Upper Mangatawhiri Dam: Spillway Upgrade
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Client: Watercare Services Ltd
Sector: Foundations
Location: Hunua Ranges
Contract Value: $2.5 million
Completed: Nov 2007
Main Contractor: Brian Perry Civil


The Upper Mangatawhiri Dam is an earthfill embankment approximately 35m high and 494m long at the crest.  It is located on land owned and operated by the Auckland Regional Council at the headwaters of the Mangatawhiri River in the Hunua Ranges. 

Work Scope

Upgrade works strengthening the spillway chute, construction of a new cut-off wall (to protect the spillway toe from erosion), and repairing the upper spillway apron slab.

Work included:

  • In-situ reinforced concrete walls (1,000m³ each side being 150m long and ranging between 4.5m to 8.0m high) to existing concrete spillway

BPC Auckland provided both the management and construction staff for this project.

The end chute works were carried out in an environmentally sensitive stream bed. 

The use of silt socks was adopted rather than silt fences.  Their performance was outstanding and the arrangement of a cascade of "dams" into which the water was discharged proved to be very effective at settling out the silt and treating the Ph.  This ultimately led to no environmental discharges.