The outlet pipe of a spring box is 32-50 mm and set 10-15 cm above the floor so that silt stays behind, and the overflow pipe is the same size or larger, set at the level of the spring eye and sized to carry the highest flow ever measured, because a blocked or undersized overflow backs water up over the eye. The overflow must never have a valve on it, the drain at the bottom gets a valve that is kept closed, and every pipe that passes through a wall is sealed so that water cannot run along the outside of it.
Part of Spring Capture
The outlet takes the box’s water to the supply pipe, the overflow takes the surplus away, and the drain lets the box be emptied and scoured. Together they are the only openings in the front wall.
Three Pipes, Three Heights, One Job Each
| Pipe | Height | Size | Job |
|---|---|---|---|
| Outlet | 10-15 cm above the floor | 32-50 mm | Supplies the community line; the sump below keeps silt out of it |
| Overflow | At the spring eye level (the Spring Capture guide sets it 5 cm below the top of the wall; see Box Design for matching the two) | Same as the outlet or larger | Carries the surplus away |
| Drain | At the very bottom | Same bore, with a valve | Empties the box for cleaning |
What the sump holds. The 10-15 cm below the outlet in a 1 m by 1 m box is 100 to 150 L of space where silt can collect. Inspect and flush it at the monthly visit, long before it reaches the outlet.
Size The Overflow For The Biggest Flow You Have Recorded
The overflow has to take everything the spring delivers that the outlet does not. If it is too small, the water level climbs above the eye and the flow falls, or the walls leak. The Spring Capture guide’s rule is overflow equal to or larger than the outlet. Check it against the highest reading in the log (Flow Records).
For a pipe flowing at velocity v, the bore needed for a flow Q is d = √(4 × Q ÷ (π × v)).
Worked example. The highest flow recorded is 20 L/min = 20 ÷ 60,000 = 3.33×10⁻⁴ m³/s. Choose a conservative velocity of 0.5 m/s, since the overflow runs by gravity with little head:
d = √(4 × 3.33×10⁻⁴ ÷ (π × 0.5)) = √(8.5×10⁻⁴) = 0.029 m = 29 mm
A 32 mm overflow passes this: its area is π ÷ 4 × 0.032² = 8.0×10⁻⁴ m², which carries 8.0×10⁻⁴ × 0.5 = 4.0×10⁻⁴ m³/s = 24 L/min at 0.5 m/s, and 48 L/min at 1 m/s. If your records show a wet-season peak of several times the dry flow, use a larger pipe. A 50 mm pipe carries 59 L/min at 0.5 m/s. The spring’s outlet flow at the same moment is smaller: a 10 L/min outlet flow in a 32 mm pipe runs at only 10 ÷ 60,000 ÷ 8.0×10⁻⁴ = 0.21 m/s.
Keep Contamination And Backflow Out Of The Pipes
- No valve on the overflow. A closed overflow is a blocked overflow: the box fills over the eye and pressurises. The Spring Capture guide makes the same point: always an overflow equal to or larger than the outlet.
- Valve on the drain, normally closed and locked; after flushing, close it, or the box empties.
- Open air end. The overflow and drain must discharge into the open, above the water in the ditch, with a gap and an insect screen or a flap, so that nothing can crawl up the pipe and water cannot be drawn back.
- Screen the outlet intake with coarse mesh so that debris and small animals do not enter the supply.
- Seal the wall penetrations. Water travels along the outside of a pipe through a wall. Cast the pipes in with a collar or a ring of mortar round them, or seal with clay in a sleeve, and test.
- Direct the overflow away. Discharge it downhill of the box, to a trough or a ditch, not onto the gravel zone of the back wall.
Monitoring The Overflow With A V-Notch
The guide suggests a V-notch weir or a measuring point at the overflow, so the flow can be checked without opening the box. For a 90° V-notch the Source Development guide gives Q = 1.38 × H^2.5, with Q in m³/s and H the head over the notch in metres (Source Development). For 10 L/min (1.67×10⁻⁴ m³/s): H = (1.67×10⁻⁴ ÷ 1.38)^0.4 = 0.027 m, 2.7 cm. For 2 L/min: H = 1.4 cm.
Flow depends on H to the power 2.5, so a reading error of 2 mm in H is 2.5 × 0.2 ÷ 2.7 = 18% of the flow at 10 L/min, and 35% at 2 L/min. A weir shows large changes well and small flows badly; the bucket is still the way to measure a weak spring (Bucket Timing).
What You Need
- Pipe in the sizes above, with sleeves or collars
- A coarse screen and a fine screen — for the outlet intake and the overflow end
- A valve for the drain
- Mortar or clay — to seal the wall penetrations
- A level — for setting the heights from the eye mark
Method: Set Heights, Seal, Screen, Test
Step 1: Mark the heights from the eye line: outlet 10-15 cm above the floor; overflow at the eye; drain at the floor.
Step 2: Build the pipes in as the front wall rises, collared and sealed.
Step 3: Fit the screens and the drain valve.
Step 4: Fill the box and check the water sits at the overflow level with no leaks round the pipes.
Step 5: Check the flow at the overflow and at the outlet with the bucket.
Common Mistakes
- A valve on the overflow. The box pressurises and cracks.
- An outlet at floor level. Silt goes down the supply.
- Unsealed wall pipes. Leaks along the pipe wash out the wall and let in surface water.
- An overflow ending in a puddle. Water and animals get back in.
Safety
Valves, pressure and the drain
Do not fit a shut-off valve in the overflow. Open the drain slowly, standing clear, since a full box discharges a jet at the pipe end. Do not put your head or body into the box to fit a pipe; work from the hatch with long tools.
What’s Next
- Learn next: Backfill & Protection — protecting the finished box
- Combine with: Air Locks — keeping air out of the outlet line