Place swales by watching where water runs in a real rain, marking the contour with an A-frame level, and spacing the swales so that each one can hold the runoff from the strip of ground above it, which is the runoff coefficient times the storm depth in millimetres times the strip width in metres, in litres per metre of swale. Build every swale level, keep it clear of buildings and wells, and give it an overflow so a storm larger than the design cannot breach it. The carrier guide spaces swales 15-30 feet apart on gentle slopes and closer on steep ones; the volume check shows what storm that spacing can actually hold.
A swale is a level, shallow trench dug along a contour that catches runoff and lets it soak in, and its placement decides whether a hillside planting is watered or washed out.
A Swale Holds A Fixed Volume, So Its Spacing Must Match The Storm
The carrier guide (Food Forest & Permaculture Design) says to dig swales uphill of tree rows, put the spoil on the downhill side as a berm, plant on the berm, connect overflow between swales, and space them 15-30 feet apart on gentle slopes and closer on steep slopes. It gives no reason for those distances. The arithmetic below supplies one.
Worked Example: How Far Apart Can Swales Be?
1 mm of rain on 1 m² is 1 litre. If a strip of ground d metres wide (measured down the slope) drains into each metre of swale, a storm of depth P millimetres on soil with runoff coefficient c delivers V = c × P × d litres per metre of swale.
Inputs (illustrative; take P from your rainfall record and c from the soil and ground cover):
- Swale section: V-shaped, 0.5 m wide and 0.3 m deep = 0.5 × 0.3 ÷ 2 = 0.075 m² = 75 litres per metre (the carrier’s range is 12-18 in deep and 18-24 in wide).
- Runoff coefficient c = 0.5; storm P = 40 mm.
Strip width the swale can hold: d = 75 ÷ (0.5 × 40) = 3.75 m (about 12 ft). With the carrier’s 15 ft (4.6 m) spacing, the swale fills at P = 75 ÷ (0.5 × 4.6) = 33 mm; at 30 ft (9.1 m), at P = 75 ÷ (0.5 × 9.1) = 16 mm. Rain during the storm also soaks in and adds capacity, so these are conservative, but a heavier storm than the swale holds will overflow it. What a heavier storm does is decided by the overflow, not by the swale.
Method
Step 1: Watch a real storm. Walk the site in heavy rain and note where water runs, collects and cuts channels. Mark each on the map (Flood Control and Drainage).
Step 2: Mark the contour. Use an A-frame level (A-Frame Level) and set pegs at equal height along the slope. Dig to the pegs, not to the ground.
Step 3: Test the soil’s infiltration. Fill a hole about 30 cm deep with water and time how fast it empties. If water stands for days, a swale will hold stagnant water and breed mosquitoes.
Step 4: Space by volume. Use the arithmetic above with your storm and soil.
Step 5: Plan the overflow. Cut a lower spillway at the end of each swale, lined with stone, leading to the next swale or a stable channel.
Step 6: Keep the slope in check. Steeper ground needs closer, shorter swales and a plan for the failure of one; see Swale Design.
Common Mistakes
- A swale off the contour — water runs to the low end and breaches.
- Putting a swale on top of a gully or an old landslide — saturated ground on unstable slopes can slide.
- Planting trees in the swale bottom — they sit in standing water. Plant on the berm.
Safety
Trench faces collapse and a swale breach releases a flood
Keep swales shallow. Do not enter an unsupported trench deeper than 1.2 m; at that depth a trench face can collapse faster than a person can climb out. Never dig alone, and keep the upslope face stable. A breach on a hillside releases the stored water onto the swale below, so keep buildings, wells and septic fields out of the flow line.
What’s Next
- Learn next: Swales and Berms on Contour — building and planting the swale
- Combine with: Access Paths and Harvest Zones — paths must not become water channels