Avalanche Control Programme

About Avalanches

Being an alpine highway – at the highest point the Milford Road reaches 940 metres above sea level – avalanches can seriously affect the road during avalanche season (usually June to November). The avalanche area covers 17 km, starting at Falls Creek, above Hollyford Road junction (91km north of Te Anau), and ending at the Chasm on the Milford Sound side of the Homer Tunnel.

An avalanche is a mass of snow falling down a mountain. Avalanches can vary in size from small to devastating, massive avalanches that are among the most powerful and complex natural phenomenon. It is difficult to predict with absolute certainty when they will come down.

When the glaciers retreated from Fiordiand valleys, the new terrain of near-vertical walls and large, steep snow basins provided an ideal landscape for avalanches. Snowfall, wind and temperature changes create complex layering within the start zone snow-packs. When the layers weaken, avalanches may be triggered by gravity, new snow, rain or deliberately cleared using explosives.

In the Upper Hollyford and Cleddau Valleys recurring avalanches have cleared vegetation from the mountain walls, flattened trees and bush and maintained large treeless areas on the valley floors.

The avalanches start so high up in the mountains that they cannot be seen from the Milford Road itself. The impact pressure of avalanches can be up to 5,000 kilopascals. For comparison, only 15kPA is needed to break a car’s windscreen.

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Avalanche Management

To keep the road safe and open as much as possible during the avalanche season, Transit operates an internationally recognised avalanche control programme which predicts and controls avalanches. A specialist avalanche control team, contracted by Transit, uses high tech equipment to manage avalanches. The specialised weather and condition monitoring equipment is based both at road and mountain level and is monitored 24 hours a day to optimise safety and minimise road closures.

Besides predicting avalanches, a crucial part of the programme is also controlling the avalanche hazard by either not allowing traffic to stop inside the avalanche area or closing the road and using controlled explosives to release avalanches before they occur naturally.

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Avalanche Hazard

The avalanche hazard is the prediction of the probability of avalanches occurring and is expressed as a level of danger. You must consider this before entering the avalanche area.

The avalanche hazard forecast is compiled from information that includes existing avalanche start zone snow-pack conditions (snow pit studies), current weather data (from automated road and high level weather stations, which transmit data from the mountain top), the weather forecast and local knowledge of avalanche activity.

When a low avalanche hazard is posted on the road condition information signs on the Milford Road, the road is open. When a moderate avalanche hazard is posted there is a risk of avalanches reaching the road and the road could be closed at short notice. When a high avalanche hazard is posted the road is closed.

Avalanche hazard	Probability of avalanches	Road status
LOW	Avalanches are unlikely	Road is open
MODERATE	Avalanches are possible	Road could be closed at short notice
HIGH	Avalanches are expected	Road is closed

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