ORC report warns of increasing flood risk at Glenorchy

An Otago Regional Council (ORC) report has warned that the frequency and magnitude of flood hazards in the Glenorchy area remains ever present as a serious risk that must be addressed through practical district planning.

The report "Natural Hazards at Glenorchy" has been prepared to facilitate and inform decision-making and the management of the community's exposure to natural hazards. It will be presented to the Queenstown-Lakes District Council and local residents.

It is based on previous technical investigations and identifies each hazard and its implications for the local community.

The chairman of the ORC's engineering and hazards committee, Cr Stephen Woodhead, said the November 1999 flood event at Glenorchy raised public awareness of the area's susceptibility to natural hazards.

""The high level of both direct and indirect hazard exposure experienced by the Glenorchy community is amplified by the pressure for new development and increased tourism ventures operating from the township," Cr Woodhead said.

The report says the frequency and magnitude of flood hazards is likely to increase because of predicted changes in rainfall patterns as a result of climate change.

It warns this will lead to an increase in the susceptibility, and ultimately the risk, to the Glenorchy community, and adds that additional land use intensification or development in hazard-prone locations would heighten this risk.

The ORC's director of environmental engineering and natural hazards, Gavin Palmer, said the continued growth of the Rees River delta into the head of Lake Wakatipu was raising the river bed adjacent to Glenorchy, which in the longer-term would assist in the river's realignment during a flood.

The report says the flood hazards posing a direct threat to the community come from Lake Wakatipu, the Rees River, the Buckler Burn and the Bible Stream.

Dr Palmer said while several of these were well recognized, the risks associated with the lateral migration of both the Rees River and Buckler Burn were considered to be the greatest in the long-term.

(Lateral migration is the process whereby channels move sideways across the wider floodplain of the river).

"The actual existence of the Glenorchy community is, in effect, a result of the natural hazard processes that are occurring in and around the community today," Dr Palmer said.

The sediments Glenorchy is situated upon were deposited as a result of hazardous debris and flood flows originating from the Buckler Burn.

While these processes have more recently been confined to the Buckler Burn fan-delta, significant aggradation within the channel could cause avulsion of the Burn down the main highway and into part of the township.

(Aggradation involves raising the grade or level of the river bed primarily by depositing accumulated sediment. Avulsion is the abandonment of a river channel and the establishment of a new channel at a lower elevation on its floodplain.)

This is the subject of a three-year, ORC-funded PhD studentship at the University of Canterbury.