Contact Energy seeks approval to dredge Bannockburn Inlet

Hundreds of truckloads of silt looks set to be removed from the Bannockburn Inlet near Cromwell and pumped further down the Kawarau Arm of Lake Dunstan towards the town.

Contact Energy is in the process of requesting go-ahead from the Otago Regional Council to dredge the bed of the lake at the inlet, to make the lake deeper at this popular swimming and boating spot. 

The energy company has submitted a resource consent application to the regional council that if granted will allow it to remove between 12,000 and 35,000 cubic metres of sediment every five years at Bannockburn.

Overlooking the Bannockburn Inlet: Contact Energy is proposing dredging the popular boating and swimming spot to rid it of excess sediment flushed down the Kawarau River and into the sheltered waterway (Image: Trip Advisor).

“The proposal is to operate a suction dredge to remove water and sediment from the lakebed of the Bannockburn Inlet and to pump it through a floating pipe into the Kawarau Arm of Lake Dunstan,” Contact’s application says. 

The pipe will shoot the sediment flurry back into the main flow of the Kawarau Arm, just outside the mouth of the inlet, close to the lake bank.

Because the natural flow is considerably more than what will be pumped into it - 120,000 to 200,000 litres per second versus 700 litres per second - the slurry discharge will be "highly diluted", it says.

The application proposes an excavator will also be used to shift sediment, and work will only be undertaken from May until October to avoid the general swimming season.

During dredging, access to the inlet for floating vessels will be limited and use of the Bannockburn jetty may also be restricted.

In addition, the application proposes the use of a digger each year, likely for no more than a two-week period, to carry out minor works to clear the shoreline of the inlet.

Contact holds several resource consents to operate the dam at Clyde and a consent condition of these requires the company to manage the build up of sediment in the Bannockburn Inlet so members of the public can continue to use it recreationally and so it continues to be visually appealing.

Because of this, there is ongoing monitoring of the depth of the lakebed at Bannockburn by Contact.

Its last scheduled look in January has revealed the threshold requiring it to clear the sediment had been reached at “many places” within the inlet.

So, it is now obligated to remove the accummulated sediment.

The proposed dredging will also ensure a 50-metre-wide channel is maintained at the inlet’s entrance.

Although clearing the inlet is a requirement of an existing resource consent, Contact still needs to apply for subsequent resource consent to dredge, so that is what it is now doing.

Contact has been granted a similar consent to dredge in 2008, but that consent had been for five years only and had expired in April 2013. 

The inlet is approximately 22 hectares and has a perimeter spanning approximately three kilometres, the consent application says.

“The inlet is naturally accumulating sediment which flows into the Kawarau Arm from the Kawarau River and is sourced mainly from the sediment-laden Shotover River further upstream.”

According to an aquatic ecology assessment report that has been prepared to support Contact's application, "dense beds" of aquatic weeds have been observed in shallower sections of the inlet.

The presence of weeds contributed to the death of a swimmer in the inlet in 2016.

Speaking with Crux today, Contact says approximately 1.2 million cubic metres of sediment comes down the Kawarau River each year - that is as more than the amount of concrete poured during the building of the 100-metre-high dam at Clyde - and "a small proportion of this enters the Bannockburn Inlet".

The ORC's acting manager for consents Alexandra King says Contact's application is currently being processed. 

It is yet to be decided if the consent application will be publicly notified or not.