Overview
ISO 8333:1985 specifies a method for measuring subcritical liquid flow in open channels using V-shaped broad-crested weirs. It applies to small rivers and artificial channels and covers site selection, installation conditions, head measurement, discharge equations, measurement errors and related requirements. The standard includes three annexes (including Annex B on “more than full” flow discharge and Annex C on modular limits and drowned‑flow reduction factors).
Key topics and technical requirements
- Scope and applicability: Intended for subcritical flows in small natural or artificial channels using V‑shaped broad‑crested weirs.
- Site selection: Requires a preliminary survey addressing channel cross‑section regularity, velocity distribution, channel gradient (avoid > 1:250), impermeability of foundation, upstream/downstream controls (dams, sluices, confluences), approach channel length and clearance, wind effects and debris accumulation.
- Approach and downstream channels: Approach must be straight and uniform (typically ≥ 10 × channel width in artificial channels); downstream conditions should not drown the weir.
- Measuring structure: Weir must be rigid, watertight and perpendicular to flow; may be fixed or movable (single or double vertical slots) to allow cleaning or regulation. Crest length L should permit nearly horizontal flow (L ≥ 2 Hm). Crest angle a typically 90°–150°. Upstream rounding radius R recommended in the range 0.1 L to 0.2 L.
- Head measurement: Use stilling well, hook/point/staff gauges or float recorders. The standard sets procedures for determining the head‑gauge datum (gauge zero) relative to the weir vertex-important for accurate volumetric discharge.
- Discharge equations and coefficients: Provides equations for “less than full” (free) flow and references Annex B for “more than full” flow. Discharge depends on crest geometry, measured head, approach velocity coefficient Cv and discharge coefficient Cd. A mean Cd ≈ 0.95 may be used for design within specified h1/L ranges.
- Practical limits & accuracy: Minimum measurable head, maximum permitted head ratios and crest angle constraints are specified; maintenance and regular verification are required to control measurement errors.
Applications and users
ISO 8333:1985 is used by:
- Hydrologists and water resources engineers for gauging flows in small rivers and canals
- Irrigation managers and civil engineers for flow regulation and measurement in hydraulic structures
- Monitoring agencies and hydrometric laboratories establishing permanent gauging stations
- Designers of flow measurement installations and consultants preparing gauging specifications
Related standards
- ISO 772 (vocabulary & symbols)
- ISO 4373 (water level measuring devices / stilling wells)
- ISO 4374 (round‑nose horizontal crest weirs)
- ISO 5168 (estimation of uncertainty in flow measurement)
Keywords: ISO 8333:1985, V-shaped broad-crested weir, open channel flow measurement, weirs and flumes, discharge coefficient, head measurement, subcritical flow, flow gauging.