Discharge estimation in natural rivers with geometrically irregular cross-sections and roughness differences between different parts of the wetted perimeter is complex. Therefore a compound channel can be used as a model of these rivers. Although assesing the behavior of rating curves in compound channels is very difficult and time-consuming, taking photographs of a floating object with a specified time interval is a method by which the velocity of the water surface can be estimated. The flow rate is first calculated using the isovels passing through the measured surface velocity. Then, the estimated discharge at a water level is applied to evaluate the rating curve. A compound channel model is built in the laboratory to carry out this experiment, and then the experimental results are obtained from rating curves. For the first time this paper uses surface water velocity to estimate the discharge and rating curve based on the SPM method introduced in 2003 and 2017, respectively. Also, an experimental setup consisting of a compound channel with a two-stage floodplain is another innovative feature of the current work that has not been previously explored. The results of discharge estimation based on the isovels and surface velocity show that the average error does not exceed 12%. Finally, the average statistical parameters of MAPE and NRMSE are obtained 6.4% and 0.045, respectively, for the estimated rating curve using the available data.

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