Alexandra man Grant Campbell, who has been following our Protecting our Water series, shares his knowledge of the topic.
Yet another aspect of the water quality issue (protecting our water) was raised in a report in this paper last week (Week in Review, 30 May).
On page 11 it was reported that the Land Information New Zealand (Linz) agent (Colliers International) recommended to the Cromwell services committee meeting that no further action regarding the build-up of sediment at the Cromwell boat ramp would be taken because it “could not be stopped”.
The boat ramp would no longer be available for use as it was becoming dangerous for users.
Following three “one-in-one-hundred-year” floods, which inundated Lower Alexandra in 1994, 1995 and 1999, it was found the sediment build-up in Lake Roxburgh caused the water level to rise during such events.
When a barrier (dam) is placed across a river and a lake is formed, the current is reduced so the sediment falls to the bottom.
As the sediment level on the bottom increases, the volume is reduced.
For any given river flow, the water level will be higher.
This occurred in the Roxburgh Gorge, causing Alexandra to flood.
During the re-consenting process for the operation of the dams, the ORC introduced a condition called “flushing”.
When the river flows are high, the lake level at Roxburgh is lowered.
This makes it more like a river, thus increasing the velocity and “flushing” the sediment past the dam.
This was introduced at Roxburgh only, not Clyde, because at that stage the problem had not arisen in Lake Dunstan.
However, it was only a matter of time before the sediment (which comes largely from the Shotover River catchment) would make its way to Cromwell.
There are other alternatives for sediment removal.
The Bannockburn inlet also fills with sediment from Lake Dunstan.
This is monitored and when required it is pumped as a slurry back into the lake.
Silting also occurs at the confluence of the Manuherika and the Clutha, necessitating regular removal by diggers and trucks.
This work is undertaken and paid for, as part of consent requirements, by the dam operator, Contact Energy.
Worldwide there are many examples of silt removal methods employed by dam operators, as this consequence is widely experienced.
That the sediment has reached Cromwell indicates the Kawarau arm is full and for any small increases in river flows, the discolouration of the river/lake will be more prevalent.
Before the dams were built the river naturally flushed silt during flooding events, and the sediment was carried out to sea.
While “flushing” attempts to recreate this, Lake Roxburgh is only lowered a small amount relative to its height.
The level chosen was a compromise to allow some generation to still occur and only tends to redistribute the sediment downstream towards the dam.
Sediment in the bottom of the lake does not affect the output of the power station – this is governed by the lake level.
The current consent conditions are being monitored by the ORC.
It could advise how effectively the current flushing method is working.
It is unlikely to be as thorough as if the lake level was taken back to its original river level, which had worked for thousands of years.
Hence, there are options for Linz to consider.
As this also affects the flooding risk to our towns and helps protect our waterways, it is important the community is involved.