Question: The base shear estimation for load condition including both wave and current cards under a single load condition is higher than estimating the base shears from wave and current separately in different load conditions.

Answer: During the drag force calculation, we consider the square of the particle velocity.

Now, in case of pure current, it shall be particle velocity due to current, say Vc and in case of pure wave it shall be particle velocity due to wave, say Vw.

But during the combination, we were trying to simply add (Vc)^2+(Vw)^2 to obtain the result. However, our calculation should use the term (Vc+Vw)^2. (I have not written 1/2*Cd*rho*A part of the equations)

Instead of algebraically adding the individual forces, we need to algebraically add the velocity components first, and then the resultant velocity shall be used to determine the combined force. This shall cater the mathematical nonlinearity of square component and consequently the difference would automatically arrive.