Johnson, J.D. 1986 Lower Clutha power investigations : engineering geological feasibility investigation of the Birch Island (C69) damsite. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 396 59 p.
Abstract: The Birch Island damsite is one of four proposed damsites being considered for development on the lower Clutha River between Roxburgh Hydro and Tuapeka Mouth. Conventional surface and subsurface investigations have been used to assess the geological feasibility of this site for a concrete gravity structure. Restrictions placed on the subsurface investigations have reduced the scope of the site assessment. Based on surface mapping and limited subsurface data, it has been concluded that rock at the Birch Island site will be adequate for a concrete dam foundation provided that: 1. the relaxed rock in the river channel area and deeply weathered rock in the right abutment area are removed; 2. the two fault crush zones which pass through the damsite are allowed for in foundation design; 3. extensive grouting across the site is carried out to seal potential leakage paths through the rock along minor defects. Because the conclusions are based on very limited subsurface information, further feasibility investigations are recommended to verify the interpretations made in this report. This report also includes comment on the seismotectonic hazard of the Birch Island damsite and on the stability of the slopes within the proposed reservoir areas. (auth)