EARTHWORKS - INSTALL SWALES

Swales are tree-growing systems and the main water strategy of a food forest. They help to capture and soak runoff water into the ground, improve groundwater recharge, and establish trees and other plants.

They are most useful on sloping sites with fine-textured, dry, compacted, or eroding soils, where runoff is an issue, especially in arid or semi-arid climates.

Essentially, a swale is a shallow trench laid out dead-level along the land’s contours. It can be anywhere from 1 to 4 feet (0.5 to 1. 2m) or more wide ,1 to 3 feet (0.5 to 1m) deep, and of whatever length necessary. The earth dug from the swale is piled on the downhill side to make a raised mound or berm roughly the same size as the trench.

Surface water and rainwater run downhill into the dug-out portion of the swale—the shallow trench, spread out along its length—and slowly percolate into the soil. This slow infiltration recharges the water table for a broad area stretching downslope of the swale, creating an underground reservoir that aids plant growth.

Planting trees or other crops on the mound (berm), or just below the downhill side of the swale, takes advantage of the infiltrated water, and improves tree survival during dry periods.

The swales fill with mulch, soil and organic matter over time and are eventually made obsolete as the mature trees take over the swale’s functions.

Swale – a raised mound and a trench holding runoff water and infiltrating it into the soil

 

What you’ll need? low-tech/low-cost option

Survey tools:

  • A-frame level
  • Landscape flags or wooden stakes

Excavation tools:

  • Pointed shovel
  • Pick ax
  • Metal rake 

Swale construction materials:

  • Compost (optional)
  • Cardboard (optional)
  • Mulch – Woodchips, Straw…
  • Miscellaneous rocks, any shape

[PRINT OUT THE CHECKLIST]

What you’ll need? high-tech/high-cost option

Survey tools:

  • Laser level
  • Landscape flags or wooden stakes

Excavation tools:

  • Backhoe, excavator or tractor with one-bottom plow 
  • Pointed shovel
  • Metal rake

Swale construction materials:

  • Compost (optional)
  • Cardboard (optional)
  • Mulch – Woodchips, Straw…
  • Miscellaneous rocks, any shape

[PRINT OUT THE CHECKLIST]

How to install swales

STEP 0: Sitting – Planning

1. Assess your slope

First, survey the site terrain to see how steep your slope is.

Note that you can only install swales on gently to moderately sloped land (0° and 20°) with less than 3:1 gradient (not more than 1 foot of drop over a 3-foot run). Any steeper than this and you risk blowout from too much water buildup, overflow, and soil destabilization.

On slopes steeper than 20° (or more than 3:1 gradient) use terraces, they act in a similar way to the swales in harvesting water on level ground.

2. Sitting your swale(s)

You can install swales just about anywhere where they are needed, as long as the slope requirement is met. However, you’ll want to avoid placing them closer than 10 feet (3m) to the house.

Also, it’s good practice to install your first swale at the highest point of your land. This is a very important principle of harvesting water, this way you slow the flow as soon as possible and manage water coming in from offsite. 

Where you begin and end your swale is entirely up to you, it can be 4 feet (1.2 m) long or extend all the way across your acreage. In most cases the restrictions of the surrounding landscape (access, property boundary, fencing) will be your guide.

3. Determine the distance between the swales

If you’re planning on digging several swales, you’ll need to determine how far apart to make them. This will depend on your site goals and rainfall and runoff conditions, but the overarching rule of thumb is: the greater the runoff, the closer the swales should be spaced.

For example, on steep overgrazed or disturbed land, you will be confronted with large volumes of fast-moving sediment-laden water in intense rainfalls so you should place your swales at close intervals.

On gentle slopes covered with thick native grass, the watershed can absorb more rainfall before significant runoff begins so you’ll need fewer, and more widely spaced, swales.

STEP 1: Stake out the contour: getting level

Now lay out the swale lines on the site, to do this you’ll need to find and mark contour lines.

For this you’ll need a leveling device – a high-tech tool such as laser level is the most accurate tool, but the lower-tech alternative, the A-frame, can also work effectively. See the Survey guide on how to use these tools.

  • Use your leveling tool to work your way along the contour of the land, marking the course of your swale with stakes or survey flags. Note that on hilly ground you may need to space your pegs closer to avoid height errors.
  • Once you’ve finished recheck the layout and markings to be sure that everything is correct before you move any earth.

