Observations Dam Levels & Water Usage; A Discussion

Flowin

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I want to see the Kurnell desal run off electricity from the 50MW hydro plant in Warragamba dam. Not because it makes any sense but it would be an interesting paradox. Releasing water from the dam into the ocean to generate water taken from the ocean which is then pumped back into the same network. Its like a small scale, man made hydrological cycle.
ha ha had to laugh at that one. the dam hydro water release could also dilute the brine from the desal.
that would be a very expensive man-made "illogical" cycle LOL
 

davidg

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ha ha had to laugh at that one. the dam hydro water release could also dilute the brine from the desal.
that would be a very expensive man-made "illogical" cycle LOL
History is literred with examples of humans doing things just because they can. I feel as though I have made quite a strong business case for my plan. Now I just need to sit back watch the profits roll in...

Sydney water really need to take a more "doin it for the memes approach" IMO.
 

Flowin

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What if the $1.6b cost to raise the Warragamba dam wall was spent on buying up and removing flood prone houses foolishly built on the floodplain? For that price you could remove and demolish around 1,600 house at say $1m each. Would that help?
Apparently the population is already 70,000 @ say 3 per house = around 25,000 houses so won't even get 10% of the way there - but would flood prone house be worth $1M each ?
 

davidg

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Apparently the population is already 70,000 @ say 3 per house = around 25,000 houses so won't even get 10% of the way there - but would flood prone house be worth $1M each ?
Maybe not but they're probably worth a bit more then $100k
 
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Rainbow Spirit

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I just did a search on the average house price in Windsor and it is $740,000, one might assume flood prone houses would cost less than that, say $600,000. That equates to 2,666 houses.
 
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warrie

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I want to see the Kurnell desal run off electricity from the 50MW hydro plant in Warragamba dam. Not because it makes any sense but it would be an interesting paradox. Releasing water from the dam into the ocean to generate water taken from the ocean which is then pumped back into the same network. Its like a small scale, man made hydrological cycle.
Sorry but the hydro plant shut years ago.If it were still there it would use 64 cumecs and could have made green power to keep the level at 1 m below FSL or 95%.
 
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Rainbow Spirit

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So we hear a lot about the bath tub effect on flooding along the Hawkesbury, a crazy idea, what if some tunnels were bored through the squeeze points. They could use some of the road tunnel machines that they use to bore all those toll roads in Sydney...
 

Majikthise

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So we hear a lot about the bath tub effect on flooding along the Hawkesbury, a crazy idea, what if some tunnels were bored through the squeeze points. They could use some of the road tunnel machines that they use to bore all those toll roads in Sydney...
Would have to be mighty big tunnels, sackville is tidal , so no gravity to assist. Contrast to mass water movement that the snowy scheme achieves.
 
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Greysrigging

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Darwin River Dam is at 90% capacity as of 25th March, so pretty much at the end of the Wet Season. Generally, this season's 'Wet' is regarded as a good one, with upwards of 2000mm in the rural districts and catchements since Oct 1st 2020. The fact that the dam is at only 90% is indicative of the two previous below average wet seasons, therefore this years 'filling' of the impoundment has started from a low base.
https://www.powerwater.com.au/about/what-we-do/water-supply/darwin-water-supply/darwin-river-dam
 

Flowin

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So we hear a lot about the bath tub effect on flooding along the Hawkesbury, a crazy idea, what if some tunnels were bored through the squeeze points. They could use some of the road tunnel machines that they use to bore all those toll roads in Sydney...
Would have to be mighty big tunnels, sackville is tidal , so no gravity to assist. Contrast to mass water movement that the snowy scheme achieves.
There is not enough longitudinal gradient to drive massive flow through tunnels - even if they were very large tunnels.
Diagrams below the simulated 1990 flood profile from Hawkesbury Nepean Flood Study Volume 2
upload_2021-3-25_11-29-41.png
 

sbm_

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Would have to be mighty big tunnels, sackville is tidal , so no gravity to assist. Contrast to mass water movement that the snowy scheme achieves.

Mmm, my first thought was yep that is definitely crazy, but I was wrong about the Niagara Falls thing, so....

The flow rates of the Snowy Hydro tunnels would be the real-world example to use, but I couldn't find anything.

