Waves occurred on the surface of water in two-dimensional wave tank are studied. Deriving vorticity equation from incompressible Navier-Stokes equation, the boundary condition on the free surface, water bottom and flap surface are discussed. Using separation of variable method, velocity potential takes the general form in terms of eigen function. To obtain the values of horizontal and vertical velocity components, the boundary conditions on the water bottom and the flap surface are used and then mass transport velocity has been derived. Finally, time average of horizontal velocity component at mean water level is generalized
