The North Sea seiches
By Professsor Michael Rowan-Robinson, Head of Astrophysics at Imperial College, London and President of the Royal Astronomical Society 2006-8
the open ocean the tides have a simple twice-daily pattern, with two
high tides and two low tides every 25 hours or so, mainly driven by
the moon. Things become much more complicated when this tidal wave impinges
on a relatively shallow, almost-enclosed basin like the North Sea.
As the North Sea responds to the twice-daily tide driving in from the north of Scotland and also through the Straits of Dover, the wave-pattern breaks up into three seiches, one stretching from the north of Scotland to the border with England, one from the border to the Wash, and the third from the Wash to Dover. At the ends of a seiche, say at the Wash and at Dover, there are dramatic tides, with 4 metres difference between high and low tide. In the middle, at what’s called the node, (roughly at Lowestoft), the tides are much less dramatic, with only 1.5 metres between high and low water. Of course we still see the twice-daily tides everywhere but the seiches are superposed on this and control the amplitude of the tides.
But that is by no means the end of the story. Because the earth is rotating, the water is subject to the Coriolis force, the force which tends to make bath water go down the plug-hole in an anti-clockwise direction in the northern hemisphere. More importantly it’s the reason the violent winds in hurricanes (northern hemisphere) rotate in anticlockwise spiral, while in typhoons (southern hemisphere) they rotate clockwise. The effect of the Coriolis force in the North Sea is that our seiche doesn’t just slosh backwards and forwards between the Wash and Dover, it also circulates anticlockwise about a point midway between Lowestoft and Amsterdam. So the sequence of high tide times goes Yarmouth, Lowestoft, Southwold, Aldeburgh, Harwich, Southend, Ramsgate, Dover, Ostend, the Hook, Groningen. This is called amphidromic circulation. It is responsible for the famous scouring tide which strips the Norfolk and Suffolk beaches and dumps them at Shingle Street.
The harbour at Blackshore can also be thought of as a shallow basin and should have a resonant period of about ten minutes. In stormy weather the wave pattern impinging on the harbour entrance could have a component of this period which has travelled long distances. This would drive the water in the harbour surging backwards and forwards every ten minutes or so in a harbour seiche, affecting moored craft. I wonder if this is known Blackshore phenomenon?
© Prof. Michael Rowan-Robinson
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