Gravity Systems

Part of Irrigation

Gravity-fed irrigation is the most reliable and energy-free water distribution system available to a rebuilding civilization. Water collected above the field elevation flows downhill through channels or pipes without pumps, fuel, or maintenance-intensive machinery. Designing a gravity system correctly — calculating the available head, sizing channels and pipes, and laying out a distribution network — determines whether you move water efficiently across kilometres of farmland or struggle with insufficient pressure and constant blockages.

The Principle of Hydraulic Head

Head is the vertical height difference between the water source and the delivery point, expressed in metres. Every metre of head provides approximately 9.8 kPa (about 0.1 bar) of pressure. In practical terms:

• 1 m of head is enough to push water through a short, wide pipe at low velocity
• 10 m of head produces approximately 1 bar of pressure — adequate for drip irrigation
• 20–30 m of head is sufficient for sprinkler irrigation without additional pumping

Head is consumed by friction as water moves through pipes and channels. The longer and narrower the conduit, the more head is lost to friction. The usable head at the delivery point equals source head minus friction losses.

Net head = Source elevation − Delivery elevation − Friction losses

Measuring Available Head

Before designing any gravity system, determine how much head you have.

Method 1: Water level measurement

1. Mark the water surface level at the intake (spring, reservoir, or diversion point)
2. Mark the highest point you need water to reach at the field
3. Measure the vertical difference using a level, clinometer, or A-frame level
4. Subtract at least 20% as a friction safety margin

Method 2: A-frame level survey An A-frame level is a simple tool — two equal-length legs joined at the top with a crossbar, a plumb bob hanging from the apex. Each placement measures one unit of rise or fall. Traverse the planned pipe route, counting level placements and noting rises versus drops. Net drops equal available head.

Method 3: Hose and tube manometer Fill a transparent hose with water, hold both ends up, and align one end with the source water surface. The other end shows the water surface elevation — move it up or down until the water level matches. Mark this point, measure the height difference from the ground.

Channel Design

Open channels — earthen, stone-lined, or concrete — move large volumes of water at low cost. They suit systems where head is limited and volumes are high.

Manning’s Equation (Simplified)

Flow velocity in a channel is determined by slope, cross-section area, and roughness. For practical field sizing, use these approximate guidelines:

Channel Slope	Recommended Velocity	Notes
0.05%	0.3–0.5 m/s	Very flat; sedimentation risk
0.1%	0.5–0.7 m/s	Good for earthen channels
0.2%	0.7–1.0 m/s	Good for lined channels
0.5%	1.0–1.5 m/s	Earthen erosion risk above 1.0 m/s
>1.0%	Line with stone or concrete

Channel Sizing

Flow rate (Q) = Cross-section area (A) × Velocity (V)

For a trapezoidal earthen channel (the most common hand-dug shape) with:

• Bottom width: 30 cm
• Top width: 60 cm
• Depth: 25 cm
• Slope: 0.1%
• Velocity: approximately 0.5 m/s

Flow rate = 0.113 m² × 0.5 m/s = 0.056 m³/s = 56 litres/second

This is enough to irrigate 5–10 hectares depending on crop water demand.

Channel Size (bottom × depth)	Approximate Flow at 0.1% Slope
20 cm × 15 cm	8–12 L/s
30 cm × 20 cm	20–30 L/s
40 cm × 25 cm	45–60 L/s
60 cm × 35 cm	100–130 L/s

Free-Board Rule

Design channels to run only 70–80% full. Leave 20–30% free-board above the water surface to allow for surge flows, debris, and slight grade errors without overtopping and erosion.

Lining Channels

Earthen channels lose 30–50% of flow to seepage unless lined. Lining options:

Material	Seepage Reduction	Durability	Construction Cost
Puddled clay	60–70%	5–10 years	Very low
Stone/brick	85–90%	20–50 years	Moderate
Mud-lime plaster	70–80%	10–20 years	Low
Concrete	95–98%	30–50 years	High
Plastic sheet	90–95%	5–15 years	Low-moderate

For most situations without concrete, puddle clay or apply a mud-lime plaster (70% soil, 30% lime by volume) trowelled 2–3 cm thick over a smooth, compacted earthen base.

Pipe Sizing for Gravity Systems

Pipes deliver more pressure than open channels and suit longer runs, steeper terrain, and pressurized outlets. The trade-off is higher material cost.

Pipe Diameter Selection

Pipe diameter determines flow capacity and friction loss. Larger pipes carry more water with less friction loss per metre, but cost more.

Pipe Diameter	Approximate Flow Capacity	Friction Loss per 100 m
25 mm	5–12 L/min	High (10–40 m head)
40 mm	15–35 L/min	Moderate (5–15 m head)
50 mm	30–60 L/min	Moderate (3–10 m head)
75 mm	80–150 L/min	Low (2–5 m head)
100 mm	180–300 L/min	Very low (1–3 m head)

These ranges assume gravity flow with 10–30 m of available head. Use larger pipes when head is limited.

