the graph on the website says 100%, that there’s spare dam capacity!

Captain RAAF12:26 pm 03 Dec 10

troll-sniffer said :

Captain RAAF said :

Sorry, but I will not believe any website, any authority or any spokesman that says such and such a dam is not full until I see it with my own eyes. Any dam that is not at 100% capacity right now is being kept that way due to some other reason, such as requiring access to certain areas for maintenace/construction etc.

Everything in the region is full, if it aint it’s because some department wants it that way.

Ah yes, the dogmatism of someone who knows more than the employees of the organisation that actually runs the water catchment infrastructure. I’m sure the engineers and other techies charged with monitoring dam levels are quakin’ in their booties at the prospect of being unmasked by someone who, through the power of self-delusional deduction, has worked out that the dams ‘must be full coz i says so, I seen tha rain cummin’ down an it’s like buckets n buckets’ etc

Um, I thought it was quite obvious I was talking about me and my own belief’s, sniffer. Can’t see anywhere in my post where it says ‘I expect you all to subscribe to my POV!’

On a related note, here’s something for everyone to ponder, how many DAYS of rain has it taken to deliver us from how many YEARS of drought?

troll-sniffer11:48 am 03 Dec 10

Captain RAAF said :

Sorry, but I will not believe any website, any authority or any spokesman that says such and such a dam is not full until I see it with my own eyes. Any dam that is not at 100% capacity right now is being kept that way due to some other reason, such as requiring access to certain areas for maintenace/construction etc.

Everything in the region is full, if it aint it’s because some department wants it that way.

Ah yes, the dogmatism of someone who knows more than the employees of the organisation that actually runs the water catchment infrastructure. I’m sure the engineers and other techies charged with monitoring dam levels are quakin’ in their booties at the prospect of being unmasked by someone who, through the power of self-delusional deduction, has worked out that the dams ‘must be full coz i says so, I seen tha rain cummin’ down an it’s like buckets n buckets’ etc

Captain RAAF11:23 am 03 Dec 10

georgesgenitals said :

Interestingly the graph of dam combined capacity provided on the actewagl website says that at 2 Dec stored amount was a a bit under 94%, but the actual graph image shows it today and only a fraction under the 100% mark.

I’ll be honest here – I didn’t expect to ever see the dam combined capacity at 100%. And yet that is what will likely happen later today.

Well George, I hate to say I told you so….but I told you so, a few months ago (maybe not you directly, but the RA in general)

I’ll say it again, slowly, this is just the start !

The dams are all now at 100%, the ground is saturated, there is now nowhere for the water to go except up!

I live on a hill, I don’t care.

georgesgenitals10:57 am 03 Dec 10

Interestingly the graph of dam combined capacity provided on the actewagl website says that at 2 Dec stored amount was a a bit under 94%, but the actual graph image shows it today and only a fraction under the 100% mark.

I’ll be honest here – I didn’t expect to ever see the dam combined capacity at 100%. And yet that is what will likely happen later today.

Captain RAAF10:47 am 03 Dec 10

Sorry, but I will not believe any website, any authority or any spokesman that says such and such a dam is not full until I see it with my own eyes. Any dam that is not at 100% capacity right now is being kept that way due to some other reason, such as requiring access to certain areas for maintenace/construction etc.

Everything in the region is full, if it aint it’s because some department wants it that way.

Holden Caulfield10:13 am 03 Dec 10

Here’s a good, quick and easy view of local dam levels: http://www.eldersweather.com.au/dam-level/act/

cleo said :

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

Lake George looked like this about two weeks ago. We have had a lot of rain since then.

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.

Funky Claude9:08 pm 02 Dec 10

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…

OpenYourMind8:20 pm 02 Dec 10

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.

Captain RAAF1:07 pm 02 Dec 10

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).

georgesgenitals12:53 pm 02 Dec 10

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!

georgesgenitals12:50 pm 02 Dec 10

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!

Holden Caulfield12:15 pm 02 Dec 10

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?

Captain RAAF11:13 am 02 Dec 10

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.

Daily Digest

Want the best Canberra news delivered daily? Every day we package the most popular Riotact stories and send them straight to your inbox. Sign-up now for trusted local news that will never be behind a paywall.

By submitting your email address you are agreeing to Region Group's terms and conditions and privacy policy.