15/05/2024
Tides and wading across the river Erme
Every now and again, someone pops up on coast path FB groups expressing concern over wading the river Erme with a predicted low tide height of 1m or 2m. So I thought I might try to explain why these depths are of no concern.
The sandy/pebby 'floor' of the Erme estuary, like any beach, slopes down into the sea – so the depth of water is different at different points. And tide tables predict the depth at one particular point, or rather along one particular line called Chart Datum.
In the case of the Erme, this Chart Datum line runs roughly across the very mouth of the estuary, from a point south of Battisborough Island to Fernycombe Beach. It is about a mile downstream of the river crossing point; and crucially, because of the aforementioned slope, about 3m lower.
Tidal ranges (and therefore tidal heights at low water) vary widely, following a lunar cycle. In mid-May 2024, low water is predicted to be about 1.6m. This means that there will be about 1.6m of water covering the imaginary Chart Datum line. But the crossing point will still be about 1.4m above the sea level. Or, to look at it another way, the surf line will about half a mile downstream, just above Muxham Point. The sea will not affect the depth of water at the crossing.
Of course, if you want to cross outside the 2hr low tide 'window', it's a different story.
