An analytical model is developed to calculate the stage-discharge relationship for emergent and submerged rock-ramp fish passes.Aprevious model has been modified and simplified to be adapted to a larger range of block arrangement. For submerged flows, a two-layer model developed for aquatic canopies is used.A turbulent length scale is proposed to close the turbulencemodel thanks to a large quantity of data for fully rough flows from the literature and experiments. This length scale depends only on the characteristic lengths of arrangements of obstacles. Then the coefficients of the logarithmic law above the canopy can also be deduced from the model. As a consequence, the total discharge through the fish pass is computed by integrating the vertical velocity profiles.A good fit is found between themodel and commonly observed values for fish pass or a vegetated canopy. The discharge of the fish pass is then accurately estimated for a large range of hydraulic conditions, which could be useful for estimating fish passability through the structure.