In October of 2017PermaBlitz #2took place at a rural home in Butte Creek Canyon. The ~8-acre property is gently sloped in some strategic spots which provides considerable water harvesting potential. When the design team looked at how water was utilized they noted an existing on-contour swale that needed a bit of help.
What is a Swale?
A swale – which is a dead level ditch on contour – stops water from rushing over the land and instead sinks the water into the soil keeping the soil moist for longer periods of time while also aiding in groundwater recharge. It’s important to regard swales not just as a method of slowing and holding water but as a tree growing system. Tree roots stabilize the landscape and moderate the saturation levels by utilizing the water held in the soil. The illustration shows water in a swale as it sinks and plumes slowly down through the land. A mound or berm is used to plant trees downslope from the swale and a level sill spillway allows the water to passively flow to a predetermined area, such as another swale, a pond, much basin, etc. One important goal in permaculture design is to re-use water as many times as possible before it flows off the land.
Existing Swale Problems
The PermaBlitz design team noticed that the back cut of the swale was too steep, the bottom uneven, and the level sill needed improvement. Additionally, if the swale were to fill up, the overflow route took the water to only one apricot tree before moving down to a mulch basin. To gain insight and to problem solve, the design team drew up a Water Overlay for the property.
Water Overlay for PermaBlitz #2. The swale that was repaired is directly above DS-5 (downspout 5).
The design team decided to reshape the existing swale’s back cut, re-level the bottom of the swale and move the level sill spillway to different location entirely. The Host told us that this swale had never filled up with water, but with a recently installed 2,680-gallon rain tank, overflow water would now be able to move into the swale. During winter, water would certainly fill the swale and overflow. Note in the Water Overlay illustration how the water will now be able to move to several trees instead of the one. This zig-zag pattern SLOWS water and allows it to SPREAD out before SINKING into the ground.
Swale Repair Process
Reworking the swale
The first 2 steps in the swale repair was to smooth out the back cut and then level the base of the swale. The back cut was reshaped with a gentler slope for stability and so the overland flow of water would slowly move into the swale rather than cascade over the almost vertical cut that had originally been made. The bottom of the swale was re-leveled to allow water to spread out and evenly fill the from the bottom up to the level sill spillway.
The final step was to move the location of the level sill spillway and to make it wider. Wider sills allow for a more passive water flow. The Host had leftover paving stones, so these were re-used to stabilize the soil on the sill. Lest we forget the Oroville dam spillway debacle, level sill spillways are a very important design element -always plan an overflow route that will move excess water without damaging the land!
The new wider level sill spillway allows overflow water to gently move to the next area
Example of water moving over a level sill
This other photo shows an example of a swale from a different property that has filled up during a heavy rain event. This level sill spillway allows water to passively overflow to a pond lower on the property. Note, again, the width of the spillway. It’s wide! If spillways are too narrow, water will gain speed and pressure over the area and increase the possibility of erosion during times of intense rainfall. Gauging how extreme rainfall may be at times is another very important aspect of rainwater harvesting. Determine the volume of rainwater collection potential by doing some basic math using the calculations found here.
Swale repair finished
Rooftop rainwater from the downspout flows through a pipe and daylights into the swale here
To Build a Swale or Not?
There are many things to consider before making a swale. Here are just a few:
- Do you need a swale, or are other methods more applicable?
- What might work better? Keyline, mulch basins, wattle across contour, etc. Earthworks are a disturbance and a smaller intervention may be best suited for the site.
- From which direction does water enter your property? How does it flow through and where does it leave?
- What is the degree or percentage of slope on the area intended? A swale can be used when the slope is less than 15%.
- What is your soil type? Has a soil percolation test been done?
- How would water flow into the swale? Where is runoff coming from? Is there enough water flow to need a swale?
- Calculate your largest one-day rain event. Will your swale be big enough to accommodate that event? Ensure the level sill spillway is wide enough to allow the water to passively flow from the swale.
- Stay at least 10-20 feet from any foundation. This distance ensures that water seeping into the ground won’t cause moisture problems with the building’s foundation.
- Always check for underground utility lines, water mains and irrigation pipes.
- Make sure to place the swale at least 20-30 feet from a steep bank.
- What will be planted on the berm and in the swale and up and down slope from them? Keep in mind, swales are a tree planting system, without trees and plants the swale could fail and cause more problems than you are trying to solve.
Remember, swales are one part of an integrated plan or design and permaculture is all about integrated design. Based on the Ethics (Earth Care, People Care and Fair Share) and the Principles we utilize the design methods to configure what is needed to meet our vision and goals. If swales are applicable, then we have to make sure we do our due diligence and factor in all the variables for success. Good design is based on rigorous observation and thoughtful design.
Permaculture Principles Applied
Use Small Slow & Solutions
Small & Slow Solutions: By moving the level sill from one side of the swale to the other side – a fairly easy job – we were able to move water to mulch basins for three more trees, rather than just the one tree. Spreading and sinking water throughout the landscape is always a key design goal in permaculture.
“Work at the human scale so you can build and repair everything. Many small strategies are far more effective than one big one when you are trying to infiltrate water into the soil.” -Brad Lancaster
Catch & Store Energy
Catch & Store Energy: This principle tells us to store energy when it is abundant. Water falls into that category and in the winter, when our rainfall is generally abundant, earthworks can store thousands of gallons of water in the soil.
“Rather than having water run erosively off the land’s surface, encourage it to stick around, “walk” around, and infiltrate into the soil. Slow it, spread it, sink it.” -Brad Lancaster