The river never sleeps

Written by      

John McCombe

The grey waters of the Rangitata River swirl sluggishly through the gorge at Klondyke in South Canterbury. A pair of wild goats graze on a rocky outcrop overhanging the river, their white faces occasionally nosing into the light breeze, alert to scent and sound.

Anthony de Joux, a hydrological officer with the Canterbury Regional Council, is winching himself along a steel rope 30 metres above the river. He is grateful for the tranquil conditions. The 60 kph blast that sometimes funnels down the gorge can turn his job into a knee-shaking ordeal. At such times, the metal cage that is his sanctuary can be blown almost perpendicular to the river.

Today, as he dangles a measuring device into the Rangitata at regular inter­vals, the torrent is tame and he gathers his data unevent­fully.

De Joux, and many like him, plays an important role in “taking the pulse” of New Zealand’s rivers.

Rivers are a vital element in the country’s geography. They drain the land, sustain agriculture, provide hydro­electric energy and are a recreational resource. When a river is healthy, the countryside and those who inhabit it prosper. When there are problems, the disruption can be profound.

A ruptured stopbank may inundate vast tracts of farmland and have fatal consequences for stock if not for people. Conversely, a low flow can close down irrigation schemes and leave water supplies depleted, with dire results. These are the river’s twin weapons: flood and drought.

Two-thirds of New Zealand’s population inhabits areas that are prone to river flooding, and 70 per cent of towns and cities with populations over 20,000 have a flood problem. Particularly vulnerable locations include Whangarei, Gisborne, Palmerston North, Hutt Valley, Motueka, Nelson, Blenheim, Christchurch, Greymouth, Mosgiel, Balclutha, Gore and Invercargill.

The threat posed by the Waimakariri River to Christchurch is the most fearsome. If the Waimakariri were to carve a new path south, up to 300,000 people could be affected, and an estimated $5 billion worth of damage wrought.

The first European settlers of Canterbury in the mid-19th century quickly encountered the wrath of rivers, and as early as 1868 the Canterbury Rivers Act had been enacted to provide a vehicle for flood relief. Similar legislation was soon established throughout the rest of the country.

By 1870, New Zealand rivers had claimed 1115 lives, and records show major flooding of the Hutt River in 1858 and of the Clutha and Waitaki Rivers in 1878.

Today’s dedicated monthly measuring of river flows aims to provide advance warning of problems that in the past wreaked their havoc unchecked.

Like general practitioners, hydrologists check for abnormalities in the “health” of their rivers. Even when they are off-duty, an elec­tronic “night-nurse” reports significant changes in the river’s vital signs.

This network of measur­ing stations is an army of silent sentinels, warning of impending crises. Nowhere is the network more useful than in Canterbury, where unpredictable braided river systems have schizophrenic personalities that threaten flood in one instance and abet drought in another.

The downstream sections of braided rivers are usually located on alluvial fans built up by the river itself. The fan is constructed by the river’s sweeping backwards and forwards across the entire fan area in a series of avulsions, or jumps.

The Waimakariri River is a classic case. It now flows to the sea just north of Christchurch, but 1000 years ago Lake Ellesmere to the south of Banks Peninsula was its estuary. Experts warn that it will be again!

The critical factor for Christchurch is that the bed of the river is at the same level as the surrounding land, and the river is contained only by stopbanks and extensive gravel extraction by roading and building companies.

In 1967, an avulsion occurred in South Westland when the Waitangitaona River near Harihari switched course to flow primarily through Lake Wahapo and out to sea at Okarito Lagoon. Subsequent downcutting caused the state highway bridge to collapse 15 years later. Hydrologists fear the river may avulse again within the next few decades, threatening the township of Whataroa.

The first indications of erratic river activity come from recording stations which send messages from to civil defence headquarters in the central city, triggering alarms when extremes of flow are registered.

Hundreds of these hydrological sites work in harmony with automatic rainfall and water-level recording instruments in the mountains, on the plains and at rivers, lakes and tidal areas.

Water temperature, a range of water-quality indicators and sediment concentrations are also measured at numerous sites. New Zealand rivers carry a staggering 400 million tonnes of sediment from land to sea every year. Sediment yields from the Cropp River in the Southern Alps and Waiapu River near East Cape are among the highest in the world. The Waiapu, for instance, carries 35 million tonnes of sediment per year the equivalent of a five-tonne truckload every 12 seconds!

Sediment load consists of inorganic material ranging in size from finest clays through to silt, sand, gravel and sometimes boulders. When deposition occurs in hydro­electric power reservoirs, storage capacity is lost. When it imposes itself in a catchment system, water quality diminishes, which, in turn, has an impact on biota and terrestrial ecosystems, public water supplies, irrigation and recreational activities such as fishing and boating.

High temperatures and low flows result in detrimen­tal growth of periphytons such as algae, and the accumulation of organic detritus. Larger plants such as milfoils and pond weeds invade and choke the water system.

Severe drought brings a death sentence to most of the waterways’ aquatic biota such as trout, salmon, native fish and and some invertebrates. There will often be a change in the species composition of a section of river. For example, snails, riffle beetles and a few species of caddisfly are particularly resistant to high temperature.

