A swale is a level trench with its spoil piled on the downhill side as a berm, and it holds a fixed volume per metre of length, so it captures all the rain only from storms small enough to fit in it. With a V-shaped swale 0.5 m wide and 0.3 m deep spaced 10 m apart, the capacity is 7.5 mm of rain over the whole slope, so a storm that sheds more runoff than that overflows. Build every swale level, plant the berm and not the trench, give it a stable overflow, and keep it uphill of nothing you cannot afford to flood.
Swales and berms on contour are paired earthworks, a level shallow trench and an adjacent mound, dug along a hillside to hold rain and let it soak into the ground.
Capacity, Not The Claim Of Full Capture, Decides What A Swale Does
The carrier guide (Drought Resilience & Water-Efficient Agriculture) describes shallow trenches 12-18 inches deep and 18-24 inches wide on contour, spoil mounded downhill as a berm, trees and perennials planted on the berm, and says that on a gentle slope properly spaced swales can capture 100% of rainfall on site. That last claim holds only for rain that the swale and the soil can absorb.
Worked Example: What A Swale System Can Hold
Inputs: V-shaped section 0.5 m wide × 0.3 m deep = 0.5 × 0.3 ÷ 2 = 0.075 m² = 75 L per metre of length. Swales 10 m apart, so each metre of swale serves 10 m² of ground.
- Stored per m² of slope: 75 ÷ 10 = 7.5 L/m² = 7.5 mm of rain.
- Storm of 40 mm with runoff coefficient 0.5 (an illustrative storm and coefficient): runoff = 0.5 × 40 = 20 mm, which is 20 L/m².
- The swale holds 7.5 of the 20, so the rest overflows unless it soaks in during the storm.
Capture of 100% therefore holds for storms with runoff up to 7.5 mm over the slope, plus whatever the soil absorbs while the water stands. Closer spacing, deeper swales or larger basins hold more. The berm adds a little. The soil must also drain: water that stands for days does not help and breeds mosquitoes.
What You Need
- A level (an A-frame level; A-Frame Level).
- Pegs, string, shovels.
- Stone or sod for overflow protection.
- Plants for the berm.
Method
Step 1: Mark the contour with the level and set pegs at equal height (Water Flow and Swale Placement covers spacing).
Step 2: Dig to the pegs. Keep the trench shallow; spoil goes downhill to form the berm.
Step 3: Compact and shape the berm, and cover it.
Step 4: Cut an overflow at the low end of each swale, lined with stone, leading to a stable channel or the next swale.
Step 5: Plant the berm with the perennials, trees and ground cover. Mulch the swale bottom.
Step 6: Inspect after every heavy rain for slumping, silt and breaches, and repair them.
Diagnosing Problems
- Water runs to one end — the swale is off contour.
- Water stands for days — soil drains too slowly, or the swale silted up.
- Berm slumping — too steep or too loose; flatten and plant.
- A swale breach — the swale above overflowed into it; strengthen overflows.
Common Mistakes
- Swales on steep, unstable ground — saturation can start a slide.
- No overflow.
- Planting in the trench — roots sit in standing water.
Safety
Trench collapse and breaches
Keep swales shallow, far less than 1.2 m, since unsupported trench walls deeper than 1.2 m can collapse onto a person. Do not dig alone and keep the upslope face stable. A breach on a slope sends the stored water rushing to the next swale or a building below, so keep structures, wells and septic fields out of the flow path.
What’s Next
- Learn next: Rainwater Collection and Storage — storing what soil cannot
- Combine with: Swale Design — volume and drainage design