A swale is a long, shallow dug-out trench, running level along the contour of a slope, with an adjacent berm on the downhill side. The “berm” is a soft mound made of loosely piled, non-compacted soil which acts as a retaining wall. On contour means following the progressive decrease in the height of the land at regular, level intervals, which mark the boundary, outline, or shape of the land.
The purpose of a swale is to harvest water passively. Over time, this will establish a permanent growing system, storing moisture in the soil for long-term food and water security. They also help deal with storm water run-off, and reduce erosion by slowing down the flow of water. As water flows downward, the berm interrupts and collects it in the level bottom. Water fills up the swale, the mound passively soaks it up, and forms an underground water lens of moist soil. This hydrates the soil and sub-soils below and boosts the effectiveness of horticulture and agroforestry. As the water percolates downward, it eventually hits the bedrock and moves horizontally, accumulating at the bottom of the lower slopes. Over time, this creates new springs, recharging aquifers and creating a natural water resource.
Swales can also be used to grow many varieties of plants, and are ideal for growing a wide range of crops on a single mound. Any trees planted on the swale will work in concert with the earthworks to open up the soil for more water and make the land drought proof. Over time, the trees will need no irrigation because they get it all from the soil. Your swale might also serve as a windbreak, privacy fence, or a native habitat for animals. You can build them along footpaths, along Hugelkultur mounds, or connect them to other swale systems and ponds to create long-lasting water reservoirs. They can even be arranged to receive water from buildings, roads, downspouts or runoff areas. Some of these swales may feed into a drainpipe that connects with a pond. You can also set up the overflow from your pond to feed into a larger swale.
When creating a pond system, build it at the highest level of your property to create water pressure that can power your water systems. Store water in your biggest pond, at a lower elevation, and pump it up to a smaller pond, at a higher elevation, when needed to increase the pressure to your water systems. Something you will notice over time is that the productivity of water wells downstream of the swale will increase. They may also produce longer into the summer, and the flow of streams below the swales will be more regular, and have a higher volume. If you want to move water faster, create contours that run against the slope to allow water to travel downhill. It can also be quite effective to simply arrange rocks and logs along a contour. Over time soil, vegetable matter, and other materials will accumulate behind them and passively form a swale.
How to Build a Swale
Step 1: Observe water on the site
First, collect information about the property to determine where the water is coming from, and where it is going.
Some observations to make about your property:
- Locate steep slopes.
- Determine quality of water sources and pollution.
- Determine annual rainfall.
- Identify springs, creeks, ponds, wetlands, wells, and cisterns.
- Identify seasonal flows or standing water.
- Identify where water is wasted.
- Observe where water is deposited.
Step 2: Identify the ideal site for a swale
Start your first swale on the highest point of your property, uphill from the areas you want to hydrate, and extend it along the contour. The key is to hold water as high as possible on the landscape for maximum absorption.
Generally, the site for your swale should follow these conditions:
- At least 10 ft. away from a building.
- Water drains away from building.
- At least 18 ft. away from the edge of a steep slope or septic drain field.
- The location is uphill from a low spot that doesn’t drain well.
- It should not directly border or intersect the lines of the property.
Once you’ve identified the proper location for your swales, you may need to fertilize the ground if it is too barren. Simply strip the land of grass and replace it with compost and mulch, allowing it to rot.
Step 3: Mark the contour lines
Look at a contour map to find the longest contour line at the highest possible elevation. Use this line for your initial swale, unless it is at the very top or bottom of the landscape.
Once you have chosen the spot where you want the swales to be, proceed to make a level line running parallel to the contours along the ground. To find the contours, use an A-frame, a water level, or a laser level to mark out the lines along the ground. Even if the land looks flat, there will always be some contour. Mark the contour line with stakes or flags every 6 ft., then mark the turf with marking paint and pull them out to repeat the process.
To test your contours, roll a small, lightweight ball (like a ping pong ball) downhill along a smooth slope. Assuming it rolls straight and is undisturbed by pits or obstacles in the ground, the trail the ball takes should intersect the contour lines at right angles. This will show the path of least resistance, and marks the passage of water as it flows downhill.
Step 4: Dig the trench along the marked contour line
After making your measurements, start digging the swale, creating mounds in between the ditches. Orient the ditches so that they run down the slope along the contour lines. For a large property, try renting an excavator to reduce the work for yourself. For smaller properties, you can do it by hand with simple tools.
When digging, dig at a downhill angle so that you simultaneously cut from the upslope and deposit the soil downslope onto the berm. If you encounter a layer of heavy soil that is not ideal for horticulture, add about 30 cm of good topsoil onto the swales to make them ideal for growing a wide range of crops.
Step 5: Adjust the sizing of the berms and trench
For a safe and stable berm, its thickness should be no less than four times its height. The average swale is anywhere from 18-24 in. wide. However, for most situations, the best swale width is around 16-18 in. The swales can be widened over time by further cutting into the upslope side, widening the trench and piling more topsoil on the berm. The trench can be as deep or as shallow as you want it, if you don’t plan to fill it with anything. However, you shouldn’t make it too deep, as it will make the berm unstable. A depth of 1 ft. is typical, although 6 in. to 1 ½ ft. deep are the standard ranges.
