A fisherman’s friend11 December 1998
Small hydro power operators should not only be aware of fish screening laws, but should choose appropriate screening technology with care. Andrew Turnpenny* explains why
Following recent changes in fish screening law applicable to hydro power developments in the UK, the Energy Technology Support Unit (ETSU) commissioned a study, on behalf of the UK Department of Trade and Industry, to help clarify the legislation. A review of the fish screening tech-nologies that are suited to small hydro power applications (less than 1MW) was also under-taken. The new study, entitled A UK guide to intake fish screening regulations, policy and best practice, was undertaken by Fawley Aquatic Research Laboratories and hydro power consultants, Hydroplan UK.
In Britain and elsewhere, many old mill sites and other water power facilities have been brought back into service using the old infrastructure of weirs and headraces with modern Kaplan or Francis turbines.
These low-head, run-of-river sites, if not properly designed, equipped and operated, can cause impacts on migratory fish species that are disproportionately large compared with their electricity generation potential. This happens because they are usually built low down on the mainstems of rivers, or on larger tributaries, at points where a very large proportion of the migratory population must pass the plant in order to complete their migratory life-cycle.
As a result, friction may arise between hydro power developers, and fisheries bodies (including, anglers, fishery owners and regulatory agencies) who have justifiable fears that already declining fish stocks might be damaged further.
Legislation on fish screening
Regulation of fish screening in the UK is not uniform but is divided into three jurisdictions — England and Wales, Scotland and Northern Ireland. The requirement to screen water intakes against fish is by no means new, and is not restricted to water power developments.
In all parts of the UK, fish screening law has been refined over many years. New and revised legislation on the screening and diversion of migratory fish has been introduced in England and Wales (effective from 1 January 1999) and separately in Scotland, which is already in force. These changes have been introduced to make fish screening more effective and to allow for technological innovation. Current fish screening legislation relevant to small hydro power developments share some key features:
•Fish screens must be fitted and maintained at the owner’s expense.
•The screens must be capable of preventing the descent through the turbines of the specified fish species.
•A by-wash (also known as a bypass channel) must be provided, where required, to ensure that fish can continue safely on downstream.
•A ‘screen’ may be interpreted as any device that will prevent the entry of fish, whether it be a physical mesh or a so-called ‘behavioural’ screen which uses a deterrent stimulus (eg light).
Developers and operators
In the ETSU-commissioned study, developers and operators of small hydro power schemes from all over the UK were questioned about their views on fish screening requirements. Most acknowledged the need for fish protection and were aware of their responsibilities under the law. In England, Wales and Northern Ireland, regulations covering small schemes have been in place for many years, although they have not always been enforced strongly. This is likely to change in England and Wales, since powers have now been passed to the recently formed Environment Agency.
In Scotland there was rather less certainty about what screening measures would satisfy the Regulations. The owners of some small, low-head schemes (less than about 300kW capacity) fear that screening will make them uneconomic. One operator of such a scheme recently installed physical smolt screens, with 12.5x12.5mm sized mesh. He found that the combined costs associated with installation, cleaning and maintenance of the screens and loss of hydraulic head amounted to about 40% of previous annual revenue from the scheme.
The regulator’s view
While the laws on fish screening within the UK offer the flexibility to allow behavioural screens, to date there has not been a consistent policy towards their use. The concern of the regulators is that behavioural screens at best may achieve 90-95% efficiency, compared with close to 100% for a fine-meshed physical screen.
What is more, the efficiency of behavioural systems is not guaranteed and may fluctuate, depending on operating flows and ambient environ-mental characteristics. The possibility of achieving close to 100% efficiencies using two behavioural screens in tandem or with a combination of stimuli, however, merits consideration.
Hydro power operators should be aware that a wide variety of alternative fish screens and barriers are available. A few examples of those which are suitable for small hydro power applications are mentioned below.
The conventional smolt screen comprises retractable flat panels, usually of stainless steel, held in a slotted supporting frame. The panels are lifted periodically for cleaning, and usually the slots are paired so that a clean panel can be inserted before the soiled one is removed.
In a well-designed system, the screen array is aligned obliquely to the flow, an arrangement that both lowers the required swimming speed for the fish to avoid impingement and guides them into the by-wash. This arrangement is highly effective, but is costly to install and maintain and can reduce the available head, a particular disadvantage on low-head schemes.
Spillway and smolt-safe screens
Two types of spillway screen have been introduced recently. The Coanda-effect screen is an ogee-profile screen manufactured from stainless wedge-wire material that is fitted to the downstream face of a weir. Water flows over the weir and falls by gravity through the screen slots to collect in a chamber below.
