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Drainage


Posted by Harry on March 13th, 2004 06:22 PM
In reply to French drain by Bruce on March 13th, 2004 03:40 PM [Go to top of thread]

If it were me, from what I can picture of the situation I would try to divert the water without a french drain.

Build the driveway with a couple percent of slope across the driveway with the house side higher. This way the water coming off the hill will collect on the far side of the driveway, and not get near your house.

Once its collected there, you also have to drain it along the driveway, one direction or the other, toward lower ground so it doesn't puddle at the driveway edge. There again, just a couple percent is neccessary. Depending on the terrain, you could put a high spot in the middle and drain it off both directions.

You are basically making a swale, with one side of the channel being the driveway, and the other side being the hill, and the flowline being where the driveway surface ends and the grass begins. Admittedly this is not the best place for the flowline because there will be water infiltration at the edge of the driveway slab which is not real good. If your ambitious with the shovel it would be better to dig further and get the flowline a foot or so away from the edge of the driveway.

I recommend this swale method over the french drain because the stormwater runoff water needs a drop in elevation to make it move. If the swale is graded with a two foot drop over fifty feet, when the water drops the two feet it is now fifty feet away. If you put in a french drain two feet deep, the water has now dropped the two feet and not gone anywhere. To make it move fifty feet away you now have to drop the elevation more. Its just more digging. It always better to deal with stormwater at a higher elevation if possible.

The french drain will also allow more water to infiltrate, which is not always the best practice. During a rain the ground becomes saturated and the rest of the water runs off. The swale, being grass, is saturated also, so all the water runs off. In the french drain the water is more likely to collect at the bottom of the trench and infiltrate.

Im not too sure why Mark was having such problems with the french drains washing out, unless they were perpendicular to a slope, in which case a swale would have worked better anyway.

Good luck.

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