Standards Development

Scope

To determine liquid flow, the following steps are necessary:

1) Measure water surface velocity with techniques using radar, laser or video images;

2) Correct the water surface velocity due to wind effects if necessary;

3) Option a: Transform the corrected velocity to a depth-averaged velocity in one segment using the arithmetic methods referring to chapter 7.2, secondly calculate each segment and then create the sum of all segments to obtain the cross-sectional averaged velocity distribution;

3) Option b: Transform the corrected velocity to a cross sectional velocity using the index methods referring to chapter 7.3;

4) Determine the area of the wetted cross section from the stage-area relationship;

5) Obtain discharge of each segment by multiplying the depth-averaged velocity in each segment by the wetted cross-sectional area of each segment. And then create the sum of all segments to obtain whole discharge in cross section.

This procedure is applicable to different kinds of channel and river sections.

Applications include:

— Rivers and streams;

— Artificial channels such as drainage ditches and irrigation channels;

— Process flows on wastewater treatment plants.

For any individual site the method to measure water surface velocity should be selected appropriately, based on the site conditions, nature of the application and uncertainty required. Take a special note that non-contact methods should not be used where a unique relation between surface velocity and depth averaged velocity cannot be established, e.g. where tidal phenomena are present. This is caused by the variations of flow magnitude and direction over depth being highly variable over time under these circumstances. Regarding backwater zones or in the vicinity of obstacles the relation between surface velocity and depth averaged velocity may be more complicated, but even here optical methods may be helpful to at least learn the situation at the surface.