Water begins to spill over Warragamba Dam

Warragamba Dam in western Sydney reached 100 per capacity just before 7pm. Picture: Craig Greenhill Source: The Daily Telegraph

AFTER days of anticipation Warragamba Dam has finally reached 100 per capacity and water is spilling over the wall.

Just before 7pm (AEDT), water began gushing down a 100m spillway into a pond below.

The pond acts as an energy dissipater, slowing the water before it flows down the Warragamba River to join the Hawkesbury-Nepean system.

Several kilometres away, the lookout observatory was packed with more than 100 spectators. Some had been waiting there for more than five hours.

"I came with my two boys to have a look," Ingrid Jouke from Glenmore Park said.

"It looks so high and I didn't expect that I'd see these gates open," she said.

"I've heard this spill from the dam can potentially create misery for some people, but maybe they can control how much is released."

Rain hammers NSW

The news comes after a downpour that put NSW on high alert, with three-quarters of the state at risk of flood.

Tumut in southern NSW might again be the focus of flood concerns tomorrow as the wandering band of torrential rain which has soaked much of NSW shifts south.

"It could be a hot spot tomorrow - they are expecting a lot of rain again as the rain band drifts south,'' State Emergency Services spokesman Phil Campbell told thetelegraph.com.au this afternoon.

"The caravan park there has been evacuated twice in three days and they might be doing it again tomorrow.''

Mr Campbell said the other areas being watched intently by the Bathurst and western Sydney's Hawkesbury-Nepean valleys.

"The rain band starting dumping on the central west but the town's really well protected by levees and it's a remote chance they'll get major flooding,'' he said.

"The river can get a half kay or one kay wide but it's not so much a threat to property - the bathurst showgrounds and ovals get covered but homes should be OK.''

Flood damage

"That could change tonight or tomorrow. The rain band is expected to shift to put more rain over the catchment for Warragamba so that would increase the chance of flooding in that north western Sydney fringe.''

The dense, slow-moving rain band is lingering a little longer then expected as it moves up and down NSW, said BoM forecaster Julie Evans.

source : www.news.com.au/

Prospect Reservoir releases water to supply system to ease flooding fears

BY NICK SOON

SYDNEY Catchment Authority is now pumping water from Prospect Reservoir into the general water supply system to prevent flooding in the surrounding area.

A spokeswoman said pumping began last Tuesday(February 28) and would continue for about two weeks to lower the reservoir level to one metre below full storage level.

Prospect Reservoir had about 30,870 megalitres and 93 per cent full while Warragamba Dam was 98 per cent full last Friday following continuous rainfall over the catchment areas over the last few weeks.

‘‘The water from Prospect Reservoir will be blended with supplies from Warragamba Dam and the Upper Canal, which takes water from the Upper Nepean System,’’ she said.

‘‘It’s routine practice that when Prospect Reservoir reaches a certain level, water is pumped into the water supply system to lower the reservoir.

‘‘This is to prevent the reservoir from spilling and cause localised flooding.’’

She said the last time water was pumped out of Prospect Reservoir into Prospect Water Filtration Plant to lower the reservoir level was last September.

Blacktown SES controller Gary Fry is more concerned about possible flooding in the Marsden Park, Riverstone and Schofields area by Eastern Creek and Bells Creek when Warragamba Dam release its water into Hawkesbury River.

He said his officers door-knocked about 15 houses at Lytton Road and Marsden Road in Riverstone and Clive Road in Marsden Park last Wednesday and would do the same at Garfield Road and Schofields Road if the situation got worse.

‘‘These low laying areas are close to Eastern Creeks linked to Hawkesbury River via South Creek in Windsor,’’ he said.

‘‘Flood will come if heavy rainfall continues for several days over the catchment areas.’’

He said about 20 officers were on duty on two shifts daily monitoring creeks’ water level in Blacktown.

Mr Fry did not think Prospect Reservoir would not pose a serious threat to residents, saying previous flooding was caused by blocked drainage.

Resident John Harrington who lives at Old Church Lane near Prospect Reservoir for 62 years agreed with Mr Fry that his property was unlikely to be flooded.

