January 9, 2022

Permaculture Notes: Swales

Swale digging is on Dan's and my winter project list. It's a subject we've been studying, so I wanted to organize the notes here as a resource. 

Notes from:

I. Definition - Swales are long, level excavations intended to capture surface water runoff (from rain, roof, road runoff, tank overflow, diversion drains, etc.) and then store it in underlying soils or sediments.

II. Function - to recharge groundwater systems.
A. To intercept and temporarily hold overland water flow.
B. Allow collected water to infiltrate into the downhill soil.
C. Nutrient delivery system, E.g. swales built under or next to a chicken house can deliver nutrients from the manure via water flow to the rest of the system.
D. Full effect will take several years to realize.
E. Can eventually recharge springs and aquifers. 
 
III. Design planning
A. Ideal landscape - 15% of the ground surface covered in water 
1. ponds
2. swales to transport water to the ponds
B. Design the system of swales and ponds for water to move:
1. by the longest path
2. over the longest time
3. with the least amount of friction
4. "The farther you lead water, the more storage you have."
 C. Placement
1. Place first swale at the property's highest elevation possible.
2. Typically 4-6 feet below highest points
D. Begin the plan by marking  the level contours of the property. This will determine the pattern of the ponds and swales.
E. Spillways direct water flow from higher to lower storage features and prevent overflow. 
  
IV. Sizing
A. Varies greatly depending on soil and climate. 
1. wider and shallower in sandy soils
2. narrower and deeper in clay soils
3. up to 6 m (20 ft) wide in deserts 
B. Should allow for slow water movement.
1. The slower the better
2. Should be able to walk the speed of the flow. 
C. Should be large enough to not blow out the spillways.
D. Width
1. Can be anywhere from wheelbarrow width to roadway width 
2. should not exceed crown spread of fringing trees
E. Examples
1. Small, front yard size - 20 inches wide and 8 inches deep 
2. Large, pasture size swales - 4-6 feet wide and 18 inches deep

V. Spacing
A. Swales typically hold moisture 30-40 feet down the slope.
B. Distance between swales can be 3-20 times the swale width depending on rainfall.
1. Large swales can be spaced 12 feet apart with average rainfall exceeding 50 inches.
2. Large swales should be spaced 60 feet apart with average rainfall less than 10 inches.
C. On slopes, use height of trees to judge distance between swales, where berms are level with the height of the treeline below. 

VI. Construction
A. Built on contour, i.e they're level to allow even distribution of water
B. Equipment
1. Depends on size and budget
2. Can be dug by hand with shovels
3. Tractor with turn plow, scrape blade, subsoiler, disc harrow
4. Backhoe or excavator 
C. Ground can be ripped first with subsoiler, for the planned width of swale and berm.
D. Often dug into the subsoil 
E. Material dug from the swale becomes the berm.
1. Berms not compacted or sealed (unlike pond walls)
F. Swale floor can be sloped to encourage water drainage in a particular direction
G. Can be constructed to be backflooded by ponds 
H. Spillways to direct overflow to another swale or pond in a series 
1. lower than berm height
a. Geoff - 50-200 ml
b. Michael - 1/3 lower 
2. can be packed clay, rocks, grass, concrete, anything that won't wash out. 
 
VII. Planting
A. Berm planting 
1. essential for the long term functioning of the swale.
2. Seeded or planted on either side after an initial soaking of rain
a. grasses for grazing
b. perennial shrubs and bushes
c. Trees 
i. Some of the collected water is stored in trees.
ii. Planted in the berm or above the swale (not in the swale) 
iii. Essential in arid climates to shade and reduce evaporation
iv. Roots increase absorption efficiency
v. Leaf drop adds nutrients and organic matter to the swale 
 
B. Swale planting
1. Since the topsoil is removed, the swale itself is not typically fertile
a. soil can be left to accumulate organic matter
b. can be planted with grass
c. can be filled with mulch or gravel
 
