Title: The Investigation and Specification of Flow Measurement Structure Design Features that Aid the Migration of Fish
Author: W R White
Author: B A Wood-Ballard
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_1268, Representation ID: 427, Object ID: 2477
This document forms the Technical Report to the Environment Agency's R and D project W6-084, commissioned in July 2002. The objectives of the overall project are "The investigation and specification of flow measurement structure design features that aid the migration of fish without significantly compromising flow measurement accuracy, with the potential to influence the production of suitable British Standards." This research project is not concerned with the hydraulic performance of basic flow measurement structures which are to be found in British and International Standards. It is concerned with the adaptation or augmentation of Standard gauging structures to aid the migration of fish and is intended to ensure that these adaptations do not significantly degrade the accuracy of the structure as a flow measurement device. The research will ultimately provide specifications for fish migration adaptations or augmentations which can be introduced without significantly affecting flow measurement performance. This Technical Report on Phase 1 of the project gives the results of a literature review of readily available information. It also contains the results of a questionnaire and some follow-up interviews with interested parties. A complementary Laboratory Proposal report (not released to public) has been prepared which interprets the findings of this Technical Report in terms of priorities for future laboratory work in Phase 2, and is summarised within this report. Thus the aim of Phase 1 is to review current knowledge in order to define the most useful and productive way forward. In particular it provides the justification for future, accurate, hydrometric modelling of possible solutions in Phase 2. Phase 1 has, in summary, provided the following information: Hydrometry 1. There is general agreement within the Environment Agency that low to medium flows should be measured with uncertainty levels no greater than +/- 5%. For medium to high flows this figure could be stretched to +/- 10%. 2. The accuracy of routine current meter calibrations in the field is generally lower than the accuracy attainable at Standard flow measurement structures. 3. The usage of Standard flow measuring devices within the Environment Agency remains extensive (>1000 installations), particularly the use of Crump, flat-V and compound structures. 4. The Environment Agency view of accuracy requirements is not necessarily the worldwide, international view. This is of relevance when promoting the results through British and International Standards. Historically, countries in which water shortages are common have argued for higher accuracies in flow measurement than currently required by the Environment Agency. Fish Biology and swimming capabilities 1. There is extensive information on swimming capabilities of fish but the subject is complex and the data is neither comprehensive nor precise. Early work was concentrated on salmon and sea trout. More recently there is a greater emphasis on freshwater and coarse fish. R and D TECHNICAL REPORT W6-084/TR1 ii 2. The swimming capabilities of fish depend upon a number of factors including: a species a length (the age of an individual fish or the collective adult size of a particular species) a water temperature a water depth a water velocity a turbulence a distance to be negotiated 3. A distinction is made between "burst" speed and "cruising" speed. Burst speed is generally anaerobic swimming while cruising or sustained swimming is aerobic. Burst speed seems to be the most relevant figure for passage over or around gauging weirs. Both burst and cruising speeds depend on the size of fish, all other things being equal. 4. As a broad generalisation, salmon and sea trout have high burst speeds, typically in the range 2.0 m/s to 3.5 m/s for mature fish. Freshwater fish have lower burst speeds, typically in the range 1.0 m/s to 1.5 m/s for mature fish. Velocities on the downstream face of gauging weirs may reach 4.0 m/s. Thus, weirs present far more of a barrier to freshwater species than to salmon and sea trout. Fish passage over or around river structures 1. There are many references to "one-off" fish pass types but the choice appears to be narrowing to three commonly used devices. These are, pool and traverse, Denil (or a derivative) and Larinier (or a derivative). 2. Scale models have been used to assess the relative merits of the different types of fish pass. These have been carried out at relatively small scales to facilitate rapid, economical construction and ease of modification. These models have provided flow characteristics which are adequate for designing fish passes but not for hydrometric purposes. 