1607 Flood

Jan 2018

1607 Flood – Storm or Tsunami?

Google ‘1607 flood’ and you will find a host of articles from august and learned bodies, such as the BBC, on the devastating Severn flood of the 30th of January 1607, and the debate about the possible causes. Eye-witness reports of ‘huge and mighty hills of water tumbling over each other’ are certainly suggestive, but what scientific evidence is available to inform our current knowledge? The January gathering of the Thornbury History and Archaeological Society invited Dr Mark Lewis from the National Roman Museum at Caerleon to pour cold water on one or other of the theories.

Being an archaeologist who specializes in all things Roman, Mark had often wondered why excavations at Caerleon always flooded at high spring tides. Surely those expert engineers of 2000 years ago weren’t stupid enough to build on flood plains? Upon investigation it turns out that rising sea levels are not a modern phenomenon. They have been rising steadily for several thousand years. The sea level in Roman times was in fact 1.75 metres below present day levels.

It’s logical that as sea levels rise floods become more common, and the Severn valley has always been susceptible. But there are floods and there are Floods. The Anglo-Saxon Chronicle tells us that in 1014 there were catastrophic floods in the Severn valley. Recent finds of debris from the Atlantic sea-bed in English bogs, and analysis of very high ammonium ratios in datable samples (comet tails contain lots of ammonia apparently) suggest that the 1014 event was indeed a Tsunami caused by a comet crashing into the Atlantic.

As sea levels rose further then defensive walls had to be built, and from Norman times it was often the monasteries that built and maintained them to protect their ever-growing land holdings. A great Severn flood in 1483 changed the course of English history when it stopped the Duke of Buckingham from transporting his Welsh army to attack Richard the third. The Duke’s rebellion failed. He was executed and his son Edward Stafford, builder of Thornbury Castle, succeeded as the third duke.

What are the chances that Henry the Eighth took flood defences into consideration when he dissolved the monasteries? Badly maintained defences probably contributed to the devastation of the 1607 event. Monmouthshire was particularly badly affected. The Bishop of Llandaff had reported that the diocese was in a parlous state, with people playing football in the churchyards during services. The flood was surely a sign of God’s wrath. The waters in the valley reached up to 9.5 metres above sea level and up to 20, 000 people were drowned. But what was the cause of this devastation? This is where it gets complicated.

The science of these events has grown in recent years and we have much evidence from recent hurricanes in the Caribbean. Reports from 1607 talk of storm followed by calm and then more storm. This is the effect of a tropical storm or hurricane going right overhead. Similar devastation occurred in Norfolk indicating a storm travelling north east. The calm eye of the storm is when the atmospheric pressure is lowest and corresponding sea level is highest. Modelling the tides of this period with a supercomputer has shown that the eye of the storm and high sea levels coincided exactly with an exceptionally high tide.

There is no evidence of earthquake or comet or other event in 1607 that could cause a Tsunami. Boulder trails in the Severn valley certainly exist, but are not dateable and are probably from previous Tsunami events. So the evidence points to an unfortunate combination of a tropical storm passing overhead with a very high tide. Incidentally, just in case you come across church brasses commemorating the catastrophic flood of the 20th of January 1606, this is because they used the Gregorian calendar and not the Julian calendar. The dates had to be adjusted before the supercomputer could do its stuff. Don’t ask me to explain. Like I said, it’s complicated.

Many thanks to Dr Mark Lewis for understanding all the science and explaining it to us in such an interesting and entertaining way.

The Thornbury Local History and Archaeology Society always welcomes new and occasional members. Details of our programme can be found on this website, the library or the Town Hall. Our meetings are on the second Tuesday of the month, held at St Mary's Church Hall beginning at 7.30pm. Visitors are always welcome at the society for the small charge of £3.