Thanks, Chris. I've just about worn out my copy of NRCS handbook 590 planning this thing for the past year and a half. The handbook suggests the pipe should be sized to handle runoff ranging somewhere between a 1 and a 10 yr frequency storm event. For a 5yr event, I'm looking at needing a barrel-to-riser ratio of something like 15:12". But then... the handbook goes on to suggest that with a drainage area less than 20ac a principal spillway may not even be needed unless there is constant flow. My drainage area will be about 15.5ac.
In an effort to cut down the cost of the project a bit, I'm planning on doing as much as I can myself - the initial clearing, for instance. I guess I'm just wondering if its a bad thing to let the auxillary spillway see some water once a year or so. Going to smaller pipe would save a bundle.
