How Does the Severn Bore Occur?
How Does the Severn Bore Occur? A bore in this sense is a tidal wave of great height and speed caused by the meeting of two tides or the rush of a tide up a narrowing estuary. Conditions particularly favorable for the creation of such a wave occur in the Severn, Britain’s longest river. The Severn ends its 200-mile journey from Plynlimon in Wales as a long estuary which widens to 10 miles where it enters the sea near Bristol.
The Severn Bore occurs every month just before and soon after a spring tide. It is most impressive at the time of the spring tides nearest the vernal equinox (March 21) and the autumnal equinox (September 23). As the tide moves up river it meets a residue of slack or ebbing water and causes a tidal wave that reaches a height of about 33 feet. The bore starts near Sheerness. At Fretherne, where the river narrows, the bore rears up like a wall, to be followed by two more waves and finally the tide itself, seething and roaring.
Daniel Defoe (1661-1731), the author of Robinson Crusoe, described the bore “rolling forward in a mighty wave: so that the stern of a vessel shall in a sudden be lifted up”. Above Gloucester the bore normally subsides. But an exceptionally large one has been known to ruffle the tide less water of the Severn at Worcester.
Bores are present on about 130 days in the year, concentrated on the days immediately following the new and full moon. The size and precise timing of the bore depend on such things as the time of high tide, the barometric pressure, the wind speed and direction, the amount of water coming down the river and how well scoured the main drainage channels are.
There are a number of viewpoints from which the bore can be seen, or viewers can walk along the river bank or flood banks. Historically, the bore has been of importance to shipping visiting the docks at Gloucester, but this was alleviated by the construction of an alternative route, the Gloucester and Sharpness Canal, which opened in 1827. Nowadays the bore is of more interest to surfers and canoeists who attempt to ride the waves.
Best places to see the Severn Bore.
A convenient vantage point for the bore is Telford’s bridge at Over but the bore is at its most spectacular on the section of river between Minsterworth and Lower Parting. Some of the best viewing points are:
Newnham-on-Severn: Standing in the churchyard on the high cliff gives a view of a three or four mile stretch as the bore negotiates the Horseshoe Bend.
Minsterworth: The road is right beside the river and river access can be gained at the Bird-in-Hand, by the old ferry or at the church.
Lower Parting: Where you can see the bore split into the two river channels.
Manor Ditch: ¾ mile downstream from Lower Parting you can see the bore, perhaps as much as ten feet high, racing around the outside of the sharp river bend.
Maisemore Bridge: Here there is the added interest of the reflex wave, maybe as high as a foot, flowing back down river 10 minutes after the bore has encountered the weir.
Whichever vantage point you choose, you are likely to be astonished by the sheer power of the Severn Bore, the increasing roar as it approaches and the dramatic change in river level once the bore has passed.