Badly built ''terraces'' - how do I stabilize them?

Hi everyone,

Me and my family have bought a land of 7.5Ha in central Portugal. It has recently burned down with the big wildfires, and almost no vegetation is left and thus a lot of erosion will happen if we don't do something soon. Some part are gradual slopes an other are steeper, the thing is that in the past they made ''terraces'' to plant industrial eucalyptus all over the land, but these terraces are almost all off contour with no retention walls. Our soil is granite based and sandy, so it is not very stable and erodes easy. As it is so big it is not very viable to build retention walls all over the place, neither to  bring back natural shape of the land. So my question is if we build swales on these terraces, would that be enough to stabilize the walls? Are there any other ways to approach this situation? Most of this land will be used for forests or other native plant species.

I've uploaded a pair of pictures to give an idea of how the terraces are. (they go from 0.5m to 3m high)

I await any response with much hope!

Thomas vd laak.

The terrace flat part should angle in towards the uphill a little bit to cut down on soil erosion and help infiltration.
The terrace current 90 degree slope should be 60degree  or so.

1) Add some soil to the current 90degree slope to make it 60degree
2) Add some diy/commercial geotextile stabilize the slope
3) Build a retaining wall with chicken wire + support post and fill with stones (temp woodchip/logs until some roots stabilize it)

Get some plants/fungi roots growing on the 90degree to 60degree slopes.

Thank you for the quick reply.
So if I understand it right I have to make it a 60 degree slope no matter what. And the I can choose between stabilizing with geotextile or with chicken wire?

I have to agree with S Bengi. You need some short-term support on those exposed edges, or I fear you are quite right about what might happen in a sudden or sustained rain event.

I would suggest on-contour sediment traps everywhere, but especially up and downhill of places you think are in danger of erosion. These can be as simple as dropped branches and trunks of trees placed on contour, positioned across the slope to trap debris and sediment from flowing downhill.

You could also do this with a number of geotextiles, as mentioned, specifically designed to do the same thing while physically holding the surface together. One that I remember looked like a rubber mat in the pattern of chickenwire. The pockets would trap the sediment, in that case.

If you seed these with a local pioneer guild as soon as there's any accumulation, their root zones will quickly stabilize the compromised features. They will also trap the sediment and debris in the runoff that will occur, whatever measures you take. The sediment traps will slow the progress of sediment from the top to the bottom of the system. If placed thoughtfully enough, you can get the water to do some of the land shaping for you.

In the most extreme spots, I love the idea of stacking a dry-set stone wall up against the exposed soil you want to support. I would do it exactly that way, but designed to support the "lean" of the hills as they erode towards the support of the walls. I would then wrap it with chicken wire, because I tend to try and overestimate the effects of extreme weather events. This would give the wall structure time to settle into place under the weight of the soil.

In your position, I would focus on what materials you have available. If you have, for instance, lots of fire-damaged standing or fallen deadwood, you probably wouldn't have a problem finding enough raw materials to do woody sediment traps on contour across your whole property, which would have a huge effect, as compared to the relative damage control you'd be doing with geotextiles in select bad spots. And the wood could potentially be free. As would stackable stone, should you have that, too, potentially for free.

If you wouldn't mind giving us some more details, suggestions could be better tailored to your circumstances.

Good luck, in any case, and please keep us apprised.

-CK

Wow! I appreciate this info so much, it really is usefull for me.
I do have a lot of standing dead eucalyptus and Acacia dealbata, so I will start right away laying them on contour. Not so much stones but maybe enough to build a dry-set wall on the most eroded parts.
Would it be helpful to set bush berms on the 60º slope?
Some more details:
-In summer it can get very dry.
-On some parts the soil is Hydrophobic.
-It is now only two months after the fires and I can see already pioneers and funghi starting to come to life, in the valleys and not so much on the ridges.
-When heavy rainfalls, water comes like a river down the slope (we are almost on top of the watershed).

Best way to get things growing again is to water it
I have had fires here in Australia and after the last one, I set up watering for a large area, within a few days of the fire.
The return of growth was remarkable compared with areas I did not water. And that is after 20 years, so quick watering makes a huge difference to the regrowth.

