Swansea barrage
Encyclopedia
Marina
A marina is a dock or basin with moorings and supplies for yachts and small boats.A marina differs from a port in that a marina does not handle large passenger ships or cargo from freighters....
at the mouth of the River Tawe
River Tawe
The River Tawe is a river in South Wales. It flows in a principally south-westerly direction for some from its source below Moel Feity in the Old Red Sandstone hills of the western Brecon Beacons to the Bristol Channel at Swansea. Its main tributaries are the right bank Upper and Lower Clydach...
extending the leisure boat facilities already being offered by the old South Dock.
The barrage structure includes a boat lock, spillway
Spillway
A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...
, fish pass
Fish ladder
A fish ladder, also known as a fishway, fish pass or fish steps, is a structure on or around artificial barriers to facilitate diadromous fishes' natural migration. Most fishways enable fish to pass around the barriers by swimming and leaping up a series of relatively low steps into the waters on...
and generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...
. The turbine serves a dual use: acting as a power generator for the National Grid as well as being used to pump water back into the Tawe river system.
The barrage scheme gave rise to a number of environmental concerns. Fish navigation
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...
up the river and dissolved oxygen levels were problems that arose after completion. Salt water that came in at high spring tides sank to the bottom and stayed there, reducing oxygen levels. The Environmental Advice Centre was commissioned to undertake an aeration
Aeration
Aeration is the process by which air is circulated through, mixed with or dissolved in a liquid or substance.-Aeration of liquids:-Methods:Aeration of liquids is achieved by:...
scheme trial in the River Tawe in the summer of 1998. The system was based on a diffuser design and propeller mixer, which proved highly effective at exporting the saltwater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
from the deep area of the trial site and raising dissolved oxygen concentrations at the bed
Stream bed
A stream bed is the channel bottom of a stream, river or creek; the physical confine of the normal water flow. The lateral confines or channel margins, during all but flood stage, are known as the stream banks or river banks. In fact, a flood occurs when a stream overflows its banks and flows onto...
.
Following the success of the trial system a more extensive system was designed for the river for installation in 1999 -2000. The first stage of the installation of the designed system was undertaken in June 1999 and comprised a combination of diffusers and a large propeller mixer.
The installation was conducted using a diving team to ensure optimal location of the diffusers within the river channel. The installation will be completed over two years to treat all the problem areas over a 4km long reach of river and ameliorate the poor water quality
Water quality
Water quality is the physical, chemical and biological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and or to any human need or purpose. It is most frequently used by reference to a set of standards against which...
presented in these areas.
Monitoring results from the system installed to date have indicated that aeration has effectively raised the oxygen concentration and assisted in the export of saline water
Saline water
Saline water is a general term for water that contains a significant concentration of dissolved salts . The concentration is usually expressed in parts per million of salt....
from the system. Following the success of this work the Environmental Advice Centre is currently examining designs for other reaches of tidal river
Tidal river
A tidal river is a river, or more typically a stretch of a river, whose flow and level is influenced by tides. An example of a tidal river is the portion of the Connecticut River flowing from Windsor Locks, Connecticut, to the Atlantic Ocean. The Brisbane River, which flows into the Pacific Ocean...
within the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
that display similar problems.
Following the success of the River Tawe aeration scheme the neighbouring Welsh
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
city of Cardiff
Cardiff
Cardiff is the capital, largest city and most populous county of Wales and the 10th largest city in the United Kingdom. The city is Wales' chief commercial centre, the base for most national cultural and sporting institutions, the Welsh national media, and the seat of the National Assembly for...
used the same method to improve the quality of the water in its newly-created bay.
Other fears that arose with the building of the barrage such as the raising of the water table
Water table
The water table is the level at which the submarine pressure is far from atmospheric pressure. It may be conveniently visualized as the 'surface' of the subsurface materials that are saturated with groundwater in a given vicinity. However, saturated conditions may extend above the water table as...
causing ground subsidence
Subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
problems in the low-lying areas around the River Tawe. Since completion, no significant subsidence problems have arisen.