Scanning through some of the more than 1100 pages of the recently completed geo-technical testing report for the Shannon Creek Dam, I have come across repeated references to "Ensuring seepage is kept to an acceptable level".WHAT IS A LEAK?
Scanning through some of the more than 1100 pages of the recently completed geo-technical testing report for the Shannon Creek Dam, I have come across repeated references to "Ensuring seepage is kept to an acceptable level".
In this case seepage is mentioned only in relation to that water expected to find its way through, under or around the dam wall itself. There is no consideration of the seepage that may occur from water finding its way through the loose jumble of sandstone that forms the valley walls, or any seepage under ground through the sandy floor of the inundation area.
I asked the Project Manager what represented an acceptable level of seepage, and was told that between 10 and 15 litres per second was the aim. However it was explained to me that seepage over and above was not an insurmountable problem as that water could be collected and pumped back.
15 litres per second equates to 900 litres per minute; 54,000 litres per hour; 1,296,000 per day; 38,880,000 litres per month, or 473,040,000 litres (473ML) per year. In short that is the amount of water that could flow forever from the Nymboida River to the sea via Shannon Creek and the Orara. Good news possibly for the Orara, not so good for the Nymboida which is currently sending huge quantities of water to the ocean via the Goolang Creek power station and the Orara. Over the past eighty years this latter operation has caused huge damage through siltation of all downstream rivers, the additional water from Shannon Creek is hardly likely to help.
North Coast Water have decreed that run-off from any rain falling within the dam's catchment will be allowed to flow on to the lower reaches of Shannon Creek and beyond. This statement is designed to give us, the public, a warm, fuzzy feeling about the environment, and satisfy down stream residents and ecologists who have concerns over maintaining river flows. In fact, I would suggest it would be impossible to gauge how much water is flowing into the dam at any one time unless there is a measuring devise installed in each of the myriad of gullies and feeder tributaries that flow directly into it.
That 473ML, should the year's acceptable level of seepage be achieved, would supply Grafton's entire water needs for over a month. I am also told that, in hot dry weather, evaporation from a full Shannon Creek dam will probably exceed 250ML per month. It would seem, therefore, the Nymboida River is not only going to be asked to supply water to the ever increasing Clarence Valley population, and much of Coffs Harbour's needs to support its projected population explosion, but an additional 473ML will need to be pumped into Shannon Creek Dam each year simply to replace losses of water from acceptable seepage, plus also whatever is lost through evaporation. In a dry year this latter could be as high as 1,250ML.
Are we truly protecting our river flows and the long-term health of our river systems, or are we being conned? Finally, once the tax-payers of New South Wales have forked out mega-bucks to build this dam (final figure unknown), what chance of getting desalination up and running any time soon?
-John