Mapping the Ruapehu Lahar

MAPPING THE RUAPEHU LAHAR

At Mt Ruapehu, in the central North Island of New Zealand, a series of eruptions in 1995 and 1996 expelled the summit Crater Lake, generating a sequence of eruption- and later rain-triggered lahars. Early in 1996, the potential for a significant Crater Lake break-out lahar was identified as the Crater Lake began to refill behind a fragile barrier of volcanic material, or tephra, deposited on the hard rock rim of the crater.

As the lake rose above the base of the tephra dam in early 2007 it became a matter of when, not if, another lahar would occur. The worst possible scenario in these circumstances was sudden collapse of the new tephra dam causing a lahar as large as or bigger than the 1953 Tangiwai lahar.

The task at hand then was to produce a highly accurate three-dimensional map of the predicted lahar path along the upper Whangaehu River both before the lahar occurred, and most critically immediately afterwards. The specific requirements of the data needed led the researchers to consider the use of LiDAR equipment. Not only would it allow them to map the probable extent of the lahar over a large area at an unprecedented level of detail but it would also give them the vital before/after scenario if data could be collected before and immediately after the event.

The pre-lahar data was flown early in 2006 and just over one year later, the tephra dam failed on the morning of 18th March 2007, releasing 1.3 million cubic metres of warm acidic lake water. The torrent poured down the steep upper gorge of the Whangaehu valley at 30 km/hr, entraining snow, ice, and sand to boulder sized particles. The lahar passed Tangiwai, 40 km downstream within 2 hours and reached the coast 155 km away in the early hours of the following morning. Thanks to a detailed inter-agency response plan and the ERLAWS lahar warning system installed by the Department of Conservation no lives were lost and infrastructural damage was minimal.

Within three weeks of the lahar the post-lahar survey was performed, on 6th April 2007. The post lahar data was captured with an Optech ALTM 3100EA again mounted in the Cessna 402b, both owned and operated by NZ Aerial Mapping Limited.

Operationally, the project was difficult from two quite diverse angles. Firstly, the area of survey spanned more than 2,000m of vertical relief which required very accurate flight planning and flying to ensure correct overlaps and point density were achieved.

Additionally, as the area is situated within a Military Restricted zone, formal clearances were required to fly within it. To complicate matters further, at this time a major international exercise was taking place which required the highest levels of cooperation and coordination between the Military, Weather Office and Aircrew.

In addition to the LiDAR data, digital imagery was captured by the co-sited medium format camera. This imagery provided both checking imagery for the LiDAR classification and also a set of orthophoto images for eventual supply.

For the processing, the LiDAR sensor positioning and orientation (POS) was determined using the collected GPS/IMU datasets and Applanix POSPac software. This work was all undertaken using NZGD2000 coordinate system.

The POS data was combined with the LiDAR range files and used to generate LiDAR point clouds in NZTM map projection but NZGD2000 ellipsoidal heights. This process was undertaken using Optech REALM LiDAR processing software. The data was checked for completeness of coverage then the relative fit of data in the overlap between strips was checked. The point cloud data was then classified into ground, first and intermediate returns using automated routines tailored to the project landcover and terrain. This and subsequent steps were undertaken using TerraSolid LiDAR processing software modules TerraScan, TerraPhoto and TerraModeler.

The data was converted from NZGD2000 ellipsoidal heights into Moturiki 1953 vertical datum using the LINZ NZGeiod05 separation and offset model.

Comprehensive manual editing of the LiDAR point cloud data was undertaken to increase the quality of the automatically classified ground point dataset. As part of this process LiDAR returns from water bodies were removed from the ground point dataset. The orthophotos were used as a backdrop when undertaking the manual editing.

Independent of the aerial acquisition work we commissioned a series of surveyed check sites in open ground, to be later used to verify the accuracy of the processed datasets. Sites were chosen outside of the lahar flow path.

The height accuracy of the processed data was checked using the provided check site data. This was done by calculating height difference statistics between a TIN of the LIDAR ground points and the checkpoints. The standard deviation statistic of the height differences was +/-0.11m.

The positional accuracy of the processed data was checked visually by overlaying the check site data over the LiDAR dataset displayed with its intensity values. The data was found to fit well in position.

Analysis of the pre and post lahar data sets has provided a never before insight into the behaviour and outcome of this lahar. Most significantly for this particular event was the realisation that the lahar was approximately 25% larger than the 1953 Tangiwai lahar.

The illustration shows graphically, the effect the lahar had on the topography of the affected areas. The landslide and all other scoured areas are shown as blue areas within the graphic which was created by subtracting one LiDAR data set from the other. Blue colouration indicated scouring whilst red indicates aggradation.

The use of the LiDAR data has allowed an accurate model of erosion and aggradation to be established along the entire route of the lahar effect. This has given researchers valuable insight into the behaviour characteristics of lahar and in this instance has raised international interest with this data being studied by scientists in Hawaii, Japan, France and the UK.

For further information about Mount Ruapehu, please visit the website below.
http://www.gns.cri.nz/what/earthact/volcanoes/nzvolcanoes/ruabookprint.htm

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