You could do a tunnel from say 8m above sea level, like a flood level, through to say Marramarra Creek on the Berowra side? But my back of the envelope playing with pipe diameters and flow rates suggest you could achieve at most tens of megaliters a day...not a scratch on hundreds of gigaliters.
 

Majikthise

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I suppose you could make it functional art. ..sit a wide mouth structure at you 1:10 flood level at yarramundi , massive wide diameter pipe deep under sydney , large decorative fountain head off Bondi with spouts that can handle tree trunks...
The fluid dynamics would be more than interesting....
 

glenesk

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With SEQ water storage levels at 59% and restrictions starting at 50% when is forecast for water restrictions to come in. If we get a average rainfall over winter?
Or will it see us through until next summer with no restrictions?
Also with 11 dams spilling over, is the over capacity go on to the storage % or do they go in at 100%
 

Rainbow Spirit

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OK, so that squeeze point tunnel idea might be too crazy, so what about an even crazier idea, a direct tunnel from Warragamba dam to the coast, bypassing the rivers and discharging stored water straight into the ocean? Thus a extra 'overflow' system that doesn't add water to any rivers or flood plains? Soon as the water starts to reach the top, let it rip, and as an added bonus a man made water attraction as it sprays/cascades into the ocean...
 

Snow Blowey

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OK, so that squeeze point tunnel idea might be too crazy, so what about an even crazier idea, a direct tunnel from Warragamba dam to the coast, bypassing the rivers and discharging stored water straight into the ocean? Thus a extra 'overflow' system that doesn't add water to any rivers or flood plains? Soon as the water starts to reach the top, let it rip, and as an added bonus a man made water attraction as it sprays/cascades into the ocean...

Consider the existing river valley as a tunnel with open top. Has the same fall as the tunnel you are proposing. Flow in the valley is proportional to the cross sectional area.

To make any significant dent in flow you'd have to increase the cross sectional area significantly. A tunnel the size of a rail tunnel say is not an overall significant increase compared to the big open tunnel that already exists (the river valley).

You could improve the efficiancy by pumping the water through the tunnel. But if you were going to do any of this, you be better off tunnelling to the west through to the abercrombie and get the water west of the ranges where it could be used.

The trouble with that idea is that excess water is only available in the Hawkesbury catchment infrequently - so the economic just don't stack up.
 

Rainbow Spirit

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You would have the extra height of the dam wall for the flow to work, mind you pumping it west would be a better irrigation option. How about another dam somewhere up stream of the stored water?
 

climberman

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OK, so that squeeze point tunnel idea might be too crazy, so what about an even crazier idea, a direct tunnel from Warragamba dam to the coast, bypassing the rivers and discharging stored water straight into the ocean? Thus a extra 'overflow' system that doesn't add water to any rivers or flood plains? Soon as the water starts to reach the top, let it rip, and as an added bonus a man made water attraction as it sprays/cascades into the ocean...
If it’s discharging a Sydney Harbour a day, that’d be some tunnel.

Which beachside electorate would you like it go to?
 

Fozzie Bear

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If it’s discharging a Sydney Harbour a day, that’d be some tunnel.

Which beachside electorate would you like it go to?

Clearly Bondi. This is brilliant.

I suppose you could make it functional art. ..sit a wide mouth structure at you 1:10 flood level at yarramundi , massive wide diameter pipe deep under sydney , large decorative fountain head off Bondi with spouts that can handle tree trunks...
The fluid dynamics would be more than interesting....
 
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Flowin

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With SEQ water storage levels at 59% and restrictions starting at 50% when is forecast for water restrictions to come in. If we get a average rainfall over winter?
Or will it see us through until next summer with no restrictions?
Also with 11 dams spilling over, is the over capacity go on to the storage % or do they go in at 100%
Average winter rain won’t help the SEQ storage levels much -:- so restrictions around next summer likely.
An odd high rain event in winter enough to produce flood flows could change that.
We have had floods in SEQ in winter before like 1983 and 1893 - something ironic about those year numbers.
 
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glenesk

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Average winter rain won’t help the SEQ storage levels much -:- so restrictions around next summer likely.
An odd high rain event in winter enough to produce flood flows could change that.
We have had floods in SEQ in winter before like 1983 and 1893 - something ironic about those year numbers.
Ironic most SEQ is flooded and soon water restrictions.
 
