Dulas has been working with developers, utilities providers and landowners to deliver hydropower projects throughout the UK and worldwide for three decades now. Our Operations and Maintenance team works closely with project operators across the UK to provide a comprehensive range of operations, refurbishment and optimisation services to ensure that each turbine is running to its full potential.

In this article, Head of the team, Dylan Roberts provides an introduction to one specific aspect of hydro: the run-of-river scheme

A design starts with the flow duration curve

A run-of-river hydro scheme is one that operates using the amount of water available in the river or stream at the time, and doesn’t involve the use of any upstream storage.  Once you have identified a suitable site, the design of a run-of-river scheme starts with the flow duration curve – once you have that information you can scale your scheme accordingly. The flow duration curve shows the statistical likelihood of river flows that can be expected over a period of time, and is either derived from a set of flow measurements, or calculated from a combination of geographical catchment information and historical rainfall data, often supplied by a third party or hydrology specialist. The flow rate is often referred to as ‘Q’, followed by a number – so for example, Q40 describes the flow rate that is matched or exceeded 40% of the time.

The lower the flow rate, the higher the Q number and vice versa, so Q0 represents the highest flow rate – effectively flows during an extreme flood – and Q100 the lowest rate, which should always (bar freak weather events, drought etc) be matched or exceeded. Most run-of-river schemes are designed to run at flows of Q35/40, giving the best economic return for a typical hydro scheme.  River flows can be very seasonal, but for the majority of run-of-river hydro schemes, over a typical year you would roughly expect to be running at full power for 3 months, off for 3 months and running somewhere in between for 6 months.


The river is an eco-system and the wildlife in the river is dependent on there being a certain amount of water flowing down it, so a minimum volume must be set aside for these considerations – this is referred to as the ‘compensation flow’.  This flow rate is set by the relevant environmental agencies and schemes have to be designed such that the compensation flow is always left flowing in the river.

Silt and sediment management is another environmental concern – both during build and subsequent operations.  Earthworks and construction must be conducted carefully to prevent mud and concrete washing into the river and having a detrimental effect on species such as freshwater pearl mussels.

In-river structures like weirs can also become obstacles to silt movement, which can have an impact on downstream environments – spawning grounds for fish for example – so proactive silt management can be required at some locations to reduce the impact on breeding fish populations.


Nearly all schemes have some level of automation these days so that turbines will switch on and use as much flow as possible and switch off when the river level reduces to the point where generation is no longer efficient, and drops down towards the compensation flow limit. Modern hydro schemes can send out warnings and alarms to let the owner/operator know if something has gone wrong or malfunctioned, or if the hydro isn’t behaving as expected.  For example, if the river level is high but the intake level is low, the scheme will let us know and our engineers can dial in and take a look remotely to investigate.  Sometimes it is a case of ringing a caretaker and asking them to have a look – in this example it could be that with the first proper storm of autumn there has been a large ‘leaf-fall’ event, which could then block the intake screen, so the remedy would be for the screen to be manually cleaned.


When it comes to optimisation, most improvements to schemes are made by looking at what is currently in situ and seeing what could be improved. For instance, if a scheme has an undersized forebay, it can drain down very quickly resulting in the turbine switching off and back on repeatedly (cycling), which is inefficient and undesirable.  The control code could be modified reduce the cycling, but we could also look at redesigning and rebuilding the forebay to make the control volume larger, making the system simpler to control.

An important element of our run-of-river schemes are our Aquashear Coanda screens. They allow scheme owners to maximise revenue by maximising water flow whilst reducing the need for maintenance.  We rarely build a scheme without them and often fit them when refurbishing/rebuilding intakes – the results are instantaneous and on one scheme where we rebuilt the intake with Coanda screens and modified the control code to resolve operating issues, we achieved a year-on-year generation increase of over 50%.

Health and safety

It goes without saying that when building or refurbishing, health and safety is paramount. While we work in all weathers, we are always mindful of the conditions – you don’t want to be halfway up a mountainside or in the middle of a river when the weather turns. Bad weather can also damage equipment – schemes are designed to deal with the sort of flood that might occur once in 50 years and mitigation measures such as lightning protection and surge protection can (and should) be put in place, but occasionally weather events may come along that just couldn’t have been predicted.

Thankfully, in the UK we don’t (at least yet) experience the sort of weather extremes that exist in other parts of the world and we have a plentiful supply of water to power hydro schemes – something I always try to be thankful for when caught in a downpour!