An artesian spring is pressurised water from a confined aquifer pushing up through a crack, so it often flows year-round and may be forceful; measure how high its water would stand, remembering that each metre of rise means 9.81 kPa of pressure, and capture it with a box whose overflow lets the water out freely and whose base is a graded filter. Never close the outlet: blocking the flow raises the pressure and the water breaks out somewhere else, taking soil with it.
Part of Spring Capture
An artesian spring is a spring fed from an aquifer trapped under an impermeable layer, so its water is under pressure and rises above the level at which it was struck. The water finds a fault or crack and flows up to the surface by itself.
The Water Is Under Pressure, And The Pressure Has A Number
In a confined aquifer the water is held down by the layer above. If a crack opens in that layer, the water rises through it, up to a level set by the aquifer’s head, which may lie above the ground at the spring. The pressure at the eye is the water’s weight per area: ρ × g × h = 1,000 × 9.81 × h Pa, or 9.81 kPa for each metre h of rise. A rise of 2 m above the eye is 2 × 9.81 = 19.6 kPa, enough to make the water well up or jet. The Spring Capture guide notes these springs often flow year-round and may be quite forceful.
What You Need
- A clear hose or tube and clay — to seal over the eye and measure how high the water stands
- A graded filter — layers of gravel and coarse sand, coarse to fine from the box outwards (Inlet Wall)
- A box with an overflow as large as the whole flow (Box Design)
- A bucket and watch — to time the flow at the overflow (Bucket Timing)
Method: Measure The Head, Give The Water An Easy Way Out
Step 1: Measure the head. Hold a clear vertical tube over the eye, sealed to the ground with clay. Note the height the water reaches above the ground. Do this briefly and release it; do not leave the eye closed.
Step 2: Time the flow at the natural outlet with the bucket method, in the dry season (Seasonal Monitoring).
Step 3: Build the box with a free overflow, set at or just below the natural outlet and sized for the full measured flow, so the water leaves as easily as it did before.
Step 4: Build a graded filter under and behind where the water enters, from coarse next to the box to fine against the soil, so that the rising water cannot carry soil with it.
Step 5: Bring the supply off from the outlet pipe, below the overflow, and never close the overflow (Outlet & Overflow).
Worked Example: When The Sand Boils
Water rising through loose sand lifts the grains when its upward gradient (head lost divided by the length of the sand) reaches the critical value i_c = (γ_sat − γ_w) ÷ γ_w. With a soil weight of 18 kN/m³ (an assumed value) that is (18 − 9.81) ÷ 9.81 = 0.83. A spring that comes up through 1 m of sand with a head of 1.5 m across it has a gradient of 1.5 ÷ 1.0 = 1.5, above 0.83, so the sand boils and the eye cannot be dug out. Digging the sand away removes its weight and makes the boil worse. A cover of coarse gravel laid over the boil weighs the sand down and filters the flow. If the gradient is 0.5, below 0.83, the sand stays put.
Common Mistakes
- Closing the outlet. The pressure rises and the water bursts out beside the box.
- An overflow smaller than the flow. It backs up.
- No filter. Fine sand and silt come up with the water and fill the box.
- Measuring the head by plugging it for long. A sealed eye may find a new path.
Safety
Pressure, boiling ground and soft banks
A strong spring can undermine its own bank: slope or shore any cut deeper than 1.2 m, keep spoil at least 0.6 m back, work with a partner, and stop if the ground goes soft or sand moves (Excavation).
What’s Next
- Learn next: Box Design — overflow height and size
- Combine with: Inlet Wall — the graded filter