Lake Waikaremoana outlet - engineering geological studies of factors related to leakage through the natural dam

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Read, S.A.L. 1979 Lake Waikaremoana outlet - engineering geological studies of factors related to leakage through the natural dam. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 336 40 p.

Abstract: Lake Waikaremoana ''the sea of the rippling waters'', with an area of 55.7 km 2 and situated 80 km north of Napier, was formed 2200 YBP by the damming of the Waikaretaheke river by a landslide. As part of the 124 MW Waikaremoana power development scheme leakage through the landslide was reduced by the placing of a sealing blanket on the lake bed between 1948 and 1951. The recent discovery of additional leakage sources in the lake bed and a proposal by Ministry of Works and Development to seal them have led to this engineering geological study of the landslide and leakage through it. The landslide, previously mapped by Ongley (1932) and Anderson (1948), has been remapped based on aerial photographs. Three phases of involvement have been recognised which resulted in the deposition of 1.9 x 109m3 of an intact block of interbedded sandstone and siltstone of Tertiary age. Sliding was obliques to the dip of bedding (16o) along a plane above the left bank with an apparent dip of 6o and more than likely occurred during a very large earthquake. Leakage from the lake is controlled by the fissures and fractures at the front of the intact block formed on its impact with the debris of the earlier phases. Leakage from the lake is divided into 'warm' (primary) or 'cold' sources according to their depth of origin. The former accounted for > 85% of the leakage prior to lake bed sealing and has been reduced to approximately 4 cumecs. Present day primary spring leakage is controlled by the same zone of highly disturbed blocks in Te Whara Whara Bay as the leakage prior to sealing. Tracer testing is planned to confirm these leakage characteristics prior to any sealing. Examination of wider areas of the lake is unlikely to discover further significant primary leakage sources. Apart from Bush Spring the 'cold' springs should not be affected significantly by future sealing. The springs in the Mangaone stream catchment do not have Lake Waikaremoana as their source. No leakage from Lake Waikaremoana was encountered outside the area of the landslide.