Protecting our cities from flooding

How many times have you heard this exact phrase (or similar):

Metservice has issued a heavy rain warning for _________. In the ___ hours from _____ to _____, up to _____ millimetres is expected to fall, with rain intensities reaching up to ___ millmetres per hour.

It is a common announcement during the weather presentations for One News and 3 News.

New Zealand has many towns built on the banks of rivers. Those towns were built there to benefit from the movement of goods up and down the rivers and to provide accommodation during the days of building big hydroelectric power projects. With the development of their floodplains there have been opportunities to expand across fertile land, but there have also been consequences moving into the flood channels of untamed rivers.

Over 150 years after it was founded, Whanganui found this out the hard way in the weekend just gone. The Whanganui River, a source of water in its upper reaches for hydro-electric power generation rose up to flood in an event that might not be repeated for another nearly 200 years. As the city cleans up, its City Council will be wondering what more could be done to protect from avoidable flooding. The tens of millions that this disaster is likely to cost might seem significantly less than the cost of flooding in post-earthquake Christchurch, but if the city is to live with an acceptable level of risk, Whanganui will have to ask itself some tough questions about land zoning.

Will it decide to turn much of the strip of land, say 50 metres wide on either side of the river, into a permanent park so as to enable the river to flood, but at the same time allow a land use for that strip where the excess can flow without affecting lives and livelihoods? Maybe. Perhaps if there are any really low lying zones, the City Council might buy the most at risk properties and relocate the dwellings. I have suggested this in Christchurch where large parts of the city sank during the 2010-11 earthquake sequence, resulting in a significantly elevated flood hazard in some parts of the city.

Prior to the weekend, flooding in New Zealand towns and cities is not new, and there have been numerous cases since European settlement (amongst others):

  • Christchurch was frequently visited by the Waimakariri River prior to its stopbanks being built
  • Invercargill was subject to significant flooding in 1984, that caused widespread damage
  • The Hutt River frequently floods parts of Lower and Upper Hutt during prolonged heavy rainfall
  •  Greymouth was subject to two 1-in-100 year flood events just four months apart in 1988
  • Heavy rain commonly causes flash flooding issues on the Kapiti coast and nearby districts

Regional Councils are often expected to judge a land use proposal and do not always get it right. Centre right politicians oppose the regulations that go with setting up land zoning, but it can be a quite effective tool for reducing risk by moving people out of areas highly frequented by strong natural events. Sometimes District or City Councils are not entirely forth coming about whether or not an appropriate Land Information Memorandum exists for particular properties.

Building extensive works such as the Manawatu flood control gates are not always successful in containing a river, especially if it has a shallow bed that floods easily. Various engineering approaches such as deliberately narrowing a river channel at a particular point to make the water go fast sometimes permits debris to be built up in unlikely places.

Improving our understanding of how floods work in individual hydrological systems is up to the Regional Council and the National Institute of Water and Atmospherics. With climate change, the potential for bigger rainfall events, longer and more intense events all exist. When heavy rain begins to fall, Regional Councils will already know how much to expect, when it is coming and they will monitor the rainfall. If they think there is a high probability of human activities such as a jet boat rally being disrupted, Civil Defence is put on notice. If a flood coming is going to pose a risk to lives and properties, Civil Defence and the emergency services will be told.

Whanganui was told that this was coming. They knew that the potential for damage existed, but no one knew what such a flood event as this one would look like. Warnings were issued and evacuations were made. But to their considerable detriment a flood event like this comes with a hefty – and muddy! price tag.

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