AS A child growing up in Newport in the 1970s, I was told, as many children born before and after me have been, that Newport had the second highest tides in the world.

Unfortunately, as I have got older and hopefully wiser, I have realised that this claim to fame of my home city is sadly spurious, as is the same claim to fame apparently told to many children of their home towns and cities across the Severn Estuary and the upper reaches of the Bristol Channel.

So how high are the tides at Newport, how high are they relative to other locations in the world, and why do people from Newport think they are the second highest in the world?

Before we answer these questions though, we first need to consider the terminology.

To start with, tides don’t really have a height, they have a level.

These levels are commonly referenced to a local datum called Chart Datum, which varies at different locations.

This means, for example, that the level that tides are referred to at Newport is higher than it is at Cardiff or Avonmouth, yet lower than it is at Swansea.

It is therefore more practical to reference a tide level to a consistent datum.

This datum is known as Ordnance Datum, which is approximately mean sea level, and is what we refer to as levels of sea walls, or heights of hills and mountains etc.

So, for example, if we say Pen y fan is 886m high, what we are actually saying is that it is 886m above Ordnance Datum.

South Wales Argus: Moody: Black Rock at Portskewett looking towards the Prince of Wales Bridge. Picture: Natalie Chaston, South Wales Argus Camera Club.Moody: Black Rock at Portskewett looking towards the Prince of Wales Bridge. Picture: Natalie Chaston, South Wales Argus Camera Club.

Another issue is what we mean by “highest tides”.

Tidal levels in many tributaries of the Severn Estuary, and the upper reaches of the River Severn are considerably higher than they are at Newport, yet nobody talks about them having the “second highest tides in the world”.

I suggest what we actually mean is the tidal range, which is the difference between the highest and the lowest level of the tide.

However, this definition is also complicated by further factors as some people will quote something called the spring tidal range, yet others the difference between the highest the tides can get to, known as Highest Astronomical Tide, and the lowest the tides can get to, known as the Lowest Astronomical Tide.

This would explain why some people may say the tidal range at Newport is about 12m, yet others as high as 14m.

However, the HAT cannot immediately precede or follow the LAT, and tidal ranges cannot be calculated, only estimated.

Different organisations will therefore come up with different values, so don’t be surprised if one person says the tidal range at Newport is as high as 15m, yet someone else only as high as 11m.

I would suggest what we are really interested in is the difference between the HAT and the LAT, and this is what I will consider.

If you look up the phrase “highest tidal range in the world” you will almost certainly be told that this is to be found in the Bay of Fundy in Canada, and a little more digging will suggest that this is at somewhere called Burntcoat Head.

This may or may not be true for two reasons.

First of all, the tidal range at Burntcoat Head is the highest where it has been measured but, may be slightly higher at a nearby location where the tidal range has not been measured.

Secondly, and more importantly, the tidal range in the Bay of Fundy may not be the highest in the world.

The place that gives it a close run for its money is 1,600km north, around the mouth of the Leaf River in the south western corner of Ungava Bay, also in Canada.

There is little between them in terms of their maximum tidal range, and many organisations suggest that the tidal range in this part of Canada is larger than the highest range in the Bay of Fundy, and they may be right.

Regardless of this though, the maximum tidal range at both of these locations is certainly higher than anywhere else in the world, which means that we now have to relegate Newport slightly, and now ask “is the tidal range at Newport the third highest in the world”?

Unfortunately, the answer to this question is also no, but not because it is not true, but because the wrong question has been asked.

The tidal range at Avonmouth is noticeably higher than at Newport, and although the tidal range at Avonmouth is commonly referred to as the highest in Britain, this is not true, as the tidal range in the Severn Estuary probably peaks about 10km further upstream of Avonmouth somewhere between the two Severn bridges.

I would therefore suggest that the question should be “Does the Severn Estuary have the third highest tidal range in the world?", to which the answer is yes.

Residents around the coasts near St Malo in France and the Channel Islands may disagree, but I think we beat them by enough within the margin of accuracy of estimating tide levels to be sure of third place.

Finally, the question needs to be asked about why does the spurious fact that Newport has the second highest tides in the world (or Barry, Avonmouth or Chepstow etc if you live there) exist?

I don’t know but I suspect at some point in the past our parents or grandparents were told that the highest tidal ranges in the world were in Canada, and the second highest tidal ranges were in the Severn Estuary.

The fact that there were two locations in Canada where the tidal range was greater than in the Severn Estuary was not appreciated, and the Severn Estuary became “where we live”.

This has been passed down through the generations, just as it appears to be around St Malo, where many children born before and after me are (spuriously) told they have the highest tides in Europe.