Understanding geomorphology for hydropower scheme design

We need to understand how the design and operation of a hydropower scheme affects river geomorphology. In this section we briefly describe what this is, why this is important and what you need to do.

What is geomorphology?

The transfer of sediment - sands, gravels, cobbles and boulders – along a river or stream is an important natural process. Rivers and streams are characterised by the way in which the flow of water creates and moves this material, shaping the bed and banks of the watercourse. The scientific study of these processes and form of a river is known as fluvial geomorphology. Geomorphological processes create and maintain the diverse range of natural channel features such as gravel beds, pools and riffles that are essential habitats for the plants and animals that live in our streams and rivers. 

The way that sediment moves along a watercourse is known as the sediment flow regime. This can be visualised as a continuous flow of sediment material along the whole length of a river system from the source to the sea. Each watercourse will have a unique sediment flow regime that depends on site geology, the steepness of the channel and the form and shape of the river bed, banks and areas of land adjacent to the river banks. The sediment flow regime and the form and shape of the watercourse are very closely linked and sensitive to change. As such if you alter one element you can expect a reactive change in another. 

Hydraulic structures for hydropower and reductions in river flows caused by abstractions can change a river’s geomorphology and disrupt the sediment flow regime. They can also fragment our river ecosystems, affecting their ability to maintain healthy populations of plants and animals by restricting their movement within a river reach.

Low impact siting and design

We will normally only consider licensing new impoundments for hydropower if they are in upper catchments and can be designed and built to replicate natural, existing channel features and present a low risk of altering or disrupting geomorphological and ecological processes.

There are two stages in doing this:

Siting your intake weir – this involves applying geomorphological principles to select where to site your intake weir at a local scale within a river reach so that it will have low geomorphological impact. This is also known as ‘micro-siting’. Read about how to do this in Siting an intake weir for hydropower.

Designing your intake weir, penstock and outfall – there are simple principles that you should incorporate into the design and construction of your hydropower structures so that they will operate in accordance with licence conditions and minimise risk to the geomorphology and ecology. Find out more about these principles in Design principles for hydropower structures.

What you need to do

Submit a geomorphology photosurvey

You will need to submit a geomorphology photosurvey to us with any application for a full or transfer licence, variation or renewal (where a photosurvey has not previously been provided to us) for a hydropower abstraction and/or impoundment.

The photosurvey is a non-technical document that uses photographs to show us the geomorphological characteristics of the river reach that will be affected by your proposed hydropower scheme.

Guidance on how to complete a photosurvey can be found in Geomorphology surveys.

If the information submitted, in combination with our desktop analysis, shows us that the river reach affected by your proposed scheme has a low sensitivity to change, and the structures have been sited in accordance with our design principles, then we are unlikely to require any further geomorphological assessments.

If we think that the location of your proposed structures still presents a risk to geomorphology and ecology and the photosurvey shows other locations close by in which your structures could be placed with lower risk, then we may require you to reposition them to those locations.

In complex cases a site visit by our Geomorphologists may be required.

Quantitative geomorphology survey

For schemes that are larger, complex , require modifications to an existing weir, or schemes in reaches likely to be sensitive to change then we may require you to submit a more detailed, quantitative geomorphology assessment. This type of assessment will require field survey and data collection and involve detailed analysis of these data to describe the current geomorphological characteristics of a reach and how these may change with construction and operation of a hydropower scheme. In most cases this will require the applicant to employ the services of a specialist geomorphology consultant.

Guidance on how to complete a quantitative geomorphology assessment can be found in Geomorphology surveys.

Cumulative geomorphological impact assessment

We do not favour the construction of multiple hydropower schemes (or other significant physical modifications) within a river reach or small neighbouring catchments. The cumulative impact of multiple abstractions and in-river barriers risk changing the characteristics of even the most stable of rivers to an extent that leads to a deterioration of ecological status under the Water Environment Regulations. Any licence application leading to multiple physical modification being present on a waterbody will need to be accompanied with a cumulative geomorphological impact assessment. In most cases this will require the applicant to employ the services of a specialist geomorphology consultant.

Detailed guidance on how to do this is can be found in Cumulative impacts.


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