3. Smaller scale models have also been used to investigate new ideas on adaptations to measuring weirs. They have been of limited value because they cannot simulate aeration of the flow and their hydrometric performance is impaired by fluid property effects. 4. Some hydraulic and fish monitoring tests have been carried out at field installations, including looking into downstream conditions and the factors which attract fish towards fish passes. There are three main categories of potential solutions to the fish passage problem: a bypass channels which could be much longer than the gauging structure in the direction of flow and which might take the form of a pool and traverse fish pass, a Denil fish pass (or derivative), a Larinier fish pass (or derivative) or a semi-natural channel. a fish passes which are combined with a Standard gauging structures to form compound units. These may utilise pool and traverse, Denil or Larinier fish passes which would be separated from the main gauging section of the structure by divide walls of some type. a adaptations of Standard gauging structures in the form of fish "aids" on the downstream face of the weir, the easement approach. The solution being tested on the Moors River at Hurn is an example of this approach. The introduction of a R and D TECHNICAL REPORT W6-084/TR1 iii Larinier or Denil fish pass to the downstream face of weirs is unlikely to be successful based on past experience. Bypass channels and compound units In the first two categories, flows down the fish pass are separated from flows over the main gauging structure. The proportion of flow taken by the fish pass can be determined / designed but this proportion will vary as the river flow varies because of the different rating curves for the gauging section and the fish pass section. This assumes that monitored and controlled variable level intakes to fish passes are not a practical proposition. The overall percentage uncertainty in the measured river flow will depend on the percentage uncertainties in the flows measured by the gauging weir and the fish pass and the proportion of the total flow taken by each. Thus, if the uncertainties in the measured fish pass flows are large and the proportion of flow taken by the fish pass is also large then the overall uncertainty will be unacceptable from a hydrometric point of view. On the other hand, if the uncertainty in the fish pass flow is not much higher than the gauging weir and the fish pass takes a small proportion of the flow then the situation becomes more acceptable to the hydrometrist. The Phase 2 testing will need to look at the basic uncertainties associated with fish passes and will need to be augmented with a desk study to formulate design methods which will ensure that the overall uncertainties of the system are acceptable. It will not be necessary to model the gauging weirs, only the fish pass sections of any compound arrangement. Indeed, it may only be necessary to model the head of each type of fish pass because this determines its hydrometric performance. The greatest challenge will be to avoid high uncertainties at the gauging section brought about by relatively high discharges through the fish pass section and consequent low heads at the gauging section. Adaptation of Standard gauging structures, the easement approach Fish passage aids of various types have, and are, being tried. They offer one possible solution to the fish passage problem. Salient points from the questionnaire / consultation exercise The questionnaire was formulated jointly by the consultant and the Project Board. Replies were evaluated by the consultant and are fully discussed in Section 1 of Chapter 3. The replies to the questionnaire, the workshop, and the discussions during the consultation process were useful in that they helped to identify current issues and gave an opportunity for Environment Agency personnel and other experts to give their views. Inevitably the views were subjective and, in some cases, contradictory. Thus, in consultation with the Project Board, the consultant sifted responses and identified those issues upon which there is a broad degree of agreement. These are listed in Section 2 of Chapter 3. Key issues which have not already been reported above and which need to be addressed in future studies include: a the need to develop methods for "retro-fitting" fish pass aids because of the large numbers of existing flow measurement structures. R and D TECHNICAL REPORT W6-084/TR1 iv a a a a a a a a a the need to address the problem of trash being caught in fish passes with consequent changes to head / discharge relationships. the need to minimise afflux (the difference between upstream and downstream water levels) over flow measurement structures because peak velocites are closely related to afflux. the need to ensure that any truncation of the downstream face of a flow measurement structure is submerged by the tailwater. the need to minimise aeration because of the reduced ability of fish to navigate and swim under such conditions. the need to minimise large scale turbulence and flow convergence both of which disorientate fish. the need to provide a diversity of flow conditions locally, which fish are able to exploit. the need to attract fish towards the downstream outlet from any fish pass. the need to provide easy approaches to fish passes to minimise the amount of anaerobic swimming that is required. the need to provide suitable flow conditions upstream of the fish pass such that fish are not swept back over the flow measuring structure. Proposed laboratory tests The Phase 2 testing will need to model the proposed solution, possibly with sectional models, and seek adaptations that have little or no effect on the hydrometric performance of the basic weir. The details of the Phase 2 testing needs to take into account the types of weir which are of the greatest value to, or see the greatest usage in, hydrometry. It will also have to incorporate those fish passes or adaptations that have a successful track record and are welcomed by the fisheries interests. On present evidence, the ranges are: Weirs: Two-dimensional triangular profile (Crump) Flat-V Compound Fish passage aids: Pool and traverse fish pass Denil (or derivative) fish pass Larinier (or derivative) fish pass Adaptations to Standard weirs (easements) Details of the proposals for laboratory testing are based upon these conclusions and are given in the Laboratory Proposal report (not released to public), and summarised below. R and D TECHNICAL REPORT W6-084/TR1 v The recommended projects for Phase 2 are: Proposal 1. Desk study of the combined uncertainties associated with the introduction of fish passage aids at Standard flow measurement structures. 2. Review of the problems of trash at fish passes and ways of minimising accumulations. 3. Laboratory tests to provide an accurate hydrometric calibration of a Larinier fish pass. Value for money and Urgency High, High High, High High, Medium 4. Laboratory testing of a Larinier and/or a Pool and Traverse fish pass with a submerged orifice upstream intake set alongside a flow measurement structure (non-specific). 5. Laboratory testing of a Larinier and/or a Pool and Traverse fish pass with a submerged orifice upstream intake set midstream at a flat-V weir. 6. Fundamental requirements for the nearcrest arrangements for baffles on the downstream face of a measuring weir. High, Medium 7. Testing of a limited range of "finalised" Hurn type adaptations to flat-V weirs. High, Medium Medium, Low High, High Notes This is an important study which will: a enhance understanding of the hydrometric implications of the introduction of fish passage aids at flow measurement structures. a provide guidelines for the design of fish passes vis-A-vis the performance characteristics of the flow measurement structure. This is an important study, which should be carried out as a matter of urgency such that any lessons learned can be incorporated in any of the design solutions modelled in Phase 2. This study will consider the basic calibration of the Larinier fish pass and also possible adaptations at the upstream end to improve hydrometric performance. It will provide information for existing installations and design information for new installations. This study will look at a combined fish pass/flow measurement installation in which the fish pass is placed alongside any Standard flow measurement structure. Flow measurement through the fish pass could be achieved by a variety of means. Fish counting would also be feasible. This study is similar, in some respects, to study 4 and will look at a combined fish pass/flow measurement installation in which the fish pass is installed midstream within a flat-V weir. The interaction with flows over the flat-V weir, particularly downstream flow conditions, will need to be investigated. The baffle arrangements on the "Hurn" type easement are designed to reduce velocities on the downstream face and to create a spatial diversity of flow conditions. The upstream baffles are those which potentially affect hydrometric performance. This study will provide a limited amount of basic information about the requirements for the location of the most upstream baffle in relation to its size. There is a problem with developing general design and performance data for "Hurn" type easements. This is because the size, spacing and location of baffles are related to fish size, not weir size. Hence easements on large weirs are not necessarily geometrically similar to easements on small weirs. This study will thus investigate a limited range of typical arrangements. Generalised information will have to be derived by interpolation. Following discussions with the Environment Agency Project Board, Studies 1, 2, 3, 4 and 6 will be progressed as Phase 2 of this R and D project. R and D TECHNICAL REPORT W6-084/TR1 vi CONTENTS ACKNOWLEDGEMENTS i EXECUTIVE SUMMARY ii 1.
Publisher: Environment Agency
Subject Keywords: Flow measurement; Hydrometry; Calibration; Fish biology; Fish passage; Weir.
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