This approximation applies only to the vertical difference fre- quency second order forces. It consists on applying Newman’s approx- imation to the vertical steady drift forces in monochromatic waves accounting for the “correct”wave setdown effects. In fact, as the dif- ference frequency goes to zero, the related second order wave ele- vation does not tend to zero. This means there is a wave setdown in monochromatic waves, which increases in magnitude as the water depth decreases and contributes to the steady vertical drift forces in monochromatic waves. Following Chen [ 15 ], the setdown includes two components. The first depends of the first order incident wave potential and it is usually considered in the calculation of vertical mean drift forces in monochromatic waves. The second component results from the second order incident wave potential and it is usually neglected in the calculation of mean vertical drift force.