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mcspero

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Not sure if this is the best forum for it as opposed to the current weather event. It looks like we may just get some reasonable inflows from the somerset catchment. The gauging stations for the stanley river are showing good river rises and instantaneous current flows around 2000 ML/day. If perhaps that area of the catchment which has so far had 80mm over the past 4 days was to get a further 50 mm tonight or more. I would estimate that somerset will hit the 80% mark and subsequent inflows would be spilt into wivenhoe. We may get another couple of percent boost to our seqld water grid storages.

If we are really lucky the wivenhoe catchment could get a widespread 30-40mm additional rain tonight and tomorrow and that would likely to cause some brisbane river rises. Esk Creek, Reedy Creek and the Brisbane river near linville are showing signs of responding to rainfall with the current event so you never know your luck.
 
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glenesk

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Not sure if this is the best forum for it as opposed to the current weather event. It looks like we may just get some reasonable inflows from the somerset catchment. The gauging stations for the stanley river are showing good river rises and instantaneous current flows around 2000 ML/day. If perhaps that area of the catchment which has so far had 80mm over the past 4 days was to get a further 50 mm tonight or more. I would estimate that somerset will hit the 80% mark and subsequent inflows would be spilt into wivenhoe. We may get another couple of percent boost to our seqld water grid storages.

If we are really lucky the wivenhoe catchment could get a widespread 30-40mm additional rain tonight and tomorrow and that would likely to cause some brisbane river rises. Esk Creek, Reedy Creek and the Brisbane river near linville are showing signs of responding to rainfall with the current event so you never know your luck.
I could be wrong, but i think they are releasing out of Somerset into wivenhoe for the last couple weeks.
just checked and water storage has gone up .9% in last 7 days.
 
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mcspero

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I could be wrong, but i think they are releasing out of Somerset into wivenhoe for the last couple weeks.
just checked and water storage has gone up .9% in last 7 days.

Yeah they are. From what I understand, there is a reduced temporary full supply level of 80% which is an AHD gauge height of 97m until the Somerset Dam upgrade project is complete.

With the current flows through the Stanley river and Kilcoy creek below the weir, Somerset dam would be about 80% by morning and I would assume they would be wanting to keep Somerset Dam below 80%. They will probably release more tomorrow as inflows continue into Somerset Dam to keep it at 80% or just below.

Based on the information from the water monitoring portal,
I'm expecting Wivenhoe to be around 38.5% by tomorrow morning
and Somerset Dam to be 80%.

If we are able to score another widespread 30mm across the catchments then I think we could really see an inflow boost, even for wivenhoe dam. The catchments are mostly saturated with the exception of a small region to the west north west near the catchment area of Emu Creek. That area of the catchment hasn't had much rain over the past week so it will probably take a further 25-35mm in that region just to saturate. It is also appears to be the only gauging station in the brisbane river catchment that has shown no rises since the rain event began and is still below the cease to flow height..

Some useful links:

https://www.seqwater.com.au/project/somerset-dam-upgrade - Somerset Dam Project
https://water-monitoring.information.qld.gov.au/ - Gauging Stations with Flows
http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDQ60286.html#Stanley/Upper_Brisbane - All Gauging Stations BOM Summary updated hourly.
http://www.bom.gov.au/water/landsca...52.40/8/Point/Separate/-27.25/152.4/2021/4/5/ - BOM Soil Moisture Information based on AWRA Model.
 

Locke

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We're going to run very short of water very quickly in Brisbane if there is not a significant inflow into Wivenhoe soon.
Current storage at Wivenhoe is at 38.8% and current usage is 14-18% per year.
It was absolute insanity to set the FSL at 75% in previous years just to reduce the flood risk for few riverfront homeowners. Better for a few homes to be flooded than have an entire city of more than 2 million people risk running out of water.
 

LDRcycles

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9 of the 12 dams connected to the SEQ water grid are overflowing, and of the remaining 3 only Wivenhoe is below 50% (albeit being the largest by a considerable margin). I agree with you on the reduction of the FSL though, the operation of Wivenhoe and Somerset contributed to the 2011 flood being a metre lower than 1974 despite the catchments above Wivenhoe receiving nearly 3/4 of a million megalitres more. I doubt reducing the FSL will achieve much, but I dare say it's more about being seen to do something.
 

