Screening for Intakes and Outfalls: a Best Practice Guide
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w w w.environment-agency.gov.uk Screening for Intake and Outfalls: a best practice guide Science Report SC030231 The Environment Agency is the leading public body protecting and improving the environment in England and Wales. It’s our job to make sure that air, land and water are looked after by everyone in today’s society, so that tomorrow’s generations inherit a cleaner, healthier world. Our work includes tackling flooding and pollution incidents, reducing industry’s impacts on the environment, cleaning up rivers, coastal waters and contaminated land, and improving wildlife habitats. This report is the result of research commissioned and funded by the Environment Agency’s Science Programme. PUBLISHED BY: Authors: Environment Agency, Rio House, Waterside Drive, Aztec West, N. O’Keeffe & A.W.H.Turnpenny Almondsbury, Bristol, BS32 4UD Dissemination Status: Tel: 01454 624400 Fax: 01454 624409 Publicly available www.environment-agency.gov.uk Keywords: ISBN: 1 84432 361 7 Screening, intakes, outfalls © Environment Agency February 2005 Research Contractor: All rights reserved. This document may be reproduced with prior Jacobs Babtie Aquatic, Jacobs UK LTD, permission of the Environment Agency. Fawley, Southampton S045 1TW Tel: +44 (0)23.8089.3513 Environment Agency Project Manager: The views expressed in this document are not necessarily those of Ian Dolben, Coverdale House, York, the Environment Agency. North East Region. This report is printed on Cyclus Print, a 100% recycled stock, which is 100% post consumer waste and is totally chlorine free. Science Project Number: SC030231 (W6-103/TR) Water used is treated and in most cases returned to source in better condition than removed. Product Code: SCHO0205BIOC-E-P Further copies of this report are available from the Environment Agency’s National Customer Contact Centre by emailing [email protected] or by telephoning 08708 506506. 2 Science Report Screening for intake and outfalls: a best practice guide Science at the Environment Agency Science underpins the work of the Environment Agency, by providing an up to date understanding of the world about us, and helping us to develop monitoring tools and techniques to manage our environment as efficiently as possible. The work of the Science Group is a key ingredient in the partnership between research, policy and operations that enables the Agency to protect and restore our environment. 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Professor Mike Depledge Head of Science Science Report Screening for intake and outfalls: a best practice guide 3 SUMMARY The aim of this Guide is to provide a description of the legal responsibilities of operators of water intakes and outfalls and, from a review of current, worldwide examples, to present a synopsis of methods that are known to work best for different species and lifestages of fish in different situations. A review of the wide range of technologies that are in common use for fish screening is provided, including physical and behavioural screening technologies. Effective screening must be targeted to the species and lifestages of fish that are to be protected. Given the diversity of screening applications and environments and the need to consider the protection of a much-enlarged list of fish species than perhaps in the past, the developer or operator is faced with a potentially bewildering array of options. This review will help to guide users towards current best practice to assist in the task of screen selection and specification. There remain a number of gaps, where the effectiveness of new techniques has not been fully evaluated or where methods for particular species or applications have yet to be identified or developed. Recommendations for further screen development and evaluation are made. 4 Science Report Screening for intake and outfalls: a best practice guide EXECUTIVE SUMMARY Background and Aims Intakes used for water supply and hydroelectric power generation may harm fish if not properly screened to prevent fish ingress and there is also a risk of fish injury at intake structures or gratings. The issues and their remedies were reviewed for the National Rivers Authority by Solomon (1992). Now, more than a decade later, there have been significant changes in the law relating to fish screening, in the regulatory framework, and in fish screening and diversion technologies. It is, therefore, timely to readdress the issues. The aim of this Guide is to provide a clear description of the legal responsibilities of operators of water intakes and outfalls and, from a review of current, worldwide examples, to present a synopsis of available methods, indicating those that are known to work best for different species and lifestages of fish in different situations. Potential impacts of fish entrainment and impingement The design, installation and operation of fish screens and barriers can add significantly to the capital and operating costs of facilities. It is important for operators to recognise the potential impacts on fish and fish communities, which justify the costs of the required mitigation measures, and consider undertaking cost-benefit analyses. Migratory diadromous species are historically recognised as being at risk as they often have to pass numerous water abstractions, as well as weirs and other hazards, on their journeys between rivers and the sea. In recent years there has been increasing recognition of the risk of entrainment into water intakes of juvenile freshwater fish during their downstream dispersal phases. Unscreened intakes on water transfer schemes may cause the unwanted introduction of new species or different genetic stock. In estuarine and coastal waters, impacts can arise from refineries, docks, and shipping but especially from thermal power stations, which abstract large volumes of cooling water. Desalination plants may also be developed in the future. Review of screening and guidance technologies This section presents a review of the wide range of technologies that are in common use for fish screening, including both physical and behavioural types. For salmonids and larger fish there are six main types of physical screening techniques: 1) traditional passive mesh screens – the most common fish exclusion method; 2) vertical or inclined bar racks; 3) rotary disk screens – originally designed for sewage treatment works but with some merits for intake screening; 4) Coanda screens –wedge-wire spillway screens mainly for upland hydropower applications; 5) the ‘Smolt-Safe’ screen – another type of spillway screen and 6) band- or drum-screens modified for fish return. For juvenile and smaller fish there are four main physical screen choices: 1) passive wedge wire cylinder (PWWC) screens – the most widely used method for juvenile and larval fish protection; 2) small-aperture wedge-wire panel screens; 3) sub-gravel intakes and wells – which use the riverbed as a filter; and 4) the Marine Life Exclusion System (MLES) – a water permeable geotextile barrier currently being evaluated in the USA. Other physical screening technologies not currently available in the UK include the modular inclined screen which is a wedge-wire screen which is tilted up from the horizontal, the labyrinth screen, which is a compact arrangement of vertical bar racks arranged in chevron formation and the self-cleaning belt screen. Behavioural technologies can be used where positive exclusion fish screening is impractical or as a supplement to more conventional screen types. The best of these can Science Report Screening for intake and outfalls: a best practice guide 5 be >90% effective against certain species when designed correctly and operated in suitable environmental conditions. As they do not provide a guaranteed barrier to fish passage, they are often used in less critical applications or where the alternative is to have no screening. There are five main types that have been used within the UK, comprising: 1) louvre screens – a semi-physical barrier; 2) bubble curtains – the most basic behavioural barrier; 3) electrical barriers – e.g. the ‘Graduated Field Fish Barrier (GFFB)’; 4) acoustic fish deterrents – which exploit the hearing sensitivity of fish; and 5) artificial lighting – either to illuminate physical structures or as an attractive or repellent stimulus (e.g. strobe lights). Behavioural technologies that are not known to have been used in the UK include: 1) turbulent attraction flow – which mimics natural river turbulence to guide fish into bywash structures; and 2) surface collectors – a bypass system which is based upon the natural tendency of salmon smolts to migrate to surface layers. Outfall screening may also be required to protect upstream migrating species. There are