Tides (or Beware Kayakers)

In the same way that seeing kite-surfers in your intended anchorage can be disconcerting, seeing a kayaking school practising through slalom poles in a channel of falling white-water that you must shortly traverse, could also be mildly alarming!

Alix and I are very used to sailing on the south coast of England, where a normal spring tidal range is about 4 metres. We have chartered a yacht on Vancouver Island (Canada), where we had to deal with a similar range, but one that creates whirlpools and eddies, as the flooding or ebbing waters are squeezed between the islands and rocks of the inner archipelago.

I have sailed around the Inner Hebrides, on a sleigh-ride of 7 knots of current, past the mouth of the Gulf of Corryvrechan, where the tides set up whirlpools and a standing wave that’s maybe a metre tall. And I’ve raced J80’s round-the-cans in the Bristol channel off Cardiff (which has the second largest tidal-range on the planet, at about 12m), where the races had to be timed at slack water. Otherwise the entrants would have ended up in Gloucester.

But in the Mediterranean, the crew of Missy Bear has tended to ignore tidal range as it’s often less than half a metre. We do experience currents, especially wind-driven ones, that can help you or hinder you on passage by a knot or so. But generally, there’s nothing really to worry about or to plan for. [Note: Where we are berthed right now, at Loutra, the tidal range is about 80cm, so you have to make your bow and stern lines long enough to cope with the rise and fall of the boat.]

That would change when we approached the narrow neck of the Evia channel at Chalcis. This channel is only about 20 meters wide, and its ancient wooden bridges were the only link between mainland Greece and the island. It was a very strategic point for the defence of Athens.

The Ottomans replaced the opening Roman wooden bridge with a fixed wooden bridge. When the Venetians saw it, they called it the ‘black bridge’ or Negroponte. The new Greeks build a metal swing bridge in the 19th century.

The 19th century swing bridge

The twentieth century saw this replaced be a sliding carriageway: each half is lowered by a foot or so and slid backwards and beneath itself. Very clever. When its working that it is.

As the bridge is so critical for road-borne traffic, the port authority only opens it once a day, and normally at night, once the car and lorry drivers are tucked up in bed. We poor yachties must book our transit, pay a fee and then sit listening on VHF Channel 12 into the night for the authorities to call us to get ready.

The other critical element for the operator's timing decision is the tidal stream – as it's much safer for yachts to travel through at slack water. The tide can rip and swirl through the channel (north-to-south then south-to-north) at up to 7 knots as the earth spins Chalcis past the moon’s full gravitational pull.

Kayakers enjoying the north-south tidal flow

There are also times of the month where the tidal stream is erratic and impossible to predict. Ancient Greek philosophers travelled here to study and explain the effect. They failed. When I talked to the Port Police the day of our intended transit, he said that he would be physically looking down into the waters to see what was going on, and then make a decision when to call us on the VHF. OK.

Alix was just about to come off listening watch at 23:00, when the Port Police radioed to say, sorry, but the bridge would not be operating tonight (Monday), but would be opening on Wednesday. Hey ho.

The two halvesdrop and slide backwards under the road

The material force of the current can be attributed to the geographical effect the island of Evia has on the mild tidal flow in the wider Aegean. Normally, the moon drags the sea water eastwards, but in the Aegean the flow heads northwards past Evia. The island splits the flood-tide: a small flow passes up the Evian gulf; and a larger flow passes, on a much longer journey, around the outer, western side and around the top of the island.

The smaller flow arrives at Chalkis first and causes the tide to rise here, and continue to flow northwards under the bridge. But then the larger flow eventually arrives from around the top of the island, raising the levels further to the north of the bridge, and then causing a huge southerly rush back through the gap. There can be a foot step-drop in water level as the northern gulf tried to empty through this tiny plug hole. The kayakers love it. And it’s a spectator sport for visitors.

And through we go

The good news was that, on Wednesday night, we received the call: “Miss Beer; get ready!” And twenty-minutes later we were motoring through, thanking the bridge operators, and waving at the onlookers high above us on the quay.