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Among the phenomena of backed up outflow, we have also the passage through bar screens (the bars length is placed in the stream flowing direction). The bars may have different sections and angles of inclination (α) on the horizontal line (Picture FG1). The head loss through a bar screen is determined by means of the KIRSHMER’s formula:

- v : water velocity without the bar screen;
- α: angle of inclination of the bar screen on the horizontal line;
- d : bars thickness;
- a: spacing between the bars;
- β: coefficient of shape, function of the bar section, as shown in the picture.

Basically, the value:

represents a head loss which causes a “backup” of water upstream or downstream the bar screen, according to whether the stream speed is slow or fast.

The result is valid for a clean bar screen, while for a clogged bar screen the head loss is remarkably higher.

Indicative values of the head losses through mechanical bar screens having different spacings and an inclination of 60°