VIII. Living Swales (Michael)
A. Living swales are planted swales, not earthwork swales
1. constructed on contour of dense, clump grasses such of vetiver
2. the grasses stop and retain overland water flow, similar to dug swales
3. common in regions with shallow or rocky soils
B. Over time, earthwork swales will fill in with plant and runnoff debris, and the planted berms will create living swales.

~~~

When I started organizing my notes into an outline, I didn't realize how many of them I had. A lot! So this turned into a larger job than I anticipated. Still, to have what I've learned organized and easy to access feels good. 

Next time, I'll show you our plan to, hopefully. put this information to good use.

8 comments:

Rain said...

Hi Leigh! ☺ That was super informative, I'm a visual person so I couldn't quite grasp it, but I did an image search and now I get it. I think it's a fantastic idea for you and Dan to build swales, lots of work and well worth it!!

Boud said...

This is a vivid reminder of how long term good stewards have to think and live.

Toirdhealbheach Beucail said...

Leigh - Thank you! The notes seem very clear and understandable in terms of function, concept, and even building techniques. I will be interested to see what your plan is as I think I know what you are building, but that will help immensely.

I find the part about recharging aquifers especially interesting as this is a very timely discussion - which begs the question, of course, why more people are not doing it (or government entities, for that matter).

Leigh said...

Rain, (and everybody!) I can't recommend the Mollison online PDC highly enough. It's available on Udemy.com www.udemy.com/course/permaculture-design-course/, where they continually offer it at a discount, usually around 20 USD or less. My only regret is waiting so long to take the course.

I'm a visual person too, so I get the need for visuals to understand. My next post will be the our start on our plan, and after that I'll show you what we've done so far. You're right about it being lots of work, but it's one-time work with years and years of benefit.

Boud, very true! My honest belief is that stewardship is meant to be, and must be, generational. Definitely not the way things are done in the modern world (of which the consequences are speaking for themselves).

TB, Bill Mollison was actually able to recharge springs on one of the properties he owned, so that they began to flow year-round. Geoff Lawton tells the story of being in the midst of a severe drought, where all his neighbors were experienced dry, parched conditions. His place was lush and green because of the system of swales and ponds he had previously built. The neighbors told him he was "lucky" to have good pasture and gardens. His response what that luck had nothing to do with it, it was all the result of good permaculture design.

This definitely begs the question of why more people aren't doing it, and why governments aren't developing programs to promote it, especially considering the dire outlook of climate change. But they aren't, and that makes no sense to me.

Rich said...

I don't have any experience with swales, but I do have experience with terraces which seem to be just another name for the same thing. All of my cropland is terraced with broad based terraces and there are a couple of small 5-10 acre terraced former fields in the pastures.

I've read about building swales and over the years I have observed how the terraces in the pasture function' and I just haven't seen the benefits that are supposed to come from building swales.

Of course, it might be that my "swales" don't work because they aren't built to correct permaculture specifications, but if it was me, I'd start extremely small with any swale building project.

Leigh said...

Rich, in the beginning, I terraced my garden, but the soil still dried out faster than I liked, and I had to do a lot of irrigating. With my swale beds, I double dug down to make a large trench in the clay subsoil. Years later, those swale beds need a lot less watering because the soil stays moister for longer!

I agree about treating it as an experiment to begin with. We've actually got our first swale started (hand dug) and have been amazed at how much water it holds after it rains. I understand that it takes several years to see the full benefit of subsoil hydration, but so far, I'm encouraged.

Toirdhealbheach Beucail said...

Thanks Leigh! That is a super interesting real story about recharging a spring. We have one in the Middle Meadow that does not flow nearly as well as it used to; I will definitely stay tuned.

Leigh said...

TB, now that's exciting. Who knows what you might be able to do with it! That could definitely be a starting point for an experimental swale.

Dan and I discovered an area two ridges down from one of our pastures that really likes to hold water. Several months ago, I dug a trench there after heavy rainfall and discovered water seeping through the soil underground. It quickly filled the trench. The trench stays full for days and days after rainfall. Right now, we're thinking it a likely place to dig a pond, because Bill and Geoff have a lot of interesting things to say about aquaculture, as well as swales.