Get the Offline ArchiveFree lifetime updates

Swale Design

5 min read

Parent
🌺
Rain Gardens and Swales
Absorbing runoff naturally
Current
🌊
Swale Design
Shallow channels that infiltrate

Design a swale by choosing its length from the runoff it must hold and the rate your soil absorbs. A swale 30 cm deep and 1.5 m wide holds between 0.225 and 0.30 cubic metres of water for each metre of length depending on its shape, so 100 m holds roughly 22.5-30 cubic metres. Keep the bottom level along the contour within a few centimetres, build the downhill side up with the spoil, and give each end an overflow so a storm larger than the design spills safely.

Part of Flood Control and Drainage

A swale is a shallow, level channel along a contour that holds runoff and lets it soak into the ground, rather than carrying it away.

A Swale Is A Long, Narrow Basin, Not A Ditch

A ditch has a slope and a destination. A swale has neither: it is level and its destination is the ground beneath it. That changes the design questions. How much water must it store? How fast does the soil take that water in? How accurately must the bottom be level? What happens when the storm is bigger than expected?

The parent article gives the starting dimensions: 30 cm deep and 1-2 m wide, along a contour line, planted with deep-rooted plants.

What You Need

Method: Design And Build A Swale

Step 1: Find the volume to store. Runoff volume = catchment area x rain depth x runoff coefficient.

Step 2: Find the storage per metre. Cross-section area x 1 m. A triangular section is 1/2 x width x depth. A rounded, bowl-shaped section is about 2/3 x width x depth.

Step 3: Divide to get the length. Length = runoff volume / storage per metre.

Step 4: Check the drain-down time. Time to empty = water depth / absorption rate. It should be short enough that plants survive and mosquitoes cannot breed.

Step 5: Peg the contour. Walk the level along the slope, setting pegs at the same height. Dig to the pegs, not to the ground surface.

Step 6: Build the berm. Use the spoil to build up the downhill side, and compact it. Lightly mulch or seed it.

Step 7: Add overflows. Cut a lower, rock-lined section at each end, or at a chosen point, so excess runs over stone rather than eroding the berm.

Worked Example: Length, Level And Drain-Down

Length. A roof of 100 m2 receives a 50 mm storm and sheds 90 percent of it. The swale is 1.5 m wide at the top and 0.3 m deep.

  • Runoff: 100 m2 x 0.05 m x 0.9 = 4.5 m3.
  • Storage per metre, triangular: 0.5 x 1.5 x 0.3 = 0.225 m3 per metre. Rounded: 0.667 x 1.5 x 0.3 = 0.30 m3 per metre.
  • Length: 4.5 / 0.225 = 20 m (triangular), or 4.5 / 0.30 = 15 m (rounded).
  • A 100 m swale of the same section holds 0.225 x 100 = 22.5 m3 (triangular) to 0.30 x 100 = 30 m3 (rounded).

Level tolerance. A swale 100 m long, 0.3 m deep, built 0.15 m out of level from end to end (a fall of 0.15 / 100 = 0.15 percent) has water at full depth at the low end and none at the high end. Half the depth is used up by the error. Level to within a few centimetres over such a length, or make shorter sections.

Drain-down. Standing water 0.3 m deep:

  • In soil absorbing 75 mm/h (the slow boundary): 300 / 75 = 4 h.
  • In soil absorbing 8.3 mm/h: 300 / 8.3 = 36 h.
  • In soil absorbing 600 mm/h: 300 / 600 = 0.5 h.

Mosquitoes take 7 to 12 days from egg to biting adult (see Mosquito Prevention), so a swale that empties within a day or two is safe for that, but a clay site that holds water for a week is not.

Common Mistakes

  • Digging to the ground surface, not to the contour pegs — the bottom follows every undulation and pools.
  • No berm — the spoil left at the edge slumps back. Build and compact it.
  • No overflow — a bigger storm cuts through the berm and the swale becomes a gully.
  • Swales on a steep slope without a plan — one breach releases the stored water onto the swale below. Keep them modest in length and use overflows.
  • Planting only shallow-rooted grass — deep roots open the soil and keep the infiltration rate up.

Safety

Steep ground and trench edges

Digging along a hillside leaves an undercut upslope face that can slump onto the digger. Keep the upslope face stable and shallow, and do not work alone. A breached swale on a slope can release a sudden rush of water downhill, so keep paths and buildings out of its flow line.

What’s Next