Swales Are Very Useful Gardening Features & Contour Swales Are The Ultimate Swale Type. But What Are Contour Swales & How Do You Build Them Level?
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Contour swales are a landscaping feature that offer a multitude of benefits in the UK where rain is plentiful and flooding becoming more common. for your UK garden.
What Are Contour Swales?
A swale is a low place that can gather and retain water. It can be natural or it can be manmade. When manmade it is more often a shallow channel with gently sloping sides designed to gather or capture water so that it is able to percolate the soil and perhaps prevent flooding. Some swales allow water to gently flow through them to a lower point.
A contour swale is all the above PLUS it is constructed to follow the contours of the land. Hence the water does not flow but sits in the swale until it percolates or evaporates. This means it has more opportunity to penetrate the soil and recharge the aquifer below.
The image on this page is an AI generated image of a contour swale. It is not perfect but hopefully provides enough visual clues to what I’m talking about. I would prefer to see the sides being a gentler slope and the berm being wider and capable of being paved. however the berm could well be planted with perennial plants and become part of the garden
How to Ensure The Contour Swale Is Level.
The trick with contour swales is ensuring they follow the contour. This can be done by careful surveying of the site with equally careful excavation of the contour swale. Various methods can be used to survey the site and ensure the swale is level. Fortunately it doesn’t need expensive equipment. Just an understanding of basic everyday physics.
Because water finds its own level we can use a water filled hose to create a U shape to find levels. Filled with water the level of water at each end is always going to be the same. So if soil level at one end is, say, a metre below the level in the hose then whatever level is a metre below the water level t the other end is exactly the same level.
Using a transparent hose obviously makes this easier. As does using a pole marked in metric or imperial measures at each end. The measures allow us to see how far off level the location is.
Another basic physics method uses gravity to find levels. A heavy pendulum will always hang in a vertical position. |If that pendulum is located within an A frame with the two sides being of equal length it will always hang exactly below the (central) hanging point. Therefore, if the pendulum is off centre it means the legs of the A frame are at different levels.
One you get the hang of it, it becomes quick and easy to run an A frame across a slope and find all the points of a corresponding level to your starting or datum point.
How to Dig a Contour Swale
It really isn’t such an arduous task.
Once have found our levels we need to mark them with a rope, hosepipe, some sand or whatever you find easiest.
Then dig a shallow trench about a metre/yard wide across the contour line.
Place all the spoil downhill of the trench to form a “berm”.
Ensure the slopes in and out of the “trench” are gentle.
Finally ensure the top of the “berm” is level so that the water doesn’t overspill at a low point. Think of it as if it were a weir. If it overflows it should do so evenly across its whole width so that nowhere has a stream or river cutting through it!
A level top to your swale can be grassed or paved and used as a path.
Why You Should Consider Contour Swales
* Tames the Downpour: Capture and retain rainwater, reducing runoff and preventing soil erosion – a common woe during heavy UK “showers”. * Flood Reduction: Swales slow down rainwater flow, reducing the risk of flash floods and protecting your property, and properties downhill of you, from damage. * Soil Saver: Maintains healthy soil structure and fertility by reducing erosion and promoting better water infiltration. * Recharges Aquifers: By retaining rainwater, contour swales help replenish underground aquifers, ensuring a reliable water source during dry spells. * Aesthetics with a Purpose: Enhances the beauty of your garden while providing valuable habitat for wildlife and plants. * Energy Efficient: Reduces the need for irrigation, saving you energy and money.
Contour Swales: The Downsides
It makes a lot of sense we look at the downsides as well as all the positives.
Effort: It takes time and effort to survey and create swales. I would however argue that so does any things we do in the garden. From installing a path or building a greenhouse effort is needed. In this case, if well maintained, a swale can last a century or more. Not many greenhouses will last that long.
Divides Plots: Where a contour swale cuts diagonally across a small rectangular plot and cuts it into two triangles it does cause me to take a deep breath and ask myself if it is worthwhile. Much depends on the size of the plot, how prone it is to erosion and how much water it could retain. The effort needs putting into proportion. What we are talking about is digging a shallow trench and heaping the spot to one side. It’s no more than we do if we winter dig a plot, the only difference being that the spoil is placed downhill and couple of feet away. That’s no big deal is it?
