ISO 18481:2017 pdf – Hydrometry — Liquid flow measurement using end depth method in channels with a free overfall.
1 Scope This document specifies a method for the estimation of the sub-critical flow of clear water in a smooth, essentially horizontal channel (or a gently sloping channel), abruptly discontinued at bottom by a hydraulic structure, with a vertical drop and discharging freely. Such an overfall forms a control section and offers a means for the estimation of flow using the end depth measurement method. A wide variety of channel cross-sections with overfall have been studied, but only those which have received general acceptance after adequate research and testing, and therefore do not require in situ calibration, are considered. This document covers channels with the following types of cross-sections: a) rectangular with confined and unconfined nappe; b) trapezoidal; c) triangular; d) circular; e) parabolic. The flow at the brink is curvilinear; therefore, the measured depth at the drop is not equal to the critical depth as computed by the principle based on assumption of parallel flow. However, the end depth and the critical depth (as in the case of the assumption of parallel flow) have unique relation, which is used to estimate the flow through these structures. 2 Normative references The following documents are referred to in the text in such a way that some or all of their content constitutes requirements 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. ISO and IEC maintain terminological databases for use in standardization at the following addresses: — IEC Electropedia: available at http://www.electropedia.org/ — ISO Online browsing platform: available at https://www.iso.org/obp
5 Principle The un-submerged flow at an abrupt end of a long channel can be referred to as free overfall. In many cases, the measurement of flow depth at the free overfall is possible and could be used for discharge estimation. Such a discharge measurement method does not generally require any obtrusive structure to be built. Many available overfall structures constructed for other reasons could also be used for the discharge measurement with minor modifications. There is a unique relationship between the flow discharge and the critical depth in an open channel. The ratio of end depth to the critical depth (EDR) established theoretically and verified experimentally offers an easy method to measure the discharge using end depth method. 6 Installation 6.1 General General requirements of overfall discharge measurement installation are described in the following clauses. Special requirements of different types are described in clauses which deal with specific types. 6.2 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 may be made to conform) to the requirements necessary for measurement by the end depth method. The potential application of this method of flow measurement is at proposed or existing water and waste water treatment plants, where flumes and channels form part of such installations. The discharge measurement using end depth can be installed on existing flumes and channels after verification that they conform to the requirements necessary for measurement by the end depth method or they can be modified to make them conform to the requirements. Particular attention should be paid to the following features in selecting the site and ensuring the necessary flow conditions.