smear of initial water surface level
 

I have problem of applied smear flow over element for my free surface flow model, as in CFX documentation I wrote the following relation in expression
rdrop = 0.3 [m]
dist = rdrop - sqrt((x-0.5 [m])^2 + (y-0.5[m]^0.5)
delta = 0.01 [m]
drop = 0.5*tanh(dist/delta)+0.5

but I have two problem, first what is rdrop, is that 0.3m standard? because this value for small model is very big.
second, is x,y horizontal and vertical component ? what about Z because my model inlet in x and z, what is it is value (0.5m) mean?
thanks