Locke

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While dams in some areas are spilling, Wivenhoe is by far the largest, and any shortfall when it drops to a critically low level would not be sufficiently covered by the other dams connected to the grid. The reduction from 100 to 75% level probably represents the loss of a 1-2 year buffer in times of consecutive years without inflows.
The SEQ grid has not really been put to the test in terms of the water usage generated by the current SEQ population. The closest we got was in the earliest part of the century but there were periods of up to 13 years without an inflow event in the 20th century and a recurrence of such a period today would certainly lead to SEQ effectively running out of water.
 
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Vinny

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From Rocky council Facebook page:


Council today announced that we have begun trucking water to Mount Morgan.
Key points from the announcement are:
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The Dam is currently sitting at 8.6%.
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The water is safe to drink but there are issues with the taste.
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In response Council has begun trucking water to Mount Morgan.
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There's currently about 6 trucks of water per day being added to the water supply, and we're ramping this up to 20 trucks over the next week.
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This will provide around 160 litres of water per person per day.
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This is a temporary measure to ensure residents have access to good quality drinking water.
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We're absolutely committed to finding a more long term solution.
Council will be meeting with residents in Mount Morgan next week to explore options with the community.
Take a look at the video below or head over to our website for more

https://www.rockhamptonregion.qld.g...4aG6UPcoA2cFVngFxOO1miCqM3Dpz_qgIZQJ6neQAs3Es

“The State Government had been covering the cost of trucking water to Stanthorpe until their recent rainfall, and we are in discussions with them to do the same for the people of Mount Morgan.
 
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Flowin

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While dams in some areas are spilling, Wivenhoe is by far the largest, and any shortfall when it drops to a critically low level would not be sufficiently covered by the other dams connected to the grid. The reduction from 100 to 75% level probably represents the loss of a 1-2 year buffer in times of consecutive years without inflows.
The SEQ grid has not really been put to the test in terms of the water usage generated by the current SEQ population. The closest we got was in the earliest part of the century but there were periods of up to 13 years without an inflow event in the 20th century and a recurrence of such a period today would certainly lead to SEQ effectively running out of water.
Wivenhoe max water storage capacity is reduced to 90% not 75%.
last time it was reduced to 75% was 2011.
 
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Mezo

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With SEQ water storage levels at 59% and restrictions starting at 50% when is forecast for water restrictions to come in. If we get a average rainfall over winter?
Or will it see us through until next summer with no restrictions?
Also with 11 dams spilling over, is the over capacity go on to the storage % or do they go in at 100%

We've hovered between 50% and 60% for the last few years, but never gotten close enough to that 50% mark that I keep hear about to trigger water restrictions. Why would next season be any different? It couldn't really get much dryer than the last couple of springs. Or is it once Wivenhoe gets below a certain point we're screwed anyway?
 
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Flowin

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We've hovered between 50% and 60% for the last few years, but never gotten close enough to that 50% mark that I keep hear about to trigger water restrictions. Why would next season be any different? It couldn't really get much dryer than the last couple of springs. Or is it once Wivenhoe gets below a certain point we're screwed anyway?
Mezo, a key influence is the level in Wivenhoe at the end of the wet season. This year is the lowest Wivenhoe has been at the end of the wet season for a over a decade.
Whether or not the 2020-21 wet season has ended is another question, but even if more rain occurs it needs a big event with substantial 200 mm or more (300 mm realistically) rain over the Wivenhoe catchment to produce a substantial replenishment of the storage level.
 
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mcspero

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I think at the moment the wivenhoe somerset catchment has an average antecedent precipitation index of about 25mm based on the bom records over the past month. About 60mm is about what is required to saturate the catchment when dry leaving about 35mm of rain to saturate the catchment at this instant.

If you take the combined catchment area of somerset and wivenhoe together of approximately 7000sq km (since somerset is for the purpose of this discussion is full at 80%) we can estimate how much rain one would need to fill wivenhoe to 90%.

Lets assume a 4mm/hour rainfall event (e.g. 100mm / 24 hours)
A continual loss of 2mm / hour (since even a saturated catchment still has continuous infiltration)
A catchment area of 7000 sq km (the combined catchment size of wivenhoe and somerset)
580,000 ML of inflow required (to get wivenhoe to 90%)

We could estimate the following:

Day 1 (35mm required to saturate the soil) 65mm available for inflows but due to the continual loss / half of the 65mm of rain is not available for runoff.

