Case studies

Free surface channel cross-sections

In the monitoring of watercourses and artificial channels, knowledge of the cross-sectional geometry is a fundamental step for any hydraulic assessment.

Today it is possible to quickly and accurately acquire the transverse profile of a channel using surveying instruments which, positioned on one bank, perform point-by-point scans of the section using laser pulses. The result is a point cloud that continuously describes the geometry of the bed and banks, providing detailed information on width and depth at each point.

Once the data have been acquired, they are processed in a calculation environment, for example using spreadsheets. The measured points are mapped in coordinates, allowing the graphical reconstruction of the section and subsequent geometric analysis.

A key step is the determination of the wetted area as a function of the hydraulic head. The term hydraulic head refers to the height of the water relative to the channel bed, i.e. the fluid level within the section at a given point.

By properly setting up the spreadsheet, it is possible to obtain the relationship between water level and the cross-sectional area effectively occupied by the flow.

This relationship strongly depends on the shape of the section:

Rectangular section: the wetted area increases in a perfectly linear manner with the hydraulic head. Each increase in level corresponds to a proportional increase in area.

Trapezoidal section: the relationship is close to linear, but shows a slight non-linearity at low water levels. As the level increases, the behavior progressively approaches linearity.

Irregular sections: the relationship between hydraulic head and wetted area becomes non-linear. Variations in geometry (variable slopes, irregular bed and banks) introduce more complex trends that must be evaluated case by case.

The availability of this information makes it possible to improve the accuracy of hydraulic analyses, particularly in the estimation of flow rates and in the modeling of flow phenomena.