Abstraction rates for hydropower

Our Abstraction Licensing Strategies set out how we licence abstractions at a catchment scale.  We use the licensing system and these strategies to make sure that abstractions from our rivers don’t cause flow to fall below our Environmental Flow Indicators (EFIs). EFIs describe the amount of water that we require in a river system to protect ecology.

Most abstractions for hydroelectric power generation in Wales are for high-head, run-of-river schemes that result in a flow depleted reach between the points of abstraction and discharge. Abstractions like this that take water out of a river or stream and then return it without loss at a point downstream are termed non-consumptive. For these we apply standards of flow protection that differ slightly to the EFI’s to account for the non-consumptive nature of the abstraction and in most cases the short reach of river with reduced flows.

Abstraction licences include conditions that detail when and how much water can be taken from a river or stream, so that enough water remains in the flow depleted reach to protect in-stream and riparian ecology, landscape, recreation and amenity.

Protecting biodiversity and resilience of ecosystems

The Environment (Wales) Act 2016 gives us a duty to maintain and enhance biodiversity and to promote the resilience of ecosystems in Wales. Our approach to licensing abstractions for hydropower recognises hydrology as the master variable, enabling proper structure and function of riverine ecosystems. It is based on limiting hydrological alteration to protect the whole river ecosystem rather than being specific in protecting target species.

Our approach to setting abstraction rates for hydropower where there is a depleted reach has three key principles. These are:

1. protecting the ecology of depleted reaches by seeking to replicate the characteristics of the natural flow regime
2. maintaining hydrological and ecosystem connectivity
3. managing the spatial impact of schemes on the wider river ecosystem according to the size and gradient of the stream and the location of that stream within a river catchment.

Where a proposed hydropower scheme risks adversely affecting flow sensitive features of a designated conservation site, protected species, supporting habitat or salmon spawning ground then an application may be refused or greater restrictions on abstraction applied.

As a rule, hydropower development in lower catchment streams and rivers presents a higher risk of disrupting ecosystem connectivity and of having a greater spatial impact than schemes that are in smaller, headwater catchments. The ecology of headwater streams can still however be sensitive to changes in flow.

The need to maintain ecosystem connectivity is particularly important to protect migratory salmonids that require access along lower catchment rivers to reach spawning and nursery tributaries where they are present in upper reaches of catchments. We therefore don’t favour developments in these areas.

Setting abstraction rates to protect the environment

Abstractions for hydropower that create a depleted reach will normally be required to protect three key characteristics of natural flow regimes. These are:

Protecting low flows

The flow rate at which abstraction should stop as streamflow falls is usually described as a ‘Hands-off Flow’ (HoF). This is an operational threshold set to ensure that low river flows will always be protected from abstraction. It makes sure that a ‘protected low flow’ is retained in the depleted reach to support in-river ecology. All hydropower abstraction licences will specify a Hands-off Flow. The protected low flow rate is usually equivalent to a low summer flow and termed a Q95 flow when described as a flow duration statistic. There may be cases where the environmental sensitivity of the depleted reach requires a higher flow rate to be protected.

Protecting flow variability

A hydropower abstraction will normally need to allow a proportion of the natural flow to pass downstream of the intake to mimic the river’s natural flow variability within the depleted reach. This flow is a proportion of the total flow in the range between the HoF and the maximum abstraction rate. The proportion abstracted is termed the ‘percentage take (% take)’.

It is important to preserve natural patterns of river flow within the depleted reach. Flow variability in the mid and low flow range inundates marginal habitats and provides life cycle stage stimuli for a range of flora and fauna as well as helping to support natural geomorphological processes downstream.

Protecting high flows

A maximum rate of abstraction above which no additional flow may be abstracted will be set. This rate is typically expressed as a proportion of the mean flow (Qmean). Having a maximum abstraction rate ensures that high flow rates necessary for in-channel geomorphological processes occur within the depleted reach.

Hydropower abstraction zones

Where a proposed hydropower scheme is ‘on-weir’ then the initial abstraction rates for schemes with no depleted reach may be applied (see Table 1).

Where a scheme will result in a depleted reach, simple site characteristics are used to classify its location into one of three zones to determine initial abstraction rates.

These zones are:

Zone 1: where a proposed scheme may affect sites designated for nature conservation, supporting habitat, certain protected species and  salmon spawning grounds (Table 2).

Zone 2: where a proposed scheme is not in Zone 1 but is a river or stream where the average channel gradient of the depleted reach is less than 10% slope  (<10%) (Table 3).

Zone 3: where a proposed scheme is not in Zone 1 but is a river or stream where the average channel gradient of the depleted reach is 10% slope or greater (=>10%) (Table 4).

