The Severn Bore

 

After Roger’s talk on the River Severn, Geoff Pook has compiled the following about the bore.

Severn Bore.  To help to understand our unique situation.

 There are features of the Severn Estuary and the Bristol Channel which cause the feature known as the Bore.  The large volume of water between Wales and Southwest England has only one place to go when the tide rises – it is forced into the funnel shaped estuary and then further upriver.

In his talk, Roger was right to say that there are two Bore’s per year – if you are talking about bores large enough to surf on.  People travel from all over the world to ride the bore so they will only come for a Good-un!  There are many factors which influence how good it will be: -

#             Wind – a strong south-westerly will help.

#             Moon _ which phase is it in i.e. full, half, crescent.

#             Sun - What season of the year is it.

There are between 250 and 260 bores a year.  They only occur when the tide exceeds a certain height.  Only tides more than 9 meters high (measured at Fethherne, near Frampton on Severn) are considered worth surfing though smaller ones can be spectacular. Most bores finish at Maisemore weir, but the very highest can overrun this.

 The orbit of the Moon around the Earth is not a circle, but an ellipse.  Also, the orbit of the Earth around Sun is an ellipse. Ironically, the Earth is closer to the Sun in the winter than summer. (See Diagram)

 The bore is caused by the Gravitational pull of the Sun and the Moon.  When the Sun and Moon are aligned, we see a Full Moon.  Their Gravitational pull acts together. These are the times of the highest waves.  The bore which occurred this year on 12th March was the largest we are likely to see for a while unless extraordinary weather conditions drive the wave higher.

                                                                                                                                                                       See also  Google- The Severn Bore Time Table – for more details.

Geoff Pook