That would mean 32.5mm of rain over 7000 sq km area = 32.5*7000 = 227,500 ML

Day 2 (Catchment is saturated) 100mm would cause 50mm of rain to be available for inflows due to the continual loss.

That would mean on day 2 ... 50mm of rain over 7000 sq km area = 50*7000 = 350,000 ML.

Over a combined 2 days that would suggest about 200mm of uniform widespread rain (at 4mm/hr) is needed to produce the required inflows to get to the 90% wivenhoe level.

Its worth also considering with this blanket approach how uneven the catchment is likely to respond. Parts of the wivenhoe catchment that are west and north west of the dam have generally had very little rain over the past month e.g. Cooyar Creek, Emu Creek, Boat Mountain etc and are still quite dry compared to the stanley river catchment and the eastern parts of the wivenhoe catchment such as Mount Byron, Mount Glorious etc.
 

Flowin

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I think at the moment the wivenhoe somerset catchment has an average antecedent precipitation index of about 25mm based on the bom records over the past month. About 60mm is about what is required to saturate the catchment when dry leaving about 35mm of rain to saturate the catchment at this instant.

If you take the combined catchment area of somerset and wivenhoe together of approximately 7000sq km (since somerset is for the purpose of this discussion is full at 80%) we can estimate how much rain one would need to fill wivenhoe to 90%.

Lets assume a 4mm/hour rainfall event (e.g. 100mm / 24 hours)
A continual loss of 2mm / hour (since even a saturated catchment still has continuous infiltration)
A catchment area of 7000 sq km (the combined catchment size of wivenhoe and somerset)
580,000 ML of inflow required (to get wivenhoe to 90%)

We could estimate the following:

Day 1 (35mm required to saturate the soil) 65mm available for inflows but due to the continual loss / half of the 65mm of rain is not available for runoff.

That would mean 32.5mm of rain over 7000 sq km area = 32.5*7000 = 227,500 ML

Day 2 (Catchment is saturated) 100mm would cause 50mm of rain to be available for inflows due to the continual loss.

That would mean on day 2 ... 50mm of rain over 7000 sq km area = 50*7000 = 350,000 ML.

Over a combined 2 days that would suggest about 200mm of uniform widespread rain (at 4mm/hr) is needed to produce the required inflows to get to the 90% wivenhoe level.

Its worth also considering with this blanket approach how uneven the catchment is likely to respond. Parts of the wivenhoe catchment that are west and north west of the dam have generally had very little rain over the past month e.g. Cooyar Creek, Emu Creek, Boat Mountain etc and are still quite dry compared to the stanley river catchment and the eastern parts of the wivenhoe catchment such as Mount Byron, Mount Glorious etc.
That is a fair and reasonable analysis. But if a rain event occurs that can produce rainfall rates greater than 20 mm/hr over a significant part of the catchment then the loss rates could be less than 4 mm/hr.
Other perspectives from the images below.
1) the latest BoM seasonal streamflow forecast for Gregors Creek gauge (about half Wivenhoe catchment and excludes Somerset catchment) for April to June does look rather dismal.
2) an important perspective for that seasonal flow forecast is past skill for mid autumn to early winter period is rather ordinary
3) the current seasonal 3-month outlook for chances of exceeding 200mm over Wivenhoe’s catchment are quite low.
Overall there are not many promising signals.
But it only takes one rogue event such as an ECL to change that, so I guess we will just have to wait and see.
Need to get past the climate ENSO autumn predictability barrier too to get a clearer picture of the second half of 2021.

