A swale is a permaculture term that probably gets thrown around a lot, often inaccurately, and at times implemented incorrectly.
There are a few reasons that we use swales in permaculture.. By putting them on contour lines and giving them level bottoms, we are able to tame water flows that might otherwise be damaging to the landscape. So we are preventing erosion, and we are capturing nutrients, silt, and organic matter. As that water is tamed, swales also spread it out over the landscape rather than having it concentrate only in specific areas, and they hold the water in place until it is able to soak into the soil.
Over the next few years, the landscape will become fully hydrated, which will make it drought-resistant, and the groundwater sources will begin to regenerate. Swales, like any catchment, are a means of stopping water runoff, especially from roads and hard surfaces, and putting it to use rather than having it drain away. Using swales for these reasons can prevent both floods and droughts, which make them a pretty amazing tool.
Water isn’t the only purpose of swales, and it is, in fact, imperative that they be regarded as a tree growing system. Swales without trees can possibly be even more damaging than the flow of water they have pacified. In some climates, they can potentially oversaturate the landscape, leaving a designer with difficult growing conditions. Trees, however, will moderate the saturation levels, utilizing the water deep into the soil as opposed to having it collect and cause problems. The other reason trees are so important to swale systems is that their roots stabilize the landscape, especially the berm, that loose pile of soil build on the downhill side of the swale, and the backside of the excavation. Swales only make sense when they are used to cultivate trees, as in a humid climate they would probably overcharge the ground with water and in a bare, arid climate the system would likely fill, erode, and evaporate very quickly.
While swales don’t have any particular size or measurements, there are certain rules that must be adhered to when building them. Swales—in permaculture terminology—are built on contour, which means they run level across the landscape. Even more importantly, they must be excavated to have level bottoms so that the water rests evenly within in them, soaking throughout the terrain rather than congregating in a particular area. The berm is on the downhill side of the swale and should be planted with both trees and groundcovers so that the soil is stabilized. The swale also must have a level spillway so that, in times when water is overabundant, it can release safely and passively in an appropriate location without damaging the berm.Where we place swales is also significant, and it’s important to remember their relationship with water when doing so. Swales are not appropriate on steep landscapes. Any area with more than a fifteen-degree slope (about 1:3.75) isn’t appropriate for installing swales, as the water saturation may cause slides, which could be dangerous. However, like dams, the higher we can get swales in our landscape the more effective they’ll be, as the water will plume beneath them, continuing to use force of gravity to permeate the subsoil at a right angle down from contour (i.e. water can’t move uphill without a pump). Lastly, swales should have trees downslope to regulate the water and stabilize the soil, which means putting a swale just uphill of a building (and its foundation), with nothing between them, probably isn’t a great idea.
Lastly, swales are used to grow trees. Even in the case of using them to extend catchments for dams or to provide an overflow system, trees should still be planted, especially on the downhill side of the swale, and it is advisable to plant more just along the uphill edge or backside (nitrogen-fixers are great for this). When used for rehydrating and reforesting, swales can more or less be left to naturally fill with sediment and organic matter as the forest grows. By the time they are full, the forest should be fully hydrated and regulating its own water needs, and the soil will have plenty of detritus from leaf-fall to prevent it from eroding and to help it with soaking up moisture. Without the trees, the swale is missing and integral part and function, not to mention that trees are equally as important to maintaining water cycles.