09-Feb-08 Rissaga on the Canterbury Coast

Introduction

On the morning of Saturday 9-Feb-2008, I was rowing our dinghy around Horomaka Island at Port Levy when I noticed something very strange. At the southern end of the island, the water was flowing across the rocks like a river rapid, then the flow slowly reduced and reversed. At the time, there was no wind and the water surface was glassy. There was a slight swell of a few cm. It was an hour or so after high tide. Some other people were water ski-ing and they reported there was a remarkable amount of drift of their boat when they stopped to pick up their skier. Later in the day, we were swimming at the beach below our bach and we were swept 50 m or so along the beach. Then a few minutes later we were swept back in the other direction. The current was strong enough to make it hard to stand up and swimming against it was difficult. On the beach, there were no waves, but water was surging parallel to the beach. The height of the surge was only a few cm.

Coast to Coast Affected

This was the day of the Coast to Coast longest-day race. On the following Monday, The Christchurch Press published the following article:

By Jane Marshall
Christchurch Press
Monday, February 11, 2008

Race director Robin Judkins may have to introduce a new leg for the Coast to Coast - a final swim to the finish line. The annual multisport race, which traverses the South Island, was left underwater at Sumner beach late on Saturday when a surging sea flowed through the finishing chute. One minute the large crowd, which had flocked to see the finish of the one-day and two-day races, was watching the Red Checkers aerial display. The next, spectators were watching their feet as the sea raced up the finishing chute. Judkins said race organisers had been expecting an early high tide but were still caught out by freak waves which washed over the finish line, causing problems for the electronic time-keeping. Nine-times champion Steve Gurney, who was at the finish line on Saturday as a spectator, said he had never seen the tide swamp over the chute during his time involved with the Coast to Coast. "The paddle is usually regarded as the kayak leg, not the finish line," he said as spectators and race competitors splashed about in the water. Officials were quick to move, with safety concerns for those watching. They moved spectators flanking the chute further up the beach and also moved the medical tent. The women's longest day field were spared a few metres as officials brought the finish line inland to avoid the surf. Some competitors had earlier used the surf for a "super slide" as they timed their finish with an incoming wave.

How Odd!

What is the explanation for these strange events? In order to answer that question, we must resort to scientific measurements of waves, but first I need to give you a few definitions.

Ocean Waves

There are various types of ocean waves. They are identified by their period .

The waves observed on 9-Feb-2008 were long waves. There are several different sorts of long waves:

Wave Measurements

There are several wave-measuring instruments on the Canterbury Coast. There are three different types, and they measure different types of waves:

You may notice that none of these is suitable for measuring IG waves. For these waves, the period is too long for a wave buoy and too short for a tide gauge. Furthermore, they require an instrument sampling continuously at 2 Hz . There are several such instruments deployed in NZ: wave radar (Port Nelson), ultrasonic (Port Taranaki), and pressure transducer (Port Taranaki), but none in Canterbury.

The map below shows the sea-level recorders (triangles) and the wave buoys (rings) on the Canterbury Coast. By clicking on the name, you will get the record for a few days either side of 9-Feb-2008.

Weather

Here is the weather pattern that led to the event.
On 4-Feb-2008, Cyclone Gene emerged from the tropics and propagated rapidly southwards east of the International Dateline, 1,400 km east of Banks Peninsula.

Here is the trajectory of the centre of low pressure of ex-tropical Cyclone Gene plotted over the bathymetry.

Postulation

Here's what I think resulted in the event of 9-Feb-2008:
The deep depression propagating southwards far to the east of NZ generated large swell waves. As these swell waves propagated towards New Zealand they coalesced into groups with the largest waves in the middle of each group. The sea level under the largest waves was slightly depressed, but it recovered in the interval between the groups - this depression and recovery of sea level results in the far infra gravity (FIG) waves that we observe in the long-wave records. The FIG waves are bound to the swell, so when the swell peaks, the FIG waves peak also.
The propagating depression also generated a rissaga. The mechanism for this is not fully understood. In shallow water, a rissaga is generated when the speed of the propagating depression approaches the shallow water wave speed, but in our case the depression was in deep water and the wave speed was about 800 kph, or more than 10 times the speed of the propagating depression.
My opinion (not supported by any science yet) is that the propagating depression is like a huge ship that pushes a bow wave out in front of it and a wake behind it. The bow wave and the wake propagate away from the depression, and these are the rissaga waves we feel at the coast.

Conclusions

Rissaga waves generated this way are probably never going to be a serious inundation hazard because they will likely be up to a metre in height as a maximum, though they could cause inconvenience, as they did at the finish of the 2008 Coast to Coast race. But for navigation of large ships, they can cause problems with under-keel clearance. As such, they are an insidious threat because the weather system that generated them is hundreds or even thousands of km away and the local conditions could be balmy (as they were in Port Levy on 9-Feb-2008). Indeed, at Marsden Point, there is a real-time long-wave monitoring system that contributes to the Dynamic Under-Keel Clearance system ( DUKC ) used to bring large oil tankers in to the refinery.

The currents accompanying the rissaga waves that we experienced swimming at Port Levy have not been reported anywhere (that I am aware). Perhaps this is because measurement of such currents is difficult. Acoustic instruments (like ADPs and ADCPs) tuned to measure waves, can measure currents up to about 25 s period, but cannot resolve currents with periods longer than this. And acoustic and propellor-driven devices tuned to measure tidal currents cannot measure continuously at 1-minute intervals. Thus, better information on the currents that accompany rissaga must await developments in instrumentation.

Derek Goring
Mulgor Consulting Ltd
Ph: +64 3 942 5452
Email

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