Hinze Dam Stage 3

Just the other day I was listening to ABC radio and heard some GCCC representative (didn't catch the person's name) making a point about how the Stage 3 of the Hinze Dam had now doubled the water supply for the Gold Coast.

Well you know, its sort of got some basis in fact but at the end of the day its what the most optimistic would call "spin doctoring". Another way of looking at it is that its just not true.

In fact the Stage 3 of Hinze dam has only added something like 12% to the water supply of the Gold Coast.

So lets have a quick look at the facts. From the SEQWater site dedicated to the project one can find out fairly quickly some basic facts:

• Completed in 1976, Stage 1 of the Hinze Dam provided a storage capacity of 42,400 million litres,
• Stage 2 in 1989 increased this to 161,070 million liters.
• The Stage 3 project will raise the Hinze Dam embankment wall from 93.5 meters to 108.5 metres, providing a total storage capacity of 309,700 million liters.

So from basic maths one would simplistically think that 309.7 divided by 161 is pretty nearly double (without splitting too many GCCC hairs) The devil of course is in the details.

background facts:

Stepping back for a moment consider that a dam is just a blockage in a river to prevent the water that flows down the river from escaping. The intention is to trap that water for ourselves, to give us a water supply. Clearly then its the stream flow of the river which is most significant here rather than just how high the wall is on the dam. It may seem obvious but building a dam on a river which has no flow will not magically produce a wonderful water supply.

Because reality is not simple. there are other ways water gets out of the dam than by the pipes we put in to slurp out the water trapped by the dam. These include:

• evaporation losses from the surface
• seepage of water out from the soil into the area
• legislated environmental flows to keep some of that river flowing (noone wants to see the Nerang river turn into a dry river bed for example).

From my research (in my Masters of Environmental Science) I found that a "rule of thumb", in Northern Australia is that: the maximum utilization of average flows that may reasonably expect from a dam is around 50% to 75%. So having a bigger container to hold the water is not the significant issue, its how much water flows that counts.


So its not only about how big the dam is, but how much water it can yield. From the points above its clear that how much we can get out depends on how much goes and minus how much we loose while its sitting around. Seems obvious when you think of it that way.

With respect to evaporation one of the major factors is surface area. So if you fill the dam more, then as the water goes up its sides the area of the surface of the dam increases; which means evaporation increases.

The diagram here shows the sort of change in the width of the water surface as water fills a channel which gets wider as it goes up (like rivers tend to do).

As it happens back some years ago some engineers were engaged to determine what sort of yield we can expect.

During the drought crisis of 2003 it was demonstrated quite reasonably that we can rely on the Hinze dam (then at Stage 2) to cope with a yield of about 161 Million liters of water per day (but it was going down quite fast without rain to keep topping it up).

During that drought we started with the Hinze Dam (then at Stage 2) at 100% capacity, but the water usage of the Gold Coast brought the dam levels down to a smidge below 30% with just one missed wet season. Red warning lights were going off on the GCCC dashboard and emergency meetings were called.

Now if we had had Stage 3 in place back at that point the only thing that would have changed is possibly just how much water was in the dam, the rate at which the dam emptied would not have changed.

Its possible that the rains of January 2001 would have topped up the storage to greater than what the Stage 2 held, but looking at that graph, not much. Either way, you can see that as soon as those wet season rains stopped the levels of the dam started to drop, and fast too.

So since the amount of water available is a factor of the river flow, not the height one would not expect that the yield of Hinze Dam could provide much more water with simply increasing storage capacity.

In fact the Engineering analysis of the Hinze dam showed that the yield of the dam in Stage 3 rose from (an estimated) 191Million liters / day to 215 Million liters / day or an increase of 24Ml per day or an increase by about 12% Nothing like double is it.

I am certain that this information should now be well known within the GCCC.

For instance , this is the table of data presented to the Waterfutures Advisory Committee by the GCCC, when they were considering the water supply options for the Gold Coast. There is some variation (depending on who did the study) as well as effects on available water depending on assumptions of the effectiveness of water restrictions.

Summary:

Ultimately I think that its important that representatives of the Council present the facts to us about the planning, about the state of our water supply and about what the real results of changes are. Putting "spin" (nice word for bullshit) on things as critical to the community as water supply should not be entertained. Misleading the community is not something we should tolerate in our Local Government. If (alternatively) the speaker had not the faintest clue about what they were talking about then perhaps they should not be in that position either.

The population of the Gold Coast is growing fast, and in the time since the 2002 drought crisis the population has grown from 425,000 to over 591,000. Thats an increase of over 28%

Since that time we've also gained the Logan Pipeline and the Desalination plant, lets hope its enough for the next drought