Landslide dambreak flooding in the Callery River, Westland

Abstract

In 1930 dambreak flooding, which initiated in the Callery River, threatened Franz Josef Glacier. This town is centred in a high-risk natural hazard zone, which includes threat from earthquakes, catastrophic landslides, and rainfall flooding from the Waiho River. The threat from landslide dambreak flooding in the Callery River has prompted this investigation.

On a geological time scale, the formation and failure of landslide dams in New Zealand is common, and usually occurs in geological settings similar to those of the Callery River Gorge. Aerial inspection of this gorge confirmed that landslide activity in this catchment is extremely high.

Seven possible landslide dam sites were identified in the Callery River Gorge. Physical modelling was carried out to predict the shape of landslide deposits on the valley floor. Four dambreak models were used to simulate the overtopping failure of these dams. Three of the models relied heavily on embankment dambreak technology, as technology for simulating landslide dambreaks is at an immature stage.

If a landslide dam break occurred in the Callery River, resulting discharges could be between 2,000 m³/s for smaller dams, and 17,000 m³/s for larger dams. For smaller dams, the flood duration was usually 4 hours, and for larger dams, 2 hours. Attenuation of the dambreak flooding was negligible. Floodwater velocities were between 14 and 19 m/s.

A landslide dambreak could immediately transport between 100,000 to 2,000,000 m³ of material into the Waiho River. However, a significant proportion of most landslide dams did not erode immediately, and could eventually be transported to the Waiho River in future rainfall flood events. Immediate deposition of sediment on the Waiho River bed, accumulation of vegetation debris on bridge piers, and residual long-term sediment delivery will greatly reduce the conveyance of the channel under State Highway 6. Long-term residual sediment delivery to the Waiho River from a small landslide dambreak could be more adverse (in reducing conveyance at State Highway 6) than the actual dambreak flood.

Structural methods for protecting assets and people from dambreak flooding at Franz Josef would be too expensive for the community. Resources may be better invested in non-structural options for mitigating the hazard. Possible mitigation methods mainly comprised physically altering the landslide dam and pre-warning for evacuation-

1. As the Callery River Gorge is not accessible by ground, access by air and use of explosives may be the only way to physically alter the dam. 2. Immediate warning of a landslide dam break may allow for the evacuation of residents. Robust telemetry installed at the Callery Footbridge, and inspection of the Callery Catchment during heavy rainfall or after a large earthquake may provide this warning.

Assuming a landslide dam is identified immediately, warning time for evacuation during a rainfall triggered landslide dam could be between 1 to 10 hours. Pre-warning for earthquake events could be between 2 days to 1 month, or between 2 hours to 1 day if a large rainfall event occurred after the earthquake. Detection of a landslide dam in heavy rainfall and/or night conditions may not be possible. In a worst case scenario, up to 30 lives could be lost.

The threat from landslide dambreak flooding to Franz Josef is very real. This threat has at least a 1% chance of occurring annually, and is outside accepted risk limits established by the New Zealand Society of Large Dams (NZSOLD). Evacuation and mitigation procedures need to be put in place immediately.