Rule of thumb: For gravity domestic water supply, use at least 40 mm pipe for runs under 200 m, 50–75 mm for 200–500 m runs. For irrigation mains feeding multiple laterals, use 75–100 mm.

Hazen-Williams Simplified Friction Loss

Friction loss in smooth plastic pipe per 100 m of length:

Flow (L/min)	25 mm pipe	40 mm pipe	50 mm pipe	75 mm pipe
10	9 m	1.5 m	0.5 m	0.1 m
20	32 m	5 m	1.8 m	0.4 m
50	—	26 m	9 m	1.8 m
100	—	—	32 m	6 m

Select pipe so total friction loss over the full route does not exceed 70% of available head.

Distribution Network Layout

A gravity irrigation network typically has three levels:

Main canal or pipe: From source to field. Sized for total flow demand. Should run at gentle grade to preserve head.

Secondary channels or pipes: Branch from the main at intervals. Serve individual fields or field sections. Sized for the flow needed for that field area.

Field channels or drip/furrow laterals: Deliver water directly to the root zone. Sized for individual furrow or bed flow rates.

Tree Layout vs. Grid Layout

• Tree layout: One main trunk branching into smaller laterals, each branching further. Simple to design, but a blockage upstream stops everything downstream.
• Grid layout: Parallel mains with cross-connections. More resilient — a failure in one section is bypassed via another route. Requires more pipe/channel but is worth it for large permanent systems.

Water Control Structures

Offtakes: Where water is diverted from a main channel into a secondary channel. Use a simple stop-plank in a notch to control flow. Standardise notch width for consistent flow measurement.

Division boxes: A concrete or brick box that receives the main flow and divides it into two or more secondary channels in a fixed ratio. Notches cut at the same height and proportional widths split flow automatically.

Drop structures: Where the channel grade exceeds erosion limits, install a step or vertical drop with an energy dissipation basin (a pool of stones or rough concrete) to slow water before it continues in a lower channel.

Air vents: In pressurised pipes, air trapped at high points causes airlock and stops flow. Install T-junctions with short vertical standpipes at all high points, capped with a loose stopper. Air escapes; water seals the vent.

Spring Capture and Intake Design

If the source is a spring, capture it properly to maximise yield and prevent contamination.

1. Excavate around the spring emergence to expose the saturated zone
2. Place a gravel filter layer (washed 10–20 mm gravel) around the spring discharge point
3. Build a brick or stone collection chamber with a cleanout access
4. Install an overflow pipe set 5–10 cm below the top of the chamber — this ensures the chamber fills before overflow, preventing sediment disturbance
5. Install a covered outlet pipe from the bottom of the chamber to the supply line
6. Cover the entire spring box with a concrete or stone slab to prevent contamination
7. Fence the catchment area above the spring to exclude animals

Spring yield varies seasonally. Size your storage and distribution for minimum dry-season yield, not peak wet-season output.

Reservoir and Storage Tank Integration

Where spring flow is insufficient during peak demand periods, a storage reservoir collects water overnight and releases it during the day.

Sizing a storage reservoir:

• Calculate daily crop water demand in litres per day
• Determine spring or source flow rate in litres per day
• If daily demand exceeds 70% of source flow, provide at least one day of storage buffer
• For unreliable sources, provide 3–7 days of storage

Example: 1 hectare of vegetables requiring 6 mm/day irrigation depth:

• 6 mm on 10,000 m² = 60,000 litres per day
• If spring flows at 1.5 L/min = 2,160 L/day, a 60,000-litre reservoir provides 28 days buffer

Source vs. Demand Balance

Gravity systems fail when demand exceeds supply. Always measure your source flow rate at the driest time of year before committing to a system design. Overestimating spring flow is the most common cause of gravity irrigation system failure.

Maintenance

• Inspect channels monthly for sediment, weed growth, and erosion
• Clean intakes and filters before each irrigation season
• Check pipe joints and connections annually; replace cracked or leaking sections
• Re-grade settled channel sections with a level and reshaping
• Clear overflow spillways before rainy season to prevent backwater damage

Gravity Systems Summary

Gravity irrigation moves water without energy input, but only if designed around available head, soil conditions, and crop demand. Measure head carefully using a water level or A-frame survey. Size channels and pipes to match flow demand without exceeding erosion velocity limits. Structure the distribution network in a hierarchy from main supply to field laterals. Include control structures — offtakes, division boxes, and drop structures — at all critical points. Capture spring sources with a filter and collection chamber, and size storage to bridge gaps between source flow and peak crop demand. A well-designed gravity system, properly maintained, can operate for decades without fuel or mechanical components.