In my Geomorphology book there is a section on uplift caused by isostatic rebound and it goes over how the viscoplastic nature of the mantle and the mechanically detached asthenosphere allow a long delayed equilibrium to be reached at the isostatic compensation depth. This is because displaced mantle must flow back into the volume that is now freed up from a crustal load having been removed. Does anyone have any idea where the very bottom equation comes from, in the picture I have provided? I am guessing it may have to due with flow equations that manipulate the Reynolds Number. I think it somehow relates the mean speed to the volume flux but I am having a hard time finding where it comes from. Any insight would be great.