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The CityEdge Alliance’s fish-friendly multi-plate culvert at Tamahere on the Hamilton section of the Waikato Expressway – the project’s tangata whenua working group visit the site

Water, water everywhere – how the NZTA is caring for wetlands along the Waikato Expressway

Water is always going to be front of mind when designing and building a four-lane highway across the Waikato. Innovative stormwater treatments, challenging stream crossings and creating wetlands are all part of the remaining three sections of the 102 km Waikato Expressway.

The four-lane Waikato Expressway has been built in sections, with the last three currently under construction: Longswamp (5.9 km) is set to open late this year, with Huntly (15 km) due in early 2020 and Hamilton (22 km) scheduled for late 2021.

Hamilton culvert in 553 pieces

The Hamilton section of the expressway, east of the city, involves 17 bridges and five interchanges, and takes State Highway 1 (SH1) across lowlands and large gullies. One of the most challenging stream crossings is at Tamahere, where a link road is being built to connect locals to the new highway at the Southern Interchange. It crosses a deep gully, without a lot of working space.

The steel culvert was bolted together in a deep ravine

The CityEdge Alliance is building the Hamilton section and comprises Fletcher Construction, Higgins, Beca, Coffey, Hick Bros and the NZ Transport Agency. Their solution for the stream crossing was to install a multi-plate culvert to channel the stream through the gully and backfill to road level some 11 m above. CSP Pacific supplied 553 curved, galvanized metal plates which were fitted together and secured with 13,000 bolts.

The largest of the four wetlands created to filter stormwater runoff on the Longswamp section is between the expressway and Rodda Road

The finished culvert is 75 m long by 5 m wide, and the stream, which was diverted during the works, is now running through a fish-friendly rock maze.

When looking at options to protect the stream while allowing an expressway to be constructed around it, engineers first considered a concrete box culvert. “A concrete culvert was initially specified, but it would never have worked here as trying to excavate out a stream bed at the bottom of a gully, and then having to pour a foundation for the concrete sections, would have been almost impossible, not to mention costly in terms of managing groundwater and slope stability,” says Richard Langley, construction site engineer for the alliance. “It’s like slop in places, and a bridge would have been far too expensive.”

The steel multi-plate was the best option. The culvert was assembled onsite by bolting together the plates. Richard and his team were careful to maintain the cylindrical shape of the structure, firstly to ensure everything fitted together and also to ensure its strength. It took a team of eight people six weeks to install the culvert.

It was gradually buried in compacted aggregate which has formed the embankment for the new East West Link Road. The immediate aggregate surrounding the culvert was tested for pH, chloride, sulphate and electrical resistivity to reduce the risk of corrosion of the galvanized plates and fixings.

“We also had to do a lot of work to stabilise the banks before we began excavating the path for the culvert,” Richard explains. “This proved to be tricky because not only were we trying to retain an already failed slope, there was a site of cultural significance with a pa site on the ridge above this slope.”

Wetland emerges from old swamp

A watery haven for wildlife and native plants has taken shape as part of work on the Huntly section of the expressway. The 4 ha wetland off Evans Road adjoins the southern end of Lake Kimihia and has been developed out of an existing degraded swamp that had been grazed by stock for many years.

“The Huntly project team are very proud of the wetland. Our job was to turn it back into a high-quality wetland that supports a variety of wildlife and wetland plants,” says Transport Agency Waikato portfolio manager Darryl Coalter.

Auckland-Waikato Fish & Game have assisted right through the transformation and will be caretakers of the completed wetland.

It was a difficult works area as it was an old kahikatea swamp forest. The old logs were left to improve ecology/nesting habitat, and preload on the perimeter bund was carried out to ensure the bund levels remain constant.

The wetland (bottom left) at the southern end of Lake Kimihia on the Huntly section of the expressway – water birds are already settling in

The wetland has several zones that can all cope with varying degrees of inundation, says Darryl. “The largest part of the wetland is permanent open water to encourage ducks and other wetland bird species to make it their permanent home. This main body of water is approximately 3.7 ha and the remaining 0.3 ha consists of differing zones of riparian planting that can be on dry land during the summer and have wet feet during the winter.”

A weir keeps the wetland above the normal level of Lake Kimihia, a large lake under the care of the Department of Conservation. A screen keeps pest fish species out of the wetland. If the weir is topped by flooding, allowing pest fish to invade the pond, the weir gate can be opened to drain the site.

Ecologists carried out intensive fishing of the old stream that needed to be diverted during construction. Nearly 300 tuna (eels) and some native fish species were caught and transferred. Pest species caught, including 98 carp, were not so lucky.

Around 35,000 cu m of clay was dug out to form the open water area. The pond can hold up to 30,000 cu m of winter water capacity. About 40,000 wetland plants are going in, currently at 80% planted. The planting is designed to filter nutrients and sediment from the farmland catchment, and help improve the water quality going into Lake Kimihia. The work is part of an ecological mitigation plan for the Huntly project, developed by the Transport Agency in collaboration with Fish & Game.

The Huntly section, which takes SH1 east of the town across lowlands and over the Taupiri Range, is being built by a Fulton Hogan-HEB Joint Venture.

Plants help filter road runoff at Longswamp

Four large wetlands have been created to filter stormwater on the Longswamp section of the expressway, which is being built by 
Downer NZ. The roading project involves widening 5.9 km of the existing SH1 north of Rangiriri and strengthening local road networks to connect over the Whangamarino Road overbridge.

“The large, shallow stormwater holding and treatment ponds and grassed swales are designed to filter stormwater runoff before it reaches waterways,” says Downer NZ project manager Iain Fletcher.

A piped drainage system collects and attenuates water flows through hydraulic splitter chambers, discharging into one of four wetlands for treatment before flowing to open drains.

The old kahikatea swamp forest on the Huntly section – some of the old logs were left to provide further natural habitat

The size of the wetland ponds is based on the road runoff catchment area. “They are designed to moderate water flows through a series of planted weirs, allowing any sediments and contaminates to be retained before water is released to the open drain environment,” says Iain.

More than 15,000 plants have been used in the four wetlands and are a mix of several different species. Planting the wetlands has occurred as they were created and is now well established at some sites. Roadside swales and embankments have also been planted over the autumn and winter months ahead of the project wrapping up later this year.

Follow progress of the construction of the Waikato Expressway at nzta.govt.nz/waikatoexpressway and facebook.com/waikatoexpressway

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