BS-ISO-3846-2008.pdf

BRITISH STANDARD BS ISO 3846:2008 Hydrometry Open channel flow measurement using rectangular broad-crested weirs ICS 17.120.20 ? Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- BS ISO 3846:2008 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 March 2008 © BSI 2008 ISBN 978 0 580 55992 1 National foreword This British Standard is the UK implementation of ISO 3846:2008. It supersedes BS 3680-4E:1990 which is withdrawn. The UK participation in its preparation was entrusted by Technical Committee CPI/113, Hydrometry, to Subcommittee CPI/113/2, Notches, weirs and flumes. A list of organizations represented on this committee can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard cannot confer immunity from legal obligations. Amendments/corrigenda issued since publication DateComments Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- Reference number ISO 3846:2008(E) INTERNATIONAL STANDARD ISO 3846 Third edition 2008-02-15 Hydrometry Open channel flow measurement using rectangular broad-crested weirs Hydrométrie Mesure de débit des liquides dans les canaux découverts au moyen de déversoirs rectangulaires à seuil épais BS ISO 3846:2008 Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- ii Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- iii Contents Page Foreword iv 1 Scope . 1 2 Normative references. 1 3 Terms and definitions. 1 4 Symbols. 1 5 Installation. 2 5.1 General. 2 5.2 Selection of site 2 5.3 Installation conditions 3 6 General maintenance requirements 4 7 Measurement of head(s) 5 7.1 General. 5 7.2 Stilling or float well. 5 7.3 Zero setting . 5 8 Rectangular broad-crested weirs 6 8.1 Specification for the standard weir. 6 8.2 Location of the head gauge section . 7 8.3 Provision for modular flow 7 9 Discharge relationships. 7 9.1 Modular flow discharge equation 7 9.2 Modular coefficient of discharge 7 9.3 Limitations for operation in the modular flow range 10 9.4 Drowned flow discharge equation 10 9.5 Limitations for operation in the drowned flow range 10 10 Uncertainties in flow measurement 11 10.1 General. 11 10.2 Statement of the uncertainty of a flow measurement in a channel. 11 10.3 Combining measurement uncertainties. 12 10.4 Uncertainty of discharge coefficient u(C) for the broad-crested weir. 13 10.5 Uncertainty budget. 13 11 Example . 13 11.1 General. 13 11.2 Characteristics Gauging structure. 14 11.3 Characteristics Gauged head instrumentation. 14 11.4 The discharge coefficient 14 11.5 The discharge estimate 14 11.6 Uncertainty statement 15 Annex A (informative) Modular limit. 17 Annex B (informative) Gauged head discharge coefficient and total head discharge coefficient 18 Annex C (informative) Introduction to measurement uncertainty. 19 Annex D (informative) Sample measurement performance for use in hydrometric worked examples 26 Bibliography. 28 BS ISO 3846:2008 Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- iv Foreword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights. ISO 3846 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2, Flow measurement structures. This third edition cancels and replaces the second edition (ISO 3846:1989), of which it constitutes a technical revision. BS ISO 3846:2008 Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- 1 Hydrometry Open channel flow measurement using rectangular broad-crested weirs 1 Scope This International Standard lays down requirements for the use of rectangular broad-crested weirs for the accurate measurement of flow of clear water in open channels under free flow conditions. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. ISO 772, Hydrometry Vocabulary and symbols 3 Terms and definitions For the purposes of this document, the terms and definitions given in ISO 772 apply. 4 Symbols A m2 area of approach channel b m width of weir crest perpendicular to flow direction C discharge coefficient (gauged head) f drowned flow reduction factor fC combined coefficient of discharge Cd discharge coefficient (total head) Cv coefficient of velocity E m encoder height from datum eb m random uncertainty in the width measurement g m/s2 acceleration due to gravity H m total head above crest level h m gauged head above crest level (upstream head is inferred if no subscript is used) L m length of weir in the direction of flow n number of measurements in a set p m height of weir (difference between mean bed level and crest level) BS ISO 3846:2008 Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- 2 Q m3/s volumetric rate of flow S submergence ratio, h2/h1 S1 modular limit 1 v m/s mean velocity in the approach channel U % expanded percentage uncertainty u*(b) % percentage uncertainty in b u*(C) % percentage uncertainty in C u*(h1) % percentage uncertainty in h1 u*(Q) % percentage uncertainty in Q Subscripts 1 upstream 2 downstream c combined E encoder t distance between the (upstream) gauged head and the encoder 5 Installation 5.1 General The conditions regarding the preliminary survey, selection of site, installation, approach channel, maintenance, measurement of the head, and stilling or float wells, which are generally necessary for flow measurement, are given in 5.2, 5.3, 6 and 7. The particular requirements for the rectangular broad-crested weir are given separately in Clause 8. 