by Giovanni Castaldo, Danielle Parsons, and Melissa Ott Fant
This series of blog posts about Rainwater Harvesting provides an overview of some of the key practices. This is the fifth in a series of blog posts.
In these articles, we’ve covered various ways to slow, sink, spread, and store rainwater. Gabions are another way to do this on landscapes that have eroded gullies or existing seasonal drainages. Brush gabions also put to good use all the branches, bushes, and small trees that are cleared to create defensible space and reduce fire fuel load in forested areas.
A gabion is a low, leaky barrier placed perpendicular to the flow of water within a periodic drainage. Seasonally, running water temporarily backs up behind the gabion and spreads out over more of the drainage’s surface before flowing through and over this permeable dam.
Gabions slow the water’s flow, which provides numerous benefits, such as infiltrating water into the soil, reducing downstream flooding, retaining soil and organic matter on the upslope side of the dam, reducing erosion, and stabilizing a section of the landscape.
Brush gabions have been traditionally used by Zuni people in the desert southwest across drainages that flow periodically. As the brush gabions catch soil and water, they provide an excellent way to fertilize soil and stabilize drainages.
Typically a gabion is made of rocks, brush, or branches. Gabions should be used across drainages that flow only in response to rainfall events, not across a perennial drainage. They work well to stabilize roads or paths crossing ephemeral drainages. Built on top of bedrock they can create ephemeral seeps or springs as water captured in the porous soil upstream slowly seeps out of the dam.
Wire-encased rock gabions (pictured below) are sometimes used further down a drainage where water flows are larger and velocities higher. They also hold up better under the frequent stomps of cattle and resist sinking into wet sand and loose soil. However, wire-encased rock gabions require buying and importing supplies into the landscape. In most cases, a more practical and economical solution is to start small and simple by creating onsite loose rock and brush gabions.
Gabion construction in a drainage that originates upslope of, or is located off your land, may require a permit, depending on how many acres of land are affected and how the regulatory branch of your local Army Corps of Engineers (ACOE) interprets the permitting process. Check out the Clean Water Act, Section 404, and your local ACOE office for more information on gabion construction.
This video shows an example of a natural brush check dam, which we can imitate for the same effect.
Gabion materials
As with any permaculture project, your choice in materials should first be based on what you have available onsite. If you live in a place with lots of excess woody vegetation and need to clear it for defensible space and to reduce the fuel load for potential wildfires, using branches and woody materials is the way to go. If you have many medium and large rocks on your property, use them in your gabion.
Brush, branches, pole cuttings
Gabions can be made out of brush, branches, and pole cuttings. Brush gabions use dead wood and brush or carefully pruned branches from trees. Fresh cuttings of some native species can also be propagated directly into wet soil to add living material to the brush gabion over time. Thick woody material will last longer than thin, young vegetation. Wooden posts or cut poles should be 3-6 inches in diameter, and 4-6 feet long, depending on the height of your gabion.
Brush gabions are a traditional method used in alluvial fans of the arid southwest U.S. and the Negev Desert. They dissipate the water’s flow with fine mats of woody material that slow, spread, and sink runoff, sediments, and the nutrients they carry over more area, and often into channel-side, dry-farmed agriculture. Typically they don’t exceed 3 feet in height and they last 3 to 5 years. Their limited durability comes with the advantage that they are quick to build, maintain, replace, and do not require an anchoring trench or stone apron.
Studies have shown that in small and moderate flows, brush check dams accumulate soil, slow down the flow of water, and protect the channel from additional incision. In large flood flows they act as pressure relief valves, giving way and avoiding major bank and channel erosions that occur when rigid structures breach.
Rocks
If you have access to rocks, they can be a great durable and long-lasting choice for your gabion. Angular rocks are better than round rocks as their angled edges better lock the rocks together. In general, rock structures placed within channels should be constructed using rocks that are larger than 90% of the typical naturally deposited rock. If you are building a rock structure in a channel full of sand with no naturally deposited rock, import rocks that have a median dimension that approximately measures 1/8th of the total channel width.
New rock structures are the weakest in the period before sediment carried by smaller water flows has been deposited in and around the structures. To avoid damage from large storms right after construction, fill in and around the larger rocks with smaller gravels or sediment to mimic the first smaller flows.
Location and spacing of gabions
A gabion should be located in a gradually sloping straight section of a drainage, not in or just after a curve where meandering water currents and erosion are likely to cut around it. In particular, locate it by crossover riffles where sediments naturally deposit in areas of slower moving water. Placing dams here will speed up their beneficial accumulation of sediments. Gabions work best in sloping drainages that are already naturally stepped. In this way, the stepped terracing created by the gabions builds on an existing pattern.
To help anchor a gabion in place, locate it on the upslope side of a tree or boulder. If there is a constriction in a drainage, place a gabion upstream of the narrowest point of the constriction so the water flow will help wedge the gabion in place.
The gabion should be built perpendicular to the channel it is placed in. This prevents the water’s flow from being directed into a bank of the channel, which can cause erosion. Before building a gabion, it is crucial to observe the frequency and magnitude of flow in order to design the proper-sized gabion.
Ideally a series of gabions should be placed heel-to-toe, where the toe of the level terrace of accumulated soil and sediment behind each downstream dam extends to the heel of the downstream-facing base of the next upstream dam.
