This thesis studies flap gates and the correlation between calculation parameters. To study this, a modelexperiment is used with plastic discs and an experiment with an extradited lightweight flap gate with a diameter of 0,31 meters from Dannozzle. From the experiment results two correlation are defined, one between opening angel of a flap gate relative to the mean velocity and one between the opening angel relative to the
resistancenumber. These two correlations are used to make a dimensioning model for flap gates. The last mentioned correlation makes it possible to calculate the headloss from a flap gate as individual energy losses. To make such a dimensioning model, there is established a moment balance for flap gates, exposed to a flow. The model can calculate the headloss from a flap gate with an arbitrary weight and therefore is used to determine the difference in headloss for a steel flap gate and and the new lightweight flap gate, where the headloss is almost half for the new lightweight flap gate.