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Grothar

What is the true frost line in PA?

9 posts in this topic

I am having difficulty finding data on how the frost line is actually determined and the annual frost line depths for PA. I have come to realize that this is a major factor in the severity of flooding during our ritual January and March thaws. Just wondering if someone out their has done a correlation with snowfall depth as well. Thanks

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Thanks- really informative. Did not realize 42 inches frost line without skirting.

based on a 25 year return period as well. A typical home may therefore see several of these deep events in its usable life.

I personally thought Lower Macungie's building code was 36", and it may be for regular homes.

Edit: Just checked, and Lower Macungie is 36" for foundations.

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The information provided here has actually been quite informative to me as well. What prompted you to become so interested in this subject?

Frost depth IMHO has a direct bearing on flood potentials in winter and early spring thaws. When the ground is frozen, it is impervious. Stormwater management modeling and civil engineering calculations use runoff coefficients that rely on pasture conditions when developing new commercial or residential developments. The engineers from 1978-2000 primarily used a 25 year return period for designing stormwater piping and stormwater basins. They are now using 100 year return period rainfall amounts but that is still not enough to prevent flooding during frozen soils, snow melt and heavy rainfall (triple play) . If we really want to reduce flooding damage, stormwater engineering must considered this triple play condition as the asphalt coefficient in the NRCS TR-55 hydrological manual when sizing Pennsylvania stormwater basins and stormwater infrastructure.

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Frost depth IMHO has a direct bearing on flood potentials in winter and early spring thaws. When the ground is frozen, it is impervious.

I agree insofar that a frozen surface will act as an impervious surface, and therefore runoff will act similarly. However, 3 inches of frozen ground acts the same as 3 feet when it comes to this issue.

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I agree insofar that a frozen surface will act as an impervious surface, and therefore runoff will act similarly. However, 3 inches of frozen ground acts the same as 3 feet when it comes to this issue.

Yes I agree that 3 inches acts the same as 3 feet but here lies the issue. When we have a thaw lets say in late January, the ground frozen only three inches will melt sooner and not create impervious conditions as long it does for three feet because the soils will remain frozen. Most late January thaws only last 3-5 days which is not enough time for the soil profile to thaw with three feet of frozen soil. This is my entire point. The chances of a heavy rain during this thaw period is usually high and with the 3 feet of frozen profile, only the first 6-12 inches in the soil profile will thaw out before the next freeze or snowfall.

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