8.6.1 Trash Screens
 The screen, or ‘trashrack’ filters out river-borne debris before it reaches the turbine. It is an extremely important component of the whole scheme, and can be one of the more expensive items. The large majority of operating problems and maintenance costs can be traced back to the screening system so investment in a robust design will pay for itself in the long run.
The first line of protection should, in most cases, be a floating boom angled across the flow upstream of the intake. This will catch large items of floating debris before they reach the trashrack. However such debris will eventually make it under the boom unless cleared within a few days.
The standard screening solution, which has been used since the days of waterwheels, is to place a rack of bars in front of the intake, with the bars spaced so that a rake can be used to drag the accumulated debris up to the top of the screen.
The screen is a hindrance to the flow and introduces a slight head loss. Therefore the bar-spacing should be the maximum that will still trap debris large enough to damage the turbine. The turbine supplier will advise on the correct dimensions.
In addition, the flow velocity approaching the screen should be relatively slow, preferably less than 0.3 m/sec and certainly no greater than 0.5 m/sec.
8.6.2 Fish-screening
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On rivers where there are important fisheries concerns, the Environment Agency will stipulate more stringent screening requirements to ensure that fish will be deterred from the turbine intake and will be diverted to a suitable by-wash. The precise fish-screening measures will be a matter for negotiation, depending on the sensitivities of the site.
Where there are salmon smolts migrating down-river, it is normal for a mesh-spacing no greater than 12 mm to be required for at least three months in the spring and early summer. A fine-meshed screen will accumulate large volumes of debris and an automatic cleaner then becomes essential to keep the turbine running.
A number of innovative methods for excluding fish from intakes which avoid a fine-meshed physical screen are being trialled. These include the use of electric currents, bubble curtains and sound waves to guide the fish away from the intake.
These methods offer significant advantages to the operator by avoiding any obstruction to the flow, but are yet to find general acceptance with the Environment Agency. |
The Bio-Acoustic Fish Fence (BAFF) uses a combination of air bubbles and sound waves to form a behavioural screen
to guide fish away from hydro intakes. | |
