THE article in October 16th Review 'Look how much the Severn's sunk in the last 20,000 years' raised more than a few eyebrows not only for its content and mathematical error but also its conclusions. In one respect Durham University is perfectly correct, the Severn has in the past flowed at a much higher level in its landscape as, coincidentally, has the Wye. However, this is not caused by the land sinking 0.3mm per year but in fact due to a combination of river bed erosion against the action of long term geological uplift across the Forest of Dean to the Cotswolds.
Post Glacial Rebound is the currently accepted explanation of UK land level uplift and tilting since the end of the last Ice Age. However, the rotational or differential movement across the Cotswolds between the Severn estuary and the Thames estuary offers no obvious explanation, but the fact is that over the last 3,000 or so years archaeological research indicates the former has been uplifted over three and a half metres in relation to the latter.
The Severn and Wye estuary is like no other river estuary in Britain and the reason why is due to some very unique forces of nature. First of all regular long term cycles of tidal progression and regression should not be underestimated as these over the last 6,000 years have had a significant impact in shaping the river we see today.
Research between Minsterworth and Westbury on Severn indicates that at Westbury on Severn and Rodley the upper river bank deposition layer, began to be laid down following 4000BC when the whole area was overwhelmed by rising sea level. This dramatic rise in sea level is identified by the almost indistinct stratification in the bottom 340mm third of this Upper deposition layer, compared with the top 760 mm, where once the sea had retreated, the stratified layers clearly show vegetation establishing itself between a succession of river flood events.
Prior to 4000BC the Upper Westbury and Upper Rodley deposition layers did not exist. This was then a post glacial landscape transformed as recently as 6,000 years ago when the whole of Severnside was inundated by the sea. Following the retreat of the sea in the late 4th millennium BC the area continued to be inundated at regular intervals by flooding (probably on a July 2007 flood magnitude) on average every 30 years, but this cycle of river inundation ceased well before the 1607 tsunami.
The real clue as to what is happening to land levels across Severnside and Forest of Dean is in the topography, the Wye's dramatic incised meander from Symonds Yat to Chepstow and the river terraces on the west bank of the Severn are clear evidence of uplift. South west of Gloucester the Severn, although tidal, flows down to Longney Point in a channel deep enough to be navigable by small craft at low tide, a distance of some seven miles overland. However, here the nature of the river changes suddenly and abruptly. The course of the Severn broadens into shallow mudflats and sand banks, pounding back the deeper channel upstream. Similarly the Wye above Symonds Yat is navigable for some distance upstream but here again the nature of the river changes suddenly and abruptly. Also equally significant are the historical tide levels reported at one recent local seminar, in spite of sea level rising elsewhere no measurable rise in the maximum height of spring tides has been recorded at Sharpness since the port was built at the beginning of the 19th century.
In this area of the Severn estuary we may well be enjoying an albeit brief respite from rising sea level, with land levels currently rising more or less in tandem. Ok for now provided the rate of sea level rise does not accelerate further.
Gordon A K McDonald FRICS, The Stenders, Mitcheldean.