1、BRITISH STANDARD CONFIRMED APRIL 1998 BS 3680-4F: 1990 ISO 4374:1990 Measurement of liquid flow in open channels Part 4: Weirs and flumes Part 4F: Round nose horizontal broad-crested weirsBS3680-4F:1990 This British Standard, having been prepared under the directionof the Industrial-process Measurem
2、entand Control Standards Policy Committee, waspublished under the authorityofthe Board of BSI andcomes into effect on 31August1990 BSI 11-1999 First published June 1981 Second edition August 1990 The following BSI references relate to the work on this standard: Committee reference PCL/3 Draft for co
3、mment 88/22888 DC ISBN 0 580 18854 X Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Industrial-process Measurement and Control Standards Policy Committee (PCL/-) to Technical Committee PCL/3, upon which the following bodies were represe
4、nted: Clyde River Purification Board Department of the Environment Department of the Environment for Northern Ireland Institute of Measurement and Control Institution of Civil Engineers Institution of Water and Environmental Management Ministry of Agriculture, Fisheries and Food National Rivers Auth
5、ority Water Services Association of England and Wales Welsh Office The following body was also represented in the drafting of the standard, through subcommittees and panels: GAMBICA (BEAMA Ltd.) Amendments issued since publication Amd. No. Date CommentsBS3680-4F:1990 BSI 11-1999 i Contents Page Comm
6、ittees responsible Inside front cover National foreword ii 1 Scope 1 2 Normative references 1 3 Definitions and symbols 1 4 Installation 1 5 Maintenance General requirements 3 6 Measurement of head 3 7 Round-nose horizontal broad-crested weirs 4 8 Discharge equations 7 9 Uncertainties in flow measur
7、ement 9 10 Example 11 Annex A (normative) Symbols and units 13 Annex B (normative) Velocity distribution 14 Annex C (normative) Example of calculation of coefficient of dischargeC D basedon the boundary layer thickness 16 Figure 1 Round-nose horizontal broad-crested weir General arrangement 5 Figure
8、 2 Combined measuring and regulating structure of a round-nose horizontal broad-crested weir 6 Figure 3 Coefficient of approach velocity, Cv 8 Figure B.1 Examples of velocity profile in approach channel 15 Figure C.1 Relative boundary layer displacement thickness 18 Table C.1 Recommended roughness v
9、alues, k 17 Table C.2 Viscosity of water 17 Publication(s) referred to Inside back coverBS3680-4F:1990 ii BSI 11-1999 National foreword This Part of BS3680has been prepared under the direction of the Industrial-process Measurement and Control Standards Policy Committee. It is identical with ISO 4374
10、:1990 “Liquid flow measurement in open channels Round-nose horizontal broad-crested weirs”, published by the International Organization for Standardization (ISO). This British Standard is a revision of BS 3680-4F:1981, which is withdrawn. The Technical Committee has reviewed the provisions of ISO 77
11、2:1988, to which reference is made in the text, and has decided that they are acceptable for use in conjunction with this standard. A related British Standard to ISO772is BS 3680-1:1983 “Glossary of terms”. A British Standard does not purport to include all the necessary provisions of a contract. Us
12、ers of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Cross-references International Standard Corresponding British Standard ISO 5168:1978 BS 5844:1980 Methods of measurement of fluid flow
13、:estimation of uncertainty of a flow-rate measurement (Identical) Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 18, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorp
14、orated. This will be indicated in the amendment table on the inside front cover.BS3680-4F:1990 BSI 11-1999 1 1 Scope 1.1 This International Standard deals with the measurement of flow in rivers and artificial channels under steady flow conditions using round-nose horizontal broad-crested weirs (seeF
15、igure 1 and Figure 2). 1.2 The flow conditions considered are limited to steady flows which are uniquely dependent on the upstream head. Drowned flows, which depend on downstream as well as upstream levels, are not covered by this International Standard. 1.3 The round-nose horizontal broad-crested w
16、eir has a good discharge range and modular limit and is appropriate for use in small- and medium-sized installations. It is particularly robust and insensitive to minor damage. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisi
17、ons of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards
18、 indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 772:1988, Liquid flow measurement in open channels Vocabulary and symbols. ISO 5168:1978, Measurement of fluid flow Estimation of uncertainty of a flow-rate measurement. 3 Definitions and symb
19、ols For the purposes of this International Standard, the definitions given in ISO 772 apply. A full list of symbols with the corresponding units of measurement is given in Annex A. 4 Installation 4.1 Selection of site 4.1.1 The weir shall be located in a straight section of channel, avoiding local o
20、bstructions, roughness or unevenness of the bed. 4.1.2 A preliminary study 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 measurement of discharge by the weir. Particular attention sho
21、uld be paid to the following features in selecting the site: a) the adequacy of the length of channel of regular cross-section available (see 4.2.2.2); b) the uniformity of the existing velocity distribution (see Annex B); c) the avoidance of a steep channel (but see4.2.2.6); d) the effects of any i
22、ncreased 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 structu
23、re 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; h) the stability of the banks, and the necessity for trimming and/or revetment in natural channels; i) the uniformity o
24、f the cross-section of the approach channel; j) the prevailing wind, which can have a considerable effect on the flow in a river, or over a weir or flume, especially when the river, weir orflume is wide and the head is small and when the prevailing wind is in a transverse direction; k) aquatic weed
25、growth; l) sediment transportation. 4.1.3 If the site does not possess the characteristics necessary for satisfactory measurements, or if an inspection of the stream shows that the velocity distribution in the approach channel deviates appreciably from the examples described in Annex B, the site sha
26、ll not be used unless suitable improvements are practicable. Alternatively, the performance of the installation may be checked by independent flow measurements. 4.2 Installation conditions 4.2.1 General requirements 4.2.1.1 The complete measuring installation consists of an approach channel, a weir
27、structure and a downstream channel. The condition of each of these three components affects 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 o
28、f control devices upstream or downstream of the gauging structure. 4.2.1.2 The distribution and direction of velocity may have an important influence on the performance of a weir (see 4.2.2 and Annex B).BS3680-4F:1990 2 BSI 11-1999 4.2.1.3 Once a weir has been installed, any changes in the systems w
29、hich affect the basis of the design will alter the discharge characteristics. 4.2.2 Approach channel 4.2.2.1 If the flow in the approach channel is disturbed by irregularities in the boundary, for example by large boulders or rock outcrops, or by a bend, sluice gate or other feature which causes asy
30、mmetry of discharge across the channel, the accuracy of gauging may be significantly affected. The flow in the approach channel shall have a symmetrical velocity distribution (see Annex B) and this can most readily be achieved by providing a long straight approach channel of uniform cross-section. 4
31、.2.2.2 A length of straight approach channel equal to five times the water-surface width at maximum flow will usually suffice, provided that flow does not enter the approach channel with high velocity via a sharp bend or angled sluice gate. However, a greater length of uniform approach channel is de
32、sirable if it can readily be provided. 4.2.2.3 The length of uniform approach channel suggested in 4.2.2.2 refers to the distance upstream of the head measuring position. However, in a natural channel it would be uneconomic to line the bed and banks with concrete for this distance, and it would be n
33、ecessary to provide a contraction in plan if the width between the vertical walls of the lined approach to the weir is less than the width of the natural channel. The unlined channel upstream of the contraction shall nevertheless comply with the requirements of 4.2.2.1 and 4.2.2.2. 4.2.2.4 Vertical
34、side walls to effect a contraction in plan shall be symmetrically disposed with respect to the centreline of the channel and shall preferably be curved with a radius R of not less than 2H max . The downstream tangent point shall be at least H maxupstream of the head measurement section. The height o
35、f the side walls shall be chosen such that the design maximum discharge can be contained. 4.2.2.5 In a channel where the flow is free from floating and suspended debris, good approach conditions can also be provided by suitably placed baffles formed of vertical laths, but no baffle shall be nearer t