Channel bed gradient

We use average channel bed gradient of the depleted reach as a simple measure to collectively describe the location of a depleted reach within a catchment and its likely hydrological and geomorphological characteristics. It is calculated by dividing the difference in elevation between the points of abstraction and discharge by the length of the river channel to provide an average channel gradient for the depleted reach.

A depleted reach bed gradient of 10% and greater is indicative of small, steep, high energy, upland streams that are responsive to rainfall, have incised stream channels and tend to be less sensitive to changes in flow. Reaches with a bed gradient of less than 10% slope are typically lower catchment rivers that have wider, shallow channels that are more sensitive to changes in flow and are important reaches for ecological connectivity within a river system.

How to use our guidance to determine an abstraction regime for hydropower

We have a two-tier approach to working out how much water is available for a hydropower abstraction. The use of a two-tiered approach will enable you to independently identify an initial abstraction regime from limited information about your site at an early stage in the development process. It then allows for further investigation to be carried out to assess whether the zoning can be revised.

Tier 1 Initial abstraction rates

You can work out the initial abstraction rates for hydropower by finding out the zone for your scheme from basic site information:

• On weir schemes and schemes without a flow depleted reach

If your proposed scheme is on-weir and does not create a flow depleted reach then initial abstraction rates in Table 1 apply.

• Schemes affecting sites of high value for nature conservation

Your scheme will be in Zone 1 and initial abstraction rates in Table 2 will apply if:

• any part of your proposed scheme including the depleted reach is within, or may affect a site designated for nature conservation
• the flow sensitive species freshwater pearl mussel (Margaritifera margaritifera) and native white clawed crayfish (Austropotamobius pallipes) are present in any river reach affected by your scheme
• it is located in a river reach that is in the upstream or downstream catchment of a Special Area of Conservation in which salmon is a notified feature
• there are gravel beds for existing or potential salmon spawning grounds present in the depleted reach or at the points of intake and outfall.

Read more details on how to find this information in Designated sites, protected species and supporting habitat.

• Run-of-river schemes in lower catchment rivers and streams

If your proposed hydropower scheme will create a flow depleted reach and the average river bed slope for that reach is less than 10% then your scheme is in Zone 2 and initial abstraction rates in Table 3 apply.

• Run-of-river schemes in upper catchment rivers and streams

If your proposed hydropower scheme will create a flow depleted reach and the average river bed slope for that reach is 10% or greater then your scheme is in Zone 3 and initial abstraction rates in Table 4 apply.

For simple, small scale, and low risk run-of-river hydropower schemes it is likely that no further assessment may be necessary and the initial abstraction rates in tables 1 to 4 may be taken forward to full licence application.

Tier 2 Site specific flow standards

Tier 2 allows additional site information to be used for a more detailed assessment of environmental risk. This could mean that a scheme may move to an alternative zone with subsequent application of that zones abstraction rates. Additional topographic, ecological, hydrological or geomorphological surveys and assessments will be needed to demonstrate that abstraction rates can still meet necessary standards of environmental protection.

For example, a proposed scheme that falls into Zone 2 at Tier 1 because it has a depleted reach gradient of 9% may move to Zone 3 at Tier 2 as further assessment shows that reach hydrology, ecology and geomorphology are characteristic of Zone 3 and its design and location within the catchment means it will not disrupt ecological continuity or adversely affect fish passage or geomorphology.

It is important to note that we do not favour hydropower developments in sites designated for nature conservation. Completion of environmental analyses can be costly and their submission in support of a licence application does not guarantee issue of a licence or movement between zones.

In exceptional circumstances it may be appropriate for abstraction rates to be further tailored to meet specific environmental or operational conditions. Applicants are advised to discuss these requirements with us at the earliest opportunity in the application process. 

Abstraction rates for hydropower

Abstraction rates for schemes with no depleted reaches

Hydropower schemes that are built on a weir, dam or natural fall (such as a waterfall or steep cascade) and discharge turbine flow into the river channel at the toe of the structure may not result in any depletion of flow in the downstream reach of river.

This type of scheme may be considered as having no depleted reach and the abstraction rates in Table 1 will usually be applied.

Q95 is a flow duration statistic that describes a flow rate for a site that is equalled or exceeded for 95 percent of the time in an average year. It represents a typical low summer flow.

Available flow is that available for abstraction after flow requirements for amenity, heritage, landscape, ecological protection and fish passage have been fully met. 

Qmean is the mean annual flow at a site.

The level of low flow protection will be based on site specific requirements to support local ecology and amenity, such as the visual appearance of flow over the face of a weir or cascade and fish passage. We usually require a protected low flow of Q95.