62FF5EE9-740D-43AF-83D4-D2B656C973F9.png
EACF6FA9-099D-42DB-8CDE-62F8465F28A6.png
76AA9C1B-072E-459E-A15C-82ACBD8E2F39.jpeg
 
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mcspero

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That is a fair and reasonable analysis. But if a rain event occurs that can produce rainfall rates greater than 20 mm/hr over a significant part of the catchment then the loss rates could be less than 4 mm/hr.
Other perspectives from the images below.
1) the latest BoM seasonal streamflow forecast for Gregors Creek gauge (about half Wivenhoe catchment and excludes Somerset catchment) for April to June does look rather dismal.
2) an important perspective for that seasonal flow forecast is past skill for mid autumn to early winter period is rather ordinary
3) the current seasonal 3-month outlook for chances of exceeding 200mm over Wivenhoe’s catchment are quite low.
Overall there are not many promising signals.
But it only takes one rogue event such as an ECL to change that, so I guess we will just have to wait and see.
Need to get past the climate ENSO autumn predictability barrier too to get a clearer picture of the second half of 2021.

62FF5EE9-740D-43AF-83D4-D2B656C973F9.png
EACF6FA9-099D-42DB-8CDE-62F8465F28A6.png
76AA9C1B-072E-459E-A15C-82ACBD8E2F39.jpeg

Hi Flowin,

I was assuming the continuous loss was 2mm/hr because I read something in one of the bom flood modelling papers. With the streamflow forecasts, I have never really found them useful since I feel like they can never represent a rain event and streamflows are really dependent on rain events. For example 100 mm in one or 2 days will produce far more inflows when compared to 100 mm spread over a month with 5 or so smaller 20 mm events.

The streamflow models I feel tend to look at general monthly averages and never capture the true likelihood of the inflows of an event.

I did some interesting analysis based on inflows derived from the http://www.bom.gov.au/waterdata/ and looking at the time series explorer for wivenhoe. There is interesting data for when inflows occur throughout the year looking back since wivenhoe was commissioned in the 80's. I filtered the available data by max recorded ML in storage value per day and captured / filtered to only give me the records when the next day had more than 10ML increase from the previous day (e.g. Net Inflow)

Below is a pivot table by Month of all the wivenhoe inflow data since 1986 through till April 2021. Interestingly the month of April is our third best month for inflows historically and having looked at the data, May and June can occasionally be big months with ECLs and on shore convergence type events.

Also keep in mind the data below is based on daily change as recorded inflows so an event like 2011 floods doesn't show up quite as significant in this data set because the dams were releasing during the event and the dataset looks at the net daily change in ML compared to the previous day. I.e. if there was 500000 ML inflow in a day but 400000 ML was released, my dataset would show a net daily change of 100000 ML and that would be the number used in the dataset below.

Month----------ML Inflow---------Percentage contribution
Jan-----------2325808.157---------14.2%
Feb-----------2656446.747---------16.2%
Mar-----------1799593.097---------11.0%
Apr-----------2184392.885----------13.4%
May----------1217079.184-----------7.4%
Jun-------------922030.250-----------5.6%
Jul -------------787614.338-----------4.8%
Aug-------------983752.379-----------6.0%
Sep-------------446138.950-----------2.7%
Oct--------------911561.664-----------5.6%
Nov-------------947120.141-----------5.8%
Dec------------1173496.256-----------7.2%
 

mcspero

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Looking at the data further. If we look at when the big inflows have occurred e.g. more than 100,000 ML positive net change per month (sum of daily net inflows per month) since wivenhoe was commissioned we have the following data:

1985 - 0 months
1986 - 0 months
1987 - 0 months
1988 - 5 months (April, June, July, August December)
1989 - 2 months (March, April)
1990 - 1 month (October)
1991 - 1 month (December)
1992 - 2 months (February, March)
1993 - 0 months
1994 - 0 months
1995 - 0 months
1996 - 2 months (January, May)
1997 - 0 months
1998 - 0 months
1999 - 3 months (February, August, November)
2000 - 0 months
2001 - 1 month (February)
2002 - 0 months
2003 - 0 months
2004 - 0 months
2005 - 0 months
2006 - 0 months
2007 - 0 months
2008 - 0 months
2009 - 2 months (April, May)
2010 - 2 months (October, December)
2011 - 1 month (January)
2012 - 1 month (June)
2013 - 2 month (January, February)
2014 - 0 months
2015 - 1 month (February)
2016 - 0 months
2017 - 0 months
2018 - 0 months
2019 - 0 months
2020 - 1 month (February)
 

