Swale dimensions and frequency

Hello all!

We are planning a series of 6 swales that will act as paddock partitions in a field that is approximately 165’x165’. As such, we have spaced the swales at about 35’ on center from one another. The slope of the field various somewhat but averages around 10-15 degrees. The top soil varies in thickness but due to the clay rich subsoil, is often water logged after heavy rains. At other times, due to the lack of trees in the field and surrounding land, it can also become dry and compacted between rainfalls. We would like to plant trees and living hedge crops on the downhill side of each swale for both livestock and human food production as well as to serve as paddock partitions. We are still finalizing which species to implement but will probably grow fruit and nut trees appropriate to our zone (5a) as well as hazel and probably Washington hawthorn for living hedge structures.
We are wondering how wide and to what depth to make the swales. We assumed a width of about 3’ would suffice and perhaps a depth of a foot or so, removing top soil and depositing it immediately down hill of each swale. We were also wondering if there is a recommended minimum weight of excavator to be used when constructing swales…i.e. does the subsoil layer in each swale bottom need to be compacted by the treads or bucket in order to effectively carry water without eroding? This is our first attempt at swale construction. Thank you for any advice or suggestions you might have, given our lack of experience! Thank you!

Welcome to Permies Georgia! I hope we can help with your project, which is a great goal.

It will be hard to fully answer your questions without more information. A general region in your profile/signature can help fill in a lot of background for informing others’ answers to your questions (ie weather, hydrology and ecology). It sounds like some serious research and planning is warranted for this due to that very steep slope (10-15% is close to the max for safe vehicle use) and waterlogged soil. A couple concerns do arise for me though from what has been provided.

1) Swale bottoms/ditches are not meant to be compacted at all. Swales are for absorption, and the bottom is often keyline/Yeomans plow ripped (not inverting/mixing the soil layers). On a small scale I have broadforked and deeply mulched the bottoms that I use as paths between berms.

2) Compacted, waterlogged clay soils are prone to collapse and landslide. Swales should infiltrate water within 72hrs, and of course if water is coming continuously then adequate overflow level sills and or monks/swivel pipes for not just max rainfall but potential snowmelt need to be installed. In zone 5 potential ice dams probably need to be accounted for.

Even here in zone 8 in NW CA, I am currently looking at nearly 3ft of snow on the ground, with more coming. That will likely melt off in a subsequent rain event, which could be very large (5” rain days are at least annual events here). I generally build my drainages to account for double the record hourly rainfall. Here that means 3”+/hr. That multiplied by the catchment area upstream and its runoff quotient equals the minimum amount of water that must be able escape, ideally over a wide, flat area of hardscape, vetiver grass, or if all else fails into a forest. I try to have at least one backup overflow into a forested area.

I’d encourage anyone to account for worst case scenarios, and this includes people and property downhill. Its also always worth asking if a given intervention is worth the cost and risk, and if it will be putting undue burden on the intervener to maintain it. What if you hurt your back shoveling your driveway (like I just did) right before a major rain event melts all the snow while you are laid up and inable to clear clogged pipes (this is why open level sills are often better than any pipe)? This is what I am looking at, and am glad I am at the top of a watershed, above forest and no people for miles, and I still build what I hope are oversized overflows.

Best of luck, and I hope I haven’t scared you off the project. It is just amazing that anyone can rent an excavator and effectively destroy their own landscape and those downstream in a very short period. They are very fun to run though. Be safe on that steep slope!

I would strongly recommend Permaculture: A Designer’s Manual by Bill Mollison (Tagari Press), and The Keyline Plan by PA Yeomans, the latter available as a pdf for free. Much has been written since, be these are foundational works that address what you are looking to do.

PA Yeomans’ “The Keyline Plan”, I believe this explicitly answers your question of swale depth and interval (but keep in mind he was in Australia with 20” of rain annually):

https://soilandhealth.org/book/the-keyline-plan/

Below, a few excerpts from Bill Mollison’s Permaculture: A Designer’s Manual (aka the “Big Black Book” or BBB). I highly recommend buying the book through Tagari Press:

This is the best video series  i've seen on swale design.  


Jeoff Lawton  @  the Weedy garden


Part 1. How to Build Swales. What is a swale?
https://m.youtube.com/watch?v=Rz6i5I42JmM

How to design Swales. Geoff Lawton visits The Weedy Garden - Part Two.
https://m.youtube.com/watch?v=I5OU7LG9Cl8&t=5s


How to Make Swales - Part Three. The Digging
https://m.youtube.com/watch?v=6zM8wBbR4RQ

Tom