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Infiltrate or drain water in small terraces ?

 
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Hello Guys,

I'm designing a small garden with about 30% of slope. I want to build small terraces in different spot and I wonder if i should infiltrate or drain the water out of the terraces.
My possible water collection from the roof is about 163 m3 annually.
The site is around 250 m2.

Thanks for your help
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Site with 30% slope
 
gardener
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I am a believer in putting water into the ground where it is cleaned and can recharge aquifers. It may be impossible to get it all in the ground though, so planning for overflow would be recommended.

My terrace does both. It infiltrates and the bottom slope is the basis for my perrenial food system.  The water moves slowly downhill and dumps into a pond system which will hold what flows out.

Once you have the terrace you can get more creative. Gabions, rock rows, or brush can slow the flow even more, creating mini dams that can hold more water allowing more infiltration.
 
pollinator
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Both. Which happens, and how, depends on volume.

I think that most people consider terraces on slopes exceeding 15%, and you're double that. I think that to increase water infiltration in any meaningful way, you'd need to terrace it.

If you didn't want to go to that extent, you could lay fallen branches and logs on-contour and stake them in place. That way, the water would infiltrate as its momentum is taken by the obstacle, its sediment gets dropped, and it tops your sediment trap at roughly the same level. You'd get increased sediment deposition on the upslope, more soil, and probably greater localised plant growth.

But terracing is the way to actually make use of the space, even if you make little fishscale terraces, little landings arranged on-contour designed to hold copses or pocket gardens against the shelter of the hill.

Let us know how it goes, and good luck.

-CK
 
Louis Romain
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Thanks for your answers Chris and Wayne.

Chris, I understand to you make a point for creating terraces versus not creating terraces. So it confirms that we are on the right path of doing terraces. But my question is more on whether to infiltrate water into the terraces or not, and if yes how to be confident that there won't be land slides or that the riser won't fall ?

Wayne, so you infiltrate water in your terraces and there's no landslide hazard ? By "bottom slope", do you mean the riser of the terrace (wall) ? or the slope below the riser ?
Regarding the creative solutions you suggest to add, do you mean to add them on the bench or on the riser ?
Thanks for clarifying this.
 
wayne fajkus
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I hope this explains it. The water cant overflow and go downhill towards the trees. It can only travel in that vee pattern down that downward path.

In that vee trough,  creative use means blocking or damming that downward flow. A row of rocks to dam the downward flow. Rocks, a tree trunk layed down and staked down. Create holding puddles to slow the flow. Water would fill puddle one, over flow, fill puddle 2, overflow to fill puddle 3, etc.

I am having no issues with landslides. I am keeping the area mulched until plants fill it in.
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Louis Romain
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Ok thanks Wayne. I understand now. Your vee feature is not on contour and the water is flowing inside it. That's what I was missing.

What's the slope in percent ?

We have 33% slope here, it's a quite important slope.

I am still not clear whether I should infiltrate water or not in the bench...
 
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The general rule of thumb, you do not want to infiltrate water in terraces. 5-10% slopes act quite different from 30-40% slopes. For 30-40% slopes, I would recommend checking rice terraces of the Philippines or terraces of Inca. In the case of rice terraces, there is an impermeable layer and any excess water is discharged by overflow. Incas fill the back of the wall with gravel or broken stones for bottom drainage and let a gap at the top for overflow. Both of them are trying to overcome the problem of the slope getting destabilized. It might not seem much, but when it rains with 100% inflatration numbers add up quickly. Soil gets saturated and heavy, it begins to move. Both Incas and rice terraces are big projects though. You might get away with the issue with small gardens, small areas won't be able to raise the water table. There are two no-need-to-engineering solutions. You can slope soil descending towards the terrace wall, so any excess rainfall will shed over the wall. You will have a 1-2% slope (depending on your soil type 0.5-4%) compared to 30%. That is one solution (classic low-cost terracing). You will still need to drain the backside of the wall if you are going to build a concrete/stone wall. The other solution to put french drains covered with geo-fabric buried under the entire terrace (thickness of 1-4 ft), works better with drainage pipes (looks like modern time adaptation of the Philippines rice terraces). Search for diagrams, you will find many inpiring projects!
 
wayne fajkus
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Louis Romain wrote:Ok thanks Wayne. I understand now. Your vee feature is not on contour and the water is flowing inside it. That's what I was missing.

What's the slope in percent ?

We have 33% slope here, it's a quite important slope.

I am still not clear whether I should infiltrate water or not in the bench...



I am not sure of the slope,  but the spot where the terrace was installed was a very steep drop so was used for the terrace. Here is an example where the terraces do a "switchback" similar to a road going up or down a mountain. That shows the scheme better. Water travels along the terrace slowly, both infiltrating the water and carrying it to a water retention pond:

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