Measuring flow discharge in rivers and open channels has always been one of the most important concerns of water sciences experts. The most common flow measurement method in open channels is velocity- area approach. An approach recently considered as a non-contact option for measuring flow is the use of surface flow image velocimetry methods. Recently, the standard PIV method has been used to measure the velocity at larger scales and on the flow surface, which is referred to as the LSPIV (Bieri et al., 2009).
The most common way of converting the surface velocity to the depth-averaged velocity is to use a coefficient called the Velocity Index (VI) which is in fact the ratio of the depth-averaged velocity to the surface velocity. In the literature, the value of the Velocity Index for river flows and laboratory flumes, which were mainly studied for subcritical conditions, is believed to be equal to 0.85, which seems to be an accepted value for this index among the hydraulics communities.
In this research, an image velocimetry technique was used to study the flow characteristics, determine the Velocity Index and investigate the surface flow pattern. The instantaneous surface velocity field was measured using the LSPIV method and then a two-dimensional time-averaged velocity map was obtained for different experiments providing the possibility of comparing the flow pattern in different scenarios. Furthermore, by means of spatial averaging of the time-averaged velocity map, the double averaged surface velocity  was obtained using which and the cross-sectional mean velocity the Velocity Index was calculated. Therefore, the effect of channel slope and relative submergence on the Velocity Index was investigated and relationships were proposed to estimate the VI in steep slopes.