NEWS: The Dart River was blocked by a landslide on Saturday afternoon, posing a risk to river users and trampers, resulting in Dart River Jet postponing operations. The river is now flowing again and GNS Science and QLDC have agreed that Dart River Jet can safely resume full operations immediately, therefore we are operating our Wilderness Jet this afternoon and will resume both the Wilderness Jet and Funyaks as normal as of tomorrow.
On Sunday 5 January 2014, Mauri McSaveney, Mark Rattenbury and Simon Cox of GNS Science/Geonet Response joined Richard Kennett (DoC Glenorchy) and Callan Grimmer (Dart River Jet) for a helicopter inspection of landslide blockage in the Dart River, Otago. Anomalous flows had been recorded on the Otago Regional Council river gauge at the Hillocks gauging station (www.orc.govt.nz) and a lake seen following a DoC inspection on the 4th January.
A well-known landslide at Slip Stream has been active in the recent rainstorms, sending debris flows down onto a debris-flow fan on the valley floor. At ~6.30pm debris was still actively flowing down a channel like very wet concrete, surging and carrying material up to boulder size down to the Dart River. Debris has been deposited across a broad front more than several hundred metres wide.
A lake approximately 4 km long has formed from Sandy Bluff upstream, covering the full width of the valley across Dredge Flat. At its deepest, the lake may be about 20m deep. The lake had grown ~800m in length from the previous evening’s inspection by Department of Conservation, and was still increasing whilst we were in the valley. Part of the Dart River track is covered by the lake.
The Dart River channel has been pushed hard across to the east (true left) side of the valley and the river is now eroding the slope. An elevated part of the track has been undercut and has fallen away. The river currently flows through the forest and further down stream covers around 1 km of the Dart River track.
A broad dam extends over at least 1km of river channel length, across which the Dart River still flows. Based on the length and low gradient of the dam spillway, the lake is not likely to disappear quickly (it is still rising). When the debris flows stop (or slow) the river is expected to start cutting down and the lake will slowly lower. No abnormally high flows should be experienced downstream (the river is currently in a fresh). The debris flows and lake pose no additional hazard in the Dart River below the dam.
A second inspection will be made on 6th January.
END 23:00 5/1/2014