Overview
ISO 4369:1979 - Measurement of liquid flow in open channels: Moving‑boat method - defines a standardized velocity‑area technique for measuring discharge in large rivers and estuaries using a moving boat. The standard specifies procedures, equipment requirements, data collection practices and computation using the mid‑section (segment) method. A complete worked example of a moving‑boat measurement is provided in the annexes. Units are SI.
Key topics and technical requirements
- Principle: traverse the channel on a preselected path (generally normal to flow) while a current meter mounted from the boat measures the vector sum of stream and boat velocities; depth profiles are recorded with an echo sounder.
- Two data‑reduction methods:
- Method 1 (angle method) - measure the angle between a vane (which aligns with water movement) and the boat path to separate boat velocity from stream velocity.
- Method 2 (distance method) - measure distance from each observation point to a fixed bank point to compute boat speed and derive stream velocity.
- Sampling and segmentation: typically 30–40 observation points per run; at least 25 segments per cross‑section are recommended to compute discharge reliably.
- Computation: mid‑section (segment) summation of segment area × mean segment velocity to approximate Q. The annex includes a facsimile computation example.
- Equipment requirements:
- maneuverable boat (stable, draft suitable for site),
- vane and angle indicator (method 1),
- current meter (propeller type preferred) with pulse rate indicator and counter,
- portable echo sounder with ability to mark observation points,
- distance measuring device (optical or electronic) for method 2.
- Limitations and constraints:
- intended for large rivers/estuaries - typically >300 m wide and >2 m deep; floats used should be in water >1 m deep,
- not suitable where under‑currents reverse direction in the vertical (tidal under‑currents affect vertical velocity distribution),
- results assume limited change in discharge during a traverse (rapidly unsteady flows require care).
Applications and users
ISO 4369 is used by hydrologists, river and estuarine engineers, water‑resource agencies, environmental monitoring teams and flood management specialists for:
- discharge measurement where fixed installations or conventional current‑meter gauging are impractical (remote sites, floods),
- short‑term rapid measurements during unsteady flow events,
- cross‑sectional surveys of large rivers and tidal estuaries.
Related standards
Standards referenced in ISO 4369 include:
For practical implementation, consult ISO 4369 annexes for the sample computation and the related ISO documents for equipment and uncertainty guidance.
