An intake structure is the point where a source enters the pipe, and it must pass the design flow through a screen slowly enough not to clog, trap sediment in a sump, draw from above the floor, and spill the excess through an overflow. For a stream intake passing 0.5 liters per second, a screen with about 50 square centimeters of open area gives a through-speed of 0.1 meters per second, and it should be made several times larger because screens clog.
Part of Community Water System
An intake structure is the built part of a water system at the source: the box, screen, sump and first valve that turn a spring, stream or well into a controlled flow entering the pipe.
The Intake Is Where A System Is Most Often Contaminated, Clogged Or Washed Out
Every later part of a system depends on the intake. If it clogs, the taps run dry. If it is built where animals, floods or people reach it, the whole supply is contaminated at the one point common to all. It has four jobs: admit the design flow, keep out solids, keep out contamination, and survive the worst flood and the lowest water.
| Source | Intake | Key feature |
|---|---|---|
| Spring, one emergence point | Spring box (Spring Capture) | Permeable back wall, watertight sides, outlet 10-15 cm above the floor, overflow |
| Spring, diffuse seep | Buried perforated pipe in a gravel trench with a collection chamber | Surrounded by clean gravel, capped with clay against surface water |
| Stream or river | Screened side intake with a sump | Trash rack, fine screen, settling sump, valve, flood protection |
| Well | Lined shaft with a sealed head and pump (Deep Wells) | Sealed top against surface water |
What You Need
- A site above the settlement and above all grazing and latrines
- A coarse rack and a fine screen, metal mesh or perforated plate
- A sump, a chamber deeper than the outlet pipe
- A valve, an overflow pipe and a drain pipe
- Stone, concrete or masonry for flood protection
Method: Build A Stream Intake In The Order Water Passes
Step 1: Choose the site. Pick a stable bank on a stretch of steady flow, upstream of every use that could pollute it. Check where the water is at its lowest: the inlet must still be covered then.
Step 2: Set a coarse rack first. It stops sticks and leaves. Slope it so debris slides off instead of building up.
Step 3: Size the fine screen. Divide the design flow by a through-speed of about 0.1 m/s to find the open area. This speed is a common design convention that I am recalling, not a figure from a source read here; the principle that slower is cleaner is what matters. A screen with half its area open needs twice the gross area.
Step 4: Add a sump. Water slows in the chamber and sand drops out. Put the outlet 10-15 cm above the sump floor, as in a spring box, and fit a drain at the bottom so the sump can be flushed (Settling Basins).
