Evans Water Engineers


+44 1566 782285 · www.evans-engineering.co.uk · email sales@evans-engineering.co.uk


For power outputs up to 500 kW

Overshot waterwheels are amongst the most fish-friendly devices, as any fish are simply 'poured' into the buckets of the wheel and are transferred with no change of pressure, or abrasion risk to the lower level. Should a fish jump out of a bucket, it will land in the tail water leading from the wheel.

The Bonnaud Noria (Overshot wheel in the form of a chain) operating as a prime-mover rather than as a 'water-lifter' will have even less impact on fish.

High breastshot wheels or Norias should be satisfactory if attention is paid to the water entry so that fish cannot become trapped between the device and the inlet flume.

The Archimedes Screw is a little different in that there is no pinch point at the entry and no 'shear' between the water and the 'concave' since the concave rotates with the screw (true Archimedes Screw). The smoothness of the outer drum and the flutes of the screw are only relevant in terms of the body of water passing over it. Should an eel or lamprey attach itself to the outer drum it will not be minced but be carried over into the next slug of water progressing down the tube.

Hydrodynamic Screws in common with backshot and undershot waterwheels are more difficult to make fully fish friendly on account of the 'pinch points' and 'shear' between the water and the 'concave'. The much-published 'hydrodynamic screws' usually pinch points at the inlet and 'shear' between the water and the concave. Provided that the concave is very smooth and the clearances between the screw and the 'concave' are very tight, the performance will be reasonable, but as soon as the surfaces become rough due to corrosion (in the case of mild steel) or abrasion (in the case of concrete) the position will be somewhat different. Even a very small gap will undoubtedly result in small fish, lampreys and eels being minced.






Fish friendly turbine

Above is a vertical shaft 'Fishglide' turbine with a cast runner.

The 'Fishglide' range

Fish-Friendly turbines are only applicable low heads because fish passing through will be subjected to a change in pressure, and this should ideally be to be kept within what a fish might experience naturally going over a waterfall or passing through rapids. My own design criteria are that it should present no greater danger to a fish than natural migration. A fish ascending or descending a river will encounter natural obstructions that are sharp and irregular, whereas the turbine passages are smooth and streamlined. The turbine has no pinch points and the guiding inlet parts are well away from the turbine runner that is very low running and has no sharp edges. The way that any fish meets the runner is more akin to passing through a revolving door, but without the possibility of getting caught between the rotating part and the stationary part.

Evans Engineering fish friendly turbines

Explanation of features:

APPROACH ANGLE. The water may approach the moving components of a device in the same general direction of rotation (with) that is a good thing or it may approach at right angles to the plane of rotation (Across) that is not so good. If the leading edge(s) are heading towards the approaching flow, it is not absolutely clear if there are any benefits or penalties.

The design objective should be to minimise the risk of blade strike.

SHEAR DISTANCE. Most turbines have stationary guide vanes to direct the water at the appropriate angle towards the runner. The distance between the downstream edges of the guide vanes and the leading edges of the runner will have a bearing on the size of fish that can safely pass through the turbine. Other devices including ‘screws’ may have fixed components such as bearing brackets that are close to rotating components.

The design objective should be to maximise the spacing and/or minimise the fish size.

LEADING EDGE PINCH. With back-shot or under-shot waterwheels in particular, the whole edge of each bucket/flight closes onto the concave, as it does with other positive displacement devices. It may be possible to design a soft edge to reduce this danger.

The design objective is to remove all potential pinch points.

LEADING EDGE. The leading edge of each blade/flight may be sharp or blunt if they are fabricated from steel plate. If the runner is made as a moulding or casting, it is possible to increase the thickness of the leading edge and radius it to minimise the damage to fish if a ‘strike’ should occur.

The design objective should be to maximise the leading edge radius.

LEAD EDGE CORNER. Where the blades/flights attach to the rotating case or ‘runner ring’ it may be an acute angle forming a ‘pinch point’, or at right angles, or curved so as to guide any fish or debris away from the corner.

The design objective should be to reduce the ‘pinch point’ as much as possible.

BLADE TIP PINCH. This mainly applies to turbines and screw devices. A propeller turbine and a hydrodynamic screw are comprised of blades/flights rotating within a stationary casing. The leading edge outer corner comes close to the casing. Very small clearances less than 1mm cannot be maintained because of engineering constraints, corrosion and distortion. Small fish, eels and lampreys cannot in my opinion pass safely through such devices because they may either attach themselves to the stationary casing or try to pass through the clearance gap that may be as large as 4mm, and possibly more.

The design objective should be to eliminate all pinch points by having the case rotate with and be joined to the blades/flights.

PRESSURE CHANGE. With waterwheels and screw devices there is no appreciable change in hydrostatic pressure as the fish transit the device. Conventional turbines operating with a head difference of 5 to 10 metres can injure or kill many fish. By limiting the pressure change and rate of change of pressure to within limits that can be shown to be safe would thus not exclude turbines.

The design objective should be to avoid injurious pressure changes.

TRANSIT TIME. It is reasonable to assume that minimising the time that fish are exposed to the dangers of ‘flight tip pinching’ and ‘abrasion’ against a stationary casing, is a good thing.

The design objective should be to minimise transit time.

SHEAR TO SURFACES. An overshot or breast wheel transfer the fish within a bucket that is relatively static. An Archimedes Screw transfers fish within a ‘slug’ of water that is subject to rotation and turbulence. A hydrodynamic Screw additionally subjects fish to a fixed casing that is moving relative to the ‘slug’ of water. A shrouded turbine subjects fish to rotation about the shaft but low turbulence and shear.

