Waves

Three and a half years ago (July 2^nd, 1998) during a particularly violent storm that lashed the upper North Island,  a single rogue wave 12.3m (40ft) high from crest to trough was measured by a deep-water wave-rider buoy at the Pokohinu (Mokohinau) Islands. Waves more than 8m (over 26ft) in height persisted at the monitoring site for 12 hours during the storm. Until now, the biggest wave expected to occur in the gulf was in the 9m range. The conclusion is that the outer gulf is a lot more energetic than was ever really expected. The highest wave ever reported occurred in the North pacific in 1933. It was 34m (112ft).

The buoy monitors and transmits wave data to the Auckland University marine laboratory at Leigh, which sends the information on to ARC. to help in the development of a wave-climate strategy. Waves are basically the driving factor in the coast and if you understand waves you understand things like coastal erosion and hazards a whole lot better. There are a lot of benefits in being able to predict coastal erosion and changes in shoreline position. Rising sea levels and more energetic storm surge activity could possibly mean that in some cases spending dollars on coastal erosion control is simply a waste of  money.

Decisions on such matters become easier with hard information and robust models.

The rogue wave had a period of 40 seconds from trough to trough. We know its height. The length, period and velocity of the wave are directly related. The longer the period, the longer the wave - and the longer the wave the greater the velocity. A working rule is that the period in seconds multiplied by three gives the velocity in knots. This wave was therefore travelling at 120 knots or 220 kph. This gives some idea of the more extreme values that can be expected to occur in the outer gulf.

The height and length of growing waves is dependant on the initial sea state, velocity and duration of wind, and sufficient distance (called 'fetch') over which to allow them to build. In general, a thousand miles is sufficient for the largest waves to build, and sort themselves into swells. Surfers at Awana know that low pressure systems south of the Kermadecs and moving south-eastward are a cause of a peculiar type of sickness that inevitably leads them to take a day or two off work. Last January, a storm system north of Hawaii sent down a week of regular swells that disrupted businesses across the Auckland region.

As this is written, a heavy swell is arriving from a cyclone around Tonga.

As stormy waves move away from their source they become more rounded, symmetrical, and regular, and move in groups of similar size as swells. As such they can travel thousands of miles. The longer swells travel faster and so arrive at a coast first in a more orderly fashion than the shorter followers. As a rule, surf builds at a beach when a storm centre is moving toward it. The first sign of a declining swell is when the larger swell sets become inconsistent and the wave periods increasingly shorter.

When a swell reaches a coast the waves get steeper, higher and shorter as the bottom increasingly shoals, though the period remains the same. Although waves may be 'reflected' off cliffs, 'diffracted' around islands, 'refraction' of the wave as it conforms to the bottom contours is the most important effect.

The underwater topography which progressively lifts the wave is vital to a good surf break. Along with an offshore wind that smoothes the surface and holds the crest from breaking until the last minute, the only effective treatment for board riders is time.

Tsunamis, if you were wondering, have extremely long periods and lengths of more than 160 km between crests with speeds up to 800kph, and there is good evidence of them striking the Barrier, but that's another story.

Don Armitage ©

First published 15^th Jan 2002