Project overview

Location Malvern Street, Dunedin

Scope $2.7M

Date of completion October 2022

The Dunedin City Council required renewal wastewater works at the base of the road bridge under the Water of the Leith in Malvern Street, Dunedin. This complex project relied on expertise to repair and reinstate the waterways and maintain the natural state and ecology of water in The Leith, an important habitat for aquatic wildlife that also aligns with our purpose, being the only construction group owned by a trust dedicated to the conservation and protection of wildlife and heritage.

Overview:

The Dunedin City Council required renewal wastewater works at the base of the road bridge under the Water of the Leith in Malvern Street, Dunedin. The existing concrete protection had eroded, exposing and allowing damage to the sewer and bridge. Considerations of a confined site and significant flood risks were mitigated with a robust temporary works approach to complete this project successfully. This complex project relied on expertise to repair and reinstate the waterways and maintain the natural state and ecology of water in The Leith, an important habitat for aquatic wildlife that also aligns with our purpose, being the only construction group owned by a trust dedicated to the conservation and protection of wildlife and heritage.

The Leith is located within The Leith Valley in the northern area of Dunedin. This waterway is fed by a large and steep catchment area which creates a volatile working environment. While The Leith generally runs (in this area) at about .3 cumecs, it is known to raise to 8 cumecs in the space of one and a half hours, with peak flows in excess of 30 cumecs. The area around the bridge is a tight and defined area at the downstream end of a bend within the river, and with a bed drop above 1.5 metres in the area of the sewer pipe. This meant our approach was to adopt Safety in Design and construction methodologies which treated The Leith as a floodway and not a benign stream.

The construction of a major concrete floodway channel to protect the sewer main (located in the bed of The Leith) within a floodway, while managing the flow of The Leith through the works presented a high degree of complexity. We achieved this by devising the use of a flume placed 2.5 metres above the work area. The flume needed to have the ability to be moved from side to side within The Leith; it also had to handle up to 12 cumecs and be easy to dismantle and remove in the event of a flood.

Works included Traffic Management, Earthworks, Civil Construction, Pipelaying, Subcontractor Management, Waterway and Environmental Management.

An award winning project for Isaac Construction: