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Where do you put the swale?

 
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I've seen a lot of stuff on swales, but I still haven't been able to figure out one simple bit of information: where on the slope is the ideal point to put the swale? For example, on my property there's a verrry gradual (even more gradual than I actually drew it here) slope downwards in the back yard, that then culminates in a far sharper angle, before leveling off in a flat lowland that floods every spring thaw or whenever it rains enough. What I'd like to do is cut a swale to collect all that rainwater coming down, and have the sill funnel the extra water into a pond I intend to put in a ravine that begins in the direction where the arrow is pointing. But what I still don't understand is the correct point on a slope where you cut the swale. Before the hard downhill slope begins, when it's still a much less extreme angle? A? Immediately before the hard downhill slope begins? B? Right dead-center in the middle of the hard angle? C? Or at the immediate bottom of the hard slope, where the hard angle meets the level ground? D?



And most importantly, WHY is the correct choice the best spot! Help me understand!


Thanks everybody for helping me cut through this mental snag!
 
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A. Usually as high up in the terrain as you can while still collecting meaningful rainfall. You picture shows a lot of capture area above A already.

And then probably at B and D as well as you work your way down. I wouldn't build one at C only because it would be hard to do with the tools I have and am too lazy to do it by hand.
 
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I'm going to give you the lazy answer -- put it where it wants to be.

The building of earthworks requires a lot of hard work moving dirt, and the lazy (and smart) man will reflect a little beforehand to figure out just how little dirt he has to move. This requires mapping out where the natural flow is now. Once you know that, take some dirt from over here, move it over there, and you can deflect the natural flow to be a little more to your liking.

I had a problem once with a house that I was renovating, that when it would rain, all the water in the front yard would flow away from the street, back towards the house, along the front of the house to the side yard, and then down the side yard back to the alley in back. Over the years, that flow had caused some rotting of the siding in front and after replacing it, I wanted to make sure that it would stay high and dry. I knew that the narrow side yard (like 5' wide) was going to be a swale, so I cut more of a channel down the middle of it and lined it with 6"-12" river rock. The side yard became a dry river bed, but even in the heaviest of thunderstorms, water would course down it, but not get up to the foundation on the side of the house.

So good, so far, but what to do in the front yard? Time to sprinkler test it. I let the sprinkler run until the flow patterns started to appear in the yard. This being a mostly dirt yard in Las Vegas, I didn't have any distracting grass in the way, all I had to do was see where the water wanted to flow over the dirt. I could see where it wanted to head for the house, and I could use the hoe and shovel to build up those areas, taking dirt from the channel I was building to divert the flow to the side of the house.

If you have a lot of vegetation, it may be a little more difficult to see where the natural flow is. You may have to wait for a heavy downpour, put on your mac, and walk your property, checking where each little rivulet and collection spot is. I had to do that with my house here in Georgia, because it was built into the side of a hill with no forethought to the drainage around it. The first heavy rain after I moved in, I could see where the water was coursing, and in my mind's eye, I could trace out exactly where the French drain was going to have to go.

This isn't rocket science. Even 5-year olds at the beach with their sand pails and shovels can figure out how to divert the waves and make a moat around their sand castle.

 
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