So far , so good. Problem is there is no such person in the
local Yellow Pages. I suspect that I'm not searching on
the right keyword. I also figure that there has to be
a professional association for drainage engineers (or
whatever they're called).

Hydrologist. Check to see if your state/county has a hydrology group.
Most do. That's a place to start. Or try the nearest large university.
They should have a geology/hydrology dept.
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