ISO 9827:1994

Overview

ISO 9827:1994 specifies methods for measuring liquid flow in open channels using streamlined triangular profile weirs. It covers steady-flow conditions and treats both free (modular) flow - dependent only on upstream head - and drowned (submerged) flow - dependent on upstream and downstream water levels. The standard uses SI units and is intended for hydrometric determinations where reliable volumetric discharge measurements are required.

Key topics and technical requirements

• Weir geometry and construction

  • Streamlined triangular profile: upstream slope 1:2, downstream slope 1:q, with a circular arc crest joining slopes tangentially and a horizontal crest at right angles to flow.
  • Construction tolerances: width and height ±0.5%, crest radius ±1%, slope ±1%.
  • Weir width should equal channel width.

• Site selection & installation

  • Preliminary survey of approach channel shape, velocity distribution and absence of steep channel.
  • Approach channel must be kept clean and free of silt/vegetation.
  • For measurements under drowned conditions, downstream channel should be straight for at least 8 × maximum head and subcritical flow.

• Head and tailwater measurement

  • Head (upstream) gauge station: recommended 3–4 × maximum head upstream from crest.
  • Tailwater (downstream) gauge station: recommended 5–6 × maximum head downstream from weir toe to avoid fluctuations.
  • Stilling/float wells: watertight, vertical, with 0.6 m margin above estimated maximum water level; connecting pipe/slot must allow quick response.

• Discharge calculation & coefficients

  • Discharge depends on measured head, weir width and coefficients C (discharge coefficient) and Cdr (drowned-flow coefficient). The discharge varies approximately with h^(3/2) (head power 1.5).
  • Modular limits and coefficient curves are provided for specified weir shapes (weirs 1–5). Weir No. 5 is restricted to modular flow; others have defined h/P limits (e.g., weirs 1–3: h/P < 1.6; weir 4: < 1.45; weir 5: < 1.4).

• Measurement limits & minimums

  • Recommended minimums to avoid surface tension/viscosity effects: h_a ≥ 0.05 m, P_a ≥ 0.15 m, b ≥ 0.30 m.

• Uncertainty & calibration

  • Systematic uncertainty in C typically within 2.5% with careful construction; combined coefficient uncertainty up to 5% for nonmodular flows. Random variability in coefficients is small (~0.5%). ISO 5168 is referenced for uncertainty evaluation and combining errors.

Applications and users

ISO 9827:1994 is used where accurate open-channel flow measurement is needed:

• Hydrologists and water-resource engineers (river gauging, streamflow monitoring)
• Irrigation and drainage system managers
• Environmental agencies (water allocation, compliance, ecological flow studies)
• Hydropower and intake monitoring
• Calibration laboratories and civil contractors installing permanent gauging structures

Related standards

• ISO 772 - Measurement of liquid flow in open channels: vocabulary and symbols
• ISO 748 - Velocity-area methods (related hydrometric techniques)
• ISO 5168 - Evaluation of uncertainties in fluid flow measurement

Keywords: ISO 9827:1994, streamlined triangular profile weirs, measurement of liquid flow in open channels, weir discharge, modular flow, drowned flow, discharge coefficient, head measurement, flow measurement uncertainty.