What the construction industry can do to protect against flood damage

​The recent spate of flooding around the country has drawn attention to the way our infrastructure copes with the effects of heavy rainfall.

Flooding is the most frequent natural hazard here in New Zealand, according to the Ministry for the Environment. What can the construction industry do to minimise the damage it causes to our buildings and other structures?

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2017: The year New Zealand flooded

Over the past couple of months, New Zealand has been inundated with intense storms and cyclones. The Bay of Plenty town of Edgecumbe was evacuated after the Rangitikei River burst its banks due to heavy rain from Cyclone Debbie. Talking to Radio New Zealand, Whakatane Mayor Tony Bonne said the resulting damage looked like it had been caused by an earthquake rather than a flood.

In April, the arrival of Cyclone Cook caused slips, flooding and power outages across much of the Bay of Plenty, Gisborne and Hawke's Bay. Parts of the Coromandel Peninsula were also evacuated to avoid the threat of flooding.

The resulting damage looked like it had been caused by an earthquake rather than a flood.

Although less intense, March's flash flooding in West Auckland did significant damage to several homes and businesses and even caused part of New Lynn's main shopping area's footpath to collapse into a huge sinkhole.

Stephen Selwood, chief executive of Infrastructure NZ, told Radio New Zealand that the rainfall Auckland experienced in March was well above what the city's stormwater infrastructure could handle. He said that bringing the infrastructure up to a level where it can cope with such rainfall would cost $15-20 billion over the next 30 years, and that Watercare had more than $5 billion of work planned.

Updating Auckland's stormwater systems is therefore going to be a massive undertaking and require a lot of construction and engineering work in the coming years – work that many companies could be involved with. 

What do I need to know when rebuilding or repairing after a flood?

When beginning repairs on a flood-damaged building, the first priority is safety. Kapiti Coast District Council warns that buildings affected by floods are hazardous because of potential structural damage and the risk of electric shocks. In the immediate aftermath of flooding, power and gas supply should be disconnected until assessments have been carried out.

Once repairs are under way, the materials used in construction will determine what needs to be done. According to Build Magazine, gypsum plasterboard in particular doesn't tolerate being soaked and will have to be replaced if the flood has reached it. Composite wood like particle board or MDF tends to handle being submerged slightly better, but if the water causes the fibres to swell it will also need to be replaced. Electrical wiring may still be usable, although fittings and sockets will probably require replacement. 

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What can builders do to help avoid flood damage?

Minimising the risk of flood damage for new structures is mainly a case of smart planning. Level, the sustainable building authority set up by the Building Research Association of New Zealand (BRANZ), says that you need to make several important decisions before building in a flood-prone area: whether you can safely build on the site at all, and, if so, where you should locate the building to keep it safe from flood waters.

According to Level, construction companies can make the most of what drainage is already available by building on the highest part of the site and keeping clear of existing natural drainage channels. Additional drainage can be added to low-lying areas to help minimise potential water buildup.

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I made it to the other side of the New Lynn crater.pic.twitter.com/gnwdLDwGxA

— Josie Campbell (@josiecampbell) 6 April 2017

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Environmental factors behind flood damage

Flooding will continue to be an increasingly important issue in New Zealand, as climate change raises the likelihood of dramatic weather events. In 2000, BRANZ released a report on the potential effects climate change could have on the construction industry – the report predicts an increase in flood damage and severity and a decrease in the respite periods between serious floods.

Climate change isn't the only environmental factor to increase the risk of floods. Earthquakes can make areas more prone to flooding by shifting the ground level, which brings structures that weren't previously at risk into the danger zone.

Climate change isn't the only environmental factor to increase the risk of floods.

This is a phenomenon that has affected Canterbury since the 2010 and 2011 earthquakes, leading the Earthquake Commission to set up Increased Flooding Vulnerability land damage claims. This policy allows people whose property has been made vulnerable to flooding to receive settlements based on the decreased value of their property and the increased risk of damage to their buildings.

Although floods are a challenge that the construction industry can't ignore, they also provide an opportunity – not just in terms of the increased number of projects dealing with repairs, but also in the opportunity to rebuild in a way that makes buildings stronger and more prepared for future disasters.

Companies that want to take on this opportunity will need experienced, capable staff who can step in to cope with the demands of new projects. For permanent or temporary labour supply that's up to the task, talk to your local Tradestaff branch today.