Debris avalanche and debris flow (lahar) in Oaonui Stream on Mt Egmont, 16 September 1998

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Hancox, G.T. 1999 Debris avalanche and debris flow (lahar) in Oaonui Stream on Mt Egmont, 16 September 1998. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 99/06 18 p.

Abstract: A large debris avalanche and flow occurred in the head of Oaonui Stream on the western side of Mt Egmont on or about 16 September 1998, apparently triggered by prolonged very heavy rain. The event was initiated by collapse of a large volume of unconsolidated pyroclastic debris flow deposits, colluvium, and blocky andesite lava from the steep cliffs at the head of the stream. Erosion of the debris flow deposits during floods in the upper Oaonui Stream now cause severe sedimentation problems in the Waiaua River, which is the Opunake water supply source. Rediversion of the upper Oaonui Stream back into its original course is unlikely to occur naturally without another major debris-transporting event, and possibly not even then. Because of the high probability of similar debris collapses and debris flows in Oaonui Stream in the future, and the extent of the diversion site, the present situation would also be very difficult to permanently reverse with engineering intervention. Another source may therefore need to be found for the Opunake water supply, or alternatively, construction of settlement ponds may partly overcome the sedimentation problem. The presence of unstable slopes at the head of Oaonui Stream suggests that similar debris collapses and flows can be expected in the future. These have the capacity to be as large or larger than the 1998 event (as in 1959). (auth/JIH)