21 April 2016

Jennie Clarke

Engineering Insight: Flood it

  • The flood protection barrier was installed as a bund and filled with 30,000 litres of water. Photos: BRANZ.
  • The water level was maintained at 300mm above the internal floor level for 24 hours. Photos: BRANZ
  • Strandboard test specimen: post-immersion, showing the resultant swelling and delamination. Photo: BRANZ.

Floods are devastating, costly and, it seems, on the rise. Scientists from BRANZ are investigating how building materials respond to flood damage in a bid to increase levels of resilience across New Zealand’s building stock.

By mid-August last year, insurers were counting the cost of extreme weather-related events across the country to the tune of $100 million. The June weather bomb and associated flooding, which affected Whanganui, Wairarapa, Horowhenua, Taranaki, the South Island’s West Coast and South Dunedin, accounted for half of that figure. Meanwhile, at the Building Research Association of New Zealand’s (BRANZ) Porirua test site, several houses were being systematically flooded and drained to learn more about the effects of water damage on building materials.

This three-year project, funded by the Building Research Levy, is the brainchild of materials scientist, Patricia Shaw. “It’s generally accepted that we’re seeing an increasing number of extreme weather events around the world and New Zealand is no exception. Tip in rising sea levels, higher tides and river levels, land level changes due to natural disasters like earthquakes, damaged or inadequate infrastructure, and infill housing that increases the amount of catchment and impermeable surface and you begin to comprehend the scale of what we’re dealing with.”

Disruptive at best, catastrophic at worst

Then there’s the human cost. “Floods are devastating,” says Patricia. “Possessions are lost, often much that’s irreplaceable. Home owners can be displaced for weeks or months at a time while houses are dried out and repaired, or demolished and replaced. Insurance provision for alternative accommodation is finite. The situation is disruptive at best, catastrophic at worst.” Surprisingly, despite these well-documented costs, a lot of current practice around drying-out and repairing houses post-flood is based on common knowledge rather than science. “Believe it or not, up to this point it’s never really been looked at in a systematic robust way,” Patricia says. “We’re now putting some science behind it, rigorously measuring the effect flood waters have on different materials.”

“A lot of current practice around drying-out and repairing houses post-flood is based on common knowledge rather than science.”

The aim is to arm designers and architects with better information about the best construction materials to use in flood-prone areas, so when there is flooding, there’s less damage and faster recovery. The same goes for home owners or builders undertaking repairs. Patricia says: “A lot of it’s about cost-effectiveness. Using a more robust Material A in your build or repairs may be more expensive than Material B but if it doesn’t have to be replaced after a flood, then you’re ahead of the game – especially when you consider if your home’s been flooded once, it’s likely to flood again.”

Improving building regulations is another key aim. While BRANZ is not a regulatory authority, it has a good working relationship with the Ministry of Business, Innovation and Employment (MBIE). As an applied research organisation, its findings often inform changes to the sector’s codes and standards.

Experiments and evaluation

Patricia and her team of scientists and engineers began with lab-based experiments on different flooring materials. It was as simple as cutting standard-size pieces and immersing them in water for different lengths of time (one hour, three hours, seven hours, 24 hours, even seven days), then measuring critical properties to determine whether or not the minimum standard for, say, strength, was still being met. Evaluation continued as the materials dried out. “While some materials performed better than others, after drying-out all regained enough strength to meet the standard,” Patricia explains. “So, then we looked at repeat immersions: if it was flooded two or three times, what then?”

At this point, the team realised they needed to broaden their scope and find out how different materials interact. “We could test insulation in isolation for its water shedding properties, which are generally very good, but the real question is, ‘What happens when it’s blocked in a cavity behind a plasterboard lining?’ We had to find a way to study building systems in their entirety: the effect of using this insulation behind this plasterboard, constructed in this way, versus other combinations.”

“This is where it all gets a little bit Mythbuster-esque. The team constructed four 2.4 metre by 2.4 metre houses to New Zealand Building Code requirements using standard building practices – then flooded them.”

This is where it all gets a little bit Mythbuster-esque. The team constructed four 2.4 metre by 2.4 metre houses to New Zealand Building Code requirements using standard building practices – then flooded them.

Two have concrete slab floors, two are on wooden piles; one of each has cladding fixed directly to the framing, the other two have a cavity system; a variety of insulation, framing, linings and claddings are used in different combinations. Questions are asked, observations made and measurements taken: how does the house as a whole dry out? What happens to the framing? Do you really need to remove the plasterboard and insulation to 300 millimetres above the water line? Is it better to force dry with heaters or dehumidifiers, or is it better to let it dry naturally? As for the flooding methodology, could the houses be craned into a pond? Would bund and steel tank options work?

In the end, the team opted for a much simpler temporary flood barrier system invented in Sweden: installed around a house, the metal frame and plastic lining keep floodwaters at bay. It had been used in Christchurch for red zone property waterproofing trials following the Flockton basin floods. In that instance, as for this project, installation was reversed; built facing the house, it allows for a temporary pond to successfully replicate a flood.

Before flooding, and to allow for data collection, the house walls are instrumented with a multitude of sensors positioned below and just above flood level, and higher up. Measurements include temperature in the internal framing cavity, relative humidity, and moisture content of the framing. “Results are even influenced by the amount of sun each wall receives, so they all must be included,” says Patricia. “It takes a good couple of weeks to get everything in place, wired and labelled.”

The house is closed up, the flood barrier erected and the temporary pond created using 30,000 litres of water to at least 350 millimetres above floor level. Leakage is re-circulated via a drainage system, maintaining the water level and creating realistic flood conditions by introducing sediment and dirt to the flow. Twenty four hours later, the floodwater is pumped out into a storage tank (topped up with collected rainwater; it’s re-used for each flood event), the flood barrier is dismantled and the six-week monitoring and data collection phase begins. Patricia and her team then go back in to collect visual results.

Further investigation

In the first go-round, over a hotter than usual February, the house appeared to be drying out. However, lining removal revealed mould growing on its framing-facing side—a real concern with damp houses. While the various types of insulation had dried out a lot, it was still wet along the bottom plate edge. Patricia and the team were surprised: “We thought it would’ve dried out more. Obviously, the water had nowhere to go when it got to that point but it prompted us to investigate drying options further. We took the lining off one wall—the classic response to flooding; on another we pulled the skirting board off and drilled holes behind, through the plasterboard and above the bottom plate; another, we left completely closed. So, will holes in the bottom of the plasterboard dry out the materials adequately? If they do, and it negates the need to replace wall linings in their entirety, remediation and a return to normality becomes easier, cheaper and quicker for the homeowner. It’s these sorts of variations we’re looking at.”

The stakes are high. Finding standard construction solutions to flood-damage problems varied by build-method, material and site might be a massive task—but the consequence of not doing so might see the economy swamped by a deluge of disruption on the near horizon.

Find out more

Learn more about this project, and see results as they become available, at www.branz.co.nz