The Totara Reserve Regional Park cliff collapse of 15 December 2006

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McSaveney, M.J.; Page, M.J. 2007 The Totara Reserve Regional Park cliff collapse of 15 December 2006. Lower Hutt, N.Z.: GNS Science. GNS Science report 2007/11 10 p.

Abstract: At about 6 pm on the 15th December 2006, a section of cliff collapsed on the right bank of the Pohangina River at the Totara Reserve Regional Park camping ground, 30 km northeast of Palmerston North, killing three of four people playing in the river. The collapse was witnessed by others, but occurred with far too little warning for any useful response by the victims or witnesses. The 50-m high cliff is made of ancient, weakly consolidated, weathered river gravels with sparse thin beds of weakly consolidated silt (Turitea Formation, about a million years old). It is prone to infrequent collapse, largely as slabs of variable thickness separating along cracks subparallel to the cliff face. The typical failures are arch failures producing rockfalls in a wide variety of sizes. The rockfall of 15 December was unusually large for the site, at about 500 cubic metres of mostly weathered gravel. At the site, failure frequency may be exacerbated by an easily eroded silt layer at the base of the cliff within the reach of floodwater, but this weak layer is not the primary cause of the failure of 15 December. The primary cause is the presence of the cliff, which is maintained by scour on the outside of a bend in the Pohangina River. Scour also maintains a water depth suitable for swimming in. There is evidence that scour was particularly severe during flooding in 2004, and the cliff may still be responding with more frequent rockfalls and increased likelihood of large rockfalls. The increase is slight, however, and events such as occurred on 15 December have always been possible. Stability within the cliff has always been deteriorating over time, and a relatively small volume of gravel behind the cliff face reached a critical point of instability at about 6 pm on 15 December 2006, resulting immediately in a rockfall of some 500 cubic metres within seconds. (auth)