ERLAWS
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ERLAWS is the acronym for Eastern Ruapehu Lahar Alarm and Warning System.
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[edit] Introduction
In 1995 Mount Ruapehu in the North Island of New Zealand erupted, leaving behind a 7 metre high dam of ash and rock - known as a tephra dam around the rim of the crater lake.
It soon became apparent that given the right conditions, rising crater lake levels could see this dam crumble and turn into a lahar not unlike that of 24 December 1953 which resulted in the Tangiwai disaster where 151 people lost their lives as the lahar swept the railway bridge away just before the express train plunged into the Whangaehu River.
In 2000 the government decided to plan, design and implement ERLAWS - a complex system of sensors and preventative mechanisms to warn of an impending lahar.
[edit] Sensors
ERLAWS consists of 3 sites at which various sensors are located - these are:
- Site 1 (Crater Lake outlet)
- three geophones to detect the vibration of the collapse and from lahars
- a buried tripwire to detect collapse of the dam
- water level sensors to detect a sudden drop in lake level
- Site 2 (NZ Alpine Club hut)
- two geophones to detect the vibration from passing lahars
- Site 3 (near Tukino skifield)
- two geophones to detect the vibration from passing lahars.
[edit] Monitoring
Signals from the sites are transmitted to the Genesis Energy power station at Tokaanu, where computers monitor the data received (via GE Fanucs Proficy iFIX SCADA HMI) and upload it to an external server and the ERLAWS website.
When incoming data meets or exceeds pre-set thresholds alarms are sent via pagers (powered by Win-911 from Specter Instruments) to police, district council staff, Transit New Zealand, Toll NZ and duty scientists who will then respond following predetermined plans. This alert can be up to two hours before a lahar would reach Tangiwai.
[edit] Transit Systems
Transit New Zealand also has systems installed on the State Highway roads surrounding the area, which alert motorists and in some cases prevent entry to the danger areas. These systems incorporate automatic barrier gates, flashing lights and electronic signs warning of flash flooding.
[edit] Events
On March 18th, 2007 a "medium" sized lahar broke through the tephra (partially destroying it) and proceeded down the mountain and into the Whangaehu River. The ERLAWS system activated at 10:47am and worked exactly as planned. Scientists estimated that 1.29 million cubic metres (1.29 billion litres) of sulphur and water had gone down the Whangaehu Valley, leaving 8.9 million cubic meters inside the crater.
[edit] Sources
Keys, Harry & Green, Paul (2004). "DOC: Mt Ruapehu Crater Lake Lahar Threat Response". Retrieved 27 April 2006.