What Are Swales? (And How to Make Them)

If you are familiar with permaculture, then you may well have heard the term ‘swales‘. Permaculture is a blueprint for sustainable design, in food production and in societal systems, which puts people care, planet care and fair share front and centre. 

When we garden with a permaculture approach, we garden for ‘permanent agriculture’ and, more broadly, ‘permanent culture’ – working not only to improve our own lives but also to work with the natural world around us, and do our part for future generations. Permaculture swales can sometimes be a part of that picture. 

What Are Swales?

Permaculture swales, or, more accurately, on-contour swales, are a type of earthwork sometimes carried out in order to manage the flow of water on a slope within a garden, or in a wider landscape. They are ditches, or trenches, that are dug into a gentle slope in order to catch water, and slow its progress downhill. 

When we say ‘on contour’ we mean that the swales are dug along level lines in the landscape. They are flat, and don’t slope up or down but rather follow lines of the same elevation across the terrain. 

Swales are used to manage water on a property, which can be beneficial where water is scarce, but can also sometimes be beneficial where excess water might cause runoff and erosion, or cause flooding issues at the base of a slope. 

When water pools in a swale, it will slowly filter down through the soil, and be taken up by plant roots nearby.

A swale and berm system is the approach usually employed in permaculture. This involves the swale itself, and a raised mound, or berm, on the downhill side, on and below which trees, shrubs or other plants are placed to take advantage of the water that collects within the swale and filters down through the soil downhill of it. 

Why Are Swales Good?

As mentioned above, on-contour swales are used as a way to keep water within the landscape, and can be particularly beneficial where there are dry periods or periods of drought. They can allow for the establishment of rich and biodiverse planting schemes where establishment can be a challenge. 

However, even in areas with higher rainfall, swales can sometimes be beneficial because they can make sure that water does not runoff, causing erosion and flooding issues downstream. 

Whether or not water is in short supply, it makes sense to do all we can to reduce our water use in our homes and gardens. Swales are one permaculture tool we can use to manage water more wisely on our properties. 

Swales catch rainwater, and groundwater flowing downhill through the soil on a site. Along with attendant planting, these earthworks or water features should slowly fill then release water into the soil evenly along the downhill side, improving the growing conditions for plants and ensuring a healthy soil environment. 

By making sure that water filtrates more evenly through the ground on a site, we can prevent that water from draining off along the surface onto streets, or municipal drainage systems. 

When water flows over the surface of the land, it can degrade the soil and also pick up contaminants and pollution which can end up in watercourses and the seas and oceans. 

When we use swales and other features which allow water to be filtered slowly through soil and plants, we essentially allow that water to be ‘cleaned’ naturally before it replenishes aquifers underground or flows down to the sea to perpetuate the water cycle. 

Where To Use On-Contour Swales

On contour swales can be used on any gentle slopes. Larger swales are used on larger properties to manage water flow on a larger scale. While smaller swales can be useful in even much smaller gardens. 

Swales can be particularly beneficial on a gentle sun-baked slope, or on a shaded slope where runoff has been identified as an issue. 

They can often be useful in creating areas for perennial food cultivation – such as forest gardens, for example. But can also be useful in a range of ways in holistic permaculture design which takes into account the conditions of a particular site. 

When Are Swales Not The Solution?

However, while swales can be a very useful solution in some cases, they are not always the right option in UK gardens. It is important to understand the site you are dealing with and to find the right solutions for the right places. 

What works well in one slightly sloping garden may not work so well in a garden with only very slight differences. Holistic planning and design is key to determining where swales should and should not be used. 

Unfortunately, permaculture approaches are often misunderstood. Permaculture solutions are not intended as ‘one-size-fits-all’ solutions. Rather, permaculture is about carefully analysing a site and determining the best planting and strategies for place. 

Swales can be a great solution for many gently sloping sites. But where the slope is more intense – more than 15% – different approaches (like terracing) must be employed. 

Where swales are used on too steep a gradient, they can be detrimental and even dangerous as they could cause land to slip due to the water infiltration. 

Swales may also cause problems where the soil is particularly wet and soggy. An extremely waterlogged soil won’t need more water to be kept around. And swales are not the solution where the water table is high. 

Indeed, the right strategy might be to gently divert water elsewhere using different earthworks – like diversion ditches or French drains for example. 

When deciding whether or not swales would be beneficial on a given site, terrain, topography, soil, rainfall levels and a range of other environmental factors need to be taken into account. 

How To Design A Swale/ Swale System

Building a swale or swale system always begins with observation of the site, and design. Leaping beyond this step to creation of a swale can lead to a range of mistakes and issues. 

Before you build a swale:

• Look at the slope – its gradient, orientation and composition.
• Think about water flow on the site as it currently exists, and the problem or problems that the swale or swales is/are designed to tackle. 
• Determine the amount of rainfall on a site, and the volume of water that a swale might be expected to tackle. This is vital in sizing swales successfully. 
• Think about any earthworks (including swales) in concert with planting – think about the planting scheme that you ultimately want to create. 
• Consider other design elements like pathways and access, to make sure swales work holistically as part of an overall design. 
• Make sure you understand where water will go from a swale if these overfill – there should always be an outlet for excess water included in any scheme. The water might spill over into rain garden areas, or a garden pond, for example. 
• Swales should typically be designed to catch water across as much of a slope as possible. 
• Swales can vary dramatically in size, but are usually around 3 times as wide as they are deep. 
• Place swales 4m or so away from any building, making sure that water drains away from and not towards any structures. 
• Make sure that swales don’t interfere with any septic systems or drainage fields. 

How to Build a Permaculture Swale

Once you are certain that you have done the groundwork and determined the best place for a swale or extended swale system:

• Use an A-frame to determine the contours on your property. Remember a swale runs on contour – with all points along the ditch at the same elevation above sea level. 
• Mark the contour line accurately. 
• Dig the trench along the marked contour line. The trench should be level on the base, with gentle slopes to each side. Remember, swales are typically three times as wide as they are deep. For example, if the ditch you make is 15cm deep, it will be around 45cm wide. If it is 50cm deep, it will be 150cm wide… Remember to size the swale according to the water and needs on the particular property. 
• Pile the soil excavated from the swale to form the berm on the downhill side.
• Create spillways that will take excess water from one swale to another, or to an overflow point such as a rain garden or wildlife pond, for example. 
• Check that the swale in on contour and observe it during a rain event to see how it performs. Make any adjustments required. 
• Add layers of organic matter to the swale which will aid in water filtration. 
• Plant up the berm with a combination of shrubs and trees, and perennial ground-cover plants. There should be a good mix of deep and shallow rooted species to help keep soil in place. 

Creating swales on your property does require some careful thought. But get it right and you will certainly be rewarded with a more resilient, sustainable and water wise garden that can look great, and provide for many of your needs. 

You can use swales or other permaculture approaches to manage water wisely on your site, and create systems that provide for local wildlife, promoting biodiversity and preventing potential harm to ecosystems in your area.

References:

https://www.tenthacrefarm.com/how-to-build-swale/

https://livingpermaculturepnw.com/what-is-a-swale-an-introduction-to-permaculture-water-harvesting/