Overview
ISO 3846:2008 - Hydrometry - specifies requirements for using rectangular broad‑crested weirs to measure the flow of clear water in open channels under free flow conditions. The standard (third edition, 2008) provides guidance on site selection, installation, instrumentation and procedures needed to obtain accurate and traceable discharge measurements using broad‑crested weirs.
Key topics and requirements
- Scope and definitions: Terms and symbols consistent with ISO vocabularies (see ISO 772).
- Site selection and installation (Clauses 5, 5.2–5.3): Criteria for choosing a gauging site, approach channel geometry, bank and bed stability, and requirements for a straight, uniform approach to ensure an acceptable velocity distribution.
- Measuring structure (Clause 5.3.3): Structural, alignment and surface‑finish requirements to ensure rigid, watertight weirs installed at right angles to flow.
- Head measurement (Clause 7): Instrumentation, stilling or float wells, and zero‑setting practices for accurate gauged head (h) readings.
- Rectangular broad‑crested weir specification (Clause 8): Geometric details, gauge location and provisions for ensuring modular flow.
- Discharge relationships (Clause 9): Equations for modular flow and drowned flow, definitions of discharge coefficient (C), modular coefficient, and conditions/limitations for each flow regime.
- Uncertainty and quality (Clause 10): Methods to estimate and combine measurement uncertainties, uncertainty budget for discharge coefficient and final flow estimate.
- Maintenance and operation (Clause 6): Ongoing maintenance of approach channel, debris control and monitoring for changes that could affect calibration.
- Worked example and annexes: Example calculations, modular limit guidance and an introduction to measurement uncertainty (Annexes A–D).
Practical applications
ISO 3846:2008 is applied by professionals and organizations involved in:
- River gauging and hydrological monitoring
- Irrigation and drainage canal flow measurement
- Wastewater and effluent channel metering
- Hydropower intake and small water infrastructure monitoring
- Flood monitoring and water resource management
Typical users include hydrologists, hydraulic and civil engineers, water utilities, environmental agencies, field technicians, and consultants responsible for establishing or maintaining flow gauging stations.
Related standards
- ISO 772 - Hydrometry: Vocabulary and symbols
- ISO 748 - Current‑meter methods (referenced for velocity distribution guidance)
- ISO/TS 24154 - Acoustic Doppler profiler guidance (for velocity assessments)
Keywords: ISO 3846:2008, hydrometry, open channel flow measurement, rectangular broad‑crested weirs, discharge coefficient, flow uncertainty, weir installation.
