Hamilton has a history of catastrophic floods.
John McCraw, founding chair of the Department of Earth Sciences at Waikato University, outlines this turbulent history in his excellent book, The Wandering River, published by the Geoscience Society of New Zealand in 2011.
McGraw explains: “The ‘plain’ that now occupies much of the floor of the Hamilton Basin was recognised by early geologists as a large, low-angle alluvial fan . . . The fan stretches from the Maungatautari gorge, near Cambridge, to Te Awamutu in the south, and to Taupiri and Orini in the north.”
Before the Oruanui eruption about 27,000 years ago, the Waikato River used to flow through the Hinuera Valley and the Hauraki Basin into the Thames Estuary.
“Over the next few thousands years, the bed of the Waikato River was steadily raised as vast amounts of eruption debris, largely unprotected by vegetation because of the effect of the cold climate of the time, were eroded into the river.
“Then the original, blocked outlet of the greatly enlarged Lake Taupo finally gave way. The lake level fell 75 metres as about 80 cubic kilometres of water poured down the Waikato River in a catastrophic ‘break-out’ flood, depositing more debris in the already filled river valley. Obstructed by the debris deposited in the Hinuera Valley, the Waikato River eventually overtopped the low divide near Piarere that separated the Waikato Valley from that of the ‘Maungatautari Stream’, a tributary of the Waipa River. The divide was rapidly lowered and the river adopted this new course through the Maungatautari gorge and Hamilton Basin,” McCraw writes.
“When the flood, heavily loaded with sediment, burst from the valley of the Maungatautari Stream . . . alluvium was deposited as flood plains that eventually reached almost every part of the Hamilton Basin.”
For thousands of years, the basin was a series of braided streams, rapidly changing course. Forest vegetation re-established in the headwaters of the Waikato River about 18,000 years ago. No longer did the braided streams regularly deposit so much sediment. Over time, watercourses began to carve into soils of the Hamilton Basin, slowly forming deeper channels that eventually became the Waikato River that he know today.
In 232AD (plus or minus five years), Taupo volcano erupted.
McCraw writes: “Lake Taupo was practically emptied by the eruption and it took about 20 years for it to refill. The outlet of the lake and the valley of the Waikato River for some distance below the lake were completely blocked by thick pumice and other sediments so that the lake waters rose some 34 metres above the present level. Then the temporary blockage was overtopped and gave way. The dammed-back water poured down the river, clearing the choked bed, and gathering yet more debris from erosion of the riverbanks. The breakout flood became, in effect, a giant mudflow with the consistency of porridge or wet concrete.
“It moved down the valley fairly slowly . . .
“At the site of Hamilton, the flood rose to within a few metres of what is now the main street . . .
“The flood probably lasted for several weeks or even months, and when it finally subsided the river valley was left partly filled with a mass of muddy pumice-rich debris, but the river quickly cut a channel down through this, leaving a layer of pumiceous gravel on the remnants of the low terrace. These are the terraces on which are situated the Fieldays’ property, the Hamilton Gardens, many Hamilton riverside suburbs, the St Andrews golf links and so on.
“We can still see the effect of this great flood in the steep, closely spaced ridges and valleys in the debris that was deposited by the flood on the low terraces at the Cambridge and Lochiel golf links.”
McCraw’s book costs $20 a copy from Bennetts Bookshop, at Waikato University. It’s much better value than a Hamilton council LIM report.
The book also gives extensive information on the instability of the soils in the Hamilton Basin. Geologically, the area is very young and still changing significantly. The Waikato River is carving a deep channel through the centre of Hamilton, which is causing riverbanks to erode. The river poses a much greater threat to the city than perhaps most residents realise.
Councillors received a warning of this ongoing riverbank erosion at their May 24, 2012, full council meeting. Council staff sought funding to do urgent engineering tests into riverbank stability below the Claudelands road and rail bridges, the council-owned Ibis Hotel, the Inland Revenue Department building at Nos 2 and 4 Bryce Street. This research will be reported back to councillors on September 27, 2012.
In light of McCraw’s research, it would be a bad idea to reorientate Hamilton towards the Waikato River, as the council intends. Millions of dollars of council money could be washed down the river in the city’s next catastrophic flood.
The Wandering River, by John McCraw (Geoscience Society of New Zealand, 2011), $20.