I would agree with previous posters on keeping with what you've got on site, when you fell the trees though, you can keep the stumps at varying heights for wildlife and for future diversity. Even stumps will create microclimates for other plants to flourish around. When you create your sediment traps with logs, remember you can probably use the burned wood both as posts and as filler to make stable banks on the terraces.

I make 90° terraces, or with a slight slope inwards to retain water, but certainly not 60°. Of course, I am on the coast of Sweden and we get huge amounts of rain pretty darned often, so it's quite different from Portugal.
I'd suggest visiting Tamera (in Portugal), the place Sepp Holzer helped plan, and also, read Sepp's book Desert or Paradise. It was a true eye-opener for me. Here's a video about Tamera

I think the gentleman suggesting you water the land may be quite right and has firsthand experience of it, and aside from that, I would say the main things are to start cover crops immediately, and also cover the soil with anything you have available to protect it. Whether that be wood chips, grass cuttings, branches, or whatever. And start planting bushes and trees, the more the better, just to get water retaining and soil stabilizing roots started. One of my mentors here in Sweden taught me that you want to create forest as soon as possible, so plant cheap trees to protect the land and the actual plants you want long term. Then, you can thin them and use them as mulch in a couple years, when your target plants are doing better.

Your place looks amazing, and I wish you the best of luck with it!

What is your climate, and specifically - do you have frosts?

If you don't have hard frosts you might consider planting vetiver - it is a grass which forms dense clumps, prevents soil erosion, and has deep taproots that stabilise banks.

Thomas, can you buy 9 or 10 300kg hay bales? I would buy them and mulch the place. I would not consider making lakes or swales unless recomended by someone who understands soil hidrology.
Something else you could do, please do not think i am joking or provoking you, is plant eucaliptus right on the edges of the terraces, they do have qualities as well and they could hold the edge soil on the next couple of years alowing you to think on a longer time scale.

When cutting dead trees, leaving the stumps taller could be useful not just to create microclimates, but to anchor the laid-down trunks and branches so they are less likely to get washed out of place by heavy runoff. In severe locations, I would anchor bigger logs, and lay small branches uphill to best trap sediment. A little sediment under and around brushy piles will give moister, shaded spots for pioneer plantings to get a good foothold.

In Australia, the dead trees are often left to stand, they may stay there for 30=40 years. They provide shade, habitat and will not interfere with any new plantings.
As a matter of interest does the wood rot in portugal if cut and laid down
It rots very quickly in Australia

A comment meant more for anyone else who reads this thread later:

 Kate McRae  I make 90° terraces, or with a slight slope inwards to retain water, but certainly not 60°.  

The 60 degree slope meant was on the face side of the terrace, to stabilize where it's eroding, from 90  |  to  60 \   not on the top surface, where you are correct, a slight slope inwards retains water.
The problem with these terraces is they have unstabilized faces that are too steep to hold themselves up, which is why they are eroding. When the slope on the face is added, that is where plants with good roots want to be planted. A 90 degree face like this needed retention walls (and even then I would have put more slope to it, being as the soil is sandy.)

And I loved all the stuff about stumps in this thread, I think I just changed a bit on my plans, I was going to remove some stumps, I think I'm working them in, thank you all of you. I love the idea of a microclimate around the stumps and my stumps were cut high (5 or 6 foot tall) to make them easy to remove with a bulldozer, thinking on it, I can use them as support posts for wires or structure to support other plants as they get growing.

I love Permies, I learn all the time!!

One way of getting some immediate erosion support is to stake-down straw wattles along the areas that are most likely to erode.  They are very easy to install: you just roll them out and then pin them to the ground with a wooden stake.

I don't know how to upload pictures to this post, or I'd do so.  I tried to attach a couple of photos.  They are remarkably effective for not being that large.  

Hey!  Look at that!  I posted a picture!  It's a small technological miracle!

Wattles work best if installed on contour (like any other erosion control).  

OK -- a few more pictures, including wattles installed after a forest fire.

I agree with the first two responses, and would use your dead trees to stabilize the terrace edges. If you cut a long straight pole to have a point, you can drive the flat top end with a post pounder. Then stack wood between the posts and the terrace. This will ultimately have a hugel bed like effect and the fungi will help the plants stabilize the steep edges. I am building something like this for my brother.