Flowin

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Looking at the data further. If we look at when the big inflows have occurred e.g. more than 100,000 ML positive net change per month (sum of daily net inflows per month) since wivenhoe was commissioned
Interesting analysis of significant inflow events McSpero.
Four additional significant inflow events need to be added to that list when a monthly positive storage change (>100,000 ML) would not have been evident because the dam was at levels close to, or at, Full Supply Level and some or all of the inflows were released - which occurred in Jan-Feb-March 2012, and May 2015.
I've taken that data from your post, added those four additional months, and overlaid the Nino3.4 SST anomaly data (sourced from NOAA) to produce the plot below.
Vertical blue lines showing the months with significant inflow events.
Demonstrates notable closely clustered inflow events aligning to La Nina (1988-89, 1999, 2010-2011, 2012) but also some La Nina episodes such as 2007-08 and 2020-21 not producing significant inflows.
Also that some significant inflow events have occurred in neutral or El Nino ENSO status.

upload_2021-4-13_6-47-15.png
 

Locke

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There was a paper I came across some time ago that was discussing a proposal to raise the wall at Borumba Dam.
In that paper, there was an observation that in the 20th century there were 3 periods in excess of 10 years where there was insufficient rainfall to provide a significant inflow event for Wivenhoe with the longest period being 13 years.
I can no longer locate the paper but I'd sure love to know how he arrived at his conclusions regarding the frequency of inflow events.
 
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mcspero

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Interesting analysis of significant inflow events McSpero.
Four additional significant inflow events need to be added to that list when a monthly positive storage change (>100,000 ML) would not have been evident because the dam was at levels close to, or at, Full Supply Level and some or all of the inflows were released - which occurred in Jan-Feb-March 2012, and May 2015.
I've taken that data from your post, added those four additional months, and overlaid the Nino3.4 SST anomaly data (sourced from NOAA) to produce the plot below.
Vertical blue lines showing the months with significant inflow events.
Demonstrates notable closely clustered inflow events aligning to La Nina (1988-89, 1999, 2010-2011, 2012) but also some La Nina episodes such as 2007-08 and 2020-21 not producing significant inflows.
Also that some significant inflow events have occurred in neutral or El Nino ENSO status.

upload_2021-4-13_6-47-15.png

Hi Flowin,

Great analysis and thanks for finding those missing months. If we look at the same data (including the missing data) as a pivot table to show the number of instances per month we get the following table

Month, Number of Occurrences of >100,000 ML Net Inflow / Month
January, 4
February, 7
March, 3
April, 3
May, 3
June, 2
July, 1
August, 2
September, 0
October, 2
November, 1
December, 3

It does show that even if we define the wet season ending at the end of April, there is still potential all year based on past history to get good inflows into wivenhoe.
 

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LDRcycles

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There was a paper I came across some time ago that was discussing a proposal to raise the wall at Borumba Dam.
In that paper, there was an observation that in the 20th century there were 3 periods in excess of 10 years where there was insufficient rainfall to provide a significant inflow event for Wivenhoe with the longest period being 13 years.
I can no longer locate the paper but I'd sure love to know how he arrived at his conclusions regarding the frequency of inflow events.

That was Trevor Herse, Ron McMah, and John Hodgkinson, they had a website with a large amount of interesting data and proposals but it hasn't been online for a while now.

Their proposal was a massively enlarged Borumba Dam (from the current 46,000ML to 2 million ML) with water being pumped to or from Wivenhoe depending on circumstances), and increasing the yield of Wivenhoe by reducing the environmental flow released. The idea there is the releases are based on an average flow skewed by including large floods, if those events were removed from the average then more water could be kept in the dam.

Raising Borumba to 200m AHD would result in a 1 million ML capacity, but the water would be backed up almost to the entrance to Baiyambora Gorge. The 300m height required for 2 million ML would have water right through the gorge to the base of Yabba Falls which is unthinkable. I'm in favour of some increase to Borumba, but not at the expense of flooding the gorge, I'm sure anyone who has been there would agree.

They had some interesting ideas with good thinking behind them, but there was more than a touch of Bradfield about it, with some overly optimistic conclusions.
 
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Locke

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I have to admit I didn't pay to much attention to the actual proposal. I was more focused on the risk they identified of very long periods without significant inflows in the Wivenhoe and Somerset catchments.

The only time this was significantly tested was in the first decade of this century and in that case, it was only 8 years without a major inflow. I'm not sure how we would cope now if we had a period in excess of 10 years.
 
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