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
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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.
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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.
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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.
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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
Open preview (PDF)Measurement of liquid flow in open channels by weirs and flumes — Streamlined triangular profile weirs
