Description
A swale is an intervention designed to retain water run-off on a slope. It is a long, small ditch on the contour lines, i.e. at the same elevation. Several swales can be constructed next to each other to increase their effect. Swales have a small berm on the downslope side, which often contains vegetation and acts as a retaining wall.
Swales are cost-effective and relatively easy to implement, preventing soil erosion, nutrient run-off, and flooding. They capture water which runs down the hill and let it infiltrate into the soil. Growing vegetation on the berm will strengthen the berm and the soil and will hold water and nutrients better. Trees, in particular, work well for strengthening berms and are watered by the water held by the swales. If possible, it is also an option to add stones to a berm to make it even stronger. In the Perma Atlas project, swales have been a great success in preventing flooding. Swales are similar to contour bunds and Fanya Chini and they can be used on a small or large scale. The greatest reduction observed when swales are introduced are in water runoff, soil loss, phosphorus runoff and nitrate-nitrogen runoff.
METHOD OF APPLICATION
Material required
- Line level/A-frame level*/laser level
- Digging instruments
- Stones, if available
- Optional: seedlings
*Making an A-frame level:
- Create two (wooden) sticks of the same length.
- Connect these sticks at the top, forming an inverted "V" shape.
- Insert another stick in the middle to create an A-frame structure, placing a mark at the center for reference.
- Hang a line with a weight or rock at the top of the A-frame.
- To find the same level on the ground: adjust the A-frame until the line aligns perfectly with the center mark, indicating a level position.
Click here for more elaborate instructions and here for a video with instructions.
Steps of implementation
1. Identify Location: Choose a suitable location for the swales, considering water flow direction and land slope (3-15%). It should be just before a place where you want to hydrate and where you want to stop erosion and run-off. Here you can find a calculator on the placing and the volume of the swales.
2. Locate Contour Lines: Ensure that the swales are aligned with the contour lines and are at the same elevation. If you are using an A-frame level, place it on the ground with the line in the middle and mark points along the contour lines for where the swales will be constructed.
If you have an A-frame level, you can determine the placement of the swale as follows:
-
- Place the A-frame on the ground, so that the line with the rock is exactly in the center at the mark.
- Keep one leg of the A-frame on the ground and place the other on the side where you want to continue the swale. While keeping this first leg in the same place, find the position with the other leg where the line is again in the middle of the A-frame. Mark these points and continue in this way.
- By connecting the marked points where the legs of the A-frame were, you will construct the contour line, i.e. the position where the swale will be placed.
See here for a different way to find the contour lines and here for more information.
3. Find Assistance: Find individuals to assist in the digging process.
4. Dig the Swales: Dig along the marked contour lines and place the excavated soil on the downhill side to create a berm. This will act as a wall to hold back the water.
-
- Width: There is no standard width for a swale, but a good width would be one where you can go in and dig from there (around 16 and 18 inches).
- Depth: 6 to 12 inches. If there is more water, the swale can be widened or deepened. The steeper the slope, the deeper the swale should be.
- Spacing between swales: This depends on the slope of the hill. With a steep slope, it is best to have the swales closer to each other. On a less steep slope, they can be further away.
5. (Optional) Stone Filling: Fill the berm with stones to strengthen the berm.
6. Verify Level: Use a level line/A-frame to check if the swale bottom is flat and level. The swale must be level so that the water does not run off to one side. Just try your best, it is impossible to do this perfectly.
7. Close Swale Ends: Dig deeper at the ends of the swale and create closing berms
8. (Optional) Fill Layer: Fill the swale with a rotting layer like leaves, wood, and straws. This will decay and provide nutrients and stabilize the swale.
9. Plant Vegetation: Plant vegetation in the berm to provide stability. The berm is also a good place for vegetation to grow as it is well-watered due to the swale system. Vegetation will strengthen the berm and hold the soil together. An additional option is to put leaves or wood into the swale to create soil as they decompose.
10. Regular Maintenance: Periodically maintain the swales to retain levelness. Remove soil or other stuff that fell into the swale to keep it open and leveled. Assess functionality against the initial plan.
A cross-section of a swale
This is what the swale will look like in cross-section. You can see how the swale basin is dug out and the soil is placed on the lower side of the slope to form a berm. The dimensions are a good example, but you do not need to copy them. The basin and the berm are more or less the same size.
Considerations for application
If there is a subsoil such as a clay layer that is not permeable to water, you should not make the swale too deep so that it remains above this layer of soil. Otherwise, the water will go under this subsoil layer and the topsoil will be dehydrated.
BENEFITS
This intervention contributes to:
CASE STUDIES
1. Perma Atlas swales project in Morroco
Description
As a result of overgrazing and tree-cutting, the region of Anguelz in Morocco has been facing severe land-related stresses in the past decade. Where once lay lush and vibrant mountains, now the landscape has turned completely arid. With the help of Perma Atlas the people from Anguelz have come together to regreen the landscape and bring it back to its former healthy state. A major issue that the area is facing is the lack of rain and seasonal flooding events. Community members have been taught in workshops hosted by Perma Atlas how to implement three interventions to help mitigate and reduce the impacts of flooding and erosion. Swales are micro-water catchment systems that collect and channel water runoff on steep slopes and help it to infiltrate into the soil. They do this whilst removing sediment and pollutants from the water flows. Locals hope that in the future and in the case of more rain events, swales can also act as a place for farming and agriculture. The project by Perma Atlas and the village of Anguelz has created a safety from flooding and heavy rains and has given the chance to the local people to learn different techniques used by their ancestors. Additionally, the implementation and maintenance of the techniques have offered job opportunities for locals
2. Swales in family-run permaculture farm in Portugal
Description:
The case study discusses the use of swales, which are level ditches dug along the land's contours, in managing water runoff in Portugal. Swales help capture rainwater and prevent it from flowing away too quickly. In the case study, ponds have been dug on terraced land to hold water and then used this water to feed swales. These swales were like small channels that helped spread water across the land, making the soil more hydrated and fertile. By using swales, the author transformed dry areas into lush landscapes, demonstrating how this technique can improve soil health and support plant growth in Portugal's climate.
FIND OUT MORE!
External sources
- Overview of considerations for deciding on the implementation of a swale by the World Permaculture Association, see here.
- For information on infiltration swale systems - please click here.
- Water catchment strategies for drylands - please click here.
- Swale Calculator (permaculturereflections.com)
Guides
