Abstraction, Impoundment, Regulation and Entrainment
Abstraction is the term for the controlled removal of water from rivers, lakes etc. Both our rivers arise in high rainfall areas and there are a series of man-made reservoirs.
(Impoundments) - the Elan dams on the Wye and the reservoirs at Crai, Usk, Talybont, Grynne Fawr and Llandegfedd on the Usk. These fill during high rainfall periods and the water so held can be used for supply, either by releases down the rivers or by piping it directly to the users. There are numerous licensed abstractions for agricultural use and some take water for winter storage.
Abstraction reduces flows downstream during dry periods and also causes the problem of Entrainment. This is the inadvertent taking of anything 'unwanted' - in this case our concern is that fish and eggs are sucked in at water abstraction sites and lost from the river. Screens to prevent this are in themselves problematical particularly for small fish, eggs and juveniles because the necessary small size of the grid required to exclude them requires constant unblocking.
On the Wye, the huge series of dams across the Elan allow water to be piped directly to Birmingham by a gravity feed. The current arrangement allows for a very small, continuous compensation flow to the Elan, which is augmented when the dams overflow. There is an arrangement that allows for additional water to be released if the lower Wye drops below a certain level at Redbrook (below Monmouth). This Regulation flow is used to support continued abstraction at Monmouth and Lydbrook and the water is transferred to South Wales and the Midlands. Additional abstractions are made along the Wye's length for agricultural, industrial and domestic use. However, the compensation flow is drawn from the bottom of the dam and is never at normal river temperature or chemistry and has few high flow events with consequential failure to replenish bedstone losses from the River Elan.
On the Usk, the Crai and Usk reservoirs store water which is used outside the catchment. The Grwyne reservoirs provide water storage and release into the river, while Talybont reservoir is piped to supply south east Wales. Water is pumped into Llandegfedd from the lower Usk via Court farm, and thence onwards for supply via the South East Wales circuit. The Usk is also abstracted at various points for agricultural and other uses.
On top of that, the Brecon and Monmouthshire canal has its main abstraction at Newton weir, Brecon, but there are additional off takes from the River Crawnon at Llangynidr and from several other tributaries downstream.
So what are the problems associated with the management of a catchment and its water resources in this way?
If managed efficiently, regulation of flow allows for:
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The extreme peaks and troughs of flow to be minimised, though some high flows are essential to good ecological status.
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Water can be provided at reasonable cost and effluent damage can be minimised.
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Some carbon free hydropower is generated.
However, the downsides include:
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Downstream of the various dams, the compensation flow has an abnormal temperature (high in winter, low in summer) and is far from ideal for hatching fish.
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The absence of spate flows can cause gravel compaction causing problems for spawning fish.
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The failure to renew sediment and gravel causes long term loss of spawning territory.
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Above the dams, valuable spawning territory is lost and often not compensated.
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Juvenile fish are lost due to problems with screening (entrainment).
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However well managed, the benefits of water saved for low flows does not fully compensate the loss, particularly if water is piped to other catchments.
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It is impossible to manage appropriate abstraction in the ace of unrestricted abstraction as in the case of the leak prone Brecon and Monmouthshire canal.
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Taking water during a medium or small spate can interfere with incentives for salmon migration.
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The current abstraction pattern on the Usk - taking a large amount of water at night rather than spreading the loss throughout 24hrs, has a very negative effect on salmon migration.
Underpinning the abstraction regimen is the so-called CAMS (Catchment Abstraction Management) process. Whatever CAMS may achieve, it is not obliged to take account of the particular needs of the migratory, Annex 2 species (Salmon, Lamprey, and Shad) for which the rivers are given their EU protected status. It is therefore quite possible for the two rivers to be managed in accordance to the abstraction regulations required by CAMS but to fall well short of their Favourable Conservation Status or Good Ecological Status as required by the Habitats and Water Framework Directives.
These needs are assessed in a separate process, The Review of Consents, which unlike CAMS does not involve any stakeholder representation. Of particular concern is the additional abstraction from the Usk to support the leaks from the Brecon and Monmouthshire canal. Under current regulations, an ancient abstraction of right ensures that more than adequate amounts of water may be drawn off at several points from the Usk without any reference to the needs of the river or any particular requirement to conserve water.