The Coanda-effect, whereby fluids tend to adhere to a solid surface (the effect that causes milk to run down the lip of a jug) encourages water to flow this way. The open area is adjusted seasonally to ensure that a residual flow passes downstream and carries fish to safety.
The benefits of this type of screen over a conventional flat-panel smolt screen are that it is self-cleaning and virtually maintenance free. Owing to the small slot size (<1 mm), fish down to fry or egg size are excluded, as well as sand, avoiding the need for costly sand traps.
The smolt-safe screen uses a rather similar principle, although it relies more on gravity and less on the Coanda-effect. The mesh size here is of a more typical smolt-screen size. Smolts are diverted into the residual flow in the same way. To date the screen has not been used in hydro power applications but it appears suitable, perhaps for medium to high-head schemes.
Louvre screens date from the 1950s in North America and were tested experimentally in Scotland during the 1960s. Despite achieving deflection efficiencies of over 90% with salmonoid smolts, their use in Europe has been very limited but is now increasing.
These are classed as behavioural rather than physical screens, as the deflecting principle is the fishes’ avoidance reaction when meeting the turbulence generated by vortex formation. They are normally fitted obliquely across a channel.
Advantages of the louvre screen over a flat-panel screen are that flow is much less restricted and blockage is unlikely. Little maintenance is required, particularly if a trash rack is positioned upstream so as to prevent trash accumulation.
Louvre screens work well at channel velocities of 1m/sec or more but efficiency declines at low velocities. This can be a problem where generating load is low during the smolt season.
The bio-acoustic fish fence
The bio-acoustic fish fence (BAFF) is an entirely new device that uses an acoustic stimulus combined with an air bubble curtain. A pneumatic sound source is housed in a low profile chassis unit that also houses the bubble generator.
A wailing sound in the sub-600Hz frequency range, the optimum hearing band for most fish, excites the bubbles. Owing to the physical transmission properties of the air/water mixture, sound becomes trapped within the bubble sheet. An approaching fish detects a ‘wall’ of sound and veers away, thus being guided towards a by-wash, much as for the panel or louvre screens. The diversion efficiency for smolts is similar to that quoted for the louvre screen.
The advantages of the BAFF are that there is no detectable head loss and no trashing problem. Any design must, however, take account of the dynamic nature of the bubble curtain within the water flow.
By-washes and approach velocity
The by-wash arrangement is a crucial and integral part of any fish screening system. This is never more true than for a behavioural barrier, where any reluctance of fish to enter the by-wash may increase the risk of habituation to the barrier, resulting in loss of efficiency. This is often the mode of failure for behavioural screens. Some key points include:
•The by-wash should be placed close to the downstream end of the screen.
•The entrance should not be too narrow: 30cm would be a minimum.
•A flared or bellmouth entrance is best.
•The water should accelerate smoothly and exceed the burst speed of the fish within a short distance in order to commit fish to the by-wash.
•The more water used, the better. At least 2% of turbine flow is normally required. By-wash performance should be tested during commissioning of the system. There is no better way than by direct observation.
The velocity of water approaching the screen is also critical. Too high a velocity may cause fish to be impinged on a physical screen or drawn through a behavioural barrier. The velocity recommended for salmon smolts in the UK by the Salmon Advisory Committee is 25cm/sec (measured perpendicular to the screen face), based on earlier studies of smolt swimming speeds, although this is a guideline and has no legal standing. The value chosen has an important bearing on the cost of screening, as the lower the velocity, the larger the screen area required.
Recent Canadian studies by Peake and McKinley (Can J Fish Aquat Sci 55, 1998, pp682-687) throw doubt on the validity of the earlier estimates of smolt swimming performance and show that Atlantic salmon smolts are capable of sustaining speed in excess of 1m/sec for periods of hours.
If this finding were substantiated more widely, it would suggest that a design velocity of at least double the present value would be suitable, even allowing for the stragglers. This would make fish screening for hydro power plants much less expensive and would have important economic implications, especially for small, marginally viable schemes.
Selecting the right screening system The choice of a fish screening system must reflect a number of factors, including water flow, available head, required efficiency, availability of electrical power and manpower and cost. The technology must also be suited to the technical ability and resources of the operators.
It is important that the designers of hydro power schemes and their consultants, as well as fishery regulators, are well briefed on the technologies available and their limitations. The ETSU-commissioned guide is intended to provide this information.