Intake weirs should incorporate environmental design features to: 

  • maintain the protected low flow (or Hands-off Flow)
  • distribute flow between the abstraction and residual (environmental) flow
  • prevent abstraction exceeding the maximum licenced rate
  • screen against ingress of fish to the abstraction
  • maintain downstream fish passage
  • provide upstream fish passage
  • provide eel passage

In this section we describe the design features to manage the abstraction and protect residual flows for downstream ecology. Illustrations of a typical arrangement is shown in our guidance section Location plans and technical drawings.

Details on screening and fish passage are provided in (Sections 9 and 10) respectively.

Protecting flows for the river environment

Abstraction licences include conditions that set out how an abstraction should be operated to ensure that a residual flow remains in the flow depleted reach to protect the river environment.

Passive abstraction control

We prefer intake weirs to be designed in a way that manages the distribution of water between abstraction and residual flow for the environment through fixed hydraulic design.

This means that the environmental flow requirements set out in the licence conditions should be ‘hard engineered’ into the hydraulic design of the abstraction weir. It provides a passive approach to maintaining downstream environmental flows under changing river flow conditions.

Passive control is most suitable for overspill type intake weirs. They are more robust in terms of their operation and reliability and allow greater confidence in their compliance with abstraction licence conditions than active flow control systems.

Active abstraction control

Active control systems utilise an arrangement of electronic river level sensors, control unit and automated intake gates or valves to manage the abstraction to deliver the proportional take as set out in licence conditions. They are much more complex than passive control systems and can require a high level of maintenance given their greater sensitivity to environmental conditions.

A flow measurement system would need to be established so that the control system could balance the flow distribution between abstraction and residual flow at short time intervals. This allows the rate of abstraction to be controlled in real time in response to quickly changing river flows.

Active control systems should also be designed to fail safe so that in the event of an equipment failure, the abstraction for power generation shuts down giving priority to protecting environmental flows for the river.

There are likely to be some sites or arrangements, such as low head weirs or non-impounding intakes, where active control systems will be required. In these instances, we advise developers to discuss the control system with us at an early stage of project development.

Protecting low flows

Your intake weir will need to pass the protected low flow rate that is specified on the abstraction licence entirely through a rectangular notch in the weir crest. The notch must have priority of flow over the abstraction and therefore be set lower than any point of abstraction. We require the full protected low flow to be discharged via a notch as this provides a route through which fish, sediment and other flora and fauna may pass, maintaining ecological continuity within the river ecosystem.

Illustrative engineering drawing showing invert of low flow notch relative to the abstraction weir crest elevation.

The low flow notch should be designed so fish can move through it without physical harm and sized to meet the specified protected low flow rate. The notch should be broad crested and have rounded or chamfered edges at the sides and base on both upstream and downstream faces to provide more favourable flow conditions for fish movement and avoid harmful sharp edges.

It should be located towards the centre of the weir so that streamflow can pass directly through the notch. Low flow notches should not be positioned at the side of a structure, at an angle to the main streamflow or where there is a likelihood that sediment will accumulate and compromise fish passage or reduce the ability of the notch to pass the required low flow.

The notch should be constructed using a slightly roughened substrate such as natural stone or scabbled concrete. As a guide, notch width and depth should have a ratio of 3:1 i.e. the width should be three times the depth. Hydraulic calculations should be used to work out the dimensions of the low flow notch.

The broad crested weir equation is used to calculate flow through a notch:

Q space equals space C B space open parentheses 2 over 3 close parentheses to the power of 3 over 2 end exponent space g to the power of 1 half end exponent. h to the power of 3 over 2 end exponent

Q = flow rate (m3/s)
C = co-efficient of discharge
b = weir crest width (m)
g = gravity (9.81 m/s2)
h = depth of upstream flow above weir crest (m)

We suggest that for a notch in a typical, smooth finish, cast concrete, broad crested weir the co-efficient of discharge co-efficient used in the weir flow calculation is:

  • 0.84 where the notch has square faces / edges
  • 0.94 where the notch has rounded or chamfered edges

Broad crested weir flow is typically where the length of the weir (L) (front to back) is 1.5 times greater than the upstream depth of water over the weir. These co-efficients are therefore valid up to a water depth of about 200 mm where the width to depth ratio of 3:1 is applied and concrete weirs for small scale hydropower are typically 300mm thick.

Where upstream fish passage is required, the base of the notch should slope downwards at an angle of about 45 degrees to the downstream face of the weir. This is to provide flow conditions better suited to upstream movement of fish and eels through the notch from the fish pass or easement. The base of the sloping notch should at least be level with the static water level in the most upstream pool of the fish pass or easement.

