Thomson, R. 1994 The 1984 Iris Burn slip : a geological appraisal . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 94/38 15 p.
Abstract: A 300 000 m3 landslide occurred on or about the 26 January 1984 in in situ rock on the right wall of the Iris Valley, eastern Fiordland. The failure was clearly triggered by a severe rainstorm. Abnormally high groundwater levels in a relaxed and open jointed rock mass probably induced shearing along a master joint that dipped at 32 degrees towards the valley. The overlying rock mass collapsed, with the consequent development of a rock avalanche which moved rapidly down a moderate slope to the Iris Burn and up a gently rising valley floor on the opposite bank. Fine, angular rock debris was spread over 0.3 km2 of the valley floor at distances of up to 550 m beyond and elevations of 40 m above the Iris Burn. A fluid wave, moving from the flooded river in advance of the rock debris, devastated the bush-clad valley floor over a 0.6 km2 area. The wave deposited timber in mounds up to 600 m beyond and 50 m higher than the river. An eroded margin and distal timber mounds attest to the wave having swung downstream to a point 900 m from the axis of failure. Landslide hazards remain in the area but the risk of further failure at the site is considered to be very low. (auth)
