Digital twins of hydrographical networks can be designed based on simulators. They aim not only at reproducing with accurate dynamics of the rivers and canals but also to analyze past events, to design, test, and compare new control algorithms. A delicate stage is an estimation of uncontrolled in/outputs along the canal during a specific event. Observers for the level variable reconstruction have been proposed in the literature. For real systems, the estimation of uncontrolled in/outputs is still challenging. Limiting uncertainties during this estimation requires accurate models of the controlled gate dynamics. Model of gates can be very complex particularly for sea outlet gates. Their dynamics are nonlinear with priming stages and flow drops by the end of low tides. The flow depends on the gate opening, its duration, the tide coefficient, and also the level in the canal. A highly operational approach consists of considering a delay at the opening of the gate, then an average flow, and finally a progressive limitation of the flow by the end of the low tide. Based on this operative model, the uncontrolled in/outputs along the canal are estimated. This method is applied on the canal of calais located in the north of france. It is strongly supplied by runoff and several pumping stations that supply the canal during rainy events. The outlet gate is characterized by very complex dynamics. It is modeled with the proposed model. Then the flow due to runoff and upstream pumping stations is estimated. The paper aims at proposing a sensitivity analysis of this estimation according to the proposed dynamical model of the outlet gates using real data.