Overview
ISO 748:2021 - "Hydrometry - Measurement of liquid flow in open channels - Velocity area methods using point velocity measurements" defines standardized methods for measuring discharge in open channels by combining cross‑sectional area and mean velocity determined from point velocity devices. The standard applies to rotating‑element current meters, acoustic Doppler velocimeters (ADV), ADCP (stationary method), surface velocity systems (including floats), and other point‑measurement approaches. It sets site selection, measurement and computation requirements and addresses uncertainty estimation, while noting that detailed deployment procedures should be taken from instrument manufacturers and local agencies.
Key topics and requirements
- Principle: Discharge is computed by Q = A × V (flow = cross‑sectional area × mean velocity).
- Site selection: Channels should be straight and uniform; recommended upstream straight length ≥ 6× channel width and downstream ≥ 3× width. Sites must avoid vortices, reverse flow, converging/diverging reaches and local obstructions.
- Cross‑section measurement: Methods for measuring channel width and depth to determine wetted area.
- Velocity measurement: Procedures for point velocity sampling, vertical integration, oblique flow handling, surface‑velocity methods (floats, non‑contact systems), and common interpolation/integration techniques.
- Computation of discharge: Arithmetic and integration-based methods including mean‑section, mid‑section, bathymetric verticals and independent vertical procedures.
- Uncertainty and limitations: Guidance on estimating measurement uncertainty (see also ISO 25377:2020 Hydrometric uncertainty guidance), contributory uncertainty sources, and statistical estimators.
- Annexes: Informative material on use of current meters, float systems, example surface systems, uncertainties, ice‑covered conditions and wire correction factors.
Practical applications
ISO 748:2021 is used for:
- River and stream gauging for water resource management and flood forecasting
- Calibration of rating curves at hydrometric stations
- Environmental flow assessments, reservoir and dam operations, irrigation supply management
- Field protocols for hydrologists, civil and hydraulic engineers, water agencies and consultants performing discharge measurements
- Reference for instrument manufacturers, researchers and regulatory bodies establishing measurement quality and uncertainty reporting
Who should use it
- Hydrologists and field technicians performing flow measurements
- Water resource and environmental agencies setting monitoring standards
- Civil/hydraulic engineers involved in design, operation and compliance
- Instrument manufacturers and laboratories validating velocity‑area systems
Related standards
- ISO 25377:2020 - Hydrometric uncertainty guidance (referenced for uncertainty estimation)
- ISO 772 - Hydrometry vocabulary and symbols
ISO 748:2021 provides an internationally accepted framework for accurate, repeatable open‑channel discharge measurement using point velocity methods, improving comparability and traceability of hydrometric data.