 

STEP 2: Dig your swales

Once you’ve marked the course of your swale, you can start digging.

Depending on the type of your soil the depth and the shape of the swales will vary:

  • In clay soils where water infiltrates slowly and soil generally holds together well, you’ll want to dig deep trenches and higher and narrower mounds.
  • In sandy soil where the water absorbs more readily and sand is less stable when wet you’ll want to build wide and shallow trenches with thicker mounds.
  • For ease of access also consider making the mounds wide (a maximum of 3 feet~1m) enough that you can easily reach the whole planted area without trampling the soil and plants.
  • Note that the bottom of the basin does not need to be perfectly even, but testing it with the A-frame or long level is helpful in ensuring that you are close.

Depending on the size of the project you can dig swales by hand or employ machinery such as a backhoe, excavator or use a tractor with a one-bottom plow.

    High-tech/impact/cost options:

    Backhoe or an excavator – this option is logistically the most complex but if you have numerous, long and large swales it’s the only reasonable choice.

    • Unless you can operate the machines yourself, you’ll need to find someone to do the earthworks for you.
    • Also consider that using these machines is a two-person job, one on the laser level checking the level of the bottom of the trench and one operating the backhoe or an excavator.
    • After the job is done, you’ll need to perform some hand-grooming of the excavated material.

    Tractor with a one-bottom plow – this is something you can do by yourself if you have the tractor and the implement.

    • When making a swale you simply lower the plow into the ground and follow the swale outline.
    • You’ll end up with uniform swales that will probably require some hand-grooming afterwards.

    Low-tech/impact/cost option:

    Digging a swale with a shovel – If your site permits and you have the physical capabilities this is the simplest and the cheapest way to dig out your swales. No prep work, just hit the ground immediately.

    • To construct your swales with a shovel, start digging a trench just upslope of your survey pegs, then use the excavated dirt to build your dirt mound downslope. Note: If your soil is exceptionally hard, use your pick ax!
    • Flip the sod upside down, this is the first layer in the forming mound. As you keep digging out the trench, pile up the earth on top of the inverted sod.

    STEP 3: Groom and fill with mulch

    Now when the excavation is complete the swales will need some grooming and shaping prior to planting.

    • You can add compost and/or other amendments on top of the roughly piled up mounds and mix everything in with this now loose soil.
    • Once mixed in, shape the mound using a short-tined metal rake. Your goal is to make the top of the berm smooth so that falling rainwater settles evenly.
    • Plant (and mulch) the mound immediately, this prevents erosion and stabilizes the disturbed soil. See Cover Cropping and Planting Bare Root Trees on how to do it exactly.
    • Fill the dug-out part of the swale – the trench, with mulch (large wood chips; mulch, pine needles, and leaves…), this will help hold and absorb water and create a nice walking surface and look. Depending on the weed pressure before putting the mulch consider putting cardboard in the bottom.

    STEP 4: Locate and dig spillways

    When installing swales, you always plan for an overflow. If your swale is hit with a storm surge or you expect it might be receiving a high volume of water then plan how to direct and catch the overflow.

    • A spillway is a low point in the mound, locate it on the end of your swales or at the places where you want a pathway across the top of the swale. Choose your best spots on the mound and remove one-third of the mound material, make it as wide as necessary for your path or at least twice as wide as your mound height.
    • When you have a series of swales, one above another, offset the spillways from one swale to the next rather than placing one spillway directly above another. This will make the excess water take the longest path to the next swale, spreading and infiltrating into the soil, instead of draining straight downhill.
    • Once you have dug your spillways, place rocks of any shape along the bottom and sides of the newly excavated path/overflow to stabilize it so fast-flowing water won’t erode the mound and land downslope.

    STEP 5: Maintenance and troubleshooting

    • When all is done, check your swales often, especially during and after storms. If there is a need to fix anything, it’s probably a simple fix with hand tools.
    • If spillways are eroding, reinforce them, if berms are blowing out, make them bigger.
    • If the swales were misplaced off contour and act as a water drain, build small internal mounds or one-rock check dams within the trench area. Position them perpendicular to the main mound to intercept the water before it drains away. Make these no taller than two-thirds the height of the main mound.

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