River Dee regulation system
Encyclopedia
The River Dee regulation scheme is a system of flow balancing and quality management along the River Dee
managed by a consortium of the three largest water companies licensed to take water from the river, United Utilities
, Welsh Water
and Dee Valley Water
; together with the regulator, the Environment Agency Wales
.
including Liverpool
and the Wirral far exceed the locally available sources of clean water. The River Dee
runs mainly in North Wales
before flowing through Chester
, England, and then returning to Wales in a man-made channel constructed to gain land from the Dee Estuary
. The Dee is the largest relatively clean river near to the North West conurbation
and without water from the Dee much of Liverpool
would be without water. However, the natural flow of the River Dee during most summers is insufficient to sustain any significant abstractions. To overcome this problem, a series of reservoirs have been constructed to store the excess water available in the winter time and release it back into the River Dee during the drier months.
This is the principle of low-flow regulation It was used by Thomas Telford
at the beginning of the 19th Century in order to guarantee a supply of water to the Ellesmere Canal
. Telford constructed sluice
s at the outlet of Bala Lake to control the flow downstream so that there was always sufficient to supply the canal where it started at Horseshoe Falls
.
The River Dee has also been used for direct drinking water supply with the Alwen Reservoir
, built in the 1920s to supply Birkenhead
with water.
In the industrial revolution many rivers in industrial areas became too polluted by effluents to be usable for water supply. The Dee however remained clean with relatively few polluting effluents in the Dee catchment upstream of Chester. Consequently, the City of Chester
has been able to directly abstract Dee water since the first Chester Waterworks Company was formed in 1826.
, which is only a short distance from the lake exit. This provided 18 million cubic metres of stored water in Bala Lake that could be controlled and used on a seasonal basis for low-flow regulation. This enables continuous abstraction from the River Dee of 235,000 cubic metres per day by six Statutory Water Undertakings and British Waterways Board
. An additional benefit was a reduction in flooding events downstream of Bala
as Bala Lake was able to contain the worst of the winter flood peaks.
, a new 81,000,000-cubic-metre capacity regulating reservoir within the Bala Lake catchment area. This was completed in 1967 by Liverpool Corporation, and designed to operate in conjunction with the Bala Lake Scheme. This enables additional Dee abstractions of 327,000 cubic metres litres per day together with additional flood control storage. In summer time the impact was to increase threefold the dry-weather flow for most of the length of the river. This development was hugely controversial at the time and remains politically fraught to this day since the construction of the reservoir involved flooding the Tryweryn Valley and the village of Capel Celyn
and twelve farms. Local people saw this a destruction of part of the Welsh culture
in order to supply England with water. This caused a great deal of controversy, resentment and protest. To try to offset some of the environmental concerns associated with the scheme, some of the stored water was specifically set aside to make special releases to help fisheries
, to provide recreational opportunities (canoeing and white-water rafting on the Afon Tryweryn
) and to disperse pollution events should they occur.
A four megawatt hydro-electric station at the dam was also included in the scheme.
. This reservoir was first filled in 1979 providing an additional 60 million cubic metres. This increased the potential for abstraction from the river in the lower reaches to nearly 900,000 cubic metres per day.
By 2002 the authorised abstractions had been taken over by three statutory undertakings and British Waterways Board with a total licensed abstraction of 850,000 cubic metres per day. In addition a residual flow of at least 364,000 cubic metres per day is maintained over Chester Weir
in all but the most testing of drought
s, safeguarding the passage of migratory fish and limiting the ingress of saline water over Chester Weir during high tides.
. This location was chosen as it was on a section of the river where flow could be readily measured and above the very flat stretch that meanders into Cheshire. The current rules state that when flow exceeds 10 m³/s no intervention is required. In practice some adjustment of Bala sluices may take place to increase storage in Llyn Celyn and conversely some releases may be made from Llyn Celyn for recreational or power generation purposes. When the flow at Manley Hall approaches 10 m³/s, additional flow is released from Llyn Tegid. If that is insufficient, flow from Llyn Celyn is used to maintain 10 m³/s at Manley Hall. In extreme situations where the flow from Llyn Celyn is insufficient to maintain the flow, releases of water from Llyn Brenig . Circumstances can arise where even this is insufficient to maintain flows and in such cases drought provisions are agreed which progressively reduce the maintained flow at Manley Hall.
