Mitigation for pumping station intakes

Design Guidance

A wide variety of fish screening systems are available to suit different requirements and environmental conditions. New types of screens are continually under development. The main types of fish screens are considered here.

Fish screens can be divided into two main categories:

  • Physical barriers (devices such as a perforated metal plate that physically prevents fishes from passing); or
  • Behavioural barriers (devices that encourage fishes to swim away).

If designed correctly, both types of barriers can be extremely efficient at preventing fish entrainment. It is generally accepted that physical barriers offer the highest fish diversion efficiencies (DTI, 1998). Both categories of fish screen can take many different forms and these are briefly reviewed below.

Physical Barriers

Physical barriers prevent fish from entering into an intake by limiting space through which fish can travel. This typically involves installing a permeable mesh across the intake which allows the passage of water.

Behavioural Barriers

Behavioural barriers use the natural response of fish to a stimulus (e.g. light, sound, vibration) to deflect the fish away from the stimulus. These systems are particularly favourable where physical fish screening is impractical (e.g. where there is a risk of fouling or where the screen would cause a hazard to navigation).

  • Acoustic – Regularly emitting a loud noise or vibration to deter fish from a certain area.  This technique is suitable for deflecting migrating fish, resident coarse fish, estuarine and marine fish.
  • Lighting – Bright/flashing light can be used in conjunction with other behavioural systems to deter fish less sensitive to sound (e.g. eels).
  • Bubble curtains – Aerating water to guiding fish to a suitable point downstream of an intake.

Design Considerations

The appropriate design of a fish screen is dependent upon a number of factors; including flow characteristics, channel form and size, as well as the species and the size of fish requiring protection. General design principles are provided below, with more specific guidance provided in subsequent sections:

  • Fish screens should be located in areas and depths of water with low concentrations of fish throughout the year. A survey may be required where insufficient data on fish populations exists.
  • Fish screens should be located away from natural or man-made structures that may attract fish that are migrating, spawning, or in rearing habitat.
  • Where possible, the screen face should be oriented in the same direction as the flow to minimise the risk of entrainment.
  • Fish screens should be located a minimum of 300 mm (12 in.) above the bottom of the watercourse to prevent entrainment of sediment and aquatic organisms associated with the bottom area.
  • For larger screens, structural support should be provided to the screen panels to prevent sagging and collapse of the screen.
  • Large cylindrical and box-type screens should have a manifold installed in them to ensure even water velocity distribution across the screen surface. The ends of the structure should be constructed from solid materials and the end of the manifold capped.

(Adapted from Department of Fisheries and Oceans, 1995 useful Guidance on Freshwater Intake End-of-Pipe Fish Screen Guidelines).

Design considerations are considered in more detail below.

Hydraulics and Flow Velocity

Physical contact with the screen may result in injury or death. For this reason, the primary objective in the design of fish screens should be to match the swimming ability and behaviour of fish to the hydraulic characteristics of the screen to minimise the probability of contact. Velocities surrounding the screen should be sufficiently low to allow fish to swim voluntarily away from the structure. This can be controlled by, among other factors, the screen orientation and the approach channel configuration. The approach velocity should be set to a level that is less than the sustained swimming speed of juvenile fish.

An approach velocity of less than 0.3m/sec is recommended (Solomon, 1992) and Murphy (2000).

Pump Intake Screen Location

Where possible, fish screens should be installed at locations with sufficient flow velocity to sweep away debris removed from the screen face. A topographic survey and flow survey may be required to determine the optimal screen location and facility layout and provide information necessary to design the screen hydraulics. Screens should be submerged to a depth of at least one screen radius below the minimum water surface, with a minimum of one screen radius clearance between screen surfaces and adjacent natural or constructed features.  A clear escape route should exist for fish that approach the intake.  For example, if a pump intake is located off of the river (such as in an intake lagoon), a conventional open channel screen should be considered, placed in the channel or at the edge of the river.  Adverse alterations to riverine habitat shall be minimised.

Intake Screen Protection

Intake screens should be protected from heavy debris, ice and other conditions that may compromise screen integrity.  Protection can be provided by using log booms, trash racks or mechanisms for removing the intake from the river during adverse conditions. Where possible, a corrosion control system should also be installed to increase the life and efficiency of the screen. An inspection and maintenance plan for the pump intake screen should be developed to ensure that the screen is operating effectively.

The following types of physical barriers are most common in the UK:

Rotary Drum Screens - Water passes through a screen mesh which covers a rotating cylinder. A key advantage of this type of screen is that the rotation of the drum also serves to continuously remove debris. Debris is carried over the screen as it rotates and is washed off the screen on the downstream side. Screen rotation is achieved by an electric motor, paddlewheel, solar panel, or hydraulic motor.

Passive Flat Plate Screen – A flat plate mesh is placed over the intake. Fixed plate screens are commonly used for industrial, domestic water supply and irrigation intakes both at pump and gravity diversions. The advantages of the fixed plate screen are that they are easy to seal, mechanically simple because there are no moving parts. For small diversions, these screens can be installed on the bank of a river and, therefore, require no bypass. However, the removal of debris is an important design consideration for this type of screen and may be more difficult than with a rotary drum screen depending on the type of debris. Fixed plate screens require a mechanical cleaning system for debris removal.

End-of-Pipe Screens – These screens are built as a chamber attached to the end of a pipe (typically box-shaped or cylindrical). The walls of the chamber are constructed from a screen mesh. The primary advantage of this type of screen is that they are easy to install, and are readily available as ‘off the shelf’ designs. However, as they are fixed structures and have moving parts, maintenance is required due to clogging or biofouling.

Infiltration Systems - Infiltration systems consist of a perforated pipe buried in the bed of the watercourse. Water is drawn through the bed or bank into the pipe. They are installed in a steep section of a channel, which is maintained free of fine sediment by hydraulic action. It is necessary to be in a location where the bed is regularly scoured and thereby cleaned rather in a depositional area. The screen area is the area of the streambed or bank through which the water flows rather than the area of the intake pipe. The streambed or bank is also the feature that acts as the fish exclusion device. The primary advantage of this type of screen is that, when successful, that the stream hydraulics rather than mechanical cleaning devices manages debris and sediment. The primary disadvantage is the risk of being plugged with sediment and/or debris.

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