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Our basement is unfinished, and since we bought it one year ago it has flooded three times. This only occurs with heavy snowfall, i.e. when a large amount of snow falls and then melts. Even the heaviest rains have not caused flooding, it's only the snow.

The house was built in 1952, but the area of the basement that floods is under the kitchen which was a later addition. I cannot see any cracks in the foundation. It's also not clear where the water is coming from, as it seemed like the water just appeared in the middle of the room.

Below is a picture of the sump pump. There are large rusty pipes leading into it, two of which do drain a lot of water when there is heavy rain fall. The largest one with the green valve on it does not drain anything, I have no idea what it is supposed to be. Are these French drains?

Any tips as to how to diagnose this water flooding with snow problem?

enter image description here

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  • I assume that ice damming during snow melt changes the rates of infiltration at different ground locations, explaining your correlation. First guess, that additional pipe was the discharge line from an old sump pump. Does it point in the direction of the kitchen area? Does it point in the direction of a plumbing stack? Where is this box located? Is it located in the pre-kitchen addition area, indicating that the drainage channels may not include the kitchen addition? Is it located in the kitchen addition area, indicating that the drainage channels include the kitchen area?
    – popham
    Commented Nov 5, 2023 at 14:42
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    What's the grading around the house like? Seems likely snowmelt is being trapped near the house more than rain is. One management technique would be to run the snowblower around the house and move it away before it melts. Possible that the "pipe with green valve that doesn't drain inwards" has a valve so it's supposed to only drain outwards, if water rises that high in the pit, but is (now) clogged further down the line and does not? Only reason I could imagine for a valve on a pipe in a sump pit.
    – Ecnerwal
    Commented Nov 5, 2023 at 14:56
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    You'll have to pinpoint how the water is getting in. Sometimes you can find out by looking for how the water stains flow, or you may just have to watch when the phenomenon is about to happen again. Usually the hard part is finding the ingress path, and the fix is often fairly easy.
    – Smith
    Commented Nov 5, 2023 at 15:12
  • Also check if the sump drain freezes in cold weather, e.g., it's not far enough below frost level. Commented Nov 5, 2023 at 18:10
  • Thank you for the helpful comments. @popham No, that pipe points directly out of the house, toward the backyard. I think your guess is a good one about it being an old discharge line. And yes, this sump pump setup is in the old house, so perhaps the addon area does not have drainage channels. Is there a way to check where there are drainage channels?
    – user310374
    Commented Nov 5, 2023 at 18:41

1 Answer 1

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Typically your first solutions would be sloping ground away and water spout drainage. The good behavior during heavy rain suggests that these solutions aren't useful in your case.

Pumping a well creates a cone of depression in the neighborhood of the well:

cone of depression

A sump pump's drainage channel is effectively a line of these wells, where instead of the circular symmetry implied by a single well, you get the same "cone of depression" at each position along the channel. (This is technically true only for an infinitely long channel that empties instantly, whereas a 30 ft channel, for instance, is more complex).

When you have a channel on opposite exterior walls of your basement, you get an interference pattern:

well interference pattern

Holding one channel fixed while moving the other channel away, you can imagine the water table height between them increasing. For large buildings, not your basement, the distance between channels can warrant deeper channels or additional channels.

For a pair of 30 ft channels, water could still find its way in between the channel ends to allow a high water table under a basement. That's only in this theoretical framework, though. Typically the ends are joined by drainage channels.

Your drainage channels may include only a subset of your basement's walls.

  • This could happen by design.
  • This could happen because of a basement addition that was excluded from the drainage system.
  • This could happen because of a kitchen addition contractor who recklessly destroyed part of the drainage system.
  • This could happen because of the normal end of a channel's lifecycle.

I suspect that your lower "rusty" pipes are actually clay drainage tile, where I'm guessing that you have clay weeping tiles for your drainage channel. Clogged weep holes is a possibility in that case.

The exterior wall with the sump basin is obviously draining fine. There are cheap endoscope on Amazon that you could use to check along the sump basin's wall for cave-ins, but that probably won't go around a corner. If the kitchen addition shares a wall with the sump basin, then an endoscope could test the hypothesis that clay weeping tiles were destroyed during construction. An endoscope could test the hypothesis that clay weeping tiles were only installed along one wall.

Ice damming during snow melt is probably changing the water infiltration rates around your house, stressing a part of the system that typically goes unchallenged. You can stress test different positions along your foundation with your garden hose. You would have to run it slow enough so that the water infiltrates instead of running off. Dig a pit to retain water and throttle your hose to match the pit's drainage rate. The pit needs to be big enough so that the flow rate will be noticeable as it enters the sump pit. The best time to do the test would be right after the sump system has been active from a heavy rain. Otherwise the surrounding ground could be so thirsty that none of your hose water shows up under the foundation.

Start next to the old foundation 5 feet away from where the new foundation begins. This will verify the pit size and flow rate necessary to register a flow at the sump pit. Now move next to the new foundation and 10 feet away from the old foundation. Replicate the pit size and flow rate to see if the kitchen registers any flow at the sump pit. No flow indicates that the kitchen probably has no functioning drainage

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