A swale is a water harvesting earthworks ditch with a raised lip usually dug on contour, with dug soil being piled towards the downhill lip of the trench, to slow, capture and help water sink into the ground. Depending on terrain, size of operation or irrigation requirements, swales may be kept shallow, deep, or wide. Swales are usually placed with enough of a gap between them, to allow water to adequately penetrate the soil, before the next trench is dug, which also allows for agricultural activities between swales.
Swales perform a number of functions, the most important if which is forming a physical barrier which prevents water from accelerating downhill and eroding topsoil. Water once held back slowly sinks into the soil and trickles away from the swale in a managed manner. The downhill lip of the swale provides a suitable raised mound on which to plant trees, shrubs and cover crops. By preventing water from running downhill unchecked, a swale also traps sediment and bio-rich material which forms a nutrient rich decomposition mix, which feeds the local flora and the macrobiotic subsoil life.
In the 1930s, great dust storms were experienced in the great plains, caused by extensive plowing of the vast grasslands (watch: The Great Dust Bowl). The condition caused topsoil to be rapidly eroded, depleting soil health which resulted in sub-par agricultural produce. Clubbed with a severe drought during the period, the dust caused farmers extreme hardship with families going hungry and the constant dust causing severe respiratory issues. A large-scale solution was required, and this came in the form of the Soil Conservation Service embarking on a massive watershed management program, spearheaded with establishing swales covering vast swathes of land and implementing native plant replanting.
Types of swales
- Standard swales are usually shallow and broad, dug into mildly sloping terrain. These are usually unlined and allow water to seep into the soil at a slow pace.
- Enhanced swales are lined with reeds or other plants planted into a bed along one or both inside lips of the trench, which rapidly filter water to remove pollutants which may have been swept along with water while flowing over the watershed area.
- Wet swales are dug and lined with a semipermeable or impermeable layer, over which a filtration bed is placed, onto which reeds and filtering plants are established. These swales are placed where runoff might contain pollutants and are meant as both a barricade as well as long-term water treatment installations.
Installing a swale
While a swale may look simple, there’s a little science that goes into getting them right. The starting point is studying the land and the topography to understand how water flows over the landscape. This allows measurements to be made and the marking of keylines and keypoints. Once this has been carried out, a section of the catchment area is identifiable, and a corresponding downhill keyline on which a swale can be dug can be identified and marked. This is followed by studying the area’s rainfall, and undertaking soil sample studies, to best estimate the width and depth of the trench to be dug; and whether it would require a liner or packed clay (if at all). Digging a swale can be accomplished manually or using heavy mechanical equipment, depending on the scale of the project.
Depending on the topography, some swales may allow or require a tank, pond, dam or bund to be constructed at a keypoint along the keyline. To allow for water to evacuate the swale once it has reached capacity, without causing an erosion triggered breach, a hardened spillway needs to be accommodated. Special elements such as crescent shaped water holding areas and donut ring water basins may also be considered while planning, based on the flora or agriculture plans for the property.
An interesting video on the long-term effects of using swales and associated earthworks to harvest water and rejuvenate the landscape is the story of how a farmer spearheaded an initiative to utilize natural methods to make farming drought-proof.