Preliminary engineering geological investigations for a flood detention dam in Korokoro Stream, Petone

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Hancox, G.T. 1979 Preliminary engineering geological investigations for a flood detention dam in Korokoro Stream, Petone. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 321 22 p. 1 map. 1 table.

Abstract: Engineering geological investigations have recently been carried out in Korokoro Stream to assist the Wellington Regional Water board with preliminary studies for a flood detention dam. A 25 or 30 metre high dam is planned at a site about 200 metres downstream of Korokoro Forks, with the aim of reducing the flooding hazard to SH3 and the Korokoro - petone industrial area. Geological mapping at the proposed site showed the foundation rocks to be closely jointed greywacke sandstone and argillite, with up to 8mm of superficial materials on the abutments and 1-2 metres of alluvium in the stream bed. The foundation rocks are moderately strong and relatively impermeable and are thought to be suitable for either a rock fill or concrete gravity dam. Colluvial deposits close to the dam site may be suitable as impermeable core material, but further investigations are required to prove the availability and suitability of these materials. Local sources of concrete aggregate or rock fill are not readily available and these materials would probably have to be obtained from a nearby quarry and carted to the site. The numerous landslides that occurred in Korokoro catchment during the intense rainstorms of 1976 and 1977 indicate that the sudden drawdown of floodwater in any future reservoir is likely to cause further instability in the ponding area. These problems can probably be avoided if the rate of drawdown is strictly controlled. In some areas, the superficial materials likely to be adversely affected by sudden drawdown could be removed. The Korokoro Fault runs through the proposed dam site. This fault is classed as not active and the possibility of fault movement during the life of the dam is judged to be very unlikely, but cannot be entirely ruled out.