Swale

What is a swale?

A swale is a ditch that goes nowhere. It is dug across a slope exactly along the contour, so that its floor is level from one end to the other, and the excavated soil is heaped into a low bank, the berm, on its downhill side. Rain running down the slope reaches the swale and stops. With no fall to follow, the water spreads along the whole length and has time to sink in. The swale is not an open reservoir but a way of filling the one beneath it: the soil of the slope below.

The word causes confusion. In North American stormwater engineering a swale, or bioswale, is a planted channel with a gentle gradient that carries runoff away. The permaculture swale, as Bill Mollison described it and Brad Lancaster documented it for drylands, is level and holds water instead. That kind is meant here; the wider practice is described under rainwater harvesting.

Laying it out on contour

Everything depends on the line. A swale that is off by a few centimetres gathers its water at the lowest point and often breaks through the berm there, which can do more damage than no swale at all. So the contour is marked before anyone digs.

Two simple tools are enough. The A-frame is three battens joined like the letter A, with a plumb line hanging from the apex. To calibrate it, it is set on two points, the position of the string is marked on the crossbar, the frame is turned round on the same two points and marked again: halfway between the two marks is level. Then it is walked across the slope, one leg fixed while the other swings until the string hangs on the mark, with a stake at each step. The water level, a long transparent hose filled with water between two marked poles, is more precise and reaches further and around obstacles. Water stands at the same height at both ends, so the second pole moves along the slope until the water reaches the same mark. The hose must be free of air bubbles.

The line of stakes is smoothed so the swale has no sharp bends, the ditch is dug, and the soil goes to the downhill side, firmed in layers so the bank does not slump when wet.

Spillway and planting

Every swale will one day receive more water than it can hold, so it needs a planned overflow: a spillway at one end, slightly lower than the top of the berm, on undisturbed ground and armoured with stones or a dense sward, so the surplus leaves at a chosen place. From there it can run to the next swale, into a pond or onto stable, planted ground. Without a spillway the water chooses its own way out, and that is usually through the berm.

The berm is the planting bed. Moisture soaking in beneath the swale spreads downhill under the bank, within reach of trees and shrubs planted on it or just below. Bare soil on a fresh berm washes away in the first heavy rain, so it is sown or covered at once; nitrogen-fixing plants and a layer of mulch protect it and feed the young trees. In time roots hold the bank, and their shade keeps the soil cooler.

Field trials point to a real but limited effect. On degraded hillsides in the Mediterranean climate of central Chile, contour ridges and infiltration trenches held more water at 20 to 60 centimetres depth than untreated ground, above all in wetter years, and young trees, cork oak among them, grew better. Survival depended more on the species than on the treatment. A swale helps young plantings establish; it guarantees nothing.

When a swale does more harm than good

A swale puts water into a slope on purpose, and that is exactly what makes it risky where the slope cannot bear it. On steep ground the soaked soil grows heavier, and where a thin soil lies on rock or dense clay, water gathering at that boundary can let the whole layer slide. The limits practitioners name for steepness differ widely, and there is no recognised engineering threshold. On steep or unstable ground other means are safer: dense hedges along the contour such as vetiver, small catchments around single trees such as the half-moon, mulch and plant cover.

Swales also fail on soils that barely let water through. On heavy, waterlogged clay the water stands in the ditch for days, roots suffocate and mosquitoes breed. A simple test pit filled with water shows it: if the water is still there the next day, the soil first needs organic matter and roots, not more water. Nor should a swale lie above buildings, roads or cellars, where water soaked in uphill can resurface lower down.

Swale, ditch, terrace and keyline

A drainage ditch has a fall and leads water away; a swale is level and keeps it. A terrace turns the slope itself into level steps held by walls or banks; a swale is only a line in the slope, and the ground between two swales keeps its gradient. Keyline design, developed by P. A. Yeomans in Australia, uses lines set slightly off contour to guide water from the moist valleys towards the drier ridges. Keyline moves water across a slope, the swale holds it in place, and on larger land the two are often combined.