Dam levels

miz 2 December 2010 43

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?


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ARightCharlie ARightCharlie 11:56 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 RAAF Captain RAAF 12:44 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!

georgesgenitals georgesgenitals 12:34 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 RAAF Captain RAAF 12:26 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-sniffer troll-sniffer 11:48 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 RAAF Captain RAAF 11:23 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.

georgesgenitals georgesgenitals 10:57 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 RAAF Captain RAAF 10:47 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 Caulfield Holden Caulfield 10:13 am 03 Dec 10

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

p1 p1 9:45 am 03 Dec 10

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.

cleo cleo 1:47 am 03 Dec 10

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

p1 p1 9:40 pm 02 Dec 10

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 Claude Funky Claude 9:08 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.

olfella olfella 8:46 pm 02 Dec 10

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.

olfella olfella 8:38 pm 02 Dec 10

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.

olfella olfella 8:32 pm 02 Dec 10

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…

OpenYourMind OpenYourMind 8:20 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?

Pork Hunt Pork Hunt 6:20 pm 02 Dec 10

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.

Keijidosha Keijidosha 4:46 pm 02 Dec 10

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.

clp clp 4:21 pm 02 Dec 10

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.

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