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

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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

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