DG3 reappraisal

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Thomson, R. 1976 DG3 reappraisal. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 257 7 p.

Abstract: Three proposed Clutha River control schemes are to be reassessed, the object being optimum development of the Clutha Valley. Each includes the planned construction of a dam about 1 km upstream from Clyde but the elevations of the respective reservoirs are different. There are no known substantial defects in the foundations of the favoured site for the dam at Clyde, DG3/1. Drilling and mapping indicate that the dam will be founded in composite schist. A number of faults are known in the damsite area. The most prominent bifurcates north-east of DG 3/1; the branches pass within a kilometer on either side of the site. Recent movement can be observed along the fault trace 4+km north-east of DG 3/1. Surface and subsurface evidence at the site precludes the existence of marginal pre-existing river channels but one may diverge from the left side of the main gorge a short distance upstream. The lowest proposed reservoir is confined almost entirely to the inner gorge. Erosion will probably be confined to the shoreline. The intermediate and highest reservoirs will affect alluvium and slope debris around the peripheries and will, to a greater or lesser degree, saturate the toe areas of several extensive deposits of slope debris. It is not known weather these deposits were formed as a result of large-scale landsliding over relatively short periods (minutes, hours, days, weeks, months or even years) or as a result of creep over a much longer period (many centuries or thousands of years). It seems that most if not all of the deposits are stable at the present time, but the possibility of their being affected by renewed movement following the raising of water levels in reservoirs can not be ruled out on present knowledge. The probability of large scale landsliding, however is judged to be low in most places. (auth)