The Severn Estuary Tidal Barrage Project

There have been many proposals and government white papers written on the construction of a tidal barrage across the Severn estuary.

The most promising and feasible being carried out in 1989 by a committee of businessmen from various construction industries known as Severn Tidal Power Group (STPG), backed up by a government Sustainable Development Committee in 2009.

The STPG proposed to build the barrier across a ten mile stretch at the mouth of the Bristol Channel, which would produce electricity on the ebb tide only, but with an output capacity of over 8000MW.

This is an article on renewable energy and in particular using the power of the tides as a source.

Here we shall have a look at two of the current tidal barrages in operation in France and Canada (other barrages are in Korea, Russia, and China) and examine the latest proposal to build a tidal barrage across the Severn Estuary, with the attendant effects on the estuary and river environment, shipping, and the local populace.

We begin then with the current operating tidal barrages in France and Canada.

As the STPG proposals recommend the technology used in the Rance Barrage in France and the Annapolis barrage in Canada, it is appropriate that we have a quick look at how they operate.

• La Rance Tidal Barrage

The Rance tidal barrage has been operating at an annual output of 600GWh being produced by 24 turbines of 10MW capacity since 1966. It has now paid for itself, and the electricity is currently being fed at a low cost to the French Grid.

• The Annapolis Tidal Barrage

The Annapolis tidal barrage was constructed across the river estuary in 1984 and has an annual output of 50GWh with the turbines operating on ebb tides only. It was built to fulfill a few purposes other than produce power, as it replaced an aging iron bridge and ferry across the river.

However the Annapolis barrage has a few environmental problems; the banks of the river are being eroded and a trapping zone has been created where several whales have been stranded behind the barrage.

However the Annapolis barrage has a few environmental problems; the banks of the river are being eroded and a trapping zone has been created where several whales have been stranded behind the barrage.

In 2007, the UK government set up a Sustainable Development Committee, which has reported back in favor of construction of a tidal barrage across the Severn Estuary.

It has been proposed that the Severn Tidal Barrage will be comprised of a ten-mile span across the mouth of the Bristol Channel, effectively joining England and Wales.

The barrage will be constructed from prefabricated steel reinforced concrete caissons which will be built in various locations in the UK and towed to the final location on the estuary.

These caissons will contain the numerous sluice gates, shipping locks, and specially designed water turbines. Once in correct location, the caissons will be ballasted, sinking slowly and accurately onto the foundations on the bottom of the estuary. They will then piled, grouted, and joined up forming the ten mile wide tidal barrage.

It is proposed that road and rail links will be laid on top of the barrage with special facilities provided where they pass over the ships locks section in the form of a bridge or a long sloping road.

The water turbines will operate on the ebb tide only this being more efficient than flood generation. This will enable the turbines to have two generating periods as the Severn estuary has a semi-diurnal cycle – two high and two low tides every 24 hours and 50 minutes.

The rise and fall of the tides are caused by gravitational forces exerted on the earth by the moon and sun, either acting separately resulting in normal tides or in combination where we get the higher spring tides.

Tides can be diurnal; one rise and fall/day, or semi-diurnal having two rises and falls a day.

A large tidal range (difference in vertical height between the ebb and full tide) is essential for the turbines to operate efficiently as they depend on potential energy to drive them. The Severn Estuary has the world’s second largest tidal range of 14 meters. It also has a semi-diurnal tidal cycle which means that the turbines have two operating periods per day.

The proposed method of operation is ebb tide generation with pumping. This entails opening the sluice gates to the incoming tide which fills the area behind the barrage known as the basin, and then the sluice gates are shut.

As the tide starts to ebb the turbines pump more seawater into the basin thus increasing the amount of stored potential energy.

Once the tide has ebbed sufficiently, the turbine inlet gates are opened and the turbines start to generate power, generation continuing until the water level is equal both sides and insufficient to drive the turbines.