The overall effect of this regulation has a marked impact on the hydrograph
of the River Dee. In dry years the hydrograph flat-lines at 10 m³/s whilst dry weather continues such as in 1990
service. The results of the analysis are made available to the four participating organisations on a routine daily basis. For each of the critical water quality parameters alert levels and action levels have been set based on past experience. If an alert level is exceeded an immediate alert (DEEPOL 1) is issued to all participants. As contamination levels increase or the contaminant plug nears an abstraction point the DEEPOL level rises to DEEPOL 2 and finally DEEPOL 3 at which time the affected abstractions are closed until the river quality returns to normal. This quality management system was developed following a severe Phenol
pollution of the River Dee in the 1980s which resulted in contaminated water being supplied to large areas of Liverpool and the Wirral. Contamination of raw water is a more significant problem on the River Dee because the River Dee is normally of exceptionally good quality and as a result the abstractions have been constructed directly from the river rather than via bank-side storage reservoirs as is more common when taking water from Rivers of less reliable quality.
River Dee, Wales
The River Dee is a long river in the United Kingdom. It travels through Wales and England and also forms part of the border between the two countries....
managed by a consortium of the three largest water companies licensed to take water from the river, United Utilities
United Utilities
United Utilities Group PLC is the UK's largest listed water business. The Group owns and manages the regulated water and waste water network in the north west England, through it subsidiary United Utilities Water PLC , which is responsible for the vast majority of the group's assets and...
, Welsh Water
Welsh Water
Dŵr Cymru Welsh Water is a company which supplies drinking water and wastewater services to most of Wales and parts of western England.It is regulated under the Water Industry Act 1991.-History:...
and Dee Valley Water
Dee Valley Water
Dee Valley Water / Dŵr Dyffryn Dyfrdwy is a company which supplies drinking water services to parts of North East Wales and parts of North West England...
; together with the regulator, the Environment Agency Wales
Environment Agency Wales
Environment Agency Wales is an Assembly Sponsored Public Body. It is that part of the Environment Agency that covers Wales. The Regional divisions of the Environment Agency are based on the concept of catchment management and administrative boundaries therefore follow the watersheds of major rivers...
.
Water demand
The water demands of North West EnglandEngland
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
including Liverpool
Liverpool
Liverpool is a city and metropolitan borough of Merseyside, England, along the eastern side of the Mersey Estuary. It was founded as a borough in 1207 and was granted city status in 1880...
and the Wirral far exceed the locally available sources of clean water. The River Dee
River Dee, Wales
The River Dee is a long river in the United Kingdom. It travels through Wales and England and also forms part of the border between the two countries....
runs mainly in North Wales
North Wales
North Wales is the northernmost unofficial region of Wales. It is bordered to the south by the counties of Ceredigion and Powys in Mid Wales and to the east by the counties of Shropshire in the West Midlands and Cheshire in North West England...
before flowing through Chester
Chester
Chester is a city in Cheshire, England. Lying on the River Dee, close to the border with Wales, it is home to 77,040 inhabitants, and is the largest and most populous settlement of the wider unitary authority area of Cheshire West and Chester, which had a population of 328,100 according to the...
, England, and then returning to Wales in a man-made channel constructed to gain land from the Dee Estuary
Dee Estuary
The Dee Estuary is a large estuary by means of which the River Dee flows into Liverpool Bay. The estuary starts near Shotton after a five miles 'canalised' section and the river soon swells to be several miles wide forming the boundary between the Wirral Peninsula in north-west England and...
. The Dee is the largest relatively clean river near to the North West conurbation
Conurbation
A conurbation is a region comprising a number of cities, large towns, and other urban areas that, through population growth and physical expansion, have merged to form one continuous urban and industrially developed area...
and without water from the Dee much of Liverpool
Liverpool
Liverpool is a city and metropolitan borough of Merseyside, England, along the eastern side of the Mersey Estuary. It was founded as a borough in 1207 and was granted city status in 1880...
would be without water. However, the natural flow of the River Dee during most summers is insufficient to sustain any significant abstractions. To overcome this problem, a series of reservoirs have been constructed to store the excess water available in the winter time and release it back into the River Dee during the drier months.
This is the principle of low-flow regulation It was used by Thomas Telford
Thomas Telford
Thomas Telford FRS, FRSE was a Scottish civil engineer, architect and stonemason, and a noted road, bridge and canal builder.-Early career:...
at the beginning of the 19th Century in order to guarantee a supply of water to the Ellesmere Canal
Ellesmere Canal
The Ellesmere Canal was a canal in England and Wales, originally planned to link the Rivers Mersey, Dee, and Severn, by running from Netherpool to Shrewsbury. The canal that was eventually constructed was very different from what was originally envisioned...
