ISO 4360:2020

Overview

ISO 4360:2020 - Hydrometry - Open channel flow measurement using triangular profile weirs - defines standard methods for measuring water flow in open channels with triangular profile weirs. Intended for steady conditions, the standard covers both modular (free) flow, where discharge depends only on upstream head, and non‑modular (drowned) flow, where downstream levels also influence discharge. This fourth edition (2020) replaces the 2008 edition and includes corrected calculations and a supporting spreadsheet (Annex C) to aid discharge and uncertainty calculations.

Key topics and requirements

The document sets out technical requirements and guidance for accurate triangular‑weir measurement, including:

• Principle: relationship of discharge to upstream head, downstream head (if drowned), geometry and flow dynamics.
• Installation: site selection, approach channel, measuring structure and downstream channel considerations to ensure representative velocity distribution and hydraulic control.
• Head measurement: locations and methods for gauging upstream and downstream heads, including crest tappings and gauge wells.
• Weir and channel geometry: dimensional and finish requirements for the weir crest, approach cross‑section and bed conditions.
• Flow regimes: distinction between modular (free) and non‑modular (drowned) conditions and how each is treated.
• Discharge characteristics: formulae framework, including coefficients such as coefficient of discharge (C_d) and velocity factors for modular and non‑modular flow; limitations on use.
• Uncertainty: detailed treatment of measurement uncertainty, combining component uncertainties, and an uncertainty budget (with an informative annex introducing uncertainty concepts).
• Maintenance and operation: guidance to maintain installation conditions and prevent changes that affect accuracy.
• Supporting tools: Annex C spreadsheet and worked examples for practical calculation and reporting.

Relevant normative references cited include ISO 772 (vocabulary and symbols), with cross‑references to guidance on velocity measurement (ISO 748, ISO 24578).

Applications and users

ISO 4360:2020 is used by professionals responsible for open channel flow measurement where triangular weirs are appropriate, including:

• Hydrometric agencies and national water authorities
• Water resources and flood‑forecasting engineers
• Irrigation and drainage managers
• Wastewater and environmental monitoring teams
• Consulting engineers and researchers conducting gauging, calibration or compliance monitoring

Typical applications: river and stream gauging, permanent monitoring stations, calibration of hydrometric networks, regulatory discharge reporting and flood management.

Related standards

• ISO 772 - Hydrometry: vocabulary and symbols
• ISO 748 - Current‑meter flow measurement guidance
• ISO 24578 - Acoustic Doppler current profiler guidance

Keywords: ISO 4360:2020, triangular profile weirs, hydrometry, open channel flow measurement, modular flow, drowned flow, discharge coefficient, head measurement, measurement uncertainty.