Crater Lake outlet area

Ruapehu Bulletin, 24 June 1997, Page 6

Crater Lake outlet area

The material underlying the outlet channel prior to the 1995-96 eruptions was relatively strong and erosion-resistant grey lava sill and blocky, lightcoloured older lava. These lava layers are now covered with at least 6.4m of tephra comprising variable bedded ash and blocky scoria, with a few layers of old (1996) snow and ice. Away from the outlet channel the 199596 ash and scoria is 5- 10m thick and is up to 20m thick in the head of the Whangaehu River. The grey lava and older lavas are thought to have been little affected by the 1995-96 eruptions. This means that there was no significant lowering of the pre-1995 outflow channel level, and that the material underlying this channel therefore remains strong and stable, with little collapse potential. However, the overlying 1995-96 ash and scoria is weak and poorly compacted, and is likely to be relatively permeable and prone to internal erosion. Although the barrier of weak material may initially retain the lake when it refills to a higher level than before the 1995 eruptions (about 2536.4m above sea

level), it is unlikely to do so for long. The lake overflow stream may gradually erode a channel in the ash barrier down to the grey lava surface, but more probably the barrier could fail rapidly (over less than an hour), due to piping (internal seepage erosion), as inferred to have done in December 1953, resulting in the sudden release of about 1 .5 million m3 of lake water down the Whangaehu River. Detailed slope stability analysis of the outlet area has not been carried out as there is good historical precedent for a crater dam breach of the type described above to show that it is a likely scenario.