In July 2022 Engineers Australia held a risk seminar at Southern Cross University. It wasn’t long after the floods so naturally the floods came up in panel discussions. An audience member suggested we needed large basins to capture the water to reduce flooding in Lismore. Not one of the engineers on the panel wanted to discuss this because they thought the idea wasn’t worth discussing.
Then, under instruction from NEMA, CSIRO went to the people asking if they wanted large detention basins modelled. Kevin Hogan was on the record saying that detention basins could take 2m off the 2022 flood height in Lismore. Coincidentally apparently, the CSIRO report that eventually came out showed that a 2.07m reduction could be achieved. Unfortunately, the report is a hydrologic model only, with no reference to reality.
The Rock Valley dam would be located 12km from the Lismore CBD with a storage of about one sixth that of Sydney Harbour. That’s a lot of water and under the NSW Dam Safety Act and Regulation it would be classified as a dam. ANCOLD, Australian National Committee on Large Dams, has guidelines which would have to be followed if that is the case.
One of these guidelines is that the dam would require a spillway to carry the probable maximum flood. That’s a big spillway. Incidentally there is no consideration in the CSIRO report of what will happen in a flood larger than the 2022 event.
The dam wall is required to hold back 40m deep water. On top of that is the freeboard so the wall would be considerably higher than 40m. The drawdown rate, i.e. the amount the water level is lowered, is nominated in the report at about 3m per day. Half a metre per day is considered high and one metre per day is considered exceptional. This is because of the danger of rapid drawdown failure.
Landslip risk
The whole of the region has experienced landslips so it is possible that the stored water would cause landslips anyway, but even if that is not the case an acceptable drawdown may well be close to 0.1m per day taking the drainage time to more than six months.
On page 18 (Fig 10) there is an artist’s impression of a dam showing a gate going the full height of the dam. That would be 40m high, but its width is not stated. This is fanciful, but let’s just take the wording used.
The gate is nominated as a sluice gate. A sluice gate is an upward opening gate that allows discharge underneath so assume there is a hole in the wall with a sluice gate on it.
At 40m water pressure on one side going to zero on the other side, the gate would cavitate enough to destroy itself and any concrete near it. Cavitation involves the formation of shock waves due to pressure changes. Because of these challenges, cone valves are how many other Australian dams of similar height release water. These bolt to a pipe.
These are just three serious engineering issues demonstrating that the image in the report is grossly misleading. The CSIRO does a great job in many areas but in this instance they appear to have looked at how the February 2022 flood levels can theoretically be reduced by the amount stated by Mr Hogan without considering whether their proposal is either practical or realistic.
Now we have a further $3M to spend on feasibility studies. These can’t be just rough statements like those above. Experts in many fields will be involved. Reports will be produced. These will be collated. It won’t happen overnight and meanwhile all those landowners involved will suffer. They will suffer emotionally, and they will suffer financially now that there is a cloud over their property.
For four decades The Echo has printed the stories some people loved, some people hated, and some pretended not to read. If you want us to keep telling the truth, the real truth, not the sugar-coated version. We’ll need your support to keep the presses rolling. 
