18 | Northland Regional Council: Awanui Flood Upgrade T T Richard Loader Project reduces flood risk to area Matthews Park – construction of high-level spillway. REGIONAL DEVELOPMENT The Northland Regional Council’s (NRC) $15 million Awanui River Flood Scheme Upgrade is now in its closing stages, and will significantly reduce risk to property and assets from flood waters that have previously ravaged the area. The Awanui River runs through Kaitāia and out to the west. A secondary channel, the Whangatane Spillway, runs southwest to northeast from Kaitāia and cuts through the settlement of Awanui a short distance away. The spillway is also included in the upgrade. Traditionally, there has been a lot of flooding in Kaitāia township. The worst recorded flooding to date was in July 1958 which produced a flow rate of 220 cubic metres per second, which saw 1-metre standing waves on Kaitaia’s main street. “There’s a lot of low-lying housing in and around Kaitāia particularly, which are quite vulnerable,” says NRC Senior River Management Engineer Barney Brotherhood. “There’s also a lot of agricultural land outside of Kaitāia CBD that take a hammering in those flood events. The project upgrades the existing LOS (Level of Service) from a 1:30 year event to a 1:100 year event in urban Kaitāia and 1:10 year event to 1:20 year in rural areas.” The upgrade project commenced mid to late 2018 and was initially programmed over nine years. The first two years were pre-covid and funded ($2m) from targeted rates. From the beginning of covid, ($8.5) million funding was obtained from Kānoa, the government’s Regional Economic Development Unit, on the basis that the remaining seven years were compressed to three years. A further $4.5 million was co-funded by NRC. Project works included extensive modifications and improvements to stabilise existing stopbanks and allow the river to carry up to 15 per cent more floodwater. It also including reducing Awanui flood overflow to the Tarawhaturoa Stream, and upgrading Awanui River and Whangatane spillway capacity to accommodate the additional flow. Conceptual and preliminary design was completed internally by NRC with the help of Tonkin and Taylor. Hydraulic modelling was completed both internally and using expertise from Tonkin & Taylor and also DHI (Auckland). Detailed design was completed by Riley Consultants, with Haigh Workman Consulting Engineers and Barfoote Construction assisting with bridge design. “Following a competitive tender process Kaitaia Contractors Ltd and Far North Roading were awarded the earthworks contracts. Both contractors were well set up for river contracts, which is quite specialised. “They were supported by a number of sub-contractors. The project works were delivered despite the vastly compressed timeframe and weather challenges throughout.” From south to north, the geographic spread of the project was about 12 kilometres and involved approximately 450,000m3 earthworks comprising; ~6km stopbanks, ~5km benching, ~2.2km spillways. There has been ~1.2km scour protection (~15,000m3 rock), and ~200m timber floodwalls. “The geometric design of the spillways and benches was largely undertaken in-house with digital designs transmitted to contractors who utilised them in machine control systems on site, avoiding the requirement for traditional set out,” explains Barney. “Pre-works and post-works survey was all undertaken with high accuracy UAV (drone) systems, all in-house. We also utilised an existing region-wide LiDAR dataset, which provided an accurate surface for a geometric design baseline.” The project will deliver significant benefits to the community and businesses. For example, Juken New Zealand have a major industrial facility that makes triboard in the area, and the Timber Mill is located on a vulnerable bank of the Whangatane spillway. The upgrade delivered an improved channel alignment, increasing Spillway capacity and reducing the flood risk to the Timber Mill. Local Marae (Ōturū and Te Paatu) undertook Kaitiaki and planting roles. Two planting days were organised by Hapū, with input from local schools and community. To date, there are about 15,000 plants in the ground. www.hoskincivil.co.nz • Project Management • Project Delivery • MSQA • Asset Management • • RAMM Data Collection • Condition Assessments • Quantity Surveying •
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