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The following note characterizes the hydraulic behavior when removing manure from alleys through the flush system in dairy freestall barns. The flush system guarantees the moving, the conveyance and the removal of manure from the alleys flooring, depending on the main manure flux characteristics. The application of the Chezys formula, using the Manning expression for the Chezy coefficient, to calculate the mean flush flow velocity is investigated to outline the importance of the proper adjustment of the value to be attributed to the alley roughness, which depends on both the flooring material and the manure quantity on the flooring. Experiments on a freestall barn, located in the Reggio Emilia in the North of Italy, which has been using this cleaning system for some years, have been performed to calculate the Manning coefficient n values once it was known the flow flush velocity. Differences in the quantity and type of solid materials laying on the floor have been found to strongly influence the roughness of the alley and the variation on the velocity values. Velocity measurements collected during flush operations both on the already cleaned alley and on the alley enriched with manure allowed us to investigate the Mannings coefficient variations depending on the manure presence on the alleys. Differences on measured and calculated velocity values obtained through the application of the Chezy formula are attributed to the choice of the Manning coefficient n value equal to 0.02. The calculated velocity values were always less than those measured, yield an under-estimation of the velocity and then a minor cleaning actions.