Construction of gabions
Gabions should not be taller than 2-3 feet, and no taller than ⅓ to ½ the depth of the channel at the dam’s center point in the middle of the drainage. It is better to place many gabions throughout the landscape than to build a few large ones that benefit only a small section of the watershed.
Brush gabions
To build a gabion, dig an anchoring trench to a depth half the width of the materials you are using to construct your dam. The trench should be dug both into the drainage bed and laterally into the stabilized portion of the banks. If the banks are unstable, cut them back to a 1:1 slope or more gradual slope, before digging the anchoring trench in them.
Cast all dirt from digging the trench upslope of the gabion to jump-start the silting-in of the dam. You can also cast it in between the gabion materials wherever there is a gap.
To create a gabion, vertical posts are placed perpendicular to the flow of water. These posts can be harvested branches or could even be living native riparian trees like willows. Depending on your stream size, the posts are approximately 3 to 4 feet long and are buried about 1 foot deep. Posts should be lower than the bank, with the middle being the lowest point. This is important because we do not want to divert water from the stream channel.
Next, two trenches are dug parallel to the support posts in order to anchor the brush check dam in place. This is where the transverse pieces will go that run perpendicular to the flow of water. The key is to use flexible pieces of wood that can be woven between the posts; young Douglas fir or redwood are great options, if available. It’s like weaving a permeable basket, allowing for the flow of air and water to move through the structure, and for the sediment to be trapped behind the structure.
A traditional Zuni method uses pungie posts. Pungie posts consist of clusters of pinyon and juniper limbs and stems set into the arroyo bed to form comb-like structures that capture woody debris and create reinforced debris jams during flow events. The posts are set at least 2 feet into the ground, angled upstream. Placement of the posts is downstream from brush-pile structures and upstream from other types to protect and reinforce them.
When building a brush check dam, you can also “stack functions” so that the dam serves multiple purposes. In addition to recharging the water table, it can be constructed to double as a small bridge. You can achieve this by stuffing the middle area between posts full of woody material. An added benefit is that it processes woody fuels on the land, transforming fire danger into water recovery.
Rock gabions
To create a rock gabion, follow the same trenching method explained above. The larger heavier rocks should be set firmly in place on the downstream face of the gabion’s anchoring trench to secure the other rocks in place. Smaller rocks should be placed in spaces between the larger rocks to prevent water from shooting through unfilled openings, which can cause erosion, and to allow sediment to build up behind the gabion.
The upstream face of the gabion should be ramped at 1.5:1 or more gradual slope to divert a portion of flowing water over the gabion rather than absorbing the full force of flowing water into a more vertical dam face.
The top of the gabion should have a banana shape. The center elevation should be significantly lower than the ends of the dam, which should extend upward to the top of each bank, preventing water from cutting around the ends of the dam, and acting as a spillway of sorts. In this way the flow will be directed down the center of the drainage. Create a banana-shaped anchoring trench to create a banana-shaped dam that is tightly anchored into the bottom of the drainage and the banks. When it’s built correctly, you should be able to walk across your dam without the rocks moving or slipping. A slow, steady, and stable approach is to build a course or layer of a gabion, let it silt up in a storm, then build another course or layer, let it silt up, and so on.
If the bottom and the sides of the channel are not bedrock, the gabion needs an apron. The apron is a platform of large (preferably flat) rocks set on the bottom and sides of the channel along the entire length of the downstream side of the gabion. It begins where the downslope side of the gabion stops. The rock apron absorbs and breaks up the erosive force of water spilling over the dam in heavy storm flows. This prevents soil erosion from under the dam.
The apron can be wrapped in wire but often it’s just made of rocks that are large enough not to wash away and that are set level with the natural bed and banks of the drainage. Check dam aprons should be at least 1.75 times as long from front to back as the check dam or gabion is tall.
If you are building a loose-rock apron, excavate a trench of sufficient depth to anchor the rocks of the apron while leaving the top of the finished apron roughly level with the natural bed and banks of the drainage. As always, toss the spill dirt on the upslope side of the gabion to speed up the silting in process.
Maintenance
Once a gabion has silted in, you can distribute seeds of plants native to the area around the dam. These will add to the seeds brought down by the runoff from upstream in the drainage. Native grasses are well suited to filter fine clay particles from muddy water and build soil in gully bottoms. The most appropriate vegetation for these locations is drought-tolerant, has the ability to lay down in periodic heavy water flows, and after getting covered with soil is able to sprout back out in more places than before the water flow. In places with a climate similar to Santa Cruz, CA, willow, cottonwood, and elderberry work well for this. Some species that do this in the southwest climate that Brad Lancaster mentions in his book are asparagus, apache plume, and seep willow.
It may also be necessary to reinforce or alter the stream bed upstream from the gabion. In the photo below, the stream channel was widened and rounded, then planted with native grasses and covered with jute netting.
Gabions should be checked regularly. If any rock is moved downstream, it is appropriate to substitute it with a larger, heavier rock. Reinforce the apron if scouring is happening on the downslope side, reinforce the apron. Adjust and reinforce the gabion so that flow goes through the middle again if flow starts scouring on the sides.
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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/