The momentum forces act on the dam by the following equation:
Unlike the hydrostatic pressure and the stagnation forces, the direction of the momentum forces is directed up-stream. In this way, the momentum forces actually subtract from the total forces on the barrier. However, because this force works in the direction easing the force on the barrier, we do not subtract it from the design force total. Instead, the momentum forces will act as an additional part of the safety factor on the design. Below are the calculations of the momentum forces.