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 Water Abstraction, inter river transfer, and their effects

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Chris Daphne

Male Posts : 141
Join date : 2010-01-28
Location : Highlands of Scotland

PostSubject: Water Abstraction, inter river transfer, and their effects   Fri Mar 26, 2010 12:19 am

Thought i would just write a bit about the above.


Simplest method involves placing a pipe in a stram and diverting some of the water down the pipe under gravity to a supply point. - Frequently used in rural areas with little effect on the aquatic environment as long as a small proportion of the flow is abstracted.

For a larger community - a lake is usually tapped into. A sluice or dam is built at the outflow to control water levels. This may impede fish movement on a migratory system unless a fish pass is built into the outflow.

Where no pond/lake is available then reservoir construction may be necessary. In its simplest form- a small catchment in a hilly area is trapped by a small dam and water diverted to a supply point with the exception of a compensation flow which passes downstream to maintain the stream environment. Again there are few environmental problems although there may be objections should the scheme flood an area of scientific interest.

Large reservoir formation means impounding the upper reaches of a river in an area of high rainfall and diverting water directly to the demand area ( Direct Supply) This may mean water is piped a large distance and may not be returned to the river. Or it may be used within the catchment and returned lower down the river but with reduced quality.
This type of system has many environmental problems - changes in flow, water temperature, water quality, obstruction, delay to migratory fish and flooding of spawning and nursery areas.

A reservoir may be sited close to a river and kept full by pumping from the river when the flow is adequate. The reservoir water may be used to supply a nearby town - Oxford from Farmoor reservoir. It may be used to supply towns that are a good distance away like Grafham. If the supply is used close to the river and the effluent returned then there may be little effect. When the supply is used outside the catchment there may be problems like the ones previously mentioned. More problems arise if the water is pumped to towns upstream and the effluent returned to the river so that some of it may be pumped into the reservoir. This recycling may accelerate Eutrophication - Effluent from Milton Keynes recycles into Grafham

Another type of reservoir which are of greater value to a river are "Regulating reservoirs". Water is released from the reservoir during periods of low river flow and abstracted downstream at the point of demand. The flow is regulated, flood flows are reduced due to some of it being impounded and drought flows increased due to supplementation. When water is required for abstraction a compensation flow is discharged to maintain the river.
Environmental problems still occurr, the river is no longer natural. Large fluctuations in river levels may occur. There may be insufficient flow to scour the river bed of silt and weed growth and assist migratory fish. Lower flows may exist below the abstraction point.

Another method is to pump water directly from the lower reaches into a storage reservoir. The river is left in a natural state until the last minute. This scheme can only operate if the water is reasonably pure and where treatment facilities exist. This is the case with LONDONS SUPPLY FROM RESERVOIRS AT STAINES AND BARNES TAKING WATER FROM THE THAMES AND LEE.

A system was developed by the old Thames Conservancy of pumping water from boreholes but the water into into rivers and using these as aqueducts to carry water to towns - Lambourn and Kennet supply water to Metropolitan London. The punping augments summer flows and may be beneficial as long as the boreholes are not overexploited. This danger point has now been reached and the system is under review.

All the above schemes are relatively simple and involve single rivers. For future demand a more integrated strategy is required - this is best achieved by INTER RIVER TRANSFER from one river to another. This has many biological, physical and chemical impacts on both rivers.

Effects of Water Transfer
In some cases the amount of water from the donor will be more than the average daily flow of the recipient leading to flood conditions leading to a need for channel modification. Enlarging the recipient channel could mean the destruction or improvement of the existing fishery or create a new fishery. Fish and food webs require specific conditions to survive. Fish will not remain where there is insufficient depth or cover. A straight trapezoidal channel with steep sides and evenly graded provides little or no cover and is poor habitat. A winding channel cut with one or more lateral shelves ( Berms ) deepened on bends and with natural cover will rehabilitate as natural scour takes place.
Seasonal transfers can be beneficial to fisheries such as in drought conditions on the recipient river. The rate of the introduced flow is important, sudden deepening may occurr or a sudden reduction may strand fish.
Changes in flow velocity- will affect river bed stability, sedimentation, survival of rooted plants and invertebrates, river channel habitats and the bankside habitat. Suspended solids may be increased leading to smothering, increased turbidity which will affect plant growth.
Changes in water temperature - A river recieving water from a reservoir may undergo severe temperature fluctuations if the water is drawn from the hyperlimnion in a stratified water body.

River transfers in Britain tend to be from NW to SE. This means soft water is moved into a hard water area. If the donor water carries pollutants or has the capacity to affect pollutants already present in the recipient river. Changes in PH are important as this may increase the toxicity of ammonia and metal cyanides.
Biological problems - Transfer of fish diseases and parasites, invertebrates, phytoplankton, macrophytes and fish.
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James Page

Posts : 2152
Join date : 2010-01-21

PostSubject: Re: Water Abstraction, inter river transfer, and their effects   Tue Apr 20, 2010 8:15 am

that's brill bro, or, just the ticket in my case, cheers buddy
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Richard Crimp

Male Posts : 1875
Join date : 2010-01-20
Age : 52
Location : London

PostSubject: Re: Water Abstraction, inter river transfer, and their effects   Tue Apr 20, 2010 11:14 pm

Very interesting Chris, and although I've read about water transfer before, it is nice to see it in a condensed form that is relativly easy to understand.


Richard Cool
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