36、o the point at which the head is measured than a distance of 10H max . 4.2.2.6 Under certain conditions, a hydraulic jump may occur upstream of the measuring structure, for example if the approach channel is steep. Provided that the hydraulic jump is at a distance upstream of not less than about 30H
37、 max , flow measurement will be feasible, subject to confirmation that an even velocity distribution exists at the gauging station. 4.2.2.7 Conditions in the approach channel can be verified by visual inspection or measurement for which several methods are available such as current-meters, floats, v
38、elocity rods, and concentrations of dye, the last being useful to check conditions at the bottom of the channel. A complete and quantitative assessment of velocity distribution may be made by means of a current-meter. The velocity distribution should then be assessed by reference to Annex B. 4.3 Wei
39、r structure 4.3.1 The structure shall be rigid and watertight and capable of withstanding flood flow conditions without damage from outflanking or from downstream erosion. The weir crest shall be at right angles to the direction of flow and the geometry shall conform to the dimensions given in the r
40、elevant clauses. 4.3.2 The surfaces of the weir and of the vertical abutments flanking the weir shall be smooth; they may be constructed in concrete with a smooth cement finish, or surfaced with a smooth non-corrodible material. In laboratory installations, the finish shall be equivalent to that of
41、rolled sheet metal or planed, sanded and painted timber. The surface finish is of particular importance on the horizontal crest, but the requirements may be relaxed beyond a distance along the profile1/2H maxupstream and downstream of the crest profile. 4.3.3 In order to minimize errors in the disch
42、arge measurements, the following tolerances should be aimed at during construction: on the crest width,0,2% of this width with a maximum of 0,01 m; on the horizontal surfaces, slopes of0,1 % (1 mm/m). The structure shall be measured on completion of construction and at regular intervals thereafter a
43、nd if it varies from the design dimensions by more than the permissible tolerances, the discharge shall be re-computed. 4.4 Movable measuring structure 4.4.1 For water management purposes, it is in many cases necessary to measure flows and also to control water levels or flow at the same location. A
44、 combined measuring and regulating structure provides the most economic means for this purpose. The movable round-nose horizontal broad-crested weir may be constructed with one single vertical slot in which the supporting plate of the weir crest can be raised or lowered according to the desired cres
45、t level. A vertical guide wall, founded on the bed of the channel and parallel to the supporting plate, acts as a watertight barrier for the movable weir.BS3680-4F:1990 BSI 11-1999 3 4.4.2 If regular sediment flushing is expected to be necessary, the weir may be constructed with two slots. The movab
46、le weir is placed in the downstream slot while a bottom gate is placed in the upstream slot. During flow measurement, the bottom gate is lowered onto the bed. To flush sediment that has settled upstream of the weir, the bottom gate is opened by connecting it to the movable weir and raising the weir
47、and gate together. 4.4.3 The most commonly used type of movable weir is that with two vertical slots. This telescopic weir consists of two sliding blades and a movable weir which are mounted on a steel guide frame in the following manner. a) The bottom gate is blocked in place under operational cond
48、itions and acts as a bottom limiter for the movable weir. b) The upper slide is connected to the bottom gate by means of two steel strips placed in the frame grooves and acts as a top limiter for the movable weir. c) The movable weir is connected by two steel strips to a horizontal lifting beam. The
49、 weir crest is horizontal in both directions. The upstream nose of the weir is rounded off in such a way that flow separation does not occur. Figure 2 shows the round-nose horizontal broad-crested weir as a measuring and regulating structure. 4.5 Downstream conditions Conditions downstream of the structure are important in that they control the tail-water level. This level is one of the factors which determines whether modular or drowned flow conditions will occur at the weir. It is essential, therefore, to calculate or observe tail-water levels over
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