The Severn Estuary and river have the second largest tidal range in the World, ie the largest difference between low tide and high tide. This fact coupled with the shape of the funnel-shaped estuary means that the rushing tide results in a wave.
The size and timing of this wave are hard to predict. Some factors can be more easily predicted; if the moon and other planets are all aligned and therefore exerting gravitational force or pull on the water then this causes a high tide. Other factors are harder, for example a South-Westerly wind will push the waves across the Sea and up the Severn-Estuary, resulting in a bigger Bore and earlier arrival by up to 20-30 minutes. The height of the wave is normally better for a higher rated bore eg 4 or 5*. However this also varies due to conditions on the day and a major factor is the height of the river because high levels result in water washing over the top of the banks and energy is lost. Once the leading wave has passed, it is interesting to see the massive volume of water following and rise in water levels. This can also result in one of the hazards of wave watching, flooding of the banks. Please think about where you stand (and where you park a car)!
In order to complete this cache you’ll need to find out when the Bores are expected. Generally these are during the Spring and Autumn, and predictions of the best Bores and further information are available on the severn-bore web site here. The picture above was taken for a 3* tide, but a 4/5* tide would be better.
In order to log this cache, you can be anywhere along the river and please:
The first is of your GPS showing the coordinates and the stretch of river that you’re watching. The second is of the Bore itself, in order to claim this cache you need to photograph a clearly visible wave (no old holiday/web photos please). The background should be roughly the same in both pictures.
1. Record in your log the expected time of the Bore AND actual time of the Bore.
2. Ideally take two photos posted on your log on this cache page or mailed to me.
Beware where you stand since the river sometimes floods as a result of the Bore and there are lots of splashes. Likewise be careful where you park (Elmore Lane West often floods).
SAFETY - There are very real risks from rapidly rising water which immediately follows the Bore. Be careful where you stand and be aware of your surroundings.
There are usually crowds of people for every decent sized Bore and this can result in traffic delays and parking problems. We parked in a layby at N51°50.072', W02°17.332 but most people just parked at the side of the road.
Please check out the Geocaching Association of Great Britain for further information about Geocaching in the UK.