1、BRITISH STANDARD CONFIRMED MARCH 1998 BS 3680-4I: 1986 ISO 8333:1985 Measurement of liquid flow in open channels Part 4: Weirs and flumes Part 4I: V-shaped broad crested weirs ISO title: Liquid flow measurement in open channels by weirs and flumes V-shaped broad crested weirs UDC 532.543:532.57:681.
2、12BS3680-4I:1986 This British Standard, having been prepared under the directionof the Industrial-process Measurementand Control Standards Committee, was published under the authorityofthe Board of BSIandcomes intoeffect on 30September1986 BSI 12-1999 The following BSI references relate to the work
3、on this standard: Committee reference PCL/3 Draft for comment 84/21095 DC ISBN 0 580 15312 6 Committees responsible for this British Standard The preparation of this British Standard was entrusted by the Industrial-process Measurement and Control Standards Committee (PCL/-) to Technical Committee PC
4、L/3, upon which the following bodies were represented: Department of the Environment for Northern Ireland Department of the Environment Welsh Office Institute of Measurement and Control Institution of Civil Engineers Institution of Water Engineers and Scientists Scottish Development Department Water
5、 Authorities Association The following bodies were also represented in the drafting of the standard, through subcommittees and panels: Anglian Water Authority British Effluent and Water Association Amendments issued since publication Amd. No. Date of issue CommentsBS3680-4I:1986 BSI 12-1999 i Conten
6、ts Page Committees responsible Inside front cover National foreword ii 1 Scope and field of application 1 2 References 1 3 Definitions 1 4 Units of measurement 1 5 Installation 1 6 General requirements for maintenance 4 7 Measurement of head 4 8 V-shaped broad-crested weirs 6 9 Discharge equations 7
7、 10 Errors in flow measurement 9 Annex A Scaling effects 13 Annex B “More than full” flow 14 Annex C Submerged flow 15 Figure 1 V-shaped broad-crested weir 3 Figure 2 Longitudinal section of three types of construction 3 Figure 3 Determination of head-gauge datum 5 Figure 4 Dimensions of weir and fl
8、ow 6 Figure 5 Discharge coefficient as a function of h 1 /L 7 Figure 6 Flow-pattern above crest for free flow with h 1 /L = 0,353 8 Figure 7 Dimensions for “more than full” flow 14 Figure 8 Modular limit for V-shaped broad-crested weirs 15 Figure 9 Submerged flow reduction factor for V-shaped broad-
9、crested weirs 16 Publications referred to Inside back coverBS3680-4I:1986 ii BSI 12-1999 National foreword This Part of BS3680 has been prepared under the direction of the Industrial-process Measurement and Control Standards Committee. It is identical with ISO8333:1985 “Liquid flow measurement in op
10、en channels by weirs and flumes V-shaped broad-crested weirs”, published by the International Organization for Standardization (ISO). Terminology and conventions. The text of the International Standard has been approved as suitable for publication as a British Standard without deviation. Some termin
11、ology and certain conventions are not identical with those used in British Standards; attention is drawn especially to the following. Wherever the words “International Standard” appear, referring to this standard, they should be read as “British Standard”. The comma has been used as a decimal marker
12、. In British Standards it is current practice to use a full point on the baseline as the decimal marker. The Technical Committee has reviewed the provisions of ISO772:1978, to which reference is made in the text, and has decided that they are acceptable for use in conjunction with this standard. A r
13、elated British Standard to ISO772 is BS3680-1:1983 “Glossary of terms”. A related British Standards to ISO4374, which is quoted for information purposes only is BS3680-4F:1981. At present there is no corresponding British Standard for ISO4373:1979 which is quoted for information purposes only. A Bri
14、tish Standard does not purport to include all the necessary provisions of a contract. Users 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 Corres
15、ponding British Standard ISO 5168:1978 BS 5844:1981 Methods of measurement of fluid flow: estimation of uncertainty of a flowrate measurement (Identical) Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 16, an inside back cover and a back cove
16、r. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS3680-4I:1986 BSI 12-1999 1 1 Scope and field of application This International Standard specifies a method for the measurement of
17、 subcritical flow in small rivers and artificial channels using V-shaped broad-crested weirs. The advantages of this type of weir are described inclause8. NOTEA comparison of the different types of weirs and flumes will form the subject of a future International Standard. 2 References ISO 772, Liqui