. Telford constructed sluice
Sluice
A sluice is a water channel that is controlled at its head by a gate . For example, a millrace is a sluice that channels water toward a water mill...
s at the outlet of Bala Lake to control the flow downstream so that there was always sufficient to supply the canal where it started at Horseshoe Falls
Horseshoe Falls, Wales
The Horseshoe Falls is an artificially created waterfall on the River Dee near Llantysilio Hall in Denbighshire, Wales, approximately west of the town of Llangollen....
.
The River Dee has also been used for direct drinking water supply with the Alwen Reservoir
Alwen Reservoir
The Alwen Reservoir or Cronfa Alwen is a 5km long reservoir near Pentre-Llyn-Cymmer in the county borough of Conwy, North Wales, held back by the 27 metre high Alwen Dam. It impounds the Afon Alwen, and the dam is 8km downstream from Llyn Alwen. It was built between 1909 and 1921, originally to...
, built in the 1920s to supply Birkenhead
Birkenhead
Birkenhead is a town within the Metropolitan Borough of Wirral in Merseyside, England. It is on the Wirral Peninsula, along the west bank of the River Mersey, opposite the city of Liverpool...
with water.
In the industrial revolution many rivers in industrial areas became too polluted by effluents to be usable for water supply. The Dee however remained clean with relatively few polluting effluents in the Dee catchment upstream of Chester. Consequently, the City of Chester
Chester
Chester is a city in Cheshire, England. Lying on the River Dee, close to the border with Wales, it is home to 77,040 inhabitants, and is the largest and most populous settlement of the wider unitary authority area of Cheshire West and Chester, which had a population of 328,100 according to the...
has been able to directly abstract Dee water since the first Chester Waterworks Company was formed in 1826.
Bala
In the late 1950s the Bala Lake Scheme was promoted to increase the available water for abstraction in the River Dee. Telford's original sluices were by-passed and the natural lake outlet was lowered . New sluice gates were constructed downstream of the confluence with the Afon TrywerynAfon Tryweryn
For the flooding of the Tryweryn Valley, see Llyn Celyn.The Tryweryn is a river in north Wales which starts at Llyn Tryweryn in the Snowdonia National Park and after joins the river Dee at Bala. It is one of the main tributaries of the Dee and has been dammed to form Llyn Celyn...
, which is only a short distance from the lake exit. This provided 18 million cubic metres of stored water in Bala Lake that could be controlled and used on a seasonal basis for low-flow regulation. This enables continuous abstraction from the River Dee of 235,000 cubic metres per day by six Statutory Water Undertakings and British Waterways Board
British Waterways
British Waterways is a statutory corporation wholly owned by the government of the United Kingdom, serving as the navigation authority in England, Scotland and Wales for the vast majority of the canals as well as a number of rivers and docks...
. An additional benefit was a reduction in flooding events downstream of Bala
Bala, Gwynedd
Bala is a market town and community in Gwynedd, Wales, and formerly an urban district of the historic county of Merionethshire. It lies at the north end of Bala Lake , 17 miles north-east of Dolgellau, with a population of 1,980...
as Bala Lake was able to contain the worst of the winter flood peaks.
Llyn Celyn
As demand for water increased it was necessary to increase the storage on the River Dee and the next development was Llyn CelynLlyn Celyn
Llyn Celyn is a large reservoir constructed between 1960 and 1965 in the valley of the River Tryweryn in Gwynedd, North Wales. It measures roughly 2½ miles long by a mile wide, and has a maximum depth of...
, a new 81,000,000-cubic-metre capacity regulating reservoir within the Bala Lake catchment area. This was completed in 1967 by Liverpool Corporation, and designed to operate in conjunction with the Bala Lake Scheme. This enables additional Dee abstractions of 327,000 cubic metres litres per day together with additional flood control storage. In summer time the impact was to increase threefold the dry-weather flow for most of the length of the river. This development was hugely controversial at the time and remains politically fraught to this day since the construction of the reservoir involved flooding the Tryweryn Valley and the village of Capel Celyn
Capel Celyn
Capel Celyn was a rural community to the north west of Bala in Gwynedd, north Wales, in the Afon Tryweryn valley. The village and other parts of the valley were flooded to create a reservoir, Llyn Celyn, in order to supply Liverpool and The Wirral with water for industry...
and twelve farms. Local people saw this a destruction of part of the Welsh culture
Culture of Wales
Wales has a distinctive culture including its own language, customs, holidays and music.Wales is primarily represented by the symbol of the red Welsh Dragon, but other national emblems include the leek and daffodil. The Welsh words for leeks and daffodils Wales has a distinctive culture including...
in order to supply England with water. This caused a great deal of controversy, resentment and protest. To try to offset some of the environmental concerns associated with the scheme, some of the stored water was specifically set aside to make special releases to help fisheries
Fishery
Generally, a fishery is an entity engaged in raising or harvesting fish which is determined by some authority to be a fishery. According to the FAO, a fishery is typically defined in terms of the "people involved, species or type of fish, area of water or seabed, method of fishing, class of boats,...
