Cook Strait’s wild seabed

Press, 11 May 1984, Page 14

Cook Strait’s wild seabed

From the Department of Scientific and Industrial Research

“Cook Strait was a far quieter place 20,000 years ago,” says Lionel Carter of the D.S.I.R.’s Oceanographic Institute. Sea level was 120 metres below its present mark, and a bridge of land stretched from Nelson to Taranaki, linking the North and South Islands. Cook Strait became the entrance to a huge bay instead of the narrow funnel between two opposing oceans. When the sea level rose again and cut the land bridge, the fierce tides of contemporary Cook Strait began to push back and forth through the narrows.

This is the harsh environment which three submarine power cables cross to bring electricity from South to North. The cables have suffered damage, and repairing them has been costly.

To understand the conditions in which the cables must survive, Lionel Carter and Keith Lewis of the Wellington-based Oceanographic Institute, have concentrated a research programme in Cook Strait, where the cable corridor stretches from Fighting Bay in the South Island due east to Oteranga Bay in the North. “People think that once you get down below sea level, everything is quiet,” says Dr Carter. But the bottom of Cook Strait is almost as hostile as the surface. The rugged hills of the adjacent dry land are matched under the Strait, where the seafloor is shattered by geological faults and eroded valleys. New Zealand lies across the

boundary of two of the vast crustal plates which compose the earth’s surface. The fault-lined landscape and frequent earthquakes result from the grinding of one plate against the other. Under Cook Strait, erosion by the powerful tides has combined with faulting to create a region of submarine cliffs and canyons which plunge to more than 1000 metres deep a few kilometres south of Wellington Harbour. At the time of the landbridge, the strong tides were absent. Rivers like the Manawatu and Rangitikei meandered across the coastal plain to dump their loads of sediment in the bay that was the ancestor of Cook Strait.

The products of erosion — mainly sand and mud — are now swept through the strait and down the canyons. Earthquake shocks can make the sediment accumulated on the canyon sides move downslope, rather like a landslide.

Sometimes the slide stays intact. At other times, slides mix with water to form a slurry — denser than the surrounding seawater — which rushes off down a submarine slope, travelling tens of kilometres before coming to rest.

The water movements in Cook Strait are controlled basically by the tides and the weather, says Dr

Carter. They gain their great power partly from a quirk in geography.

The narrow strait acts rather like the nozzle of a water pistol to constrict and accelerate the tidal current. Similarly, the winds speed up as they squeeze between the mountain ranges of the North and South Islands.

The speed of the tides is also affected by differences in the times of high and low water around New Zealand. When it is high tide in western Cook Strait, it is nearly low tide on the opposite side. As a result, water rushes “downhill” to the eastern side.

On the surface, the tides flow at up to six knots — the pace of a fast river. Even 300 metres down, water is moving along at two knots.

In 1982, divers from the Oceanographic Institute had to fight those currents when they helped to find a leak in one of the cables. They pinpointed the leak 1675 metres out from the North Island terminus at Oteranga Bay. Here, the cable was suspended above the seabed for 15 metres between two rocks. The tidal currents had swung the cable back and forth by at least a metre, wearing down a patch of rock. The weather — especially storms — can act on the tides in various ways. A wind blowing in the direction of the tidal flow strengthens current speed. Blowing against the tides, wind reduces the speed of the current and builds up large waves. Winds may also pile up water on one side of the Strait, thus reducing or increasing the “downhill” current effect. A severe storm — especially a southerly — generates a swell which can stir sediment off the seabed down to 120 metres below.

Lionel Carter is well acquainted with weather in the Cook Strait. On his last research cruise there in 1982, the D.S.I.R. ship Tangaroa received no less than 60 gale and storm warnings in two weeks. Nevertheless, scientists still manage to collect a wealth of information on the waters and floor of the strait, using a wide range of instruments and techniques. The echo sounders shoot an electrical sound pulse down from the

ship, record how long it takes to bounce back up, and so trace the depth of the sea floor. Seismic survey systems put out a pulse powerful enough to penetrate the bottom sediments until it bounces off harder substances below.

Such surveys are important in providing a picture of strata beneath the seabed. They enable researchers to map in detail the thickness of sediments covering the seabed, and to find the faults and folds disrupting the sediment cover.

The scientists can tell that the landbridge opened and closed at least five times in the past two million years, as climate changes alternately froze the world’s water into the polar ice caps, or released it across the earth again. On each occasion a layer of sediment accumulated, which was later tilted by earth movements. When the water rose again, currents sheared off the sediment, marking each cycle of the rising and falling oceans.

The research .ship also tows sidescan sonar — a submarine device which sends out sonic pulses across the seabed. These pulses reflect off irregularities to produce the submarine equivalent of an aerial photograph. Dr Carter says that sidescan sonar was used successfully to map the sea floor near the entrance to Wellington Harbour, where an extension is proposed to the Moa Point sewer outfall.

The research ship typically takes up station at a score of selected sites around the Strait to study the seabed in more detail. Deep sea cameras recorded the life and structure of the seafloor, which ranges from gravel-strewn biological deserts in the current-swept narrows to muddy zones rich with life in “quiet” backwaters. Grabs and dredges take sediment samples from the seabed and bring it up to the ship for geological and zoological analysis. More sophisticated is the corer. A steel barrel with a plastic sleeve

is lowered and rammed into the seafloor with a 400 kg weight. It is drawn to the surface holding a cross-section of the top five metres of seabed. From the minute fossils encased in the core layers, scientists can read off a history of the seabed over the past 20,000 years or longer. To study the effects of the tides and storms, instruments are moored for up to three months at selected sites on the seabed. They record current speed and direction at different depths, together with water temperature and salinity, and take time-lapse photographs of the bottom. The entire mooring lies beneath the surface, anchored to a 1500 kg anchor weight by an acoustic release. The ship retrieves the instruments by sending out a coded sonic signal which commands the release to cut the equipment loose from the weight. It’s always a relief when the mooring appears on the surface,

says Dr Carter. Two sets of instruments were lost before acoustic releases were available.

The continuing investigations into Cook Strait show that the seabed is being altered by powerful geological and oceanographic forces. How and when these processes operate are vital questions. A submarine pipeline, for instance, carrying natural gas from the North to the South Island, would be subjected to strong tidal currents, which could scour the seabed beneath the pipeline or bury the structure under sand.

Earthquake shocks could not only stress the pipeline but also trigger damaging submarine slides. Dr Carter says the risk must be

kept in perspective. It is probably acceptable to build a pipeline with a design life of 25 years close to a fault which moves once every 500 years or so. But a risk remains. Nature does not work to a strict timetable. A fault could move this year, move again next year, and then remain dormant for the next millenium. Lionel Carter was due to make another research voyage in Cook Strait last month, but Tangaroa’s hull is now unseaworthy after years of buffeting around the waters of New Zealand. She has been laid up. Further investigations of Cook Strait have had to be delayed for lack of a suitable vessel.