The Eastern Ruapehu Lahar Alarm and Warning System (ERLAWS) is a lahar warning system that was installed on

Mount Ruapehu Mount Ruapehu (; ) is an active stratovolcano at the southern end of the Taupō Volcanic Zone and North Island volcanic plateau in New Zealand. It is northeast of Ohakune and southwest of the southern shore of Lake Taupō, within the Tongari ...

, New Zealand following

volcanic eruption Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are oft ...

s in 1995–1996. The system successfully detected and warned of an imminent lahar in March 2007. The system is being expanded to detect the wider range of lahar threats now expected on Ruapehu.

Introduction

The 1995–1996 eruptions of Ruapehu in the North Island of

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

left a 7-metre high dam of

tephra Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they re ...

, consisting of

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

and rock, around the rim of the

crater lake Crater Lake ( Klamath: ''Giiwas'') is a volcanic crater lake in south-central Oregon in the western United States. It is the main feature of Crater Lake National Park and is famous for its deep blue color and water clarity. The lake partly fill ...

. It was realised that as the lake refilled and rose above the level of its normal outlet, the tephra dam would eventually collapse, causing a large lahar. Such a lahar resulted in the 1953

Tangiwai disaster The Tangiwai disaster occurred at 10:21 p.m. on 24 December 1953 when a railway bridge over the Whangaehu River collapsed beneath an express passenger train at Tangiwai, North Island, New Zealand. The locomotive and the first six carriage ...

when 151 people died as the lahar swept a railway bridge away, causing a passenger train to plunge into the

Whangaehu River The Whangaehu River is a large river in central North Island of New Zealand. Its headwaters are the crater lake of Mount Ruapehu on the central plateau, and it flows into the Tasman Sea eight kilometres southeast of Whanganui. Water is diverted ...

. 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.

Sensors

ERLAWS consists of three 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 and upload it to an external server and the ERLAWS website. When incoming measurements meet or exceed pre-set thresholds, alarms are sent via

pagers A pager (also known as a beeper or bleeper) is a wireless telecommunications device that receives and displays alphanumeric or voice messages. One-way pagers can only receive messages, while response pagers and two-way pagers can also acknow ...

to police, district council staff,

Transit New Zealand Transit New Zealand (Māori: Ararau Aotearoa), which existed from 1989 to 2008, was the New Zealand Crown entity responsible for operating and planning the New Zealand state highway network (10,894 km, about 12% of New Zealand's roads). It ...

, KiwiRail Network staff, and duty scientists who will then respond following predetermined plans. This alert can be up to two hours before a lahar would reach

Tangiwai Tangiwai is a census area and a small rural community in the Ruapehu District of the Manawatū-Whanganui region of New Zealand's North Island. It is located east of Ohakune and Rangataua and west of Waiouru on State Highway 49. In 2018 37.5% ...

Transit systems

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 A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing ov ...

Events

On 18 March 2007, the

dam collapsed, causing a lahar which flowed down the mountain and into the Whangaehu River. The ERLAWS system activated at 10:47am and worked as planned. Scientists estimate that 1.29 million cubic metres (1.29 billion litres) of sulphur and water went down the Whangaehu Valley,YouTube video of 2007 lahar
leaving 8.9 million cubic meters in the crater lake. This lahar was at least 50% larger than the 1953 lahar that caused the Tangiwai disaster.

Expansion

Following the 2007 lahar, the risks of future lahars were reassessed and judged to affect more valleys than before. The ERLAWS system will be broadened in response to the wider threat.

References

* Keys, Harry & Green, Paul (2004).
Mt Ruapehu Crater Lake Lahar Threat Response – Crater Lake issue – a management dilemma
{Dead link, date=August 2019 , bot=InternetArchiveBot , fix-attempted=yes ''. New Zealand Department of Conservation. Retrieved 23 April 2007. * Mike Watson,

, ''The Dominion Post'', 6 September 2007. Emergency population warning systems Volcanism of New Zealand