Drought has been an economic curse to New Zealand since its earliest days. Ironically, one of the first droughts on record occurred in an area re­nowned for heavy rainfall the West Coast of the South Island. In 1887, goldminers were obliged to stockpile paydirt as there was insuffi­cient water to conduct their sluicing operations.

Since then, there have been infamous droughts in various parts of the country. The Northland dry spell of 1945-46 pushed dairy production figures 76 per cent below the norm. In the last 25 years, Canterbury farmers have confronted critical drought conditions in five growing seasons.

Hydrological stations also provide data for research on the environment and resource management. Their information is essential in providing a factual basis for those seeking to make decisions on flood protec­tion, ecological concerns, forestry, agriculture, electricity generation and a wide range of commercial and recreational matters.

Half of the country’s hydrological sites are managed by the National Institute of Water and Atmospheric Research (NIWA), and the remainder by regional councils. Regional and district councils work in partnership with NIWA on research projects, and are clients for its specialised services.

The Canterbury Regional Council’s group manager of environmental monitoring, George Griffiths, sees his region’s 240 water-level recorders and automatic rain gauges as “an elaborate nervous system, sensing the condition of the water resource throughout the region at any given time.”

Staff are always on standby in case an alarm bell sounds to warn of an impending flood. They are rostered to take calls at any hour of the day or night, so that a flood-control centre can be swiftly set up, and information and advice rendered to civil defence personnel and police.

Those directly affected by a forthcoming flood, particularly farmers, are provided with an early warning.

“The procedures are well established. In round figures, we have 24 hours from the time a flood-inducing rainfall peak occurs in the mountains till the floodwaters reach the coast,” says Griffiths. “In a year, we probably send out more than 20 flood warnings. Many of them occur in November, during Canterbury’s nor’west season and the spring snow melt”.

At these times, hydrological staff patrol stopbanks looking for signs of break-outs which could threaten surrounding areas.

“The big cause for concern in Canterbury is the Waimakariri, because Christchurch is right down the gun barrel from it, so to speak,” Griffiths says. “However, also at risk is Rangiora, because of the Ashley River, or Ashburton, because of the Rakaia River. In fact, any town located on a river plain benefits from the security network that has been set in place.

“Many Canterbury rivers are not entrenched. If the floods are substan­tial enough, these rivers can simply change course. Braided rivers, in particular, present a great worry and a danger. They can return to an old channel occupied hundreds of thousands of years ago. The Waimakariri has changed course many times in its geological lifetime. For our convenience, we have chosen to confine it to one particular channel on its fan. We are doing everything physically, economically and socially to stop it but, one day, it will do a Houdini on us and break out, arriving in Kaiapoi or Christchurch.”

River Data had Been collected in one form or another for 100 years. The first water-level recorders were imported in 1898 by the Public Works Department. Today, there are close to 1000 permanent water-level stations in operation.

Manual raingauges, read once daily, have been in use for more than a century, while chart-recording instruments, which measure rainfall continuously, were introduced in the early 1900s. They have since been replaced by electronic measuring devices, of which there are more than 300 around the country.

Radio telemetry of data from recording stations was widely adopted during the 1980s and, with the development of cost-effective compu­ter systems, data processing and storage can now be done on personal computers.

Most of the data collected by NIWA and regional councils is archived in a national database in Wellington and is publicly available.

While accessing the results is a simple matter, gathering the data is not without risk. In some areas, the most efficient method is by jetboat. Using a small winch on the boat’s bow, the hydrologist can take measurements of river depth and velocity that would otherwise be impossible to gather.

For one NIWA employee, how­ever, this task nearly proved fatal. While working on the Tekapo River, his jetboat was driven on to rocks by the fast current. The craft disinte­grated around him and he was sucked beneath the water. Unable to fight his way to the surface, he inhaled water and lost consciousness. Miraculously, with his lifejacket keeping him afloat, he was washed ashore at a horseshoe bend about 6 km downstream. Bruised and battered, he was lucky to have escaped with his life.

Anthony de Joux, too, knows the wrath of a river rampant. As he surveys the unthreatening flow meandering past him, he recalls the enraged Rangitata of 1995, whose level rose 10 to 15 metres above its norm.

“It’s running at about 200 cumecs [cubic metres per second] now. At the time of the ’95 flood, it was around 2000 cumecs,” he says.

“There is a flood risk to farmers downstream at anything from 800 cumecs to 1000 cumecs. Once it exceeds 1500 cumecs, it can start going through stopbanks into pad­docks and causing widespread damage.”

The Rangitata tops 1000 cumecs once every two or three years, but only once a century is it likely to rise to the 2000 cumec level.

Hydrologists operate on the principle that there is comfort in knowing the extent to which a river can inflict harm.

“If you know what a river can get up to, it enables you to give more warning to those downstream,” says de Joux. “With big floods, riverbeds may change, and flows can subse­quently change for various levels. That’s why we gauge them every month. We want to make sure the riverbed hasn’t altered.”

Month by month, the file on rivers like the Rangitata swells, and year by year their personality is profiled. The story which unfolds makes fascinating reading for those who can read between the waterlines.

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