ISO INTERNATIONAL STANDARD 4377 Fourth edition 2012-08-01 Hydrometric determinations Flow measurement in open channels using structures Flat-V weirs Determinations hydrometriques - Mesure de debit dans les canaux découverts au moyen de structures Déversoirs en V ouvert Reference number ISO 4377:2012(E) @ ISO 2012 No reproduction or networking permitted without license from IHS Not for Resale ISO 4377:2012(E) COPYRIGHTPROTECTEDDOCUMENT @ISO2012 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, ISO's member body in the country of the requester. ISO copyright office Case postale 56 : CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 0947 E-mail copyright@iso.org Web www.iso.org Published in Switzerland @ ISO 2012 - All rights reserved or networking permited withoutlicense from IHS Not for Resale ISO 4377:2012(E) Contents Page Foreword 1 Scope.. 2 Normative references. 3 Terms and definitions .. .2 4 Symbols.. .3 5 Characteristics of flat-V weirs... .4 6 Installation... 6.1 Selection of site.. 6.2 Installationconditions.. 6.3 Weir structure ... 6.4 Downstream conditions.. .8 7 Maintenance .8 8 Measurement of head(s). .9 8.1 General ...... 8.2 Stilling (gauge) wells.... 8.3 Zero setting ... 11 8.4 Location of head measurement sections.. 13 9 Discharge relationships.... 14 9.1 Equations of discharge.... 14 9.2 Effective heads... 9.3 Shape factors.. .16 9.4 Coefficient of velocity . 16 9.5 Conditions for modular/drowned flow .... 18 9.6 Drowned flow reduction factor .... 21 9.7 Limits of application.. 28 10 Computation of discharge ... 29 10.1 General 29 10.2 Successive approximation method 10.3 Coefficient of velocity method. .31 10.4 Accuracy..... 32 11 Uncertainties in flow determination ... 32 11.1 32 11.2 Combining uncertainties .... 11.3 Uncertainty in the discharge coefficient u*(CDe)68 for the flat-V weir ... 11.4 Uncertainty in the drowned flow reduction factor u*(Cdr).... 11.5 Uncertainty in the effective head . 35 11.6 Uncertainty budget... .35 11.7 Variation of uncertainty with flow and uncertainty in mean daily flow and the daily flow volume.... 12 Examples..... 12.1 Example 1 - Computation of modular flow at low discharge.. .37 12.2 Example 1 - Uncertainty in computed discharge... 39 12.3 Example 2 Computation of drowned flow at high discharge. 12.4 Example 2 Uncertainty in computed discharge... .43 Annex A (normative) Velocity distribution. 46 CopyrightInterational OrganizatonfrStandardizatiorights reserved ili No reproduction or networking permitted without license from IHS Not for Resale