Information and Advisory Note Number 37 Back to menu

Ecological impacts of hydro schemes on Scottish fresh waters

1. Introduction

1.1 Scotland has some of the most favourable topography, geology and climate for hydropower in Britain and the available resources have been well utilized over the last 100 years. This note describes the main types of schemes and some of the potential impacts on the physical, chemical and biological nature of Scottish fresh waters. It will become clear that further research is required to understand existing impacts and identify others. The note also discusses mitigation measures and the roles relevant organisations have to play A glossary of terms (words are highlighted in bold in the text) and suggested further reading are provided at the end.

1.2 Hydropower is a form of energy generation which, unlike coal, oil and nuclear power, does not normally involve the discharge of polluting chemicals into the environment.

1.3 Hydropower is regarded as a renewable source of energy and existing Scottish plant has a capacity to produce 1200 MW of electricity (excluding pumped storage and independently owned plant). This accounts for about 10% of Scottish demand and about 2% of the total UK generating capacity. A scheme with a capacity of 100 kW would typically supply enough electricity for about 60 homes.

1.4 Government policy is to stimulate the development and use of renewable sources of energy (which includes tidal, wave, small-scale hydro and wind energy), and to work towards 1500 MW of new renewable electricity generating capacity in the UK by the year 2000. In Scotland, this policy will be implemented through the Scottish Renewables Obligation by Orders made by the Secretary of State for Scotland under the Electricity (Scotland) Act 1989

1.5 Although hydropower is regarded as a clean source of power generation, it can have numerous ecological impacts In the past, the major concern in Scotland has been for the survival of salmon stocks where hydropower dams have presented barriers to migration. Additionally, small weirs and dams have partly or wholly blocked off many small streams, and water transfers have reduced flows.

2. Types of hydropower scheme

2.1 Storage
Most Scottish schemes are of the storage type in which water flowing from the catchment is regulated by a reservoir Water levels in the reservoir and flows from it are controlled to provide the driving energy for the turbines and to maintain stream flow below the dam. A good example is the Rannoch power station on the northern shore of Loch Rannoch (45 MW capacity). The amount of electricity to be generated is proportional to the volume of water discharged and the difference in height between the upper reservoir and the turbine station. These schemes can be very different in size and power output depending on the climate and terrain

2.2 Pumped
'Pumped storage' schemes have been developed in Scotland from the mid-1960s. Water is stored at high altitude and is released to a lower loch or reservoir to drive the turbines and provide power at times of peak demand or to control frequency on the grid. When demand is light the water is pumped to the upper reservoir for future use. There are two examples of pumped storage schemes in Scotland' Cruachan by Loch Awe (400 MW) and Foyers by Loch Ness (300 MW).

2.3 Small-scale
Since the late 1970s the simpler small-scale schemes, generating up to 5 MW, have become more common Typical small-scale schemes are of the 'run-of-river' type, which rely on the natural flow of rivers to generate electricity by utilizing the head of a number of small rivers in steep upland reaches They generally consist of a small reservoir or head pond, an intake weir, a short pipeline, and a small power station (Figure 1) They do not store water in wet periods for use in dry spells, so the output is not constant and only occurs when there is sufficient water in the river. Small-scale schemes are becoming favoured over traditional schemes because they are more economical to construct and maintain, and their impacts on the environment (though still requiring research) are regarded as more localised than traditional schemes.

2.4 Different hydro power schemes are designed to serve various types of demand in Scotland. Storage schemes occur throughout Scotland. They are designed for peak demand and require modest reservoirs. Pumped storage schemes were developed in the north for domestic and agricultural use, and often require reservoirs storing about 30-40% of annual run-off to cater for seasonal variations of rainfall. Small-scale schemes are constructed to supply remote areas in the north-west and the islands of Scotland, and are the most likely type of hydro scheme to be developed in the immediate future.

3. Impacts of hydro schemes

The impacts of hydro schemes are only predictable in general terms Hydro dams vary
in size and operation, and impounded water may be released from the reservoir surface, near the bottom, or both These differences influence the impact on the river The impacts of dams are not exclusively associated with hydropower and can apply to other schemes involving river regulation, e.g. water supply. For large-scale hydro schemes assessment should concentrate on the impacts of scheme operation and maintenance, since it is unlikely that there will be future development of these schemes in Scotland For small-scale schemes it is also important to consider impacts during the construction phase, such as siltation (see paragraphs 3 1.8 and 3.2 4).

