Background reading · Social Sciences

When the river rose: the 2015 Whanganui flood

What made the Whanganui River flood in June 2015, what happened to the town, and how people protect floodplains.

On 19 and 20 June 2015, a slow-moving storm dumped heavy rain over the central North Island for hour after hour. The ground was already wet from earlier rain. All that water ran off the hills into streams and tributaries, and on 20 June the Whanganui River rose to the highest level anyone living could remember.

What happened

States of emergency were declared in Whanganui, Rangitīkei and South Taranaki. In Whanganui town, the river spilled over its banks into streets along Anzac Parade and other low-lying areas, and more than 100 households were evacuated. Upriver, slips closed roads and cut off river communities and marae. Farms lost fences, stock and soil. The clean-up took months.

Why so much water?

Rain amount and duration: a lot of rain falling for a long time over a big catchment.

Wet ground: soil that is already full of water cannot soak up any more, so rain runs straight off.

Land cover: forest slows rain down; bare or grazed steep hills let it rush into streams, carrying soil with it.

Hard surfaces: in towns, roofs, roads and car parks are impermeable, so rain pours into drains and streams within minutes.

Protecting a floodplain

Stopbanks: long banks of earth that keep the river in its channel. They protect towns from most floods but can be overtopped or break, as one did at Edgecumbe in 2017.

Forecasting and warnings: MetService forecasts heavy rain, the regional council watches river gauges, and civil defence sends alerts so people can move in time.

Slow it, spread it, sink it: planting hills and stream banks, restoring wetlands and building rain gardens let more water soak in before it reaches the river.

Floodplains were made by floods. The question is not whether the river will flood again, but how ready people will be, and how much room the river is given.

Sources and further reading

Written for Kōkiri Learn students in our own words. Check facts against the sources.

Used in: Te Awa Tupua: Rivers, Floods and People