Mohaka River power investigations : step 2 damsite (Willow Flat or Maungataniwha site) : preliminary engineering geological mapping and proposed investigations

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Hegan, B.D. 1981 Mohaka River power investigations : step 2 damsite (Willow Flat or Maungataniwha site) : preliminary engineering geological mapping and proposed investigations. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 357 11 p.

Abstract: Preliminary engineering geological mapping of the Step Two damsite near Willow Flat, as part of the investigations to determine the hydro-electric generation potential of the Mohaka River, has been completed. The area mapped was extended to cover an additional damsite downstream from the first choice at a prominent gorge (M42) cut through sandstone near the Maungataniwha farm. The downstream site will become an option if there is too great a loss in head between the M42 powerstation and the tailwater level behind the Raupunga dam. The geology of the area mapped consists of thick mudstone overlain by conformable sequence of sandstone, interbedded mudstone and sandstone, and in places, shelly limestone. The strata have a NE-SW strike and dip 20° SE. Three faults of local, but no engineering significance have been mapped near the M42 damsite. Landslides, identified upstream and between the two damsites, will probably pose the most serious constraint on the viability of the proposal. In this respect the lower damsite is seriously disadvantaged by being sited on what is at present judged to be a landslide, with a 1 km wide landslide immediately upstream. The upstream (M42) site is affected by only one smaller landslide 1 km upstream from the dam site. Future detailed geological mapping will cover both damsites and all landslides, but subsurface investigations are recommended for the upstream (M42) site only and the landslide at M43. One drive is recommended at the M42 site, but its exact location is dependant in part on ease of access. (auth)