ISO 8368 pdf download.Hydrometric determinations — Flow measurements in open channels using structures — Guidelines for selection of structure
1 Scope
This International Standard gives guidelines for selection of a particular type of flow-gauging structure for measurement of liquid flow in open channels. It sets out the factors, and summarizes the parameters which may influence such a selection. NOTE In general, a flow-gauging structure is used when high accuracy is required for continuous records of flow.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this International Standard. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents listed below. For undated references, the latest edition of the normative document referred to applies. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 772, Hydrometric determinations — Vocabulary and symbols. ISO 1438-1, Water flow measurement in open channels using weirs and Venturi flumes — Part 1: Thin-plate weirs. ISO 3846, Liquid flow measurement in open channels by weirs and flumes — Rectangular broad-crested weirs. ISO 3847, Liquid flow measurement in open channels by weirs and flumes — End-depth method for estimation of flow in rectangular channels with a free overfall. ISO 4359, Liquid flow measurement in open channels — Rectangular, trapezoidal and U-shaped flumes. ISO 4360, Liquid flow measurement in open channels by weirs and flumes — Triangular-profile weirs. ISO 4362, Measurement of liquid flow in open channels — Trapezoidal profile weirs. ISO 4371, Measurement of liquid flow in open channels by weirs and flumes — End depth method for estimation of flow in non-rectangular channels with a free overfall (approximate method). ISO 4374, Liquid flow measurement in open channels — Round-nose horizontal broad-crested weirs.
5 Factors affecting choice
5.1 General The factors which affect choice can be considered under the following headings: a) purpose; b) range of flow; c) afflux; d) size and nature of channel; e) channel slope and sediment load; f) operation and maintenance; g) environmental impact; h) passage of fish; i) cost. 5.2 Purpose Table 1 tabulates the various structures and indicates some of the purposes for which they may be applicable, together with guidelines to their limitations. The purpose for which the structure is required will determine the range of flows and accuracies which is necessary. The accuracy in a single determination of discharge depends upon the estimation of the component uncertainties involved. In broad terms, thin-plate weirs will have a range of uncertainties from 1 % to 4 %, flumes and certain types of weirs will have a range from 2 % to 5 % and end methods and other weirs will have a range from 4 % to 10 %. Deviations from the construction, installation or use as laid down in the appropriate International Standard will result in measurement errors. 5.3 Range of flow It is necessary to consider the relation between maximum flow and minimum flow when deciding which type of structure to use, and an indication of the range of some typical structures is given in Table 2. For the best overall accuracy over a wide range of small discharges, a thin-plate V-notch weir should be used in preference to a thin-plate rectangular notch or rectangular full-width weir. For a wide range of larger discharges, a trapezoidal flume, a flat-V weir or a triangular-profile weir should be used in preference to a broad-crested weir, free overfall or rectangular-throat flume. 5.4 Afflux The rise in level immediately upstream of, and due to, a structure may interfere with the flow system and cause drainage problems, or limit the effectiveness of irrigation systems, or cause extra pumping costs. In addition, the aquatic habitat upstream of the structure may be adversely affected.