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Gravity Springs

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Parent
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Spring Types
Understanding different spring formations
Current
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Gravity Springs
Water flowing downhill through rock

A gravity spring is water flowing downhill through a water-bearing layer until the layer meets a slope or an impermeable base and the water comes out at the surface; it is the most common and the easiest spring to capture. Capture it with a spring box that holds the water no higher than the natural outlet, because the flow is driven by the head in the hillside and a box that stands water above the outlet reduces the flow in proportion, or sends the water out somewhere else.

Part of Spring Capture

A gravity spring is a spring whose water is driven out purely by gravity, flowing horizontally or downhill through permeable ground until it meets the surface. It is the usual hillside spring and the one the spring box is designed for.

The Water Comes Out Where The Permeable Layer Is Cut By The Slope

Water soaks into permeable ground such as sand, gravel or fractured rock and flows downhill along the top of a less permeable layer, such as clay or solid rock. Where a valley or hillside cuts through the contact, the water comes out. The outlet, the spring eye, is where the top of the impermeable layer meets the surface. This is the contact spring of the Phase 0 guide (Spring Location); fracture and depression springs behave the same way for capture.

Gravity springs are among the most dependable for a settlement because they need no pumping, and the Spring Capture guide calls the water-bearing layer meeting a slope the most common form and the easiest to capture. Their weakness is that they are often shallow and respond to rain: flow can rise after wet weather and fall to a trickle in the driest months.

Keep The Box Water No Higher Than The Eye

The aquifer behind the eye has a water level above the eye, which is the head that drives the flow. Water moves through porous ground in proportion to the head difference (Darcy’s law, which says flow is proportional to the difference in water level over the flow path). If a box fills until its water stands h above the eye, the head available to push water out falls from H to H − h, and the flow falls in the same proportion.

Worked example. Suppose the water in the hillside stands H = 0.5 m above the spring eye. A box with its overflow set 0.25 m above the eye gives (0.5 − 0.25) ÷ 0.5 = 0.5 of the natural flow, half. Set the overflow at the eye and the full flow comes out. With a higher box the water may also find another path round it and come out beside the box, which is what the Spring Capture guide’s warning about cracking walls and backed-up water is describing (Box Design works the overflow height out for the standard box). The heads are rough guesses unless you measure them, and the proportional rule is a simplification, but the direction is certain: lower is safer.

What You Need

Method: Find The Layer, Watch The Rain, Then Build

Step 1: Find the contact. Dig gently, working uphill, until you see the water-bearing layer sitting on the less permeable layer that carries it. The eye is at the top of the lower layer.

Step 2: Record the weather response. Measure flow and clarity before and after rain. Water that clouds up within hours after rain is shallow and connected to the surface.

Step 3: Measure the dry-season flow and design for it.

Step 4: Decide the overflow height relative to the eye, not to the wall top.

Step 5: Protect the catchment above with fencing, diversion and clean land use (Fencing, Uphill Diversion).

Common Mistakes

  • Building a high box. Raising the water above the eye cuts the flow or shifts the outlet.
  • Judging yield from the wet season. Gravity springs often fall steeply as summer goes on.
  • Digging into the impermeable layer. Cutting through the base lets the water sink away.
  • Ignoring the catchment. Shallow springs carry what falls on the ground above them.

Safety

Wet banks and cold water

The cut face above a spring is wet and slides. Slope or shore any excavation deeper than 1.2 m, keep spoil at least 0.6 m from the edge, and do not work alone or in heavy rain (Excavation).

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