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foundation drain - how much water comes out

994 Views 5 Replies 4 Participants Last post by  concretemasonry
I have a walkout basement with an exterior foundation drain tile. The grading around the house is pretty good (gets water away) and the downspouts are all routed at least 20 ft away from the house where no water will run back toward the house. How much water should be expected to come out of the drain tile in a normal everyday rain shower?

I have water trickling out during/after just a small rain....and pretty soon after the rain starts. I didn't expect to see any water coming out unless we had a huge rain where the ground is completely saturated and water table has risen. I especially didn't expect to see it trickling out just after a small rain starts.
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not to sound smart, but if this is what you have coming out then this is what you can expect. Subsurface drainage can vary from one area to the next. Could be you have a higher ground water table so it does not take much rain before it reaches the level of your trench drain. And the amount of rain can be misleading, for example if you have a 20 square roof (square = 100 s.f.) then you have 2,000 s.f. surface area. In a 1-inch rain event your roof will shed about 1200 gallons of water, which is about 5 tons of water. This does not count rain falling onto the ground or other impervious surfaces.

just my humble thoughts ....
Small amount of rain small amount of water coming out, HMM.
I'd be more concerened if there was no water coming out.
I see. I just assumed that if water was being shed appropriately, I would see no water coming out under normal conditions.

One more thing (separate question really), the drain tile is only exposed on one side of the house, the other side was just buried (no outlet). I also seriously doubt there is any pitch from the buried side to the open side...not really possible. Is that common?
yes as water seeks it own level. If water runs into the drain tile on the other side of the house it will still flow (speed depends on volume) and reach the outlet.
same basic principle as water comes out of the faucet in your bathtub, yet the entire bathtub fills up, not just the area directly below the faucet.

It is not uncommon to have only one outlet going to "daylight" (running on the ground).
bomber -

Do you water your grass?

When you water the grass, the soil (depends on the type) may hold/absorb water and it does not act like a reservoir to the point where any small rain can go to the drains and show up as an effluent. This covers a lot more area than just the roof of a house.

Water does not just go down, since it can flow laterally in soil and upward (capillary action).

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