The Tideline Moves Twice a Day and Nobody Told the Map Makers
Twice a day the Severn takes back two hundred metres of what the map calls land. The Ordnance Survey prints one coastline; the people who work the estuary keep another.
The mean high water mark is a legal object as much as a geographical one. It decides who owns what, who may build, and who carries the loss when the water arrives anyway. It is drawn as a line because a line is what a map can hold.
On the Severn the difference between that line and the water is measured in hundreds of metres. The range at Avonmouth runs to about fourteen metres on the biggest spring tides — the second largest in the world — and it moves the edge of the land twice a day, and has been doing so for a great deal longer than anyone has been drawing it.
What the survey records
The first detailed survey of this stretch was made in 1873 by a man named Pritchard, working from a rowing boat with a lead line and a theodolite set up on the sea wall. It is held at the county archive in six sheets. It is beautiful. Where it is wrong, it is confidently wrong.
Pritchard drew the saltmarsh edge as a firm line and the channels behind it as fixed. Both were reasonable things to believe in 1873 and neither has been true since. The main channel at Sheperdine has moved roughly 300 metres east in the intervening century and a half, and moved back about half that distance in the four years after the 2013 surge.
A map is a claim about what will still be true tomorrow. Out here it is a claim with a shelf life of about six hours.
What the tide records
The people who work this coast keep their own version. It is not written down in any form a surveyor would recognise, and within its scope it is considerably more accurate. Ask a fisherman where the channel is and you will get an answer referenced to a pylon, a gate and a state of the tide, which is three pieces of information a grid reference does not carry.
What that knowledge covers, roughly:
- Where the channel has moved since last winter, and which way it is still going
- Which paths are passable at which state of the tide, and how long the window is
- Which fields flooded in 2013 and in 2019, and therefore which will flood again
- Where the old sea wall runs under the marsh, and where it has gone
None of it is legally admissible. All of it is what people actually use.
The part that goes to court
In 2019 a dispute over a field boundary near Oldbury turned on where mean high water had been in 1954. The claimant had aerial photographs. The defendant had Pritchard, or rather a plan derived from a plan derived from Pritchard. Neither party had anything from the tide itself, because the tide does not keep records; it only keeps changing the thing the records describe.
The judgment ran to forty pages and turned, in the end, on the position of a hedge.
The map is not the territory, but it is the thing the court will look at.
This is not an argument against surveying. It is an argument for reading a survey as a dated document — a claim made by a particular person, in a particular boat, in a particular year, about something that had already started moving before he got back to shore.