IIRC 12:1 is a pretty dry number. 10:1 is dry snow. I believe 6:1 is closer to the average. Whatever, the numbers are on the weather sites (snowfall vs. precipitation); scale accordingly.
But that "normal drainage" is off the path only to re-freeze somewhere else. Some may go down the drain, some may just cause a skating rink in the road (the towns around here will get mighty pissed).
I assume you mean w/o. ;-) 10'? Yikes! We had 5' in three storms over 10 days (early December) last year. That was 'nuff. Fortunately we didn't get much more until March.