Severn Estuary

The Severn estuary at Beachley, Gloucestershire

The Severn Estuary (Welsh: Môr Hafren) is the estuary of the River Severn, the longest river in Great Britain. It is at the mouth of four major rivers, namely the Severn, Wye, Usk and Avon and other smaller rivers. It has the highest tidal range in the world after the Bay of Fundy on the coast of Canada, which has left it under threat of being harnessed for the development of tidal energy.

Geography

Definitions of the limits of the Severn Estuary vary, for there is seamless transition from the River Severn, to its estuary and on to the Bristol Channel, with not obvious division between these names. A narrow definition adopted by some maps is that the river becomes the Severn Estuary after the Second Severn Crossing near Severn Beach, Gloucestershire and stretches to a line from Lavernock Point (south of Cardiff) to Sand Point near Weston-super-Mare. A wider definition is that the estuary extends upstream to Aust, the site of the Severn Bridge. The estuary is about two miles wide at Aust, and about nine miles wide between Cardiff and Weston-super-Mare.

On the northern side of the estuary are the Caldicot and Wentloog Levels, on either side of the city of Newport, Monmouthshire; and, to the west, the city of Cardiff together with the resort of Penarth, both in Glamorgan. On the southern side, are Avonmouth in Gloucestershire and Portishead, Clevedon, and Weston-super-Mare in Somerset.

Denny Island is a small rocky island of 0.6 acre, with scrub vegetation, approximately three miles north of Portishead.

Map of the Bristol Channel and Severn estuary (shown here as "Mouth of the Severn")

The estuary has one of the highest tidal ranges in the world — about 50 feet. The estuary's funnel shape, its tidal range, and the underlying geology of rock, gravel and sand, produce strong tidal streams and high turbidity, giving the water a notably brown colouration.

West of the line between Lavernock Point and Sand Point is the Bristol Channel, which in turn discharges into the Atlantic Ocean. The islands of Steep Holm and Flat Holm are located close to that line, in the middle of the estuary.

Sometimes the term Severn Estuary is used to include the tidal upstream stretch between Gloucester and Aust. During the highest tides on the upper reaches of this stretch, the rising water is funnelled up the estuary into the Severn bore, a self-reinforcing solitary wave that travels rapidly upstream against the river current.

Conservation and SSSI status

The tidal range results in the estuary having one of the most extensive intertidal wildlife habitats in the United Kingdom, 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.

The estuary is recognised as a Special Protection Area (SPA) under the EC Directive on the conservation of Wild Birds. The estuary is recognised as a Special Area of Conservation (SAC) under the EU Habitats Directive.

Parts of the estuary have also been designated a Site of Special Scientific Interest. The SSSI includes most of the foreshore upstream from Cardiff and Brean Down and most of the upper estuary as far as Sharpness. The Upper Severn Estuary SSSI covers the tidal river between Purton and Frampton on Severn.

The Severn Estuary SSSI original designation overlaps into Somerset, Gloucestershire, Monmouthshire and Glamorgan. The Severn Estuary SSSI designation overlaps individual site designations for separate sites in Somerset and Gloucestershire (Spring Cove Cliffs, Middle Hope, Portishead Pier to Black Nore, Aust Cliff) and Glamorgan (Penarth Coast). The 1976 designation includes two sites previously notified in 1952 (Brean Down and Uphill Cliff). The SSSI forms the major part of a larger area which includes the Taf-Ely Estuary and Bridgwater Bay (as well as the Upper Severn Estuary)

The Upper Severn Estuary SSSI designation is in Gloucestershire.

Both SSSI citations provide detail of the geological and biological interest and of particular note is the international importance for wintering and wading birds of passage, and of estuarine habits of outstanding ornithological significance. It is stated that the estuary supports over 10% of the British wintering population and is the single most important wintering ground for Dunlin, and for significant numbers of Bewick's Swans, White-fronted Geese and Wigeon. Nationally important wintering populations are supported such as Gadwall, Shoveler and Pochard.

There are notably seven species of migratory fish which pass through the estuary in both directions. These include significant numbers of Atlantic Salmon and Common Eel.

Tidal power

Diagram of a plan to harness tidal power on the Severn c 1921

Barrage locations considered over the years

A huge tidal range and high level of surrounding industry and population have long made the Severn Estuary and Bristol Channel a focus for tidal energy schemes and ideas. Plans for a Severn Barrage — running 10 miles across the Bristol Channel from Lavernock Point near to and south west of Cardiff to Brean Down near and just south west of Weston-super-Mare in Somerset — would generate a massive 8640 MW when the tide flows, and have been discussed for several decades now. The power generated would come from a lake of 185 square miles with a potential energy depth of 46 feet. Tidal power only runs for around ten hours a day, but by using the enclosed lake as a reservoir of potential energy more hours of operation could be achieved. Other energy sources, such as wind and solar power, also create electricity at times that do not always match when it is needed. Excess power could be stored by pumping water uphill, as is already done at a variety of other installations in the British Isles.

The scheme would however destroy the whole nature of the estuary, its wildlife, its mudflats and its wild beauty, imposing in their place a giant, manmade, tideless loch with concrete sluices and walls.

The Government shelved the plans in the late 1980s due largely to cost issues and local environmental concerns. In April 2006 the Welsh Assembly approved the idea of utilising the tidal power, but the Royal Society for the Protection of Birds raised serious concerns about the effect on the mud flats, that have European Environmental protection status, and the government Energy Review published later in the year did not endorse the scheme.

Even so, the proposal is repeatedly raised in various quarters. A further feasibility study began on 25 September 2007, and got nowhere.

Advocates say that the Severn Estuary has the potential to generate more renewable electricity than all other estuaries in the United Kingdom. If harnessed, it could create up to 5% of the United Kingdom’s electricity. It is being opposed by environmental groups.

Severn Estuary Partnership

The Severn Estuary Partnership (SEP) was set up in 1995 as an independent initiative to focus the activities of local government, statutory authorities and interested parties such as farmers and fisherman. Its stated aim is To bring together all those involved in the development, management and use of the Estuary within a framework which encourages the integration of their interests and responsibilities to achieve common objectives. In 2001 SEP published the Strategy for the Severn Estuary, which sets out a plan for the management of the estuary.

SEP uses a geographically extended definition of the Severn Estuary, beginning at the tidal limit of the River Severn in Gloucester and ending at a line drawn between Hurlestone Point near Minehead and Nash Point in Glamorgan.

Archaeology

The archaeology of the Severn Estuary is richly varied and of considerable importance, reflecting both the varied nature of the topography and the importance of the river for both fishing and as a maritime waterway. The archaeological resource within the estuary is under threat from natural processes such as coastal erosion, exacerbated by the high tidal range and strong tidal currents, and from threats such as ongoing development pressure along the shoreline, marine aggregates extraction and new coastal defensive and realignment measures as well as proposed major infrastructure projects. An archaeological aerial survey report of the aerchaeology on the English side is available to view and download from the reference link.

View of Severn Estuary from the Newport Wetlands

Outside links

("Wikimedia Commons" has material about Severn Estuary)

• The Severn Estuary Partnership
• Natural England (SSSI information)