Installing swales in a mature forest?

Hello friends! This is my first post to Permies. Thanks for compiling and contributing to such a wonderful wealth of knowledge.

I've looked for a tread about installing swales into mature forests, but have not found much. Does anyone have experience putting swales through a mature forest?

A few questions: If there is a tree 'in the way' of the swale, what is the easiest way to get around it? I assume in this situation you could alter your % grade change a few degrees to drop below the tree, then bring it back to a 1% drop.

Secondly, is it possible to place a silt fence on the keyline as a very quick, cheap way to get something on the ground? A number of improvement would follow to make it more permanent.

I've got a lots of additional questions on this topic and would like to consult with someone with experience/insight into this situation.

If there is interest in this topic I could provide more detail one the specific scenario that I'm in at the moment. In a nutshell: I have the opportunity to bid on project located on a nice forested slope (~10% slope) located below a forthcoming subdivision installation that would involve lots of grading and erosion. I am pitching this as a short term environmental protection project that could be converted into a unique built environment feature that the future neighborhood could utilize, (i.e walking trail, forest farming, etc). I could go into more detail if folks are interested.

Anyway, just wanted to cast a line out there to see if anyone has thought on this.
Many thanks,
adam

Hi Adam, welcome to permies.com!

I haven't seen or heard of any examples of swales in mature forest. My gut cringes away a bit at the implications of root disturbance...

The closest I've seen was a swale/hugel installed in a ~40y/o mixed regrowth(3rd growth I'm guessing) forest near Eugene, at the Lost Valley intentional community. It looked like a pain to get done. They had a very compact tracked excavator onsite that looked like a very good fit for the job, but still looked pretty fiddly. Much of the stuff planted was in rough shape, but then this was a young system, and it was a very dry year, and I was seeing it this October just before the rains finally kicked in. In this case it looked like a major goal was to add edible plantings; I didn't get a chance to discuss the system to hear about other primary considerations. It's worth noting that much of their predominantly doug fir overstory is visibly failing after multiple summer droughts, and the remainder already doomed to a slow death, according to the consultants they've had in, so their new plantings will eventually have much more light for this reason.

Your goals sounds rather different; more info about your scenario would be great. Unless you expect your forest to vanish as well, extra care would be needed to cope with lower light levels...

One of the main challenges I see with pockets of forest left after/around subdivisions is that these trees often come down in the next rough winter, once they are bereft of the shelter of their neighbours. Since the trees on the edge of the forest take the brunt of the wind, they grow accordingly stout, while the more sheltered trees strain for light... take away the outer trees and you're often left with a log pile waiting to fall.

If you dip down below the grade that the swale-bottom is following, and then back up, you've made a pond of sorts. Seems fine to me as long as you consider the impact of a pond/mud pool there, and make sure it won't be washing out.

If you're really expecting the sort of flow/erosion that a silt fence would be used for, I don't see why it couldn't be part of your start; would this approach be specific to an area expecting outflow from the construction zone?

If other areas are being cleared for subdivision, perhaps there will be logs available, which could be pegged into the ground as combination erosion control and future nursery logs or hugelkultur beds?

Welcome, Adam.

I have to ask questions in response to your questions, sorry

Why do you want to put in swales? I ask because if we understand the purpose of the proposed swales, it helps better in finding an alternative that is better suited to the situation.

I agree with Dillon, digging swales through the existing forest means disrupting tree roots. It also means destabilizing soil that is currently relatively stable (I am assuming it is a reasonably healthy forest) and that seems to me contrary to preventing erosion.

I get the impression that you are looking at a piece of land that will not be directly subject to development, but is in a position that will be impacted by secondary aspects of the development of an adjacent area. Specifically, you refer to erosion due to run off from the development. Am I understanding the situation correctly?

How large is the watershed area feeding into this forest?

Why are you planning to slope your swales, rather than putting them in on contour? Are you thinking in terms of keyline management and trying to distribute water out to dryer ridge areas?

I think installing stone or logs "berms/check dams" on contour or your 1% grade through the system might be a better fit in this scenario. It sounds like you are wanting to mitigate erosion and run off, while slowing and spreading the moisture. Doing a surface installation rather than digging in and disturbing the existing trees might be a better fit.

I have the opportunity to bid on project located on a nice forested slope (~10% slope) located below a forthcoming subdivision installation that would involve lots of grading and erosion. I am pitching this as a short term environmental protection project that could be converted into a unique built environment feature that the future neighborhood could utilize, (i.e walking trail, forest farming, etc).

So this to me sounds like you just want a project to work on and not develop this piece of land for self use?
In the case of a 10% slope with a subdivision being built up hill, things like fertilizer run off, poison leaching and runoff (herbicide, pesticide) from new owners in the subdivision need to be considered as well.
There are many problems with trying to dig swales into an established forest floor. The root disturbance issue has been mentioned by others, there must also be consideration of the effects of the resulting water plume.
At a 10% grade there is a very real possibility of setting into motion the very things that create Mud Slides and Hill side collapses, so legal issues should be addressed before any sort of earth works begin.

As has been mentioned, in an established forest it is usually better to build on top rather than dig down. There is already an established order of soil life and you need to determine if that is worth disturbing or killing since one or the other would be the result.
erosion from above should be prevented as close to the site of erosion, meaning right at the edge of the subdivision in this case. With out having control measures in place where the troubles could begin, you are inviting a failure of control of erosion.
Built up berms would be the best use of effort in this case.
Wood would need to be left on top of the soil with no covering so there would be some leakage along the log/ soil contact line.
In any case I recommend you study hydrology before setting out to attempt any in the woods constructs just so you have a good idea of what could go wrong and plan accordingly.