18、d flow measurement in open channels Vocabulary and symbols. ISO 4373, Measurement of liquid flow in open channels Water level measuring devices. ISO 4374, Liquid flow measurement in open channels Round-nose horizontal crest weirs. ISO 5168, Measurement of fluid flow Estimation of uncertainty of a fl
19、ow-rate measurement. 3 Definitions For the purposes of this International Standard, the definitions given in ISO772 apply. 4 Units of measurement The units of measurement used in this International Standard are SI units. 5 Installation 5.1 Selection of site A preliminary survey shall be made of the
20、physical and hydraulic features of the proposed site to check that it conforms (or may be made to conform) to the requirements necessary for measurement using the weir. Particular attention shall be paid to the following features in selecting the site of the weir: a)the availability of an adequate l
21、ength of channel of regular cross-section; b)the existing velocity distribution; c)the avoidance of channels having gradients greater than1 in250; d)the consequential effects of any increased upstream water level due to the measuring structure; e)the consequential conditions downstream, including su
22、ch influences 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 sealing in river installations; g)the
23、 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; j)the clearance of rocks or boulders from the bed of the approach channel; k)the effects of wind (wind can have a considerabl
24、e effect on the flow in a river or over a weir, especially if these are wide and the head is small and when the prevailing wind is in a transverse direction). If the site does not possess the characteristics required for satisfactory measurements, it shall be rejected unless suitable improvements ar
25、e practicable. If a survey of a stream shows that the existing velocity distribution is regular, then it is assumed that the velocity distribution will remain satisfactory after the weir has been built. If the existing velocity distribution is irregular and no other site for a gauge is feasible, due
26、 consideration shall be given to checking the distribution after the weir has been installed and to improving it, if necessary. Several methods are available for obtaining more precise indications of irregular velocity distribution; velocity rods, floats or concentrations of dye can be used in small
27、 channels, the latter being useful in checking conditions at the bottom of the channel. A complete and quantitative assessment of velocity distribution may be made by means of a current-meter. 5.2 Installation conditions 5.2.1 General The complete measuring installation consists of an approach chann
28、el, a measuring structure and a downstream channel. The parameters of each of these three components affect the overall accuracy of the measurements. Installation requirements include such features as weir finish, the cross-sectional shape of the channel, channel roughness and the influence of contr
29、ol devices upstream or downstream of 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.BS3680-4I:1986 2 BSI 12-1999 Once an installation has been designed, the use
30、r shall eschew any changes which could affect the discharge characteristics. 5.2.2 Approach channel For all installations, the flow in the approach channel shall be smooth, free from disturbance and shall have a velocity distribution as normal as possible over the cross-sectional area. This can usua
31、lly 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. Unless otherwise specified in the appropriate clauses, the approach channel shall comply with the general
32、 requirements outlined below. The altered flow conditions due to the construction of the weir may have the effect of building up shoals of debris upstream of the structure, which in time may affect the flow conditions. The likely consequential changes in the water level should be taken into account
33、in the design of gauging stations. In an artificial channel, the cross-section shall be uniform and the channel shall be straight for a length equal to at least10times its width. In a natural stream or river, the cross-section shall be reasonably uniform and the channel shall be straight for such a
34、length as to ensure regular velocity distribution. If the entry to the approach channel is through a bend or if the flow is discharged into the channel through a conduit of smaller cross-section or at an angle, then a longer length of straight approach channel is required to achieve a regular veloci
