The Mega Tides of Monmouthshire:
why so high and why here?
John Burrows
Speaking to a record-sized audience (81) for the group, John Burrows gave us a well thought through and effectively illustrated talk on this topic of mega tides and their local coastal impact.
First we were introduced to spring and neap tides and shown images of these at Chepstow bridge, where the tide can rise 14 m in 4 hours. The spring and autumn equinoxes give the maximum tidal range. The Bristol Channel has the second highest tidal range in the world, only exceeded by the Bay of Fundy in Canada.
Syzygy is the name given when the earth, moon and sun are in alignment. The effect of this alignment is to create a bulge of water on opposite sides of the earth; it is caused by the gravitational pull of the moon (66%) and the sun (33%). Although this gravitational pull affects both land and sea, only water is seen to move, producing a rise in the oceans of around 0.5m. However, when spread over the whole ocean, this means the tide brings 5 billion tons of water into the Severn Estuary between Cardiff and Sharpness. This huge body of water is 45 km across in the west but narrows to less than 10 km by the time it reaches Clevedon. When this 0.5m ‘wave’ reaches the coastline, it is magnified to 1 or 2 metres high as it funnels up the Severn Estuary. This is what causes the Severn Bore.
John then analysed the components of the two types of tide – the astronomical tide and the storm tide. Within the former, the four elements that affect the size of the tide are gravity, the continental shelf, the constriction of the coastlines and the resonance (or energy) involved. There are also four components in the latter, namely, the scale of North Atlantic ocean, SW wind movements, low air pressure and excessive rainfall. As the Severn Estuary faces the North Atlantic, when all these elements combine there is the potential for extreme tides, such as when an additional meter or two of storm tide is added to an equinox tide of 14m. John considered that this is what caused the extreme historical tides on coastal Monmouthshire, such as the major flooding and casualties of the 1607 event.
Finally, we were shown visual and chart information that illustrated the impact of tides at various locations up the Severn estuary. Cross section profiles showed how our sea defences protect against the 5-billion-gallon tidal surge twice every day. He also referenced recent interest in tidal power and barrage schemes, but considered these are unlikely to ever be built. Surprisingly, he said there was relatively little local and tourist interest in this mega tide phenomenon, except for the limited impact of the Severn Bore.
John Burrows has measured and studied the tides on the Monmouthshire coast at various times over the past 20 years.
Jim Handley
Image by freepik