, to provide recreational opportunities (canoeing and white-water rafting on the Afon Tryweryn
Afon Tryweryn
For the flooding of the Tryweryn Valley, see Llyn Celyn.The Tryweryn is a river in north Wales which starts at Llyn Tryweryn in the Snowdonia National Park and after joins the river Dee at Bala. It is one of the main tributaries of the Dee and has been dammed to form Llyn Celyn...
) and to disperse pollution events should they occur.
A four megawatt hydro-electric station at the dam was also included in the scheme.
Llyn Brenig
Further statutory powers were gained in 1973 to construct another major regulating reservoir in the Brenig valley - Llyn BrenigLlyn Brenig
Llyn Brenig is a reservoir located in North Wales, in the heart of the Denbigh Moors, on the border between the counties of Conwy and Denbighshire...
. This reservoir was first filled in 1979 providing an additional 60 million cubic metres. This increased the potential for abstraction from the river in the lower reaches to nearly 900,000 cubic metres per day.
By 2002 the authorised abstractions had been taken over by three statutory undertakings and British Waterways Board with a total licensed abstraction of 850,000 cubic metres per day. In addition a residual flow of at least 364,000 cubic metres per day is maintained over Chester Weir
Chester Weir
Chester Weir is a weir which crosses the River Dee at Chester, Cheshire, England, slightly upstream from the Old Dee Bridge . The weir and the associated salmon leap have been designated by English Heritage as a Grade I listed building....
in all but the most testing of drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
s, safeguarding the passage of migratory fish and limiting the ingress of saline water over Chester Weir during high tides.
Operating rules
The operating rules for the current system are agreed with all the participants and define the circumstances in which flow will be managed and precisely managed. The key measuring location is at Manley Hall, a gauging station near ChirkChirk
Chirk is a small town and local government community, the lowest tier of local government, part of Wrexham County Borough in Wales. It has a population of over 4,000....
. This location was chosen as it was on a section of the river where flow could be readily measured and above the very flat stretch that meanders into Cheshire. The current rules state that when flow exceeds 10 m³/s no intervention is required. In practice some adjustment of Bala sluices may take place to increase storage in Llyn Celyn and conversely some releases may be made from Llyn Celyn for recreational or power generation purposes. When the flow at Manley Hall approaches 10 m³/s, additional flow is released from Llyn Tegid. If that is insufficient, flow from Llyn Celyn is used to maintain 10 m³/s at Manley Hall. In extreme situations where the flow from Llyn Celyn is insufficient to maintain the flow, releases of water from Llyn Brenig . Circumstances can arise where even this is insufficient to maintain flows and in such cases drought provisions are agreed which progressively reduce the maintained flow at Manley Hall.
The overall effect of this regulation has a marked impact on the hydrograph
Hydrograph
A hydrograph is a graph showing the rate of flow versus time past a specific point in a river, or other channel or conduit carrying flow...
of the River Dee. In dry years the hydrograph flat-lines at 10 m³/s whilst dry weather continues such as in 1990
Quality management
The River Dee regulation system also manages a water quality monitoring and alerting system that includes real-time quality monitoring for a wide range of chemical parameters supplemented by daily fixed site monitoring with analysis provided in near real-time by a dedicated laboratoryLaboratory
A laboratory is a facility that provides controlled conditions in which scientific research, experiments, and measurement may be performed. The title of laboratory is also used for certain other facilities where the processes or equipment used are similar to those in scientific laboratories...
service. The results of the analysis are made available to the four participating organisations on a routine daily basis. For each of the critical water quality parameters alert levels and action levels have been set based on past experience. If an alert level is exceeded an immediate alert (DEEPOL 1) is issued to all participants. As contamination levels increase or the contaminant plug nears an abstraction point the DEEPOL level rises to DEEPOL 2 and finally DEEPOL 3 at which time the affected abstractions are closed until the river quality returns to normal. This quality management system was developed following a severe Phenol
Phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
pollution of the River Dee in the 1980s which resulted in contaminated water being supplied to large areas of Liverpool and the Wirral. Contamination of raw water is a more significant problem on the River Dee because the River Dee is normally of exceptionally good quality and as a result the abstractions have been constructed directly from the river rather than via bank-side storage reservoirs as is more common when taking water from Rivers of less reliable quality.