Overview
EN ISO 748:2021 - Hydrometry: Measurement of liquid flow in open channels - velocity area methods using point velocity measurements - is the fifth edition (ISO 748:2021) adopted by CEN in 2021. The standard specifies validated procedures for determining cross-sectional area, measuring velocity at points, and computing discharge (flow rate) in open channels. It covers common instruments such as rotating-element current meters, acoustic Doppler velocimeters (ADV), the ADCP stationary method, and surface velocity techniques including floats and other surface velocity systems. The document updates previous editions, reflects technological developments, and incorporates guidance for measurement under ice conditions.
Key topics and requirements
- Principle of velocity–area methods using point velocity measurements for flow computation.
- Site selection and demarcation: guidance on choosing appropriate measurement reaches and marking cross-sections.
- Cross-sectional area measurement: methods for measuring width and depth to determine wetted area.
- Mean velocity determination:
- Procedures for point measurements in verticals and integration methods.
- Handling oblique flow and surface-to-mean velocity relationships.
- Surface velocity measurements (floats, non-contact systems) and their limitations.
- Computation of discharge: arithmetic methods (mean-section, mid-section, bathymetric verticals), independent verticals, and horizontal plane approaches.
- Uncertainty analysis: systematic approach to estimating measurement uncertainty, contributory uncertainty components, and practical limits.
- Informative annexes: practical guidance on current meters, floats, example surface systems, uncertainty examples, ice conditions, and wire correction factors.
- Note: EN ISO 748:2021 does not give detailed deployment or instrument-specific setup - users should consult instrument manufacturers and public agency guidelines.
Applications
EN ISO 748:2021 is intended for practical flow measurement tasks in:
- River and stream gauging for hydrology and flood forecasting
- Water resource management and irrigation canal monitoring
- Environmental flow assessments and ecological studies
- Hydropower intake and tailrace monitoring
- Regulatory compliance and water-quality load calculations
Using standardized velocity–area methods ensures consistent, comparable discharge data for modeling, reporting, and operational decision-making.
Who uses this standard
- Hydrologists and field technicians
- Water resources and civil engineers
- Environmental agencies and utilities
- Researchers and consultants conducting streamflow measurements
- Instrument manufacturers and calibration labs (for design-conformant devices)
Related standards
- ISO 9196 (measurement under ice conditions) - integrated into this edition
- Manufacturer guidelines and national hydrometry agency protocols for instrument deployment and local statutory requirements
Keywords: EN ISO 748:2021, hydrometry, open channels, velocity area methods, point velocity measurements, ADCP, ADV, current meters, discharge measurement, flow measurement, uncertainty.