Concern over Clutha river gravel build-up

There is concern a build-up of gravel in the Clutha River underneath the Balclutha Bridge may increase the possibility of flooding. Photo / Samuel White

Greater urgency is needed to deal with a build-up of gravel in the Clutha River underneath the Balclutha Bridge before the town itself gets ''wet feet'', an Owaka resident says.

Ben Souness, of Owaka, contacted the Otago Daily Times to say he had observed a ''huge amount of gravel'' build up near the Balclutha bridge since the 1978.

Mr Souness recalled seeing the water level ''only inches from going over the top of the floodbank'' during a flood at this time.

''Water was bubbling up through the ground behind the Memorial Hall.''

He believed the build-up of gravel over the past few years had reduced the effectiveness of the floodbanks.

Mr Souness also believed the high river bed would restrict water flow under the bridge in a flood.

Greater urgency was needed to extract the gravel before Balclutha got ''wet feet''.

''It might not stop the town from going under but at least we can say we have done everything we could to prevent that from happening. ''

Clutha Mayor Bryan Cadogan supported Mr Souness's concerns.

''People who have lived beside the Clutha River all their lives are increasingly expressing their concerns at the obvious changes to the Clutha River.''

It was only prudent now, before an emergency event, answers were given and the wider public had a confidence and understanding of the level of risk and options available, Mr Cadogan said.

The river, while in the Clutha district, is managed by the Otago Regional Council (ORC).

ORC engineering hazards and science director Gavin Palmer said the council regularly monitored changes in the bed of the Clutha River between Roxburgh Dam and the coast.

The most recent report presented to the ORC technical committee in March showed that, while part of the river bed near the railway line in Balclutha had risen, part of the bed had fallen.

''The ability of the river at that location to convey floods has not reduced,'' Dr Palmer said.
He confirmed there was no added risk the floodbanks were likely to breach in the event of a flood due to the gravel build-up.

''ORC has no plans to undertake gravel extraction at that site but will continue to monitor the site.''

The latest morphology report of the Clutha River between Roxburgh Dam and the Pacific Ocean provided by the ORC stated extraction was taking place on the Koau branch of the Clutha River at Balclutha and downstream of the Clydevale bridge.

The point at the which the Clutha River divided into the Koau branch and the Matau branch had eroded by about 80m in 10 years.

Two large mid-channel gravel bars beneath the railway bridge also migrated a ''few tens of metres'' in recent years.

The report stated it was unclear whether extraction helped stop bank erosion or exacerbated it.