For schemes proposed within sites designated for nature conservation or supporting habitat, lower levels of abstraction may be required if a scheme is considered acceptable.

Abstraction rates for schemes with a depleted reach

Abstraction rates are set to provide the residual flow within a depleted reach that is necessary to protect in-stream ecology according to the sites’ importance for nature conservation, its location within a catchment and its catchment characteristics.

Zone 1 - Designated sites, protected species, supporting habitat and salmon spawning areas

These sites are recognised for the presence of nationally and internationally important flow sensitive species and habitats and are afforded a high level of legal environmental protection. They are described in more detail in Chapter X.

For hydropower schemes in Zone 1 the following abstraction rates apply:

We may refuse a licence application if we consider the risk of impact to flow sensitive features from a proposed hydropower development as being too high.

We will restrict abstraction to 10% of available flow where extremely rare or flow sensitive features are present, if we consider any development to be appropriate in the first instance. In other cases, abstraction will be normally be restricted to 40% of available flow.

Zone 2 – Lower catchment streams and rivers - gradient less than 10%

These are sites that do not fall within Zone 1 but where the average gradient of the flow depleted river reach is less than 10%. These are typically shallow gradient rivers and streams found in lower catchments. Hydropower development in this zone has the potential for large spatial impact caused by physical and hydrological barriers that may interrupt the longitudinal connectivity within a river ecosystem.

For hydropower schemes in Zone 2 the following abstraction rates apply:

Zone 3 – Upper catchment streams and rivers - gradient 10% or greater

These are sites that do not fall within Zone 1 but where the average gradient of the flow depleted river reach is 10% or greater. These are typically steep gradient, upland catchments.  Hydropower developments in Zone 3 are likely to have a limited spatial impact on river ecosystem structure and function and present a low risk of disrupting ecological connectivity within a catchment.

For hydropower schemes in Zone 3 the following abstraction rates apply:

Factors affecting movement between Zones

We have shown above that an initial abstraction rate for hydropower can be determined from knowing whether a scheme may affect a site of high value for nature conservation and the river bed gradient of the depleted reach. In some cases it may be possible to consider additional physical and environmental characteristics of a site that could support movement of a scheme to a different zone.

This may be possible where:

• a proposed hydropower scheme is located within a site of high value for nature conservation (Zone 1) but it can be clearly demonstrated that the proposal will not affect any flow sensitive features of that site. The scheme may then be re-assessed as a Zone 2 or 3 scheme dependent on bed gradient of the depleted river reach.

• A Zone 2 scheme with bed gradient less than 10% may be re-assessed as a Zone 3 scheme where it can be shown that the site has the hydrological and geomorphological characteristics of a steep upland catchment and will not disrupt ecosystem connectivity.

• A scheme initially in Zone 3 (with bed gradient greater than 10%) may be re-assessed as a Zone 2 if the depleted reach is in mid catchment and abstraction from it risks disrupting ecosystem connectivity to the upper catchment.

We call this Tier 2 in our process and describe below the additional catchment characteristics that we will consider in an assessment to support a change of zone. Applicants will need to provide us with this information in support of any change of zone from that established in Tier 1 of the process.

Responsive hydrological regime

We recognise that abstraction for hydropower has proportionally less impact on flow variability in small upland catchments with flashy hydrological regimes. A flashy hydrological regime is one where the catchment has a rapid response to rainfall runoff through steep rise and fall of flow.

We assess this using the ratios of flow duration statistics Q₉₅/Q_mean and Q₁₀/Q_mean  where values of 0.1 or less and greater than 2.3 respectively indicate flashy hydrology.

Upstream catchment characteristics

If the average gradient of the principle channel in the catchment upstream of the point of abstraction is 10% or greater and the depleted reach has a bed gradient close to 10% then it is likely that it will have the hydrological and geomorphological characteristics of Zone 3.

Upstream catchment size

If the catchment upstream of the point of abstraction is 5 km² or less then it is likely to represent a headwater tributary and development of a scheme may present a low risk to ecosystem connectivity within the river catchment.

Contributing inflows

Significant tributary inflows to a depleted reach may help to reduce the impact of an abstraction for hydropower by introducing flow volume and variability.

Developers intending to incorporate tributary inflows into their mitigation proposals should submit a hydrological assessment quantifying the contributing inflows and demonstrating that the residual flow standards for the zone are being met along the course of the depleted reach.

Barriers

A scheme will not be permitted to create a barrier to migratory salmonids by reducing flows over an existing structure, such as a natural cascade or man-made bridge footing, that is currently passable under the existing hydrological regime. During the licence determination process we will consider the flow impacts on downstream barriers and may need to restrict the rates of abstraction for hydropower schemes proposed upstream. 

Read about hydropower schemes which affect protected sites