3.1 Physical and chemical impacts of hydro schemes.

3.1.1 Flow changes
Many large-scale dams have transformed shallow, free-flowing rivers into wide, deep reservoirs which may be several kilometres in length Reservoirs attenuate flood peaks, so that flow magnitude is reduced and natural river flow variability downstream is moderated (Figure 2) If flows are reduced from their normal summer levels, large areas of river bed can become exposed and spray from waterfalls can be reduced Water transfer between rivers can result in the donor river periodically drying out, e.g. River Garry, Perthshire

3.1.2 Hydropower releases can result in repetitive and rapid changes in river levels (Figure 3), e g the River Tummel below Rannoch power station. Changes in river flow pattern can be observed across daily, monthly, seasonal and annual timescales In Scotland, hydropower releases usually enter lochs where water level fluctuation is small. On the River Tay, however, hydropower releases have resulted in stage changes 60 km downstream at the tidal limit.

3.1.3 During operation of some small-scale schemes the natural flow regime is altered by water intake, resulting in locally reduced flood peaks and reduced flows over waterfalls.

3.1.4 Compensation flows
The compensation water released from a dam is set so that the river flow downstream does not drop below its natural (unregulated) minimum However, this does not necessarily mean that it is set to benefit or maintain ecological interest. Compensation releases do not always reflect natural fluctuations in river flow, and can result in higher flows during dry periods than would naturally occur.

3.1.5 Compensation flows are often expressed as a 'Q' value. The 'Q' value is a percentage which equates to the natural flow exceeded 'Q'% of the time. Hence, a Q95 release is equal to the natural flow which is exceeded 95% of the time Compensation flows can vary seasonally. 'Q' values are usually based on long-term annual flow figures, which can be provided by SEPA (Scottish Environment Protection Agency).

3.1.6 Freshets
Where flood flows are considered important, particularly in rivers with migratory fish, artificial flood flows known as 'freshets' may be released at the request of local DSFBs (District Salmon Fishery Boards) Freshets help to encourage movement of fish upstream and mitigate ecological damage caused by artificially prolonged low flows, such as drying out of spawning grounds Freshets can also be
requested by SEPA to dilute and flush out pollution.

3.1.7 Sedimentation and erosion
Reservoirs act as major sediment traps which interrupt the natural transport of sediment Rarely has sedimentation led to a loss of reservoir storage capacity, though some reservoirs infill with sediments behind dams and have to be purged (e g Fort William scheme). As a result of sediments being trapped in the reservoir, sediment-free water is released downstream of the dam at high velocity. This leads to altered patterns of local and downstream erosion and sedimentation. Scouring of fine sediments from the river beds and banks is intensified in the reach immediately below a dam, and high discharge releases can lead to armouring of the stream bed.

3.1.8 In small-scale schemes, the risk of increased sedimentation downstream is usually greatest during the construction phase. Siltation is not an immediate problem in steep upland reaches, as silt deposits are likely to be quickly washed away, but silts can collect lower downstream and alter river morphology.

3.1.9 Temperature
Dams can affect water temperatures both in the reservoirs and in the downstream channels, especially if the water in the reservoir is stratified or layered In Scotland the magnitude of temperature modification is suppressed by the prevailing uniform climate and stratification is unlikely to be a common occurrence. Some deep Scottish lochs only stratify intermittently in summer, exhibiting gradual gradients in oxygen and temperature profiles. This can result in water temperature differences between the reservoir and river. For instance, small streams may fluctuate by 6-8°C on sunny summer days, but larger water bodies may vary less than 2-3°C. Such temperature differences in the reservoir will affect the river downstream when water is released from the dam.

3.1.10 Water chemistry
Hydro schemes involving water transfer may bring about a change in the water chemistry of the recipient river.

3.1.11 Pollution incidents during construction and maintenance of different schemes, such as oil spills from transformers and ancillary equipment, can have potentially adverse effects on rivers Oil spills are toxic to biota, and cause a reduction in light penetration and increase in biological oxygen demand.

3.2 Potential biological impacts of hydro schemes

3.2.1 The magnitude and type of effects are dependent on the kind of scheme Some effects may be permanent, and may occur at various distances from an impoundment.

3.2.2 Macrophytes
Some storage reservoirs have resulted in the permanent loss of terrestrial, wetland and riverine habitat through upstream inundation. Where a fast-flowing river has been impounded the reduction of light penetration, sedimentation, and chemical changes in deep water have caused a loss of macrophytes adapted to the original riverine habitat.

3.2.3 Changes in riparian plant populations may occur downstream as a result of less frequent and reduced flooding and spray/humidity Water discharged from dams attenuates natural peak flows which may result in stranding and desiccation of aquatic flora that require moist conditions Periodic flooding is a vital component of wetland ecosystems, and the removal or altered timing of natural flooding can change these highly productive areas into less productive terrestrial habitats. Generally, colonization by macrophytes below a dam is dependent on the frequency and magnitude of discharge fluctuations The less extreme the artificial flow regime, the more successful the retention or establishment of macrophytes

3.2.4 During construction of small-scale schemes, river-bank engineering, e.g. digging trenches for pipelines, may result in the disturbance and permanent loss of riparian vegetation. In upland reaches removal of bankside vegetation reduces leaf fall, shading and ambient humidity, and may lead to a reduction in important bryophyte communities.

