The Severn Estuary – in Welsh Môr Hafren is the estuary of the River Severn, the longest river in Great Britain and forms the boundary between Wales and England. It is at the mouth of four major rivers being the Severn, the Wye, the Usk and the Avon and of other smaller rivers.

Definitions of the limits of the Severn Estuary vary. According to some maps the river becomes the Severn Estuary after the Second Severn Crossing near Severn Beach, South Gloucestershire, and stretches to a line from Lavernock Point (south of Cardiff) to Sand Point near Weston-super-Mare. 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 old Severn Bridge. The estuary is about 2 miles (3.2 km) wide at Aust, and about 9 miles (14 km) 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; 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 0.24 hectares (0.6 acres), 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 administrated by Monmouthshire, Wales.

IMPORTANCE TO WILDLIFE

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 and it is recognised as a wetland area of international importance, with many designations to protect its diverse wildlife. Parts of the estuary on both the Welsh and English sides have also been designated as a Site of Special Scientific Interest. 

Of particular note is the estuary’s 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, European white-fronted geese and wigeon. Nationally important wintering populations are supported such as gadwall, shoveller 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 RANGE

The estuary has one of the highest tidal ranges in the world — about 50 feet (15 m). 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. 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.

The 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. The Severn Estuary has the potential to generate more renewable electricity than all other UK estuaries. If harnessed, it could create up to 5% of the UK’s electricity, contributing significantly to UK climate change goals as well as European Union renewable energy targets. The proposal for a hydro-electric barrier, the Severn Barrage, to generate 8.6 GW and meet five percent of Britain’s power needs, is being opposed by some environmental groups. The Barrage  would run 16 km (9.9 mi) 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. The power generated would come from a lake of 185 square miles (479 km²) with a potential energy depth of 14 metres (46 ft). 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, but 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 UK.

The UK Government shelved the plans in the late 1980s due largely to cost issues and local environmental concerns. However, this was before recent huge rises in the price of energy, and before global warming had started to be taken seriously. In April 2006 the Welsh Assembly approved the idea of utilising the tidal power, but the RSPB has raised serious concerns about the effect on the mud flats, that have European Environmental protection status, and the UK government Energy Review published later in the year did not endorse the scheme. Opinion is still divided on the benefits of a proposed barrage.

In the news on 7 January 2015 

New tidal lagoon could be built in the Severn estuary between Cardiff and Newport

The project is in its early stages but Tidal Lagoon Power said that it is looking at the “environmental and engineering feasibility” of the scheme .

A tidal lagoon built in the Severn estuary between Cardiff and Newport could one day provide energy for hundreds of thousands of homes in south-east Wales. The company behind plans to build a similar lagoon in Swansea Bay – Tidal Lagoon Power – has confirmed that it is considering a larger project further upstream. The project is still in its early stages but Tidal Lagoon Power said that it is looking at the “environmental and engineering feasibility” of the scheme and gauging “local community opinion.”

No plans have yet been submitted, but following conversations between Tidal Lagoon Power and the Planning Inspectorate outline details of the scheme have appeared on the National Infrastructure Planning website. The outline details describe a lagoon with a potential generating capacity of between 1.8 gigawatts and 2.8 gigawatts.

As in the Swansea scheme, the lagoon would be attached to the shore, with one end around 2km from the entrance to Cardiff Bay and the other 2km from the mouth of the River Usk at Newport. At the widest point it could extend as much as 8km out into the Severn.

An application could be submitted to the Planning Inspectorate in the spring of 2017.