The Eastern Ruapehu Lahar Alarm and Warning System (ERLAWS) is a

lahar A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock, pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a valley, river valley. Lahars are o ...

warning system that was installed on

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

, New Zealand following

volcanic eruption A volcanic eruption occurs when material is expelled from a volcanic vent or fissure. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior h ...

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 The North Island ( , 'the fish of Māui', historically New Ulster) is one of the two main islands of New Zealand, islands of New Zealand, separated from the larger but less populous South Island by Cook Strait. With an area of , it is the List ...

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

left a 7-metre high dam of

tephra Tephra is fragmental material produced by a Volcano, 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, ...

, consisting of

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

and rock, around the rim of the crater lake. 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

. Such a lahar resulted in the 1953

Tangiwai disaster The Tangiwai disaster was a deadly railway accident that 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 loco ...

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. Due to the high ...

. 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. The system began operating in 2002.

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 Tokaanu is a small settlement close to Tūrangi at the southern end of Lake Taupō. The Tokaanu Thermal Pools and the easy access to Lake Taupō make it a popular lakeside holiday destination. A short walking track through the Tokaanu thermal ...

, 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, 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.

Transit systems

Transit New Zealand has systems installed on the

State Highway A state highway, state road, or state route (and the equivalent provincial highway, provincial road, or provincial route) is usually a road that is either Route number, numbered or maintained by a sub-national state or province. A road numbered ...

s 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 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 metres 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