ERLAWS

ERLAWS, the Eastern Ruapehu Lahar Alarm and Warning System, is a lahar warning system installed on Mount Ruapehu following volcanic eruptions in 1995–1996.

Introduction

The 1995–1996 eruptions of Ruapehu in the North Island of New Zealand left a 7-metre high dam of tephra, consisting of volcanic ash and rock, around the rim of the crater lake.

It was realised that some time after the lake refilled and rose above the level of its normal outlet, the tephra dam would collapse, causing a large lahar. Such a lahar resulted in the 1953 Tangiwai disaster when 151 people lost their lives as the lahar swept the railway bridge away, causing a passenger train to plunge 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.

ensors

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.

Monitoring

Signals from the sites are transmitted to the Genesis Energy power station at Tokaanu, where computers monitor the data received (via GE Fanuc's 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, ONTRACK and duty scientists who will then respond following predetermined plans. This alert can be up to two hours before a lahar would reach Tangiwai.

Transit systems

Transit New Zealand has systems installed on the State Highways 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.

Events

On March 18th, 2007, the tephra dam collapsed, causing a "medium" sized lahar which flowed down the mountain and into the Whangaehu River. The ERLAWS system activated at 10:47am and worked exactly as planned. Scientists estimate that 1.29 million cubic metres (1.29 billion litres) of sulphur and water went down the Whangaehu Valley, leaving 8.9 million cubic meters in the crater lake.

References

* Keys, Harry & Green, Paul (2004). " [http://www.doc.govt.nz/templates/MultipageDocumentPage.aspx?id=41782 Mt Ruapehu Crater Lake Lahar Threat Response - Crater Lake issue - a management dilemma] ". New Zealand Department of Conservation. Retrieved 23 April 2007.