Image

Let The Water Flow: Why These Bridges Are Designed To Be Swamped

7 April 2021

Bridges traditionally sit above the water at all times, providing continuous access even in severe weather.

But the potential for flash flooding in two streams in the Kopua Forest in Maraetaha, Gisborne, meant an innovative solution was required to allow Aratu Forests Limited to carry out its harvest operations.

Existing fords across both streams did little to provide reliable day-to-day access. The catchment area receives about 2m of annual rainfall and up to 290m³ of water can flow down the Purupuruwhaka every second during a once-in-a-century flood.

After Bridge It NZ visited the sites and assessed the terrain, it was agreed that submersible bridges that were strong enough to withstand an extreme hydro load, were the most practical and cost-effective solution.

In this instance, building a bridge that will be overtopped by water multiple times a year, offers several benefits. It was far cheaper to build (about half the price) compared to a full height bridge designed to fully clear a 1/100 year flood. That option would have required extensive abutments involving costly materials and lots of time on site to construct.

The connecting forestry roads must have a practical gradient for heavy forestry machinery, so the submersible bridges were carefully designed to work in with the surrounding roading infrastructure.

Aratu Forests Roading Manager, Scott Weston-Arnold, says the company wanted a solution that had longevity and wasn’t going to get washed away easily.

“When we looked at a low-level submersible bridge option for the Kopua North site, we could see it better suited the characteristics of the catchment, with its rock boulder nature and high gradient. 

“It was a no brainer that, 12 months later, we also went for the same bridge in Kopua South.”

It goes without saying that both bridges needed to be incredibly strong – able to withstand the weight of heavy forestry equipment and fast-moving flood waters.

The bridge deck units for both bridges were poured offsite at Bridge It NZ’s precast yard in Mount Maunganui. Each bridge was then constructed on site, over two and a half weeks. 

For Kopua North, a 10.5m-long submersible bridge weighing 140 tonnes was designed with skewed abutments so it can be perpendicular to the stream and help channel flood water through a spillway. The cast in-situ abutments used 40m³ of concrete, weighing approximately 100 tonnes. The bridge beams consist of approximately 14m³ of heavily reinforced concrete weighing 40 tonnes.

At Kopua South, Bridge It NZ constructed a 10m-long submersible bridge with a seven percent grade applied to help tie into the road, making it more appropriate for fully-loaded truck and trailer units. Specifications for the cast in-situ abutments, bridge beams, and weight were similar to that for the Kopua North site. 

The Bridge It NZ team looked after the whole process for Aratu Forests – from design and consent through to construction.

“Having an engineer on site, conducting the catchment analysis, looking after the resource consent process and enabling a smooth resource consent authorisation gives us confidence that the jobs have been done right,” says Scott.

“The beauty of Bridge It NZ is they’re essentially a one-stop shop, with cost-effective, reliable pricing, and at the same time they allow us room to collaborate and work in partnership with them.”

The company worked with Aratu Forest’s roading contractor, BBL Contracting, and used their excavators to help install the bridge beams. BBL Contracting also completed the concrete block retaining work.

“We’re stoked with the results for both bridges —Bridge It NZ has provided us with permanent bridging solutions that helps us future-proof our operations,” says Scott. “Their team was good to deal with, they delivered a great solution and we’d definitely work with them again.”

Source: Bridge It NZ

Image Credit: Aratu Forests Ltd

Read More
Image

Australian timber framing industry gets a boost

Forest and Wood Products Australia (FWPA) is set to spearhead an initiative highlighting the advantages of timber framing in construction, promising a significant boost to its promotion efforts. First established in the Australian Forest Products Association’s Softwood Manufacturing Chamber, and funded by ...
Read more
Image

Who’s transforming our economy with renewable resources?

The opportunities for converting forestry fibre to renewable resources to replace fossil fuel based products are growing every year. The adoption of bioenergy and related technologies / products in New Zealand and internationally, sourced from forestry and wood products companies ...
Read more
Image

New trade agreement a boost for NZ wood processors

Increased export opportunities through the removal of tariffs for 41 wood products included within the ‘Agreement on Climate Change, Trade and Sustainability’ is welcomed by New Zealand’s wood processing and manufacturing industry. ‘Under brand New Zealand we have a leading story to ...
Read more
Image

Södra to become the largest producer of kraft lignin in the world

As a step in the strategy of getting more out of every tree, Södra is now establishing a new business to become a world leader in the production of kraft lignin. Södra’s production facility will be the first commercial facility ...
Read more
Image

NZ National Exotic Forest Description survey – data needed

On 1 April 2024, the Ministry for Primary Industries (MPI) sent out the 2024 National Exotic Forest Description (NEFD) survey. The survey is critical for informing wood availability forecasts, which underpin infrastructure and wood processing investment decisions, as well as contributing to a ...
Read more
Image

Unlimited potential of virtual reality for forest thinning

How a virtual reality (VR) and gaming engine could add another tool to the forestry training kit. A future where new foresters are trained to thin trees from the safety of a virtual reality forest before more advanced real-world training could be ...
Read more
Image

Growing the potential of NZ’s forestry sector in partnership

Forestry Minister, Todd McClay, has announced the start of the Government’s plan to restore certainty and confidence in the forestry and wood processing sector. “This government will drive investment to unlock the industry’s economic potential for growth,” Mr McClay says. ...
Read more
Image

North American DEMO International running in September

The Canadian Woodlands Forum (CWF) is excited to announce that registration is now open for its flagship event, DEMO International®, to be held outside the Ottawa/Gatineau area in the beautiful region of Venosta, Quebec, Canada, from September 19-21, 2024. This world-class event, ...
Read more
Image

Hydrogen powered airships collecting forest data

Kelluu, a Finnish company founded in 2018, offers commercial enterprises and government agencies with data-as-a-service, specialising in monitoring and providing real-time, actionable intelligence on power lines, roads and railways, as well as surveying and monitoring potential environmental issues associated with ...
Read more