THE VALUE OFVALUEENGINEERINGACHIEVING THE REQUIRED OUTCOMES IN PINEHAVEN STREAM IMPROVEMENT WORKS

K. Sliepen (GHD Ltd)

ABSTRACT

The Pinehaven Stream in Upper Hutt, Wellington, has a long history of flooding. The largest recorded flood event was in 1976, and in more recent years multiple properties and garages experienced flooding.

In 2016, Wellington Water commenced the Pinehaven Stream Improvement project. This project is being delivered on behalf of Greater Wellington Regional Council (GWRC) and Upper Hutt City Council (UHCC) which form the project’s Steering Group.

The aim of these improvement works is to ensure that during a 1-in-25-year flood event, floodwater stays within the stream banks. Additionally, during a 1-in-100-year event, flood water does not impact habitable floors. Detailed stormwater modelling was completed to assess various options and confirm the preferred approach. Following a challenging consent process, consents from GWRC and UHCC were granted. The scope of works included the removal of three flood prone properties to improve the flow path, 1,230 metres of stream channel widening, (which includes 321 metres of new retaining walls) two new culverts and 14 new domestic bridges.

The project budget was established in 2018 and as the project moved through the design and procurement phases, it became clear that the project budget was significantly lower than the amount required to complete the full scope of intended works. With the impacts of inflation on material supply and construction costs becoming an ever-increasing issue, value engineering can play a key role in achieving successful project outcomes. For the Pinehaven Stream Improvement project, several value engineering workshops were held to assess various initiatives with subsequent stormwater modelling used to validate potential options. It was through this process that large portions of proposed stream improvement works could be removed from the scope without adverse effects on the required project outcomes. This reduction in scope provided both cost savings and a more environmentally sustainable solution.

The project Steering Group considered the stream reaches that had the largest impacts on property flooding and prioritised the construction works using a staged approach. As additional project funds were made available, additional stages could commence. Stage 1 (Culvert Installation) was completed in June 2022, Stage 2 (Willow Park) is due to commence in February 2023 and Stage 3 (Blue Mountains Road) in July 2023.

This paper details the value engineering process GHD led with the support of the Steering Group to assess and prioritise the works. Through this process, the project team were able to achieve significant flood protection benefits while working within the available budgets. The sustainability of the approach also improved significantly without increasing the risk of flooding to other areas.