3.2.5 Invertebrates
A major problem in reservoirs is drawdown. The rate at which the water level drops depends on the storage capacity of the reservoir in relation to generation requirements. In extreme situations most of the stored water is used for power generation at one time and the drawdown zone of the reservoir is wide. Invertebrates may then become stranded or cannot colonize these areas permanently, though re-colonization by chironomids and amphipods may be rapid.

3.2.6 Daily flow variation caused by power station operation can reduce available habitat and be lethal for species which only survive within specific flow limits. This can cause a decline in species diversity and abundance. Flow fluctuations also affect the natural cycle of invertebrate drift. Artificially high flows can increase invertebrate drift, whereas lower flows may reduce drift.

3.2.7 Altered flows downstream of small-scale schemes can result in reduced habitat diversity and lead to changes in instream biota which are highly adapted to local flow conditions. For example, low flows can cause a change from lotic mayfly species (e.g. Baetis and Rhithrogena) to lentic mayflies, such as Cloeon and Paraleptophlebia species.

3.2.8 Fish
Alterations to invertebrate and macrophyte communities, caused by hydropower schemes, can lead to reduced shading and food availability for fish

3.2.9 Dams clearly present a formidable physical barrier to the movement of migratory fish, such as salmon, sea trout, eels, and lampreys, both upstream and downstream. The Atlantic salmon has an ability to leap about 3.3 metres which is not nearly enough to surmount most major hydro dams in Scotland. Fish passes for such dams are therefore essential for fish to reach their spawning grounds and nursery areas.

3.2.10 Migrating fish can become trapped or delayed in pools when a power station stops operating and may be killed in summer by overheating or become more susceptible to predation by, e.g. pike, goosanders, and mink. Small-scale schemes also present restrictions to the movement of fish where there are longer periods of low flow, or where small weirs and intake pipes block upstream migration.

3.2.11 When a power station is in operation strong currents occur which can draw downstream migrating fish into the turbine intakes, rather than safely passing through turbine outlets and fish passes Approximately 10-40% of salmon smolts may be killed or injured after passing through turbines, depending on the type and size of turbine blades, and difference between upstream and downstream water level Smolt screens are used to prevent migratory fish entering the turbines. However, smolt mortalities may occasionally occur if fish are trapped against the screens by the force of water being drawn off at the intake.

3.2.12 Salmon and sea trout require clean, fast-flowing water through clean gravel or small stones to spawn successfully. Reservoirs in upstream areas may have inundated spawning areas and created silted, static conditions. It is therefore essential that below a dam compensation flows are adequate to maintain spawning reaches.

3.2.13 Reservoirs can provide an opportunity for brown trout fisheries to expand and become more available for exploitation. On the other hand, coarse fish such as perch or pike may colonize these new and then prey on or compete with trout and salmon.

3.2.14 Birds, mammals and amphibians
Water level fluctuations in reservoirs can have indirect impacts by removing habitat and reducing invertebrate and fish food sources for birds such as black-throated divers Drawdown can have direct effects on wildfowl breeding, through nests being stranded and possibly abandoned by adults, or the incubating adult being more susceptible to predation.

3.2.15 Otters are predominantly piscivorous and changes to fish populations could affect the carrying capacity of the habitat for this species. However, there is relatively little information on the impacts of hydropower schemes on amphibians or riparian mammals, though reduced food supply, changes to natural water level fluctuations and reduced cover will influence their distribution and abundance.

4. Mitigation measures

4.1 Contingency and management plans can significantly reduce the impacts of pollution events or prevent accidental release/spillage of pollutants. Sediment traps can be used to prevent and remove unwanted silt and debris, and oil interceptors prevent machinery oil from entering drainage systems.

4.2 Most research into mitigating the impacts of hydropower schemes on fresh waters has concentrated on the important Scottish fishery and fish stock aspects There has been less effort to investigate practical conservation measures for mammals, amphibians or birds (though floating islands or nesting rafts for some bird species have been used to reduce the impacts of water level changes on their breeding behaviour and success).

4.3 Fish passes and other solutions for migrating fish
There are many designs of fish pass and several methods for deflecting fish from turbine intakes, all of which have limited success The 'fish ladder1 is a series of small watery steps which the fish actively surmount when migrating upstream. A good example can be seen at the Pitlochry Dam.

Borland 'fish lifts' carry the fish upwards as the lift is filled with water, and the fish continue their upstream journey from the top of the lift. Examples are at Torr Achilty Dam in the Conon catchment, and Aigas Dam on the River Beauly

4.3.1 Encouraging fish migrating downstream to enter a fish pass can be a problem if currents are stronger from turbine intakes, as fish are attracted to faster currents. One design solution is to locate the fish pass so that downstream migrating fish encounter it before the intakes' immediate influence become apparent.