‘‘My neighbour’s property was hit by flood about six years ago because it is a low lying area and the drainage system were blocked by rubbish,’’ he said.

Daily dam levels - weekly changes and trend for the last month. Stats updated each week.

2012-03-01	Warragamba	Other Dams	Overall
Available GL(%)	1132.21 (55.86%)	1248.29 (224.98%)	2380.5 (92.20%)
Weekly change GL(%)	6.25 (0.31%)	125.75 (22.66%)	132.00 (5.11%)
Monthly change GL(%)	-12.20 (-0.60%)	307.70 (55.46%)	295.50 (11.45%)
Yearly change GL(%)	-80.66 (-3.98%)	589.16 (106.18%)	508.50 (19.70%)

Overall dam levels and usage for the last month.

In the month up to 2012-03-01 the dam levels have gone from 2081.5 GL (Gigalitres) to 2380.5 GL. Usage and inflow for the last month:

You can compare this situation to what it was like a year ago.

Current Trends

• At the overall rate of water gain in the past week in the dams, they will fill in 10 days, less than 1 years and will overflow on 2012-3-11 .
• At the overall rate of water gain in the past month in the dams, they will fill in 19 days, less than 1 years and will overflow on 2012-3-20 .
• At the overall rate of water gain in the past year in the dams, they will fill in 248 days, less than 1 years and will overflow on 2012-11-4 .
• To fill the dams we would need about 87 mm of rain over a fortnight.

Overall dam levels since November 2001

Sydney's overall dam level has gradually dropped since it was last full in 1998. This figure shows the data since November 2001 for overall dam levels.

You can see this in more detail in the year-by-year figure below. This figure shows the overall dam levels for the last few years. Each line represents how full the dams are for that year. From this figure we can see that the large drops in dam levels (since November 2001) came in the second half of 2002 and first half or 2004. The rest of the time the levels appear fairly steady. As no two lines meet each other (other than a brief period in early 2006), we can say that in every 365 day period since 2001, the dam levels have gone down.

To what extent are the overall dropping dam levels due to dropping dam levels for Warragamba Dam, as opposed to the other dams?

This graph shows the water available in each dam. You can see that Warragamba dominates Sydney's water supply, and the drop in water levels.

It is interesting also to note that since 2001 the amount of water in total in the other dams has remained fairly constant, whilst Warragamba has dropped. This can be seen more clearly in the following figure.

There are various reasons for this. For example, Sydney water is choosing to take water from Warragamba Dam in preference to the other dams. Also, in general El-Nino droughts do not hit so hard in areas nearer the coast.

What about inflow versus outflow?

The reason a dam's level drops is that more water is used (outflow) than flows in (inflow). The following figure shows cumulative outflow versus cumulative inflow for the entire system, measured daily since November 2001. In that time the overall usage is 5280.77 GL, whilst the cumulative inflow is 5425.27 GL (note that the overall capacity of the system is a little under 2400 GL).

Note that when measuring inflow, this is net inflow (after evaporation is taken into account). This is why net natural inflow can sometimes be negative. You can see that although inflow has not equaled outflow, it is still significant. This is why rainwater tanks in Sydney are still valuable in drought conditions.

Day by day, however, the inflow to the dams is not very consistent. The following figure shows the daily usage and daily inflow since November 2001. From the usage we can see that the summers of 2001/2 and 2002/3 had high usage, and otherwise there is a slight downward trend on water usage, presumably due to water restrictions, water saving devices and general water consciousness on the part of consumers.

A version of this figure with a one-year rolling average is presented here.

Once again these aspects are shown more clearly when we look at the data year by year. The general trend of each year to consume less water than the previous year is quite obvious.

We can also look at the net gains (or losses) of the dams (i.e. inflow minus usage) from above. The following figure shows this result as well as showing a breakup between Warragamba and Sydney's other dams.

How do rain and temperature affect water usage?

We have already seen the effects of the 2001/2 and 2002/3 summers on water use. What about the effects of rain and temperature on daily consumption? The following graphical representation shows water usage versus Sydney rainfall on the previous day. On low rainfall times the usage is dependent on various factors, but when rainfall is high, the water use is quite consistently low. The really wet day (130+mm) that was of average usage was in 2002 just prior when strict water restrictions came into operation.

