I hope this is in the right place, tell me if it's not! We bought or little future homestead in central Portugal at the end of last year and have just returned from our first month getting to grips with the land. I'm a little bit worried about our driveway! Sometime in the last couple of years someone has hired in some heavy machinery to make the drive. We're in quite a steep hilly area and to make the drive one side has been chiseled out of bedrock, and the loose stones have been piled up on the other side to level off what is basically a rock terrace. Trouble is, the edge of the pile that slopes down has nothing holding it up but a few old medronho bushes that survived being buried. It starts off at about 60 degrees and tapers to about 40 degrees at the house. The drive dips down in the middle and you can see where the water has come down the valley above, washed the topsoil off the top of the bedrock on one side, pooled on the driveway for a little while and then washed away a hole in the loose rocks on the other side. Considering this drive isn't that old the hole is already just under a meter in diameter and is about half a meter deep. I haven't got any pictures of the hole but this was taken near the house where it isn't quite as steep. There has to be some soil in there somewhere, as the previous owner has planted a couple of fruittrees in there that are still looking reasonably healthy. What else could I do to help hold this up? I'm kind of hoping that I can get away with not building a retaining wall, would it help to build some hugel terraces and plant those up, or would the whole lot eventually just slide off? I'm thinking it might be wise to dig a small channel through the dip to allow the water to drain away rather than collecting there. Any advice would be appreciated, I'd like to try and get it sorted when we next visit at the end of summer.
That slope is not in a very safe equilibrium but probably the volume of loose rock and dirt above the original slope is not to great. Rocks getting loose can be a problem down slope.
The best you can do is re-establish some planting. That will take time.
I would broadcast seeds from your locally available plants. Annuals followed by deeper rooting perannuals and trees. Planting shrubs or so along the road (where you can water the plantings) may accelerate things. Look what the locals use as hedging plants.
You come from the UK ? so i guess you know or can find out about hedging to stabilize slopes and erosion. There are plenty UK-sources about hedging. Don't know about such in Portugal.
You could use blocks of wood laying around to build some temporary retaining structure untill your slope stabilises when plants take hold.
My strategy is to throw rocks/boulders into holes, and size the rocks so that are large enough to not get washed away by the flowing water. I look at the largest rocks that didn't get washed away from the existing hole/ravine, and then use plenty of that size and larger rock in filling up the hole, and the area slightly downstream of the hole.
I completely trust roads like that which are cut through bedrock.
The locals seem to use drystone walls as hedges, not hawthorns or beech like I see at home! I don't recall seeing any hedges around to be honest, maybe I'm not looking hard enough!
Next time we're out there I'll pay more attention if I can peel my eyes away from the views!
Somebody posted some info about making seed balls on my other thread, they would probably be my best bet for spreading some seeds around.
There's plenty of pine logs laying around that I could use to lay on contour for some little mini terraces / retaining walls so I can put a bit of soil down to help get some trees started.
I'll throw some rocks in the hole when we get back out there, there's a pile at the end of the drive that will do nicely. Although it's been cut from the bedrock, there's actually only 2 sports on the drive where you can see the bedrock, most of the surface is just loose rocks and gravel. I don't have much faith in the bedrock myself, it's all schist in our area and it just crumbles away once it's been weathered for a while, I can walk up the drive and pull chunks out of the wall by hand! hopefully having the loose rocks on top will help protect the bedrock underneath so I just need to worry about the slope and occasionally clearing up whatever has fallen from the uphill side.
If you don't get frosts then take a look at vetiver grass hedges, planted on contour. They are used for bank stabilisation, terracing, erosion control etc... The roots are DEEP and STRONG and will tie that whole slope together. They also give you masses of biomass that can be used for mulching crops.
It is kind of hard to tell from the picture, but the problem seems to lie in the fact that the drive is not built properly, and any short term remedial efforts are really not going to be adequate or long term. It really does not matter where anyone lives, if there is a slope to the road surface, water has to be accounted for. There is nothing wrong with shelving out a road on a steep slope, cutting the material from the uphill side to the downhill side is a very common and effective method of building a road along a slope. However, to last there has to be drainage, and that is where this road seems to fall short.
The original builders appeared to have chosen the more inexpensive route of outsloping the road; that is, even though they shelved the road from the high side to the low side, there is some slope (1-2%) to allow water to run off the surface of the road and down the hillside. This is a cheap way to build a road, but not a very good way. If the area does get snow, this can make winter travel treacherous in the winter, but probably not an issue for you in the United Kingdom. However, because the road winds its way up the hillside, as well as across it, means there is a lot of water coming down the road. In a perfect outsloped road, this water would get to the edge of the road quickly and do little damage. With vehicle compaction and less than ideal grading however, what most often happens, and probably what you are getting, is large volumes of water flowing down the vehicle ruts and to the inside of the roadway. This water is gathering momentum and volume the further it gets down the hillside until it finds a spot where it can cross the road and then wash out. A VERY common issue with outsloped roadways.
The best way to cure this is with a ditch on the uphill side of the road. This can be an issue if the area is solid rock, but it is the only real alternative. You can get this ditch in one of two ways, dig a ditch if it is not solid rock, or build your roadway up so there is a ditch. Then the roadway is sloped to the uphill side. Now as rain comes down, the water flows off the road and into the inside ditch, and with rock check dams, the water is slowed and thus does not erode. The water travels down the ditch every few hundred feet until a culvert is located across the road where water is allowed to escape. To prevent erosion at the outlet of the culvert, it is armored with rock.
As with any erosion control measures, it really is a matter of slowing down the flow of water. Reducing the volume by discharging the water as quickly as possible.
If the roadway does not get plowed for snow removal, you could try wipers, swales and channel culverts, placed diagonally in the roadway to divert water coming down the roadway without turning your outsloped roadway into an insloped roadway, but I am not sure how well they really work. It might be worth trying only because it would be so less expensive then the true insloped fix. A word of caution on the swales though. While they are great for fields and food plots, driving over a swale kind of sucks, especially if pulling a trailer or making a few runs up the roadway everyday. The rolling nature of them gets old pretty fast. I am putting in a road this summer up a sleep hill and it has to be soil engineered, but I am hoping above all hopes that she does not call out for swales in my roadway as they really suck to drive over.
We haven't been out there in the winter yet but from what I've heard it can get very cold and nasty, I wouldn't be surprised if we get some snow, frost definitely (land is in central Portugal, we're living in the UK at the moment) so that might not be the best option, I'll have to keep a lookout for hardier alternatives.
As for the ditch, that's going to need some easier alternatives! The drive is far too long, and the rock rocky for me and my Pickaxe! We're on the low budget, miles from civilisation, make do with what we've got kinda scenario and a ditch is out of the question at the moment, as is building up the level of the drive. however, I'm sure with a few extra hands we could place rocks just in from the uphill edge of the drive, if they were laid in such a way as to direct the water down without allowing it to slip through (I'm thinking roof tiles, laid on their side rather than flat) most of the drive seems OK erosion wise, can't see much evidence of water running down except at the dip so there's probably less than 100m in total where it needs diverting / slowing
Travis is spot on.
Its no good having a project on a tight budget and having the access road washed out.
If its not possible to create the ditch on the hill side, at lest create some culverts or spoon drains that divert the water off the road at regular intervals.
These spoon drains need to be lined with rock to eliminate ponging and scouring.
As Travis stated, as water picks up speed to more than about 3 feet a second, it starts to erode.
The spoon drains will reduce the speed possible along the road, so be aware of that.
John Daley Bendigo, Australia
The Enemy of progress is the hope of a perfect plan