Paterson, B.R. 1982 An engineering geological assessment of the Manuherikia Irrigation Water Race, Central Otago (G42/934). Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 362 9 p.
Abstract: The irrigation water race in the Manuherikia gorge traverses the toe of a series of extensive, deep-seated, ancient landslides which are defined by a long arcuate scarp truncated in the north by geological fault. Along the right bank of the river wpere the water race is located, the slide debris has been eroded to form slopes steeper than those that occur in the upper part of the landslide. It is the continuing retreat of sections of the lower slopes that is causing problems for the water race, combined with rockfalls, deterioration of concrete, and settlement of the water race foundation. Although large-scale failure of the slide debris is unlikely to occur in the foreseeable future, shallow failures that have been active for a long time will continue to be a problem particularly following periods of abnormally high rainfall. These areas occur mainly along a section of the lower slopes above which the.land- slide debris is less stable, and the slopes are steeper than the remainder. If it is decided to retain the existing route for the water race, the alternatives are limited to upgrading the existing race, or shifting it to the eastern (left) bank. If the former alternative is chosen, several steps may be taken to reduce maintenance during the next ten years or so. These measures include: a) tree planting in the unstable areas -this would take several years to become effective; b) extension of the piped sections of the water race (particularly upstream); c) reduction of overtopping of the race. (auth)