Now, do we have all the information’s? Yes, because u is nothing but so this is the drag force component half into C D into rho into D into absolute of u into u. Please, recollect that absolute of u is considered herein to take care of the direction of wave propagation. In a steady flow, you do not need that, can just have u square that is why you calculate the force on a structure on a pile, when it is exposed in a river. So, you do not have that absolute value. But in the unsteady case like a wavy flow, you need have that absolute value and then, your u is shown as given here.