35、ty distribution. Baffles shall not be installed closer to the points of measurement than10times the maximum head to be measured. Under certain conditions, a standing wave may occur upstream of the gauging device, for example if the approach channel is steep. Provided this wave is at a distance of no
36、t less than30times the maximum head upstream, flow measurement is feasible, subject to confirmation that a regular velocity distribution exists at the gauging station. If a standing wave occurs within this distance, the approach conditions and/or gauging device shall be modified. 5.2.3 Measuring str
37、ucture The structure shall be rigid, watertight and capable of withstanding flow conditions without distortion or fracture. It shall be at right angles to the direction of flow and shall have the dimensions specified in the relevant clauses (see alsoFigure 1). The V-shaped broad-crested weir may be
38、constructed with either a fixed crest or a movable one with vertical slots (seeFigure 2). 5.2.4 Movable measuring structure The movable V-shaped broad-crested weir can be constructed with one vertical slot in which the supporting plate of the weir crest can be raised or lowered according to the desi
39、red crest level. A vertical guide wall founded at the channel bottom and parallel to the supporting plate acts as a watertight barrier for the movable weir. If regular flushing of sediment is expected to be necessary, the weir can be constructed with two slots. The movable weir can be operated in th
40、e downstream slot while a bottom gate is placed in the upstream slot. During measurement of flow, the gate is closed at the bottom. To flush sediment that has settled upstream of the weir, the gate can be opened by connecting it to the movable weir and raising the weir and gate together.BS3680-4I:19
41、86 BSI 12-1999 3 Figure 1 V-shaped broad-crested weir Figure 2 Longitudinal section of three types of constructionBS3680-4I:1986 4 BSI 12-1999 5.2.5 Downstream channel The channel downstream of the structure is usually of no importance as such, provided that the weir has been designed so that the fl
42、ow is modular under the operating conditions. The altered flow conditions due to the construction of the weir may have the effect of building up shoals of debris immediately downstream of the structure, which in time may raise the water level sufficiently to drown the weir. Any accumulation of debri
43、s downstream of the structure shall therefore be removed. 6 General requirements for maintenance Maintenance of the measuring structure and the approach channel is important to ensure accurate continuous measurements. It is essential that the approach channel to weirs be kept clean and free from sil
44、t and vegetation as far as practicable for at least the distance specified in5.2.2. The float well and the entry from the approach channel shall also be kept clean and free from deposits. The weir shall be kept clean and free from clinging debris and care shall be taken in the process of cleaning to
45、 avoid damage to the weir crest. 7 Measurement of head 7.1 General The head upstream of the measuring structure may be measured by a hook-gauge, point-gauge or staff-gauge where spot measurements are required or by a float-operated recording gauge where a continuous record is required, and, in many
46、cases, it is preferable to measure heads in a separate stilling well to reduce the effects of surface irregularities. Other head-measuring methods (for example bubble tubes) may be used, provided sufficient accuracy is obtainable. The discharges given by the working equation are volumetric figures,
47、and the liquid density does not affect the volumetric discharge for a given head provided the head is calibrated in liquid of identical density. If the gauging is carried out in a separate well, a correction for the difference in density may be necessary if the temperature of the liquid in the well
48、is significantly different from that of the flowing liquid. However, it is assumed herein that the densities are equal. It shall, however, be ensured that the gauge is not located in a pocket or still pool, but measures the piezometric head. 7.2 Stilling or float well Where provided, the stilling we
49、ll shall be vertical and shall extend at least0,6m above the maximum water level estimated to be recorded in the well. It shall be connected to the approach channel by an inlet pipe or slot, large enough to permit the water in the well to follow the rise and fall of the head without significant delay. The level of the inlet pipe shall be at least0,1m below the lowest point of the crest (vertex). The connecting pipe or slot shall, however, be as small as possible insofar as is consistent with ease of maintenance, or, alternatively, it shall
