Measuring flow discharge in open waterways has always been one of the most important concerns for water experts. Flow surface image velocimetry method as a non-contact option has recently been widely utilized to measure discharge in open channels. One of these methods is a type of PIV method named LSPIV which has been paied more attention due to the elimination of laser application. In this study, LSPIV was used to measure 2D velocity field over the surface of a steep supercritical flow. Totally 24 experiments including three different slopes (2, 6 and 10 percent) and eight different dimension ratios (7 to 20.5) were conducted and in each experiment the flow surface was recorded with a frequency of 125 fps for 60 seconds. The Velocity Index was calculated bt the obtained velocity data. This index is used for convering the surface velocity to average velocity of the cross section and subsequently for determination of flow discharge. The effect of different dimention ratios on VI was investigated. The average value for VI was obtained to be 0.701 which is differ from the established value (0.85) by about 17%. Moreover, the flow surface time-averaged velocity patterns were quantitavely compared for different flow conditions. Since, the velocity index has been so far studied for subcritical conditions, the results of this study may be helpfull for measuring supercritical flows.