ISO 4374:1990

Overview

ISO 4374:1990 - "Liquid flow measurement in open channels - Round-nose horizontal broad-crested weirs" specifies requirements and guidance for measuring steady open‑channel flow using round‑nose horizontal broad‑crested weirs. The standard applies to rivers and artificial channels where discharge is uniquely dependent on upstream head (steady, non‑drowned conditions). It is a technical revision of the first edition (1982) and focuses on weir geometry, installation, measurement practice and site selection to achieve reliable flow gauging.

Key topics and technical requirements

• Scope and flow regime: Intended for steady flows where discharge depends only on upstream head; drowned flows are excluded.
• Site selection: Require straight approach reach free of local obstructions, with a recommended approach length of about five times the water‑surface width at maximum flow for uniform velocity distribution.
• Approach channel and velocity: Symmetrical, even velocity distribution is essential; disturbed or asymmetric approach flows require remedial works or alternative measurement methods.
• Weir geometry and tolerances:
  • Upstream nose radius r ≥ 0.2 Hmax (Hmax = maximum head referenced in the standard).
  • Horizontal crest length not less than 1.75 Hmax; crest length plus nose radius not less than 2.25 Hmax.
  • Construction tolerances (examples): crest width ±0.2% (max 0.01 m); horizontal surface slopes ≈ 0.1% (1 mm/m).
• Weir structure and surface finish: Rigid, watertight construction; smooth crest and abutments (concrete or smooth non‑corrodible finishes) to minimise flow disturbance.
• Movable weirs & sediment management: Guidance for telescopic or slotted movable weirs and arrangements for sediment flushing (two‑slot arrangements, bottom gates).
• Head measurement:
  • Preferably measured in a separate gauge well to damp surface waves. Minimum well height 0.3 m above maximum water level.
  • Connecting pipe/slot invert at least 0.06 m below the lowest level to be gauged. Typical field connecting pipe diameter 100 mm; laboratory precision may use 3 mm. Floats must have clearance (≥ 0.075 m from well wall).
• Maintenance: Keep approach channel and gauge well free of silt, debris and aquatic growth; re‑measure structure if dimensions deviate beyond tolerances.

Applications and users

ISO 4374:1990 is used for:

• Flow gauging in small‑ to medium‑sized rivers, irrigation channels and water‑management works.
• Combined measuring/regulating installations where a movable crest is needed.
  Primary users include hydrometric engineers, water resource managers, irrigation authorities, environmental consultants and designers of flow‑measurement structures.

Related standards

• ISO 8368:1985 - Guidelines for selection of weirs and flumes for open‑channel discharge measurement.
• ISO 772 and ISO 5168 referenced for vocabulary, symbols and uncertainty estimation.

Keywords: ISO 4374:1990, round‑nose horizontal broad‑crested weirs, open channel flow measurement, weir installation, head measurement, flow gauging, gauge well, approach channel.