We are unlikely to accept designs that include thin plate notches, thin plate weir crests or where the protected low flow is delivered via a piped orifice.

The low flow notch should be carefully constructed to the dimensions agreed by us and detailed in the approved engineering drawings that are included as a condition in the licence.

For most sites the minimum dimensions for which a low flow notch is considered effective for downstream fish passage are 150mm wide by 50mm deep, which delivers an approximate flow of 3 litres/second (l/s). Notches smaller than this are also prone to blockage from woody debris and sediment. Licence applicants will need to discuss options with us for delivering protected low flows and fish passage if a protected low flow notch smaller than this is being considered.

The low flow notch should discharge into a plunge pool for downstream passage or into a suitably designed pool enabling upstream fish passage. This design requirement is detailed in our guidance section Fish passes for hydropower weirs.

The low flow notch must always be kept free from obstructions. Failure to pass the protected low flow as set out in abstraction licence conditions is likely to lead to enforcement action.

Protecting low flows - key principles

  • Use a broad, rectangular notch to convey the protected low flow.
  • Ensure that the notch has been properly sized to deliver the required protected low flow.
  • The depth of the low flow notch should be set from the level of the intake and residual flow crests of the weir.
  • Locate the notch towards the centre of the weir allowing direct conveyance of streamflow.
  • Ensure that both the upstream and downstream faces of the notch have curved or chamfered edges.
  • Ensure that the base of the low flow notch slopes downwards to the downstream face of the weir where upstream fish passage is required.
  • Use stone or scabbled concrete to provide a rough surface in the sides and base of the notch.
  • Use the ratio of 3 wide:1 deep to size dimensions for the notch
  • Use a minimum notch size of 150mm wide x 50 mm deep.
  • Avoid use of V notches, thin metal plates, pipes or orifices.
  • Always keep the low flow notch free from obstructions.

Protecting flow variability

An abstraction for hydropower will normally be required to allow a proportion of the natural river flow to pass downstream of the intake to mimic natural flow variability within the depleted reach. For this type of scheme, the flow split is achieved by having the proportions of screened and unscreened weir crest length appropriately dimensioned to deliver the flow split as required in the abstraction licence.

This passive system ensures that as upstream river flows naturally rise and fall then the required flow split between the abstraction and variable residual river flow is maintained throughout the flow range until the maximum abstraction rate is exceeded. Flow splits can be achieved with active control systems although this approach is more complex.

Weir crests should not include protruding metal plates with sharp edges, or concrete piers to separate residual and abstraction flow sections of the weir. These will tend to snag debris, causing blockages, reducing the discharge efficiency of the weir and causing an imbalance in the designed flow split. Exceptions to this may apply where it is necessary to separate abstraction and residual flow to enable operation of a fish pass or easement.

Protecting flow variability - key principles

  • Use an overspill type intake design that allows operation to be ‘hard engineered’ into the hydraulic structure for passive control.
  • Ensure that the lengths of the residual flow and abstraction sections of the weir have been correctly proportioned to meet the licensed flow split.
  • Ensure that the residual flow and abstraction sections of the weir crest are precisely the same level to allow the flow to split to operate as licensed.
  • Avoid use of piers, walls or metal upstands to divide the residual flow and abstraction sections of the weir where possible.
  • Always keep the residual flow and abstraction sections of the weir free from obstructions.
  • Ensure that both the upstream and downstream faces of the weir crests have curved or chamfered edges.

Protecting high flows

A maximum rate of abstraction is specified on abstraction licences in cubic metres per second (m3/s) or litres per second (l/s). In our hydropower guidance we expressed this as a proportion of the mean flow (Qmean). The maximum abstraction rate can be controlled by one or more design factors including intake capacity, and hence sizing, of the intake screen, the conveyance capacity of the penstock and/or the operation of turbine valves.

Designing for flood flows

Constructing an intake weir in a river channel will reduce the ability of that channel to convey flood flows.

If an in-river structure has not been well sited and designed there is the risk that flood flows will flow out of bank to by-pass it. This could lead to erosion of the bank and river bed around the structure, potentially destabilising the structure or leading to the need for additional bank stabilisation measures.

Developers are advised to keep weir crest height as low as possible and avoid narrowing the channel so that any reduction to the conveyance capacity of the channel is minimised. A simple hydraulic assessment should be carried out for most schemes to understand how a proposed weir will operate under flood flow conditions.

Read about intake screening for fish

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