Temperature also has an effect on water usage. This graphical representation shows that low temperatures mean low water usage. High temperatures do not guarantee high water usage, as rain can also occur as a thunderstorm on the afternoon of a hot day.

What effect would a change in our water usage have on dam levels?

The following table and graph examine what the current dam levels would be under various alternative water use scenarios. For example what would have happened if we had been using more water since November 2001, for example with no water restrictions or a higher population? Or what would have happened if we had been using less water in that time? For example, if everyone had rainwater tanks for their irrigation and hand watering of their gardens, usage would probably drop around 20%.

Hypothetical usage since 11.2001	Available water	% full all dams	1 year change GL(%)
20% More	1324.3	51.29	427.86 (16.57)
As is	2380.5	92.20	508.50 (19.70)
20% Less	2581.8	100.00	20.64 (0.80)
50% Less	2581.8	100.00	7.52 (0.29)

These calculations are just on figures since November 2001. If we had been using less water since the dams were last full in 1998 then dam levels would be even higher!

What effect would a desalination plant have on our dam levels?

The government has recently announced the possibility of a desalination plant that could produce 500ML per day (or, alternatively, a smaller one producing 100ML/day). If this had been in place and operating since November 2001, a significant difference in our dam levels would be noticeable, as the following data indicate.

Desalination plants do use a lot of electricity. One way to reduce the electricity load is to only turn on the desalination plant whenever the water level drops below 50%. We have also included this possibility in our figures and table below.

Hypothetical since 11.2001	Available water (GL)	% full all dams	1year change GL(%)
As is	2380.5	92.20	508.50 (19.70)
100 ML/day	2581.85	100.00	371.35 (14.38)
500 ML/day	2581.85	100.00	12.50 (0.48)
500 ML/day when levels < 50%	2706	104.81	508.50 (19.70)

The 500ML/day option appears to be able to hold the dam levels close to steady under the current drought conditions, hence may only need to be operated when dam levels are low. A 100ML/day plant would need to operate continuously to have much effect. Hence, should rainfall conditions return to the 1960-2000 average and usage not increase much, the 500 ML/day plant may use significantly less electricity overall as it could be turned off most of the time! In other words, as the dams (historically) often vary between 80% and 100% full, a 100ML/day plant may waste energy delivering water when the dams may soon fill, while not providing sufficient water should the dams fall below 50%. Of course, if the current extended dry spell is more permanent, then the 500ML/day plant would be required (given current usage) to maintain Sydney's water supply.

What effect would the Shoalhaven Transfer scheme have on our dam levels?

A scheme has been announced in which flows from the Shoalhaven Scheme to Sydney would be increased. The basic idea is that the Shoalhaven catchment is rather large and close to the coast (hence inflows are large) and Tallowa Dam often overflows, even under drought conditions (link provides graphs and stats on Tallowa Dam). If this had been in place and operating since November 2001, a significant difference in our dam levels would be noticeable, as the following data indicate. We also examine the combination of such a scheme and a 500ML/day desalination plant. Note that we have estimated the effect of the stage 1 and 2 Shoalhaven scheme by 80,110 GL/year respectively. The actual net gain of water in Sydney's dams is much harder to calculate as it concerns the amount of water which currently overflows from Tallowa that would otherwise have been captured. As noted above, in the past few years of drought conditions, Tallowa has still often overflowed and hence this scheme would have helped in this drought.

Hypothetical since 11.2001	Available water (GL)	% full all dams	1year change GL(%)
As is	2380.5	92.20	508.50 (19.70)
Stage 1	3201.53	124.00	588.21 (22.78)
Stage 2	3508.95	135.91	618.07 (23.94)
Stage 2 + desal	2581.85	100.00	3.47 (0.13)

If Stage 2 of the Shoalhaven transfer scheme were in place in late 2001, as well as a 500 ML/day desalination plant (both in full use) then the dams would be sufficiently full that there would be not even a thought of a dam level problem. Even without a desalination plant, this proposed scheme would have had a big impact on our dam levels.