Rainwater Harvesting: Berms and Swales

by Giovanni Castaldo, Santa Cruz Permaculture Design Course participant

This series of blog posts about Rainwater Harvesting provides an overview of some of the key practices. The series is informed and guided by the book Rainwater Harvesting for Drylands and Beyond, Brad Lancaster’s approachable and inspiring work on the topic of rainwater harvesting. This is the fourth in a series of blog posts by Giovanni Castaldo.

Resting in a newly constructed berm and swale during the Santa Cruz Permaculture Design Certificate course. The berm is the elevated portion that everyone’s leaning against, and they are all sitting in the swale.

Simple water harvesting earthworks–shaping the earth to slow and infiltrate the flow of rainwater on a slope–do not cost anything more than a shovel and your labor. When it comes to rainwater harvesting strategies, they can provide significant benefits to the landscape and your local watershed.

Berms and swales are a simple water harvesting earthwork technique that are laid perpendicular to land slope, designed to intercept rainwater running down the slope and infiltrate this water in a localized area. 

Image source: RiverLink.org (This resource has some useful calculations, too.)

The swale is an excavated basin that precedes a berm, which is a raised bank. The berm can be made of earth excavated to form the swale, or it can be made from brush, rock, or additional earth. The swale holds water, and the addition of the berm allows it to retain even more water. 

Create berms and swales on gradually sloping areas above the break line or below the key line (learn more about breaklines and keylines in this blog post). Some swale-builders recommend building on slopes no steeper than 25% grade, some say no more than 15%, and others say 5%. In addition to the safety element, building on too steep of a slope can cause landslides as water saturates the soil along the steep hillside. The question of how steep is too steep is a point of contention among swale-builders, and one rule of thumb is to only build on sloping areas where you feel comfortable getting on the shovel to build it. 

But fear not friends with steep slopes – when you are working on a slope that is too steep for constructing a swale/berm system, or at the upper limits of what you feel is appropriate, you can always build a berm on top of the soil surface using straw-waddles, straw bales, myco-incoulated bunker-bags, or piles of woody material.

Construction & Features

Calculating Size and Spacing

Berms and swales should be placed and sized in order to hold all the runoff that will drain into them in a typical storm. Use length, width, and depth of the projected basin to calculate the water-holding capacity of the coupled berm and swale. 

The lower the water-holding capacity and the greater volume of expected runoff, the closer together these earthworks should be, resulting in a smaller micro-watershed serving each. Vice versa, the greater the water-holding capacity of the berm and basin and the lower the volume of expected runoff, the farther apart these earthworks should be, so a larger micro-watershed serves each one.

To avoid damaging your berm and causing erosion during a significant storm, be sure to make your berm large enough for the potential rainfall that may take place on the landscape. As a rule of thumb, make the base of the berm portion at least four times as thick as the height of the berm and basin, measured from the bottom of the basin to the top of the berm. This will give a berm with a 2:1 slope, though a more gradual slope of 3:1 will create a still more stable berm. 

Building a swale during the Santa Cruz Permaculture Design Certificate course. Photo credit: Steve Kurtz

In sandy soil and gopher country, berms should be thicker. Berms should be wider where they function as raised access-ways. The top of the berm needs to be level so there won’t be any low points that become unexpected erosive spillways. Check that it’s level with a bunyip or A-frame level (learn more about these tools on this blog post).

Compact your berms, but not your basins. If there are ample rocks at hand, they can be laid on the downslope side of the berm for additional stabilization. Place the rocks as close together as possible to prevent rainwater from creating concentrated rivulets and eroding the berm. 

Spillways

Spillways are indented about ⅓ to ½ down from the top of the berm. A spillway is a low point in the berm, which diverts excess water in the case of higher-than-expected rainfall. Rather than overflowing your entire berm and causing erosion, spillways allow water to spill out of the berm in a strategic way. 

They should be located where slopes are gradual, vegetation is well established, and soil has had little disturbance so you can easily control and utilize the overflow. When there is a series of berm and swales one above the other, offset the spillways from one to the other rather than lacing one spillway directly above another. In this way, overflow will zig zag across the landscape slowly, spreading and infiltrating into the soil. 

Build the spillway wide, at least twice as wide as your berm height, measuring from the bottom of the basin to the top of the berm. The spillway will then be less likely to clog with debris, and the water flowing through it will be less constrictive and erosive. 

