So, this one-seventh power formula it comes from oceanography, where this is widely adopted. So, you will see that this gives the tidal current velocity at any depth z, which is equal to U T S, that is the surface current and z is the distance above the bottom measured positive upward. So this, in this definition of this formula, the definition of your z axis is just the opposite when compare to what we have done or what we have seen under the wave mechanics.