‘Correct’ is a loaded word, but for designing roof gutters NRCS used to use the max 25-year 5-min rainfall, assume the water all instantly ran to the outlet, and for your stone lot, you could conservatively assume zero infiltration.  This means that in any given year there is a 4% chance that during the hammering beating highest intensity rainfall you would have more water coming down than you designed to take away.

A 5 acre lot, if square, is 466x466.  So not a lot of time for water to run from one end to another but 5 minutes isn’t a lot of time either.  Kind of on the bubble between assuming instant flow and using a watershed time of concentration method to reduce the peak estimated flow.

The 5 minute 25-yr rainfall for Alpena is 0.506”.  If you want to be a bit less conservative, you could do the 10-yr 5-min rainfall which is 0.425” and a 10% chance any given year it rains harder than that.  The 1-yr 5-min rainfall is 0.259”, which means statistically every year it will rain that hard at least once.

So, 5ac x 0.25” in 5 mins is 15 cfs which means a 24” dual wall at 0.5% will not be able to drain the lot at least once most every year.  A 30” dual wall (0.5” in 5 mins) will drain the lot without overflowing and has only a 4% statistical chance of overflowing any given year.  Remember these are conservative assumptions, welcome to engineering.

The real issue is going to be getting the water shoved down into the tile with so little elevation change.  Orifice limitations and such.

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