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Thar she blows! New Cotter Dam overspilled

By 1 March 2012 76

The Cotter Dam Expansion Dam Cam is showing the worst case scenario of flood waters rising over the under-construction wall as the capacity of the river diversion is comprehensively overwhelmed.

Let’s hope the raging torrent doesn’t undermine the wall or this will become a disaster of 2003 bushfires proportion.

We certainly hope that doesn’t happen, but it’s not like this was meant to happen either.

UPDATE 01/03/12 09:10: Thanks to dpm for pointing us at the above video compression of the overspill

cotter dam overspill

[Photo by Androo]

UPDATE 02/03/12 8:18: And she’s still going very strong this morning:

cotter dam

UPDATE 02/03/12 09:31: This in from ACTEW supremo Mark Sullivan:

MarkS
2012/03/02 at 8:21 am

This morning observations are that the dam structure is sound. Heavy machinery has remained safe and cranes are fine. Have lost some forms and scaffolding and some light equipment. Biggest concern is the washing out of earthwork on the downstream side (looking at damcam it is on the left abutment or the right hand side looking at the dam). Possibly be a week before we get back on the dam and looks like a three week clean up. Flood is a one in one hunred year event so top marks to the engineers. Now we wait till tomorrows renewed assault.

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76 Responses to Thar she blows! New Cotter Dam overspilled
#1
chewy147:37 pm, 01 Mar 12

Uh oh spaghettios.

#2
MarkS7:43 pm, 01 Mar 12

Lets keep this right. A 3metre diversion tunnel is going thru the dam on the left hand abutment. This was built in 2011 and is performing well. You cannot fully divert a river. The threat to the dam is neglible. It is a very stable mass concrete mass. What is at risk is the dam construction infrastructure.

#3
johnboy7:56 pm, 01 Mar 12

Thanks for the clarification Mark.

So even with no spillway you’re not worried about the flow undermining the dam wall? (what spillways were invented to prevent)

The water coming down the sides doesn’t look great from here either.

#4
Devil_n_Disquiz7:56 pm, 01 Mar 12

The sound of the water flowing over the wall is quite something else. Impressive to say the least.

It would appear (from DamCam) that water is flowing over the entire structure now.

#5
rohbo8:03 pm, 01 Mar 12

johnboy said :

Thanks for the clarification Mark.

So even with no spillway you’re not worried about the flow undermining the dam wall? (what spillways were invented to prevent)

Is that not the point of the stepped wall? To break up the water and take away most of it energy before it hits the bottom?

I’m wondering if any of that scaffolding will till be there in the morning? Looks like a fair bit of water coming over.

#6
chewy148:08 pm, 01 Mar 12

johnboy said :

Thanks for the clarification Mark.

So even with no spillway you’re not worried about the flow undermining the dam wall? (what spillways were invented to prevent)

The water coming down the sides doesn’t look great from here either.

I didn’t think the dam was going to have a constructed spillway?

The stepped face is designed to dissapate the energy as it overflows.

#7
arescarti428:13 pm, 01 Mar 12

johnboy said :

Thanks for the clarification Mark.

So even with no spillway you’re not worried about the flow undermining the dam wall? (what spillways were invented to prevent)

The water coming down the sides doesn’t look great from here either.

I think it is safe to assume that the dam is sitting up against fairly solid rock (at least I certainly hope it is) so a one off event like this is unlikely to undermine it.

If it was an earthen dam, then the lack of a spillway would be tremendously concerning, but that concrete isn’t going anywhere in a hurry.

#8
Mr Evil8:19 pm, 01 Mar 12

A couple of questions.

Why were vehicles left parked on the topside of the dam when there had been pretty ample warning that NSW/ACT was going to receive a lot of rain in the next few days, and numerous flood warnings had been publicly issued?

Was any attempt made to secure loose material on the worksite before it became apparent that the waters were going to breach the top of the current wall? Whilst watching some of the webcam footage an amount of timber and what appeared to be an upturned watertank could be seen floating down the river in front of the dam – what measures are taken to prevent this material flowing down the river to become a problem further downstream?

#9
rohbo8:23 pm, 01 Mar 12

Mr Evil said :

A couple of questions.

Why were vehicles left parked on the topside of the dam when there had been pretty ample warning that NSW/ACT was going to receive a lot of rain in the next few days, and numerous flood warnings had been publicly issued?

If you know how to get some very heavy machinery off a 40m high wall i’m sure ACTEW would love to hear from you. Mark S from ACTEW said they got off everything they could with the cranes and the rest was too heavy so had to stay.

#10
MarkS8:27 pm, 01 Mar 12

Some good comments. The dam will have a constructed spillway but steps are designed to dissipate energy. Heavy machinery cannot be lifted of the dam or driven off. Most lighter material was moved. Tanks difficult to lift so tied down. Already seeing upstream debris and this will increase and may cause some problem. While dam is keyed into the abutment it is gravity that holds the 185,000 cm of concrete in place. Scaffolding and formwork the big issue as flow increases. Will try and contribute but things getting busy. Bloody rain

#11
johnboy8:29 pm, 01 Mar 12

And if any of the avid Dam Cam watchers see something dramatic be sure to get a screenshot at send it in to images@the-riotact.com

#12
johnboy8:30 pm, 01 Mar 12

No concerns about the cranes Mark?

#13
Camelicious8:34 pm, 01 Mar 12

I guess ACTEW is pumping as hard as they can out of Corin and Bendora, since I believe the pipes out of Cotter aren’t connected. Will be interesting to have a look at the Dam Cam in the morning to see what’s left there because there’s a lot of water coming over the wall right now.

I wonder how long the ungrouted abutments will remain firm before hydrostatic pressure works its magic.