The width of the trench or basin is a product of swale spacing and the predicted annual rainfall. Excessive rains can cause the swale to overflow. Prepare for this by making your swale big enough to hold most of the rain it is likely to get during the heaviest possible storm. To prepare for possible floods or overflows, set a line to mark the fill-line, or the level of maximum capacity, to show where the water can rise up to before it spills out. This is called the spillway, and should be about 2/3 the height of the berm. To calculate the individual swales water holding capacity, take ½ of the width, times the depth of the trench, times the length of the overall swale to equal the total, approximate volume.
Space the berms out to accommodate the size of the trees you will plant. For large trees, the swale and berm should be approximately 10 ft. wide to have enough room for the width of their canopies. Separate the berms by about 4 m (or close to 13 ft.) to allow for a sidewalk or walking space to be laid down between them. You can space them closer together if you’re planting smaller trees.
Step 6: Test and adjust the swale
For swales to function correctly, they must be completely level. If they are not level, then the water within them will move through the system too quickly, eroding your swales. Once you have measured and dug the swale once, then scrape the bottom with a square-edged shovel to get it nice and flat. Work the A-frame along the bottom of the trench a second time to test whether it is level, and fix any uneven spots. You can also crack the bottom of the swale with a spading fork to aid in percolation.
Next, observe the swale during a heavy rain event to assess its performance. If it overflowed, you can either: make the swale deeper, wider, or longer; or, dig another swale 18 ft. or so below that one along the contour. To prevent rapid overflow of the berm during large watershed events, make a sill (or an opening, like a low-level, horizontal shelf, forming the lowest section of the supporting structure of the mound) in the berm by cutting out a low-level passageway. This will provide an escape for the water. The sill can be however deep or wide as you want it, but it should not be so large as to compromise the integrity of the ground around it, acting as a framework. After cutting out the sill, fill it with sandbags to plug it up when you don’t want the water to escape.
Step 7: Begin filling the swale
Try not to leave the ditch empty, as if unfilled, it is unsightly, may be hazardous, and makes your swale susceptible to erosion. One option is to fill the trench with gravel to create a walkable pathway to use in dry times. To do this, first, lay down some weed blocker fabric in the trench (on the bottom and along the sides), and then add a layer of gravel. Alternatively, you can give the berm a rock border to turn it into a raised bed.
If you don’t need a walkable path in the ditch, you might consider planting nitrogen fixing plants inside, instead, like clover seed. If not, then fill it with organic material to improve the soil conditions and moisture holding capacity for other plants, to be planted later. Lay down a layer of mulch (like slashed grass and leaves) on the bottom of the trench, and add some bulk material, like spongy rotten wood. Hydrate with a few buckets of water, and then add a final layer of straw on top to reduce evaporation. This will slowly decompose and make for a fertile layer of soil that constantly re-hydrates itself and stores water, making it great for water-loving and aquatic plants.
Step 8: Plant the berm
Plant the berm as soon as the swale is level and ready in order to keep unwanted plants from invading. Dig more swales until you’ve covered the necessary area. Try to make at least several swales, running adjacent to each other, so that runoff can’t get through to create a gully.
Plants to use on the berm
Plant deep-rooted perennials, herbs, and ground covering plants to stabilize the soil, and protect the mound from erosion. You can also use the swales as the basis for a food forest by planting downslope and upslope of the swale to rehydrate the land, making it suitable for tree growth. Place water-loving trees inside the ditches, on the bottom, at the lowest point. Place large trees at the top of the berm, with smaller dwarf-trees, shrubs, and bushes on the sides. You can also use a variety of nitrogen-fixing plants on the berm, interspersed throughout the other plants.
Put large trees (like fruit and nut trees) at the top of the berm, with asparagus, chamomile, chicory, dandelion, comfrey, Solomon’s seal, Good-King-Henry, jostaberry, chickweed, ramps, strawberries, blueberries, rhubarb, and thyme growing underneath, and on the sides. Make sure to include nitrogen fixers, pioneer species, nutrient accumulators, pollinators, and predator-insect attractors. More information about these can be found in the section on Guilds.
The ratio of nitrogen fixers to productive species to include on the swale depends on the climate, your personal needs, and your goals. If you plan to tend to the trees and fertilize them regularly, then put more productive species into your system. If you want less upkeep, then plant a higher rate of nitrogen fixers. If you are in a desert climate, or the tropics, then use lots of nitrogen fixers – as many as 15:1. However, if you are in a temperate climate with steady rainfall, you can get away with as little as 1:1. If you’re in a dry climate, plant a tree inside the ditch, and use heavy mulches and ground cover on the level bottom and the berm to store water for longer. The more organic matter you have, the more water will be stored for longer.
After establishing a permanent basin of water, you can put small fish into your swales in order to control algae and mosquitoes. Buy a bag of goldfish in a plastic bag, and let it float in the swale to allow the temperature of the water in the bag to acclimate to the water in the swale. After about an hour or so, release the fish in the water. They will eat the algae and mosquitoes in the water, and help control other pests, as well.