Mercury Bay – The Tidal Frontier
By AHSO Kevin Tunley
It was 0600 on a rainy Tuesday morning in early June, when the Deployable Hydrographic Survey Unit (DHSU) departed for Whitianga. Our aim: to set up and calibrate two tidal sites, one in Whitianga, the other in Port Jackson.
Once in Whitianga, we checked in to our motel then made our way to the ferry wharf. The rain poured down as we assembled the 5m tide pole and programmed our automatic tide gauge. Once we got these in the water, this gauge will be measuring the tide here for the next nine months.
But how do we get them in the water? It was too deep for us, so we employed the much appreciated aid of the Operational Diving Team. They set the pole and the tide gauge transducer in the water and secured them with some Bandit wire to the base of the wharf, while POHST Karen Foster used her gymnastic abilities to secure them at the top.
The tide pole and gauge were set-up and running in no time, and after we said goodbye to the Divers, it was now up to the four members of DHSU to complete a 26 hour tidal calibration. That meant we sat out in the wind and rain to watch the tide, taking readings from the tide pole of how high the water was, every half hour - and every ten minutes around the turn of the tide!
Despite some quite unfavourable weather (the road to Whitianga flooded and closed behind us after we arrived) and a bit of swell, we still managed to nail the calibration. The next day, the weather eased and we were able to establish new benchmarks and finish our levelling runs, just in time to complete the tidal calibration. We downloaded the tide gauge, reprogrammed it, then pressed on to the town of Coromandel for the night. We spent that night sorting out the forms and processing the data - yes, we do work after hours!
Thursday morning came about and we were off, leaving civilisation and heading to Port Jackson at the very top of the Coromandel Peninsula, over some, shall we say, “less-than-optimal” roads. At least the weather seemed to improve.
We unloaded our bags into the shearer’s quarters, where we were going to stay, and made our way to the old concrete wharf. LHST Aaron Coster and I had to carry the two 6m poles from the farmer’s house to the end of the wharf; this was becoming a familiar task.
We put the tide pole together, put in some benchmarks and prepared the tide gauge. But we had no divers this time; that meant - don wetsuits and get in! It wasn’t too cold. There were a number of difficulties to overcome - the only adequate place to set up the tide pole faced out to sea. And the only accurate way to read the tide pole at high water was to climb a nearby goat track and read it through a set of binoculars. At night, and in the wind and rain.
We levelled all our benchmarks and sat tight through the worsening weather, as the calibration started. Twenty six hours later, the tidal calibration was done and with all our data gathered, it was thankfully time to head home. Just in time too, because the weather was getting worse. It was 4WD mode back to Coromandel dodging rockslides before we got on the sealed roads. We got back to Auckland late that Friday night, ready for the weekend!
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