ISO 4369:1979

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:

• ISO 748 (velocity‑area methods),
• ISO 772 (vocabulary and symbols),
• ISO 3454 (sounding and suspension equipment),
• ISO 4366 (echo sounders),
• ISO 5168 (uncertainty calculation).

For practical implementation, consult ISO 4369 annexes for the sample computation and the related ISO documents for equipment and uncertainty guidance.