ISO 4362:1999

Overview

ISO 4362:1999 - Hydrometric determinations: flow measurement in open channels using trapezoidal broad‑crested weirs - defines a standardized method for steady‑flow measurement with trapezoidal broad‑crested weirs under both modular and non‑modular conditions. The standard covers use of the weir in rectangular and trapezoidal channels, installation and maintenance requirements, head and tailwater measurement practices, discharge determination (including coefficients and limits), and guidance on measurement uncertainties.

Key topics and technical requirements

• Weir geometry and construction
  • Trapezoidal broad‑crested weir with upstream and downstream slopes meeting a horizontal crest; crest width equals channel width and surfaces must be smooth, rigid and watertight.
• Site selection and approach channel
  • Requires a regular cross‑section approach; artificial channels should be straight and uniform for at least 10 × channel width; special guidance for bends and entry conditions.
• Head and tailwater measurement
  • Head (upstream) measurement section recommended at 3–4 × maximum head (h_max) upstream of the upstream toe; tailwater measurement at 5–6 × h_max downstream.
  • Use of stilling or float wells is recommended for continuous records; wells must be adequately sized and connected to the channel to avoid delays or oscillations.
• Discharge determination
  • The standard provides working equations that relate volumetric discharge to measured head, crest width, gravity and empirical coefficients: coefficient of discharge (C_D), approach velocity coefficient (C_v) and drowning/ submergence considerations.
  • Graphs, tables and correlations are provided for commonly used upstream/downstream slope combinations and for calculating modular limits (when tailwater begins to affect flow).
• Installation, maintenance and limits
  • Structural stability, impermeability, debris control and prevention of bank instability are emphasized. Limitations on weir use are specified in clauses 7.6 and 8.6.
• Uncertainty and examples
  • Clause on uncertainties in flow measurement and worked examples to apply the method in practice.

Applications

ISO 4362:1999 is used for:

• Routine and calibrated flow gauging in open channels, canals and irrigation works
• Hydrological monitoring, water‑resource management and flood measurement
• Design and verification of field weirs for regulatory compliance and engineering studies
• Laboratory and field calibration of discharge measurement devices

Who should use it

• Hydrologists and water resources engineers
• Field technicians and gauging station operators
• Civil engineering consultancies and utilities managing open‑channel flow
• Manufacturers of hydrometric structures and instrumentation

Related standards

• ISO 772:1996 - Hydrometric determinations: vocabulary and symbols (normative reference)
• Relevant guidance on current‑meter use referenced (e.g., ISO publications on velocity measurement)

Keywords: ISO 4362:1999, trapezoidal broad‑crested weirs, flow measurement, hydrometric determinations, open channels, discharge measurement, head measurement, stilling well, modular limit, coefficient of discharge.