The design objective should be to minimise ‘high shear’ within the water.

INTERNAL VELOCITY. As the head increases and the water velocity within a turbine will increase, the transit time within other devices will also increase.

The design objectives become more important as the head is increased.

SURFACE FINISH. Manufacturing and maintaining a very smooth internal surface finish will reduce both turbulence and abrasion risk. The use of stainless steel or smooth plastics will reduce the damage to fish if they should come in contact with the casing or blades.

The design objective should be to maintain the smoothest practical finish.

TURBULENCE/SPLASH. High shear, turbulence and splash will all contribute to stress for the fish.

The design objective should be to maintain as smooth a flow pattern as possible.

ABRASION RISK. Some improvements can be achieved by the use of smooth internal surfaces. Abrasion risks are mainly reduced by reducing transit time and the relative velocities between fish and the components of the device.

The design objective is to is to limit these factors as much as possible.


Explanation of other features:

NOISE. Waterwheels produce quite a lot of splashing noise that is either pleasant or annoying to mill owners. Hydrodynamic and Archimedes screws can also produce splashing and slapping noise depending on the level of fill and speed of rotation. Turbines should be almost totally silent because they are submerged. All devices have the potential to generate noise from the transmission and generator if care is not taken.

PHYSICAL SAFETY. Serious injury or even death can be caused by contact with waterwheels, screws or other such devices. Enclosing or fencing them from animals and the public can result in very unsightly installations. Turbines are generally much safer in that the working components are either submerged or relatively easy to enclose within the main turbine room/shed.

EASE OF INSTALLATION. Waterwheels and Screw devices are large and cumbersome. Wheels and Norias can be assembled in place with a couple of people and light lifting equipment.

HEAVIEST COMPONENT. Screw devices have a large main component that requires an enormous crane to gain the necessary reach and lift. The access to the site may well present a major problem and expense. The process may well have to be repeated at regular intervals to carryout maintenance, particularly if the components are not manufactured in stainless steel and a smooth internal surface finish is to be maintained.

A critique of fish-friendliness of each device:

OVERSHOT WATERWHEEL. With this I include ‘pitch-back’, ‘breast wheels’ and Noiras because they are essentially the same type of device where water pours from a launder into the wheels buckets. There should be no pinch points as the water flows straight onto the top of the wheel, but there is a lot of turbulence as the buckets fill, and the smoother the inside of the buckets the less likely the fish are to be injured. The transit phase is a ‘fish-friendly’ as one can ask for, and the exit should be relatively danger free if the buckets are smooth.

Summary: No pinch points but turbulence at inlet and smoothness of surfaces should be considered.

UNDERSHOT WATERWHEEL. With this I include ‘back-shot wheels’ where the water is contained between paddles/floats and a masonry concave behind the wheel. The main dangers to fish are from the ‘pinch point’ across the full width of the paddles and the radial clearance gap between the outer edge of the paddles and the concave. Both these areas can be improved, the first by installing a soft edge at the inlet to the concave and secondly by maintaining minimal clearances against a smooth concave,

Summary: Significant problems with ‘pinch points’ but with some scope for improvement.

DYDRODYNAMIC SCREW. Developed as a pumping device for contaminated water and sewage, these devices exhibit the same problem as back-shot waterwheels with the fit to and the smoothness of the concave. In addition the leading edges of the flights are straight, thin and are not contained within a ring to remove the tip pinch point.

Summary: Being marketed as fish-friendly is somewhat misleading when compared to other devices including the ‘True Archimedes Screw’ that it simply isn’t.

ARCHIMEDES SCREW. Similar in construction to the hydrodynamic screw, but it differs in a number of important ways. The screw/flights rotate together with an outer drum so that there is no problem with fish and eels being crushed in the clearance gap between the flights and the concave. The inlet edges of the flights present the same danger to fish as in the hydrodynamic screws, but the tip pinch points are removed because they are contained within the drum. The slugs of water passing through the device are subject to turbulence as the drum rotates and it is important that the internal surface (though rotating with the flights) are kept smooth.

Summary: Significantly more fish-friendly than a hydrodynamic screw but the inlet requires improvement and the surface finish must be smooth and remain smooth.

FISHGLIDE TURBINE. This turbine has been developed from basic principals to give the best protection for migrating fish without compromising the hydraulic performance. Many of the design criteria for modern turbines such as the Francis or Kaplan designs make the safe passage of fish almost impossible (high specific speed). Traditional designs that were abandoned because they were slow speed and difficult to govern can once again be of value because of their potentially ‘fish-friendly’ features.

The ‘FISHGLIDE' is essentially a low specific speed mixed-flow reaction turbine. The standoff between the guide vanes is maximised. The vanes are few in number and fixed so that the inlet is not obstructed. The water and any fish are directed with the rotation as the runner. The lower the head and larger the turbine, the less difficulty there will be with passing fish safely and issues including the rate of pressure change are well within the limits considered to be harmless. The ‘runner’ leading edge is rounded to reduce the possibility of injury and curved so as to encourage fish into the centre of the water passage. There is an outer ‘runner ring’ that eliminating the pinch-points associated with ‘hydrodynamic’ screw designs. All surfaces are very smooth being stainless steel or resin coated and the transit time is very short.

Summary: The design objective was to produce a turbine that presents less of a risk to migrating fish than the natural features of the river ascending or descending the same drop. Zero injuries to all species and sizes of fish that can enter the turbine is the primary and achievable objective.


Rupert Armstrong Evans

Managing Director