#14
I-filed8:35 pm, 01 Mar 12

Don’t worry! CEO Mark Sullivan said on 666 earlier that the workers had “tied everything down” … I’m sure on the basis of this competent chap’s assurance that everything we (not Mr Sullivan) have paid for will be safe ………….

#15
MarkS8:37 pm, 01 Mar 12

Cranes are very solid. Currently pinned more than forty metres into ground. At the bottom of my list for now

#16
thatsnotme8:40 pm, 01 Mar 12

Mr Evil said :

A couple of questions.

Why were vehicles left parked on the topside of the dam when there had been pretty ample warning that NSW/ACT was going to receive a lot of rain in the next few days, and numerous flood warnings had been publicly issued?

Was any attempt made to secure loose material on the worksite before it became apparent that the waters were going to breach the top of the current wall? Whilst watching some of the webcam footage an amount of timber and what appeared to be an upturned watertank could be seen floating down the river in front of the dam – what measures are taken to prevent this material flowing down the river to become a problem further downstream?

So just where were the vehicles supposed to go? There’s no road leading off the top of the dam – if it was as easy as just driving off the top, do you not think they would have done that? So the alternative is craning the vehicles off of the dam. I don’t profess to know anything about crane operations, but I’d imagine that lifting something like the vehicles that have been left up there wouldn’t be easy, or necessarily safe.

And honestly…timber floating down the river? Maybe a water tank? If you can manage to differentiate between what naturally flows over the dam in an event like this, and what may be construction material, based only on the dam cam images, then you’re obviously watching a different dam cam to me.

With no connection to this project at all, I think I can safely say that yes, there would have been significant efforts made to secure as much as possible on the site. Everything that’s lost down the river costs money, and I have enough faith in the people in charge of this project to believe that they’re not going to deliberately throw money away if it can be easily avoided. I’m sure this site has been a hive of activity over the last few days, preparing for the worst. Time will tell whether those preparations were good enough.

#17
Russ8:48 pm, 01 Mar 12

johnboy said :

And if any of the avid Dam Cam watchers see something dramatic be sure to get a screenshot at send it in to images@the-riotact.com

Been grabbing shots every 30s since shortly after it started to overflow (took a few moments to write a script). Might compile a time-lapse video when it’s all over.

#18
johnboy8:49 pm, 01 Mar 12

Cheers Russ, be sure to share!

#19
Russ8:50 pm, 01 Mar 12

MarkS said :

Already seeing upstream debris and this will increase and may cause some problem.

If anything really bad is going to happen, I suspect a log-jam of debris caught at the top of the dam that breaks away suddenly will be what causes it.

#20
dpm8:53 pm, 01 Mar 12
#21
OpenYourMind8:56 pm, 01 Mar 12

Let me just say how cool it is to have the local expert tell us how it is on RiotACT. Thanks MarkS

#22
jawm8:58 pm, 01 Mar 12

…almost tempted to pop out and have a look how Cotter Bridge is holding up (the one next to the pumping station)… but then again, that could be a really dumb idea… hopefully the bridge will still be there once all this is over…

#23
caf9:02 pm, 01 Mar 12

Camelicious said :

I guess ACTEW is pumping as hard as they can out of Corin and Bendora, since I believe the pipes out of Cotter aren’t connected.

There is no pipeline to Corin – water released from it just flows down the river to Bendora.

#24
Bramina9:06 pm, 01 Mar 12

How exciting

#25
el9:11 pm, 01 Mar 12

All I can say is “holy crap!”

And thanks to MarkS (Sullivan?) for keeping us up to date.

#26
MarkS9:11 pm, 01 Mar 12

While dam cam stays up lets stay away. Some of the time lapse stuff is incredible.A slight abatement in flow last hour brings some hope. AS for I-filed my stake in this thing is not insignificant and I remain positive. No pumping going on. Bendorra over by a metre or so so and adds to woe. To my spys on the left abutment stay warm and dry

#27
Mr Evil9:14 pm, 01 Mar 12

rohbo said :

Mr Evil said :

A couple of questions.

Why were vehicles left parked on the topside of the dam when there had been pretty ample warning that NSW/ACT was going to receive a lot of rain in the next few days, and numerous flood warnings had been publicly issued?

If you know how to get some very heavy machinery off a 40m high wall i’m sure ACTEW would love to hear from you. Mark S from ACTEW said they got off everything they could with the cranes and the rest was too heavy so had to stay.

Umm, maybe the same way they got them up there in the first place?

Anyway, maybe I should have expressed myself better – the really heavy stuff isn’t the problem, as obviously if a 40 or 60 tonne object is going to be swept off the wall by the overflow, then I would suggest that there’d be bigger issues to worry about with the dam itself at that point. It’s some of the other smaller vehicles that appeared to be still on the dam in the photo that Mark submitted earlier this morning.

The point I was making is that we have had several days warning that the rainfall was going to be high, but some people seem to have been caught slightly ill-prepared by it all.

And as to my comments about objects seen floating down the river; yes, I understand that there will always be plenty of stuff that is washed down a river in flood, as witnessed on LBG last time we had a flood like this – but isn’t it a good idea to try and ensure that if at all possible that we don’t add to the debris pile unnecessarily?

#28
Bananabanana9:18 pm, 01 Mar 12

Oooo, I wish that video had sound

#29
Aquila9:20 pm, 01 Mar 12

Hmmm… In the video at 0:38 it looks like something big and round wasn’t “tied down” that well.

#30
caf9:21 pm, 01 Mar 12

Mr Evil said :

Umm, maybe the same way they got them up there in the first place?

They got them up there by building a dam underneath them. Unbuilding the dam to get them down seems a little drastic.

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