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.
Guest post By Jasper Hoeve, visiting student from the University of Twente, Netherlands
I am a third year Civil Engineering student at the University of Twente, the Netherlands. As part of our studies, we have to do an internship at a company relevant to our field. I thought this was a good excuse to travel to the other side of the world. My family did not share my enthusiasm but I went anyway. NIWA invited me for a 10 week long internship to develop and apply a methodology for high river flow estimation under El Niño Southern Oscillation (ENSO), Interdecadal Pacific Oscillation (IPO) and climate change under the supervision of Roddy Henderson.
After some research on the internet I found out that NIWA is actually a rather big company. I was expecting that a company of this size would have a formal interaction culture. However, this was not the case. I could just walk into the office of my supervisor to ask any question. He would always have time to answer my questions or discuss some results. When my supervisor did not know the answer to a question, he would send me to one of his colleagues. I could simply enter their office as well, introduce myself and then ask my question. Everyone had time for me; it was a really nice working environment. The informal working environment resulted in me telling about my weekend, what parties I visited and what trips I undertook to my colleagues and supervisor. This was all received by them with a lot of enthusiasm and interest. It was not like what I expected when I started planning this internship.
Predicting high flows and flooding in catchments relies on historical runoff data. A stationary climate, one where there are fluctuations but no long-term trend, is assumed when calculating, for example, the 100-year return period floods. However, changes in climate can distort these calculated high flow events. This may cause errors within the assumed level of security against floods. This is the reason why I studied the effects of ENSO, IPO and climate change on the high flows in New Zealand rivers.
The figure below is an example of my work. Flood frequency curves for the Ahuriri River are plotted for different scenarios of climate change in 2090. As you can see, floods are generally expected to become more extreme. The most important conclusions from my work were that a lot of catchments in New Zealand are affected by the IPO and ENSO phenomena and 20 years of flow data is not enough time to discern the effects of climate change.
I really enjoyed working here and I hope I can visit this beautiful country another time, preferably summertime.