Severn estuary

The Severn Estuary () is the estuary of the River Severn, flowing into the Bristol Channel between South West England (from North Somerset, Bristol and South Gloucestershire) and South Wales (from Cardiff, Newport to Monmouthshire).Natural England Severn Estuary SSSI information on the citation, map and unit details
/ref> Its very high tidal range, approximately , creates valuable intertidal habitats and has led to the area being at the centre of discussions in the UK regarding renewable tidal energy.

Geography

Definitions of the limits of the Severn Estuary vary. In pre-modern times the area was commonly referred to as the River Severn, or the Severn Sea. Today, at the upstream boundary, the normal tidal limit of the river is at Maisemore weir (on the West Channel) and Llanthony Weir (on the East Channel), close to Gloucester Docks, although exceptionally high tides can overtop these weirs.

Downstream, the estuary transitions into the open sea of the Bristol Channel. The distinction between the estuary and the Bristol Channel is ambiguous, with some sources treating the estuary as part of the Bristol Channel or drawing no distinction between the two. The Severn Estuary Partnership encompasses an area upstream of Hurlstone Point west of Minehead and Nash Point south of Bridgend. The Living Levels Partnership use a definition that draws the line between Sand Point (north of Weston-super-Mare) and Lavernock Point (south of Penarth), marked by Steep Holm and Flat Holm islands. Lavernock Point is also used as the boundary between Marine Character Areas 28 and 29, named Bristol Channel and Severn Estuary.

A narrower definition adopted for navigation purposes by some charts includes only the area downstream of the Second Severn Crossing near Severn Beach, South Gloucestershire. The definition used on Admiralty Chart SC1179 and the Bristol Channel and Severn Cruising Guide is that the estuary extends upstream to Aust, the site of the Severn Bridge.

On the north-west (Welsh) side, the rivers Wye and Usk flow into the estuary, and on the south-east (English) side, the River Avon joins at Avonmouth. West of the Wye, the estuary forms the boundary between Wales and England. On the northern side of the estuary are the Caldicot and Wentloog Levels which are on either side of the city of Newport; and, to the west, the city of Cardiff together with the resort of Penarth. On the southern, English, side, are Avonmouth, Portishead, Clevedon, and Weston-super-Mare. Denny Island is a small rocky island of , with scrub vegetation, approximately three miles north of Portishead. Its rocky southern foreshore marks the boundary between England and Wales, but the island itself is reckoned administratively to Monmouthshire, Wales.

The estuary is about wide at Aust, and about wide between Cardiff and Weston-super-Mare. It has one of the highest tidal ranges in the world — about . This funnel shape, large tidal range, and the underlying geology of rock, gravel and sand, produce strong tidal streams and high turbidity, giving the water a notably brown coloration. During the highest tides, rising water is funnelled up the estuary into the Severn bore, a self-reinforcing solitary wave that travels rapidly upstream against the river current.

The Bristol Channel in turn discharges into the Celtic Sea and the wider Atlantic Ocean. The islands of Steep Holm and Flat Holm are in the middle of the estuary. The Ordnance Survey refers on its published mapping to a section of the estuary seaward of the two estuarine motorway crossings as the ''Mouth of the Severn''.

Conservation and SSSI status

The tidal range results in the estuary having one of the most extensive intertidal wildlife habitats in the UK, comprising mudflats, sandflats, rocky platforms and islands. These form a basis for plant and animal communities typical of extreme physical conditions of liquid mud and tide-swept sand and rock. The estuary is recognised as a wetland area of international importance and is designated as a Ramsar site