Stabilize spillways with rock, dense vegetation, or other durable materials. Establish vegetation such as native grass as soon as possible to anchor everything in place. Rocks can be undercut if big storms hit before plants become established. To prevent this, place erosion-control fabrics or a layer of organic mulch under the rocks.

You can design your spillway to flow toward a pond, well-draining orchard, or some other place downhill of your berm that will allow you to utilize the excess water without causing damage to your landscape.

Proper Vegetation

Water-harvesting earthworks like berms and swales can support native plants that are adapted to the local climate and can be more neglected on the periphery of the property. This hardy vegetation can shelter the less hardy plants, such as the fruit trees closer to the home, from sun or wind. Hardy native plantings placed along the periphery support more native wildlife, which appreciates less interference from us. 

Berms and swales on a hillside at Wild & Radish in Pinole, CA. Photo credit: Melissa Fant.

Living berms and swales covered in vegetation grow and strengthen themselves. In basins, plant species tolerant of temporary flooding in arid climates. A low-water-use native landscape placed in water-harvesting earthworks is easy and inexpensive to establish and maintain. 

In wetter climates, plant larger perennial vegetation atop the berms for better drainage, or uphill of the basins. If working in a well vegetated area, speed up revegetation by favoring the already established vegetation with three methods:

  1. Leave a 1-foot wide undisturbed vegetated strip of land between the berm and the basin;
  2. Break up the basin into a series of smaller basins separated by undisturbed, vegetated strips of land;
  3. Bring in soil to create berms without the basins. This method works especially well in landscapes with established vegetation, so roots are not disturbed when digging. 

Cover the sale with either organic or inorganic mulches to help form a spongy and porous welcome mat to lure water into the soil and create a sheltering cover that reduces loss of soil moisture to evaporation. In addition, mulches protect the soil from the erosive force of falling raindrops and buffer temperature extremes at the soil surface. 

Using carbon-rich organic mulch in the swale ensures it will decompose slowly, enhancing rather than depleting soil nitrogen. Organic mulch in general increases the soil’s organic matter content as the mulch decomposes. This feeds beneficial microorganisms, which boost soil fertility and water-holding capacity. 

Give different plant types their preferred type of organic mulch to attract their specific beneficial microorganisms (woody long-lived plants favor woody longer-lasting mulches, annual short-lived plants favor lighter more ephemeral mulches like straw). 

Mulch, coupled with passively harvested water and newly planted multi-use perennial vegetation, transforms once-degraded urban soils into rich soil ecologies equivalent to those found in healthy regional forests. Trees associated with mulched water-harvesting earthworks are able to grow 33% larger than those without, more than doubling the trees’ potential sequestration of atmospheric carbon, passive cooling, and food production. 

In addition, research shows that the presence of more organic matter in the soil enables the soil itself to sequester additional carbon and the natural pollutant-filtering/bioremediation ability of the soil mulched with organic matter was 10x greater than that of rock- or gravel-mulched soil.

Santa Cruz Permaculture Design Certificate students pour woodchips into a freshly dug swale. The berm is shown covered in jute fabric to prevent erosion. Photo credit: Steve Kurtz.

Maintenance

To reduce unwanted sedimentation of earthworks, revegetate the watershed above the earthworks using the sediment as a soil-building resource. Wherever possible, direct the flow path of sediment-laden water across well-vegetated, gently sloping landforms before the flow enters basins and swales so it isn’t washed away.

So that the earthen berm won’t erode away in heavy rains, scatter native low-water-use groundcover or grass seeds on the berm itself (seeds of local plants that naturally grow on exposed dry areas like the berm). 

Reinforce spillways if they are eroding. If the berms blow out, build them bigger or create other water-harvesting earthworks above them. If the berm and swale is off contour and acts as a water drain, build small internal berms or one-rock check dams within the basin area perpendicular to the original berm to intercept the water. For a contour berm and swales, curve both ends of the berm uphill to avoid water eroding the ends of the swale. 

Learn More

In this series, we’ve covered some of the basic concepts and information to get you started on your rainwater harvesting journey. Want to go deeper with all of this? Join us for our Permaculture Design Certificate Course! We spend an entire weekend focused on Regenerating Watersheds & Soils, and part of your design project includes assessing a landscape for rainwater harvesting. Learn more and register today at santacruzpermaculture.com/permaculture-design-course/

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