Being an alpine highway, avalanches can seriously affect the road during avalanche season (usually June to November). The avalanche area covers 17km, 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 crashing down a mountain. They vary in size from small to devastating – massive avalanches that are among earth’s most powerful and complex natural phenomenon. It is difficult to predict with absolute certainty when they will strike.
When the glaciers retreated from Fiordiand valleys, the new terrain of near-vertical walls and large, steep snow basins provided an almost perfect landscape for avalanches. Snowfall, wind and temperature changes create complex layering within the start zone snow-packs. When the layers weaken, avalanches can be triggered by gravity, new snow, rain, or deliberately cleared in managed way with explosives.
In the Upper Hollyford and Cleddau Valleys recurring avalanches have swept vegetation from the mountain walls, flattened trees and bush and created 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.
To keep the road safe and open as much as possible during the avalanche season, the Transport Agency operates an internationally acclaimed avalanche control programme that predicts and controls avalanches. A specialist avalanche control team employs high tech equipment to manage avalanches. The specialised weather and condition monitoring equipment is based both at road and mountain level, and is monitored around the clock to maximise safety and minimise road closures.
Besides predicting avalanches, the programme also controls 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.
The avalanche hazard is the prediction of the probability of avalanches happening and is expressed as a level of danger. You must consider this before driving into 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, that 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. If a moderate avalanche hazard is posted there is a risk of avalanches reaching the road and the road could be closed at any time. When a high avalanche hazard is posted the road is closed.
|Avalanche hazard||Probability of avalanches||Road status|
|LOW||Avalanches are unlikely||Open|
|MODERATE||Avalanches are possible||Road open, but may close at short notice|
|HIGH||Avalanches are expected||Road is closed|