Non-intrusive measurements of shallow water discharge

Waste-water channels or physical hydraulic models often convey shallow water flows with depths around 5 cm. Such free surface flows can in principle be measured using standard measuring flumes or thin-plate weirs, but proper employment of these is often practically impossible, e.g. due to limited space. To avoid this, various flow meters with contact probes (i.e. »area-velocity« probes) are employed instead, but in reality this often results in inaccurate measured values of discharge. This paper presents an effective way to determine discharge of very shallow flow without intruding the flow. Our approach is based on computer aided visualization, namely on the quantification of the field of vectors representing local velocities on the water surface of the flow. In contrast to other studies, this method does not require complex measuring equipment, special lights or special devices for the seeding of particles. Experiments were conducted in 0.5 m and 1.06 m wide rectangular channels, made of glass and concrete, respectively, and they show that this method could be employed both in hydraulic laboratories and in the field. Measurements showed that velocity on the surface of the shallow water flow differs from theoretical average mean flow velocity in the observed cross section, and further that this difference increases with the decrease of water depth. This suggests that the assumption, which states that in shallow water flows the surface velocity is similar enough to the mean flow velocity, is not necessarily correct.