Ruapehu Lahar Progress

Summary: 5.3 metre lahar recorded at Tangiwai flood gauge. Bund protecting spill into Waikato Stream intact. Road and rail traffic suspended in the area. Minor damage. Tephra dam not completely destroyed. Management plan successful.

The lahar on Mt Ruapehu this morning has been described as “moderate” as reports from emergency and scientific staff begin to come in. Data from sensors and cameras show a spectatcular flow as servers groan under the load from the Internet.

A number of government agencies are involved in managing the effects of the lahar. The police are responsible for traffic management and closed State Highway 1 between Waiouru and Rangipo for several hours, detouring traffic through National Park. Some local roads remain closed, as does State Highway 49 until structures have been inspected and the lahar has completed its passage.

The North Island Main Trunk Railway has been closed delaying north-bound and south-bound Overlander trains and freight services until the lahar flow has eased. Passengers will be moved around the rail line closure by bus to complete their journeys. It is expected that the rail traffic will be resumed shortly once the Tangiwai railway bridge has passed inspection by Ontrack and other agencies.

The Department of Conservation (DoC) is responsible for much of the conservation estate in the area, and has installed and managed the monitoring equipment which detected and traced the progress of the lahar.

The Horizons Regional Council manages the natural and physical resources of the Wanganui and Manawatu regions and is responsible for local matters such as public safety, Civil Defence and maintenance of services. The Council maintains a webpage with information on Ruapehu’s lahar risk.

Early reports indicate that today’s lahar flow was of moderate size. The contents of Ruapehu’s crater lake began flowing when the tephra dam began breaking up at 10:47 this morning, the 18th March 2007. Over the next 45 minutes the dam quickly eroded as the crater lake dropped toward the level of the south-eastern rim. However, latest reports indicate that the tephra dam created by the last two eruptions of Ruapehu volcano has not fully collapsed.

The lahar has flowed safely under most bridges over the Whanagehu River and, although material is still flowing mid-afternoon, the peak has passed. The flow has been contained within the Whangaehu River and only minor damage has been reported, DoC having lost a footbridge over the river.

The bund which was constructed to prevent the lahar spilling into the Waikato Stream has held.

The Tangiwai flood gauge reported that the Whangaehu River was running at about 1.2 metres during the morning, slightly above normal. An initial rise of 30 cm was recorded at 12:13 before the river abruptly rose to 4.07 metres at 12:39 where it stayed for several minutes. Another abrupt rise to 6.4 metres was recorded at 1:03 p.m. before the peak flow passed through bringing the river to 6.53 metres at 1:04 p.m.

The river level dropped 3 metres over 3 minutes at 1:17 p.m. and stayed at that level until dropping to 2.33 metres over several minutes at 1:40 p.m. Minor increases have been recorded since, but the river was at 2.3 metres at 2:19 p.m., just over a metre above its level prior to the lahar’s passage.

The Horizons Regional Council webcam upstream from the Tangiwai Bridge in the Whangaehu Valley recorded the lahar as it passed through early this afternoon.

At this stage, it appears that the management plan put together by the government agencies responsible has successfully managed Ruapehu’s latest lahar event.

One Response to “Ruapehu Lahar Progress”

  1. Chris says:

    What’s really awesome is that no one was killed – and no one was hurt. And I had just been wondering where the lahar threat was at….

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