Assessing the cumulative and in-combination impacts of hydropower schemes

When assessing licence applications for a hydropower scheme we need to understand whether it will have cumulative or in-combination impacts with other developments, operations or activities. This includes developments that may still only be at a planning stage.

Cumulative impacts are the effect of similar impacts from multiple developments on a river reach or catchment. In-combination effects are where a river may be affected by one or more developments but in different ways.

We are likely to require a greater amount of supporting assessment for licence applications where there are potential cumulative or in-combination impacts than for a single, isolated hydropower scheme.

The term in-combination also has a specific meaning in the context of the Habitats Regulations where any plan or project likely to have a significant effect on a Special Area of Conservation (SAC), Special Protection Area (SPA) or Ramsar wetland site will be subject to an Appropriate Assessment either individually or in-combination with other plans or projects.

We will carry out the Appropriate Assessment, but applicants have a legal obligation to provide us with the information that we require to do this. 

See our guidance on Designated sites, protected species and supporting habitat for more information on this.

Individual hydropower schemes must meet the standards of environmental design and operation we set out in our guidance for us to consider their licensing. Where multiple hydropower schemes or other developments are present, we may require new developments to meet higher environmental standards to minimise the risk of cumulative or in-combination impacts.

The cumulative and in-combination impacts most often associated with high head, run-of-river hydropower schemes are associated with changes to river flows caused by abstraction and creation of in-river barriers through construction or modification of intake weirs.


We may restrict rates of abstraction for new schemes where one or more hydropower schemes, or other abstractions, are already present on the main stem of a river in a tributary or catchment or in multiple, separate rivers within a catchment.

We will usually licence abstractions for hydropower to the flow standards set out in our guidance Abstraction rates for hydropower. For a single scheme, these allow an abstraction to cause river flows to fall below our normal levels of flow protection but only for a limited length of river after which flow is fully returned to the same channel. In most cases this is unlikely to have an adverse impact on riverine ecology due to the limited spatial extent of impact and protection of flow volumes and flow variability in the depleted reach due to the restrictions on abstraction we set out in licence conditions.

Multiple abstractions for hydropower and other uses could cause river flows to fall below our normal levels of flow protection for extended lengths of river reach either on a single river or on adjacent streams in a small catchment. This means that having less flow for a greater length in our rivers becomes a significant risk to the structure and functioning of river ecosystems.

The normal restrictions we apply to consumptive abstractions will leave enough flow in our rivers to meet the standard of environmental protection for Good Ecological Status (GES) as described under the Water Environment Regulations (Water Framework Directive). We have a statutory duty to achieve GES in most Welsh water bodies and to prevent the environmental standards of water bodies already at GES from deteriorating.

Our Zone 1 flow standard for hydropower, allowing abstraction of 40% of available river flow above the protected low flow, provides the same level of flow protection as the river flow objectives set out in our abstraction licensing strategies to achieve Good Ecological Status.

We will allow non-consumptive abstractions, such as those for hydropower, to cause river flows to fall below this level for a limited length of river reach provided they are consistent with our flow standards for hydropower and do not present a high risk to in-stream ecology.

We do not favour development of hydropower schemes in high ecological status water bodies.

We will normally limit abstractions for hydropower from causing flows to fall below our standards for good ecological status to 15% of the length of the main stem of streams and rivers within catchments or their tributaries.

Many hydropower schemes in Wales are in very small headwater sub-catchments of less than 2 km2 within a minor tributary catchment. In these situations, we may allow up to 100% of the stream length within the headwater sub-catchment to fall below our flow standards for good ecological status, providing that we are satisfied it will not affect the structure and functioning of the river ecosystem when considered cumulatively with impacts of other hydropower schemes within the wider catchment.

If you think the abstraction for your scheme may have a cumulative impact on river flow, then you may submit a pre-application enquiry to us. Alternatively, we will advise you on receipt of your abstraction licence application if we think the amount of water available for power generation will be restricted due to the presence of multiple hydropower schemes or other abstractions.


We are unlikely to licence new impoundments in lower catchment rivers or streams. We do not favour construction of multiple barriers in steep, upland river reaches unless it can be clearly shown that a new weir can be sited and designed to protect ecosystem connectivity and have low risk of impact on river ecology and geomorphology.

Where a new impoundment is proposed in a reach or catchment where other structures and abstractions are already present, we are likely to require applicants to carry out quantitative geomorphological and ecological assessments to help us understand potential cumulative and in-combination impacts.

What do you need to do?

You should assess the features of the river reach and catchment in which you propose to locate your hydropower scheme and research plans and projects to decide whether your proposal will have any cumulative or in-combination impacts. You should consider:

  • other water abstractions and discharges
  • operating or redundant weirs, dams or any other form of impoundments or in-river structures in the catchment in which your scheme is proposed
  • sensitivity of the ecology and geomorphology in the river reach to changes in flow and construction of new barriers
  • land use within the catchment (e.g. forestry, mining, agriculture)
  • river dredging, flood defence works, transport links
  • impact on sites designated for nature conservation
  • water body ecological status as set out under the Water Framework Directive

Most of this information can be found on the Welsh Government Lle Geoportal.

You should contact us in advance of your application if you are considering developing a new impoundment in a catchment where multiple in-river structures or hydropower schemes are already present.

We will advise if we think a cumulative impact assessment is needed and if so, what we would expect it to assess.

Completion of a cumulative impact assessment for a new impoundment does not guarantee that we will issue a licence for it.

In some cases, no further abstractions or new impoundments will be permitted as the cumulative and in-combination impacts may compromise our statutory duties to meet Water Environment Regulations (Water Framework Directive) objectives.

Last updated