Esshup .. Great info as always on this site .. the inside diameter is 48" the height is 129" .. keep in mind though (and I'm not sure that this makes a difference) that the there is a hole in the bottom to accomodate the 24" discharge pipe .. I originally, mistakenly, thought that this would equalize the pressure .. Obviously I was incorrect but I would imagine that it has some bearing on the calculation ..

FishinRod .. I scrapped the waterfall design because water travels the path of least resistance and can be diverted with a leaf .. and when water is diverted so goes the sub-base .. and once the erosion started the problem magnified times 1000 .. if 5% of the water going over the falls diverted behind one of the stones toward the top of the dam .. eventually the base became saturated which in turn softened the base structure which allowed for settling which changed the pitch of the rock which allowed more water to veer off course .. and so on and so on ..

No such thing as a problem .. just opportunities