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Safe Outlets

4 min read

Parent
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Drainage Ditches
Channeling surface water away
Current
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Safe Outlets
Where to send diverted water

A safe outlet is a place that can take the whole flow of a ditch without eroding, flooding a neighbour or fouling a well: a stable stream or river, or an infiltration area large enough to soak the storm runoff. Every ditch must reach one. An infiltration outlet in slow soil can need about 208 square metres for a 1.5 hectare settlement, so check the soil before choosing it and fall back to a stream outlet if the area is not there.

Part of Flood Control and Drainage

An outlet is the downstream end of a drainage system, the place where collected water finally leaves. A ditch is only as safe as the outlet it ends in.

A Ditch With No Safe Outlet Moves The Flood, It Does Not Stop It

Water collected by ditches does not disappear. It must go somewhere, and where it goes decides whether the system works. The parent article lists the options: a stream, a river or an infiltration area, and it insists that every ditch reach a safe discharge point.

What Makes An Outlet Safe

  • Stable — it will not erode or cut back under the added flow.
  • Big enough — it has capacity to take the flow without overtopping.
  • Away from people — the water does not flood a neighbour’s land or a building downstream. Discharging drainage into a neighbour’s area causes conflict and downstream flooding.
  • Away from wells and latrines — drainage water carries contamination and can pollute a well. Keep outlets well clear of water supplies.
  • Protected — the point where the ditch meets the stream is rock-lined, because the water arrives fast. See Inlet and Outlet Protection.

Method: Choose And Build An Outlet

Step 1: Look for a stream or river first. A natural waterway with a stable bed is the simplest outlet. Enter it at a downstream angle, not square on, set the ditch bed above normal water level so the stream does not back up the ditch, and line the entry with rock.

Step 2: Otherwise plan infiltration. Choose flat, open ground downhill of the settlement with soil that drains. Test it as in Soil Drainage Capacity.

Step 3: Size the infiltration area. Use the arithmetic below. If the area needed is more than you can spare, use a stream outlet or reduce the flow with swales upstream.

Step 4: Spread the flow. Let the water leave the ditch across a wide, level, grassed strip rather than from one point. Concentrated flow cuts a gully.

Step 5: Check after the first storms. Look for cutting, ponding or flow toward buildings, and fix it before the next.

Worked Example: How Big An Infiltration Outlet Needs To Be

Assumptions (all stated): settlement area 1.5 ha = 15,000 m2; runoff coefficient 0.5; a design storm of 50 mm over the whole area; the water allowed 24 hours to soak in.

  • Runoff volume: 15,000 m2 x 0.5 x 0.05 m = 375 m3.
  • Slow soil, infiltrating at the 75 mm/h boundary: in 24 h the ground absorbs 0.075 m/h x 24 h = 1.8 m of water depth. Area needed = 375 / 1.8 = 208 m2.
  • Fast soil, at 600 mm/h: 0.6 x 24 = 14.4 m of depth. Area needed = 375 / 14.4 = 26 m2.

The area needed in slow soil is eight times the area in fast soil (208 / 26). These rates come from the soil test in Soil Drainage Capacity, and the storm and runoff coefficient are assumptions you should replace with local values. Where the soil is slow and the land is small, use a stream outlet.

Common Mistakes

  • Ending a ditch on bare ground — the flow cuts a gully and the ditch has no true outlet.
  • Pointing the outlet at a neighbour — this causes flooding downstream and conflict.
  • Putting the outlet near a well — runoff carries contamination into the supply. Keep it far downhill and clear.
  • Infiltration in clay — the area needed is large, and the water ponds. Use a stream.
  • Entering a stream square on — the stream bank scours. Angle the entry downstream and line it.

Safety

Outlet banks and fast water

Where a ditch discharges into a stream the water arrives fast, the bank is often undercut and the edge can give way under a person’s weight. About 15 cm of fast-flowing water can knock an adult off their feet. Inspect from the bank, keep back from the edge, and never wade in a stream carrying storm flow.

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