River Dee regulation system
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River Dee Regulation Scheme
The water demands of North West England including Liverpool and the Wirral far exceed the locally available sources of clean water. The River Dee, which runs mainly in north Wales before entering the Irish Sea in Cheshire, England, 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 Shropshire Union Canal. Telford constructed sluices 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.
[edit] Bala
In the late 1950's 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 Tryweryn, 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 m³/d 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.
[edit] Llyn Celyn
As demand for water increased it was necessary to increase the storage on the River Dee and the next development was Llyn Celyn, a new 81,000,000 m³ capacity regulating reservoir within the Bala Lake catchment area. This was completed in 1965 by Liverpool Corporation, and designed to operate in conjunction with the Bala Lake Scheme. This enables additional Dee abstractions of 327,000 m³/d together with additional flood control storage. In summer time the impact was to increase three-fold 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 and 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, provide recreational opportunities (canoeing and white-water rafting on the Afon Tryweryn) and to disperse pollution events should they occur.
A 4 MW hydro-electric station at the dam was also included in the scheme.
[edit] Llyn Brenig
Further statutory powers were gained in 1973 to construct another major regulating reservoir in the Brenig valley - Llyn Brenig. This reservoir was first filled in 1979 providing an additional 60 million m³. This increased the potential for abstraction from the river in the lower reaches to nearly 900,000 m³/d.
By 2002 the authorised abstractions had been taken over by three statutory undertakings and British Waterways Board with a total licensed abstraction of some 850,000 m³/d. In addition a residual flow of at least 364 m³/d is maintained over Chester Weir in all but the most testing of droughts, safeguarding the passage of migratory fish and limiting the ingress of saline water over Chester Weir during high tides.
