Technical Information

Design Process

Behavioural fish deflection systems can be installed at a wide variety of locations and for a wide variety of applications, but typically systems are installed on intakes that are already in use, proposed to be reused as part of the redevelopment of a site, or on a new intake that is already planned.

Ideally we will be involved in the design of the intake to ensure it is as ‘fish friendly’ as possible, but more often than not we are asked to specify a system for an existing intake. This is generally possible, although certain factors, such as high intake velocities, can result in a larger behavioural system having to be specified than might have been necessary if we had been involved at an earlier stage.

When designing a system for a site we need to have an understanding of the site layout, as well as any regulatory or operational requirements. The information we generally require is outlined on a separate page ‘Discuss a Project’. If all of the information is not available then we are usually able to make assumptions so that we can provide a preliminary specification, which can be amended at a later date as more information becomes available.

Once we have the available information we can review the optimum locations for the Sound Projectors, and then determine the optimum number of projectors that may be needed to specify an effective system. In order to achieve this both hydraulic and acoustic modelling may be required.

Hydraulic Measurement and Modelling

Water flows are always important in acoustic deterrent design. The key reasons are:

  1. The current velocities must not be high enough to sweep the fish across the barrier line irrespective of the deterrent effect of the acoustic system, and so it is important to ensure the maximum intake velocity is below the maximum swimming speed of the fish. The maximum intake velocity for most fish species is assumed to be 0.3m/s, so when the intake velocity exceeds 0.3m/s the fish will need to be deflected before the intake velocity exceeds 0.3m/s.
  2. Water flows are important for guiding fish to safety. Migratory fish or fish moving on tides will need to be directed back into safe flows so that they can continue their journeys.
  3. For BAFF installations, current flows will determine where the bubbles will surface and hence the location on the river, lake or canal bed where the BAFF system needs to be placed. As a working figure, a bubble will rise in water at a rate of 0.25m/s.

Direct hydraulic measurements using a propeller current meter can provide very useful information, but may only be practical on smaller projects. Where possible values should be taken on a rectangular grid pattern upstream of the proposed barrier, at 60% depth, or preferably averaged from 20%, 60% and 80% depth. Remember that these need to be taken for representative flow conditions, or else to cover the 'worst case', i.e. most extreme conditions of flow.

On larger schemes, or projects where a new intake is proposed but hasn’t been built, predicting the flows using hydraulic modelling will be more likely. 2D or 3D modelling can be carried out, but 2D models are usually quite adequate for specifying a system, and are inexpensive to run. Their chief advantage over measurements is that more extreme conditions that might not often occur, and are therefore difficult to measure, can be simulated. In addition, for development projects any number of 'what-if?' scenarios can be simulated.

Acoustic modelling

Once the flow streamlines have been determined this information can be used to specify the minimum area in front of an intake that a SPA acoustic system needs to protect. The aim is to produce a smooth even sound field with a sound level high enough that enables the fish to detect the deterrent signal, react and turn away before the current potentially exceeds the maximum swimming speed of the fish, drawing the fish into the intake.

For smaller sized intakes FGS has the experience from previous installations to be able to specify a suitable system without the need for modelling however, on larger intakes the sound field can be very complex, with not only interactions between the Sound Projectors themselves, but also with the water surface, river bed and surrounding structures. When these interactions are not understood nulls in the sound field can be created, which can lead to the failure of a system, as has been reported elsewhere for systems not supplied by FGS.

FGS is able to optimise the number and positioning of the Sound Projectors by using its in house proprietary acoustic model PrISM, to predict the most effective sound field. Using PrISM enables FGS to model numerous scenarios, including the impact of different water depths on the sound field, the impact of maintenance on an operating system and also the degree of redundancy that may be required to meet the requirements of a regulator.

Usually the system is run to predict unweighted sound levels, providing a system suitable for a wide range of fish species, but when a key species of fish is identified, and an audiogram for that particular fish is available, FGS can run PrISM using the dBht function, which predicts the sound level as it will be heard by the key fish species. The resulting system is therefore optimised for the key species, but can still provide protection for other species that may be present.

In comparison to SPA systems, BAFF systems require no acoustic modelling as the sound is encapsulated within the bubble curtain, the acoustic field is a property of the BAFF system design and not of the local environment. However, since the position of the bubble sheet is strongly influenced by water flows, a good understanding of the hydraulics is especially important for BAFF systems.

Time Required for Design

We will always work with a client to try and meet the required specification and delivery schedule, but it is advisable to consider the following time scales when planning the installation of a behaviour barrier. It generally takes 1-2 weeks for the available information on a site to be discussed with a client and fully understood, at which point FGS is usually able to provide a preliminary design for a system. For smaller systems, where modelling is not required, the system is usually fully specified and quoted within four weeks.

If hydraulic modelling is required then this can take a further 2-8 weeks, depending upon the complexity of the required modelling and work load when a request is made. The acoustic (PrISM) modelling typically takes an additional 4-6 weeks for the preparation of a full report, although preliminary results are usually available in the first 2-4 weeks. As a result if acoustic modelling is required then it is generally advisable to allow a minimum of two months to fully specify and quote for a system, and longer if hydraulic modelling is also needed.

For further information on specifying a fish deflection system please contact FGS.