Waiwhetu Stream

 

The lower Waiwhetu Stream is currently undergoing a major clean-up. The project has four goals:

• Removal of contaminated sediment
• Increased flood protection measures
• Infrastructure upgrades
• Ecological enhancement of the lower Waiwhetu environment

The project is a joint venture between Greater Wellington Regional Council, Hutt City Council and the Ministry for the Environment.  Construction company Brian Perry Civil has been selected to perform the full works.  CPG and URS are the lead consultants on the project.

What will the works involve?

The proposed clean-up works will take place in the section of Waiwhetu Stream between Bell Road and Seaview Road. Approximately 12,000 m³of contaminated sediment will be safely excavated from the stream bed and disposed of at Silverstream Landfill. The stream bed will be tested to ensure the material requiring remediation has been removed.

The flood protection works will involve widening and deepening the stream channel from Bell Road Bridge to the stream mouth, to allow a greater volume of water to be conveyed.  At the same time, we will take the opportunity to upgrade related infrastructure, such as sewer crossings and stormwater outlets.

The channel widening and deepening works (along with the removal of contaminated sediment) will reduce the flood risk to surrounding properties.

Native trees and shrubs will be planted along the banks and a wetland for fish habitat will be created to increase the biodiversity of the area and encourage the development of a healthy ecosystem.

What are the risks?

Release of contaminants into the stream

Excavation of the contaminated sediment will be undertaken in one ‘cell’ at a time.  The cells are created by sheet pile walls constructed right across the stream channel. These will contain any mobilised sediments within the excavation cell.  Three sedimentation cells will be constructed downstream of the excavation cell to let any sediment that has been stirred up settle to the stream bed, so it will be kept out of the stream flow.

Silt curtains will also be installed upstream and downstream of the work cells to trap any remaining sediment.

Flooding

We have deliberately timed the clean-up works (which requires blocking-off the stream flow) to coincide with the historically lowest annual median flow of the river. The channel widening and deepening works downstream of Seaview Road Bridge will be undertaken in the wet (without cutting off the stream flow), therefore flood risk will be minimised. We will constantly monitor the weather forecasts during the works, and if a large rainfall is predicted, a Flood Risk Management Plan will be activated to reduce the flood risk to the local area.