What is ISO 18320 about?

ISO 18320 specifies methods of determining the stage–discharge relationship for gauging stations. ISO 18320 also specifies an accuracy for defining the stage–discharge relationship based on a sufficient number of discharge measurements, complete with corresponding stage measurements.

ISO 18320 considers stable and unstable channels and includes brief descriptions of the effects

on the stage–discharge relationship of the transition from in bank to overbank flows, shifting controls, variable backwater and hysteresis.

Who is ISO 18320 for?

ISO 18320 on measuring liquid flow in open channels is relevant to:

• Manufacturers of open channel flow measurement devices
• Installers of ultrasonic level sensors
• Authorities of flow measurement
• Hydrologists

Why should you use ISO 18320?

The relationship at a gauging station between stage and discharge is commonly referred to as the stage–discharge relationship, rating curve or rating.

ISO 18320 is about stage–discharge relationship which has been developed for the user to enable the future production of a time series of discharge based on continuous stage measurements at the gauging station. It is generally much easier to continuously measure stage than it is to measure discharge. Hence, once a stable stage–discharge relationship has been established at a gauging station, the creation of a record of discharge is greatly simplified. The stage–discharge relationship for open-channel flow at a gauging station is governed by channel

conditions at and downstream from the gauge, referred to as a control. Two types of control can exist, depending on channel and flow conditions. ISO 18320 provides a record of the discharge of the open channel or river at which the water level gauge is sited. This is achieved by measuring the stage and converting this stage to discharge by means of a stage–discharge relationship which correlates discharge and water level.

Overall, ISO 18320 is helpful as it assists the user about the relationship between stage and discharge which is defined by plotting measurements of discharge with corresponding observations of stage, taking into account whether the discharge is steady, increasing or decreasing, and also noting the rate of change in stage. This can be done either manually by plotting on paper or automatically using computerized plotting techniques.

What’s changed since the last update?

BS ISO 18320:2020 replaces ISO 1100-2:2010, which has been technically revised. The main changes compared to the previous edition are as follows:

• Major revisions have been made to Clause 5, including a new figure of a stage–discharge relationship and shift curves
• Clause 7 has been revised to be consistent with new standards on uncertainty