Evans Water Engineers


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Environmental Impacts of Hydropower


Small and micro-hydro projects do not have anything like the local impacts of large multi-megawatt storage hydro schemes. There are arguments for even these large scale developments in the right locations because they do save a lot of carbon emissions, but the potential for serious local environmental and social conflicts is very real. Saving CO2 emissions and reducing our dependence on fossil fuels must take a high priority, but installing many smaller ‘run of river’ hydro plants can capture almost as much energy but without the conflicts. So why hasn’t this been done in recent years? Well, simply because our financial and political ‘lords and masters’ would rather be photographed ‘cutting the ribbon’ at the opening of a 1000MW hydro plant than a 100kW plant in the ‘back of beyond’! When I started proposing such schemes in public meetings where I shared the platform with the pro-nuclear government representatives of the day, I was chided with the accusation that I wanted a return to some ‘Halcyon Hibridean Hovel’, times have certainly changed in the rhetoric if not in the actual deeds.


The impact of a small-scale hydropower installation may be significantly less than for a large scheme but they should non the less be examined. It should also be noted that with good design there can be positive benefits to the environment in addition to the CO2 reductions. The main impacts relate to the ‘system’ itself and the physical structures and items of plant. There are impacts that relate to the on-going operation of the plant, the construction phase and decommissioning in the unlikely event of the plant no longer being required. The most significant impacts can be on the fish population, so I will deal with it in detail. At risk of offending some fishermen, it should be pointed out that fishing in a parallel commercial interest to hydro and whilst the hydro engineer’s intention should be to minimise the impact on fish, the very nature of angling is to catch and usually kill fish. Having been brought up with a fishing rod I have no particular argument against sport fishing, but I will take issue with anyone who suggests that fishing interests have any particular environmental interests other than to further their own sport. That is not to suggest that the majority of anglers don’t hold strong environmental views, because I know that they do. A senior EA official put a value of a single salmon on the Thames at £10,000. I can well understand this because it is the ‘expectation’ of catching this elusive creature that spurs anglers to pay their licence fee, but at that price make damn sure you don’t kill it if you do catch it or it will be the world’s most expensive lunch!


Migratory fish, as I have already mentioned are a major consideration. With regard to migratory species the main question is whether they can pass unharmed through the particular waterpower ‘device’ or whether they will have to be excluded from the device and diverted by another channel. If the fish are excluded by means of a suitable screening system, then the nature of the turbine is not relevant to any fishery questions but the method of avoiding it is.

Intake screens are straightforward on high head piped projects since there is no possibility of fish passing through the turbine so have to be excluded right at the start of the pipeline. Projects with leats (channels) and a pipeline are more complex since it is necessary that fish moving down stream that are drawn into the lead, have a safe route back to the main stream without the risk of becoming stranded half-way.

Leat schemes with a fall greater than about 10 metres may have a problem with finding a safe route that cannot dry out, but each situation should be considered on its own merits. Remote screening systems may be difficult to maintain and power if they require mechanical operation, so it may also be appropriate to have a screening system part way along a leat where it is accessible. Care will necessary if any portion of the channel is drained for maintenance or by accident, again so fish aren’t stranded.

Fish-passes and ladders are a fairly complex matter to get right, not least because fish often do not do what is expected and move up and down a stretch of river or channel irrespective of what their general migration may be. I have also witnessed sea trout passing freely over obstacles deemed impassable by fishery officers and others swimming up through turbines whilst they are running at full power.

Fish diversion devices are in my opinion useless, and this is simply because they can fail if there is a loss of power to the acoustic device or bubble screen. A success rate of even 90% would mean that if several hydro projects were present on a particular river, the chances of a fish surviving all the plants would be small. I leave it to others to argue the case in favour of such systems, but I remain to be convinced.

Fish screens must be of fine enough mesh to physically exclude any fish of a size and/or type that would be killed or injured if it went through the turbine. The water velocity approaching the mesh of the screen needs to be sufficiently low for the fish to be able to escape to a ‘safe passage’. Failing this there is a case that if a fish was sucked onto a screen, for example a ‘rotating drum screen’ they should be washed off again without injury in the shortest possible time.

Back-washing screens are the only ones where fish can survive after being drawn onto the mesh and any mechanical cleaner would obviously injure or kill the fish. Overflow screens of the bar or Coanda type need to have enough surplus water to at least keep the surface wet and free from debris at the bottom edge, otherwise fish may slide down and get tangled in a mass of debris at the foot of the screen.

Depleted reaches of a river will have flows that are agreed and controlled by the EA licence agreements. It is my opinion that very simple systems should be employed to maintain and measure flows. Calibrated notches that give the agreed compensation flow for a particular river level or a calibrated intake that allows the licensed flow to be taken according to the flow in the river. Care needs to be taken if the flow being taken by the turbine changes rapidly and causes fish to become trapped in pools as the water level recedes.

The construction of a new leat or the restoration of an old one can have significant benefits for the aquatic habitat. The increased wetted perimeter offered by the new channel acts as new habitat, and combined with the hydraulic necessity for low velocities and an alignment that usually has more sunlight, the weed growth and associated ecology is abundant during the summer months when the hydro plant may not be functioning at all. There has been the tendency to specify that leats be closed down completely or to a very small ‘sweetening flow’ in the summer, and this may be a mistake. The decision should be taken on ecological grounds as opposed to bureaucratic ones as is sometimes the case.


Overshot waterwheels are amongst the most friendly devices, as fish simply fall into the buckets and are transferred with no change of pressure to the lower level. Should a fish jump out of a bucket, it will land in the tail-race. By the same token the Bonnaud Noria operating as a prime-mover (waterwheel in the form of a chain) 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.

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.

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.

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.


My personal objectives, and I accept that they are my own feelings, are that a well-engineered hydro scheme, whether high or low head, should be almost unnoticeable to the untrained eye. This is relatively easy for high head projects where the powerhouse can literally be buried, leaving only an access door visible. The tendency to build a square modern structure with a token slate roof and a security fence is a tragedy. If a building has to be above ground, then it should fit in with the locality, and if this is rustic dry-stone construction, that’s what it should be, not a square, flush pointed building like a public lavatory. As for erecting large signs about it being an environmentally friendly project, what about the signs! Low head sites are more difficult, partly because of the need on many sites to house equipment above the maximum flood level, though tanking the powerhouse so that it can be submerged isn’t usually a problem.

Waterwheels when installed in conjunction with an existing mill house or a new building of traditional design, can add to the general aesthetics of an area. I regret that installing any ‘Hydrodynamic Screw’ or ‘Archimedes Screw’ is a total abomination in most picturesque areas. The only visual parallel, along with the obligatory safety fencing is a sewage treatment works. Problems that have already been encountered including safety of the public, vandalism blockage and noise, rather outweigh the supposed visual fascination of watching it go round. If it is to be an item of entertainment or education in an urban environment, there may well be a good case for their use, but in quiet rural locations the only true aesthetic solution is to bury whatever you choose to install.


Any heavy construction work will bring with it a host of potential environmental impacts from water pollution, noise, dust and general annoyance to neighbours. I can only suggest that great attention is paid to how the project is carried out and that you give as much warning to neighbours about what to expect. The safety of the construction staff and the public at large is dealt with in more detail under ‘project management’.

Rupert Armstrong Evans

Managing Director