4.3.2 Many power station intakes are fitted with deflector screens or very fine mesh screens to prevent entry by smolts In extreme cases, migrating fish are caught and transported by road around some dams, in the Conon River system smolts migrating downstream were at one time trapped and transported for 32 km by road to a downstream release point; 99.8% of the smolts survived transport. Recent developments in turbine design and slower starting-up procedures have minimised fish damage at many sites. On small streams, downstream migration causes fewer problems because damage is generally less frequent and escape over spillways or passes is easier. Some research has involved the possible use of noise screens, loud underwater noises or even sounds of fish themselves to deter fish from entering turbine intakes Other research has concentrated on visually attracting fish to passes and lifts.

5. Legislation and roles of statutory organisations

5.1 Planning permission
New hydro developments below 1 MW , capacity require local authority planning permission Schemes above 1 MW capacity require consent from the Secretary of State (Electricity Act 1989). An environmental assessment may be required for schemes up to 10 MW (Environmental Assessment (Scotland) Regulations 1988) The Government has also produced planning guidance and advice for renewable energy developments. The planning consultation process provides SNH with an opportunity to comment on impacts affecting the natural heritage.

5.2 Discharge consents
SEPA provides information regarding the setting of compensation flows and sets discharge conditions (under the terms of the Control of Pollution Act 1974, amended by the Water Act 1989), and should be consulted for all proposed hydro developments. The discharge consultation process again allows SNH to comment on likely impacts on nature conservation interests.

5.3 Nature conservation
SNH is a statutory consultee for hydro development proposals at sites of international conservation importance (Special Areas of Conservation and Special Protection Areas), under the Conservation (Natural Habitats, etc.) Regulations 1994 Where a proposal may affect a nationally important site (e.g. Site of Special Scientific Interest) SNH
should be notified and consulted For proposals likely to affect these sites an environmental assessment will normally be required SNH policy regarding renewable energy developments is set out in an SNH Policy Guidance Note (see further reading)

5.4 Fisheries
Careful planning is required for the protection of all species of fish, particularly migratory salmon and sea trout Throughout the planning process the local DSFB (or in its absence, the Scottish Office Inspector of Salmon and Freshwater Fisheries) should be consulted, especially in the design and location of fish passes and exclusion devices, such as intake screens The Scottish Office has published a guidance note on the construction and maintenance of fish passes and screens for the safe passage of salmon and sea trout This guidance is provided to aid those who have to implement the Salmon (Fish Passes and Screens) (Scotland) Regulations 1994. Fish passes may require planning permission from the local authority, and advice should be sought from SEPA in avoidance of pollution risks during construction

6. Future research

6.1 One of the major areas for research is the effects of reduced flows on invertebrates, marginal plants, fish production, and distribution of birds, amphibians and mammals (especially in the light of possible re-introduction programmes, e g European beaver)

6.2 As the UK electricity industry continues to evolve, it is possible that some existing schemes may, in the future, be decommissioned. The potential ecological effects of any such proposals should be investigated, and this may require additional research before any decommissioning plans are implemented.

Glossary

Ancillary equipment - transformers, storage tanks, etc

Amphipods - members of an order of marine and freshwater crustaceans, e g freshwater shrimp (Gammarus sp)

Armouring tight compaction of the stream bed substrate by high dam discharges

Biological oxygen demand (BOD) - measurement of the amount of organic pollution in water (high BOD indicates heavy pollution)

Bryophytes - mosses and liverworts

Carrying capacity - the maximum number of individuals that can be supported indefinitely by a given part of the environment

Chironomids - family of true flies whose larvae develop in fresh waters

Drawdown - the fall of water in a reservoir, which can lead to a sterile reservoir margin

Invertebrate drift - the downstream movement of invertebrates in running waters, which shows natural daily patterns

Lentic - standing water, e.g. a loch or pond

Lotic - running water, e.g. a river or bum Macrophytes - freshwater plants visible to the naked eye

Piscivorous - fish-eating

Riparian - anything pertaining to the banks of a watercourse

Spawn - to deposit eggs and sperm in prepared gravels (redds)

Spawning grounds - the riverine gravels used for spawning

Stage changes - flow changes recorded on hydrograph

Authors

Nikki Wood, Scottish Natural Heritage

Dr Terry Langford, Hopehead Ecological Services, Milford-on-Sea, Hampshire.

Contacts for further advice and information

Nikki Wood
Freshwater Conservation Officer

Dr Philip Boon
Head of Aquatic Environments Branch

Located at
Scottish Natural Heritage
Research and Advisory Services Directorate
2 Anderson Place
Edinburgh EH6 5NP
Telephone: 0131-447 4784

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