By Daniel Collins
It would have been hard to miss the deluge that washed over the country this past week. Otago and Canterbury were particularly affected, with flood damage in Waitaki District estimated at about $1 million. Minor flooding also occurred around Christchurch, with the lower reaches of the Avon, Heathcote and Styx Rivers overflowing their banks.
To put these high Christchurch flows into context, on late Tuesday afternoon I drove out to two flow gauging sites on the Heathcote River, one at Ferniehurst St (managed by Christchurch City Council) and one downstream at Buxton Tce (managed by Environment Canterbury and Christchurch City Council) and filmed the flows. See the footage below (top = Ferniehurst; bottom = Buxton). In many places, the water levels reached just shy of the top of the bank or beyond; a portion of the road near Buxton Tce had been flooded. There’s some background material on Heathcote River flooding here.
The river continued to rise until about 9 pm, when it surpassed 8000 L/s at the Ferniehurst site and nearly reached 20000 L/s at the Buxton site. During the 28 seconds I was filming at Buxton, almost eight 40-foot shipping containers’ worth of water passed under the footbridge.
The hydrographs below (data thanks to CCC and ECan) show how the discharge in the two Heathcote River sites rose over the over the week prior to the 14th, and how these peaks compare with historical data. Flows this high don’t last for long. For only 0.02% of the record time have flows ever been higher at Buxton. In the last 20 years, there have been five other flood peaks of this size or larger at Buxton Tce.
You might have noticed that the Buxton Tce site has more than twice the peak flow of Ferniehurst, which responds quite gradually. The Buxton Tce site is a few km downstream of Ferniehurst; Bowenvale Stream joins the Heathcote River between them, and is the main source of extra flood water. Bowenvale Stream drains a steep Port Hills valley that reaches up to the Summit Road and includes part of Victoria Park. Flood waters run off rapidly from the Port Hills, and cause most of the rapid rise and fall at Buxton Tce.