5.2 Selection of site A preliminary survey shall be made of the physical and hydraulic features of the proposed site to check that it conforms (or can be made to conform) to the requirements necessary for accurate flow measurement by the weir. Particular attention shall be paid to the following features in selecting the site for the weir: a) the availability of an adequate length of channel of regular cross-section; b) the existing velocity distribution; c) the avoidance of a steep channel, if possible (see 5.3.2); d) the effects of any increased upstream water level due to the measuring structure; e) the conditions downstream, including influences such as tides, confluences with other streams, sluice gates, mill dams and other controlling features, which might cause drowning; f) the impermeability of the ground on which the structure is to be founded, and the necessity for piling, grouting or other means of controlling seepage; g) the necessity for flood banks to confine the maximum discharge to the channel; BS ISO 3846:2008 Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- 3 h) the stability of the banks, and the necessity for trimming and/or revetment in natural channels; i) the clearance of rocks or boulders from the bed of the approach channel; j) the effects of wind, which can have a considerable effect on the flow in a river, or over a weir, especially when the river or weir is wide and the head is small and when the prevailing wind is in a transverse direction. If the site does not possess the characteristics necessary for satisfactory measurements, the site shall be rejected unless suitable improvements are practicable. If an inspection of the stream shows that the existing velocity distribution is regular, then it may be assumed that the velocity distribution will remain satisfactory after the construction of the weir. If the existing velocity distribution is irregular and no other site for a gauge is feasible, due consideration shall be given to checking the distribution after the installation of the weir and to improving it if necessary. Several methods are available for obtaining a more precise indication of irregular velocity distribution. These include velocity rods, floats or concentrations of dye, which can be used in small channels; the last is useful to check the conditions at the bottom of the channel. A complete and quantitative assessment of the velocity distribution may be made by means of a current-meter or other point velocity instruments. More information about the use of current-meters is given in ISO 748 1. Further information on measuring river velocities using acoustic Doppler profilers can be found in ISO/TS 24154 3. 5.3 Installation conditions 5.3.1 General The complete measuring installation consists of an approach channel, a measuring structure and a downstream channel. The conditions of each of these three components affect the overall accuracy of the measurements. Installation requirements include features such as the surface finish of the weir, the cross-sectional shape of the channel, the channel roughness, and the influence of control devices upstream or downstream from the gauging structure. The distribution and direction of velocity have an important influence on the performance of a weir, these factors being determined by the features mentioned above. Once a weir has been installed, the user shall prevent any changes which could affect the discharge characteristics. 5.3.2 The approach channel On all installations, the flow in the approach channel shall be smooth, free from disturbance and have a velocity distribution as satisfactory as possible over the cross-sectional area. This can usually be verified by inspection or measurement. In the case of natural streams or rivers, this can only be attained by having a long straight approach channel free from projections into the flow. The following general requirements shall be complied with. a) The altered flow conditions owing to the construction of the weir might cause a build-up of shoals of debris upstream of the structure, which in time might affect the flow conditions. The likely consequential changes in the water level shall be taken into account in the design of gauging stations. b) In an artificial channel, the cross-section shall be uniform and the channel shall be straight for a length equal to at least 5 times its water-surface width. BS ISO 3846:2008 Copyright British Standards Institution Provided by IHS under license with BSI - Uncontrolled Copy Licensee=Boeing Co/5910770001 Not for Resale, 08/14/2008 22:01:54 MDTNo reproduction or networking permitted without license from IHS -,-,- 4 c) In a natural stream or river, the cross-section shall be reasonably uniform and the channel shall be straight for a sufficient length to ensure a regular velocity distribution. d) If the entry to the approach channel is through a bend, or if the flow is discharged into the channel through a conduit or a channel of smaller cross-section, or at an angle, then a longer length of straight approach channel may be required to achieve a regular velocity distribution. e) Baffles shall not be installed closer to the points of measurement than a distance 10 times the maximum head to be measured. f) Under certain conditions, a standing wave may occur upstream of the gauging device, e.g. if the approach channel is steep. Provided that this wave is at a distance of not less than 30 times the maximum head upstream, flow measurement is feasible, subject to confirmation that a regular velocity distribution exists at the gauging station and that the Froude number in this section is no more than 0,6. Ideally, high Froude numbers should be avoided for accurate flow measurement. If a standing wave occurs within this distance, the approach conditions and/or the gauging device shall be modified. 5.3.3 The measuring structure The structure shall be rigid and watertight and capable of withstanding flood flow conditions without distortion or fracture. It shall be at right angles to the direction of flow and sha