The Case Against the Mitchell Dam

For a long time, in fact too long to freely admit, I have been posting on sites such as Andrew Bolt's or TBBWP regarding the naive argument for building a dam on the Mitchell River in Gippsland. I thought it was time to post a wrap of my position including some facts and figures to counter those often made up by pundits.

The Catchment

The Mitchell River catchment is quite large covering an area of 4,870 square kilometres made up of mainly woodland and forest. The main rivers in the catchment include the Dargo and Wonangatta. Over the last few years the catchment has been subject to severe bushfires and drought conditions.

Recent Flows

The Mitchell catchment has been subject to well below average flows for an extended period of time. This is the same as the majority of catchments in Victoria. The average flow out of the total Mitchell catchment is 1,355,000ML per year.

As it can be seen even with close to average rainfall the flows from the catchment have been well below average. Looking at the data more closely:

As it can be seen there has been 4 months with above average flows in the last 24, of which, only one (June 2007) was above 125% of the average monthly flow. In the same corresponding timeframe there have been 4 months with flows below 5% of the average flow and 17 months with flows below half of the average.

The June 2007 flood was the result of 3 days of intense rainfall in the catchment. If that event is not included the flows within the system look dire.

Draining Out to Sea?

One claim about the Mitchell River is that it 'just drains out to sea'. This of course is false as the Mitchell River supplies about 30% of total inflow into the Gippsland Lakes System. We have all heard and read about the devastating effects removing the Murray River inflows from Coorong estuary systems in South Australia, so to think that it just 'flows out to sea' is naive.

Last summer the Gippsland Lakes were subject to blue-green algae blooms.

The History of Dam Proposals

There has long been proposals to construct a dam on the Mitchell River in Gippsland. The reason for this construction was not to supply water to Melbourne or to reduce downstream flooding, the main reason was to supply water for a proposed irrigation district on the flats near Sale.

In fact there was a dam constructed across the river in the 1880's but it was washed away and never replaced.

The Current Proposal

Pundits have talked about constructing a dam similar in size to the Thomson Dam.

It would have a storage of around 500 GL and a dam height of around 80m. The SKM report suggests that the annual environmental flow from the dam to supply the river downstream should be around 491 GL or about 37% of the annual flow. This does not take into account any needs of the Gippsland Lakes.

Since the Mitchell catchment supplies a large prtion of flows into the Gippsland Lakes it would be anticipated that the required environmental flows would be much larger than the 37% noted in the report. This increase would reduce the yield of the dam significantly when you consider the average yearly flows in the table above.

The estimated capital cost for the proposal in 2005 dollars $1.35billion (equates to $1.5b in 2008 dollars). In this cost there is an allowance of $480M for a 119km pipeline. This estimate is very doubtful as indicated by a 'very low' confidence level in the report and based on recent costs of $750M for the 70km long North - South Pipeline which has similar diameter pipes.

Based on a similar unit rate as the North - South pipeline the pipeworks would cost in the order of $1.3 Billion. It shold be noted that the pipeworks associated with the Mitchell proposal include a tunnel section through sensitive environments which would dramatically add to the cost.

By revising the 2005 estimate to 2008 dollars and using a current unit rate for the transfer system it would be expected that the Dam would cost in the order of $2.3 Billion. This still includes contingency on the dam construction costs.

Summary

The proposal to construct a dam on the Mitchell River in Gippsland is short sighted.

The capital costs of such a scheme would be closer to many other current alternative supply proposals than what is being reported.

The reliability of the supply is not as high as some would think with only an annual yield of 86GL/year at 95% currently and once the CSIRO climate change effects are taken into account the yield could drop to 69GL/year by 2050.

The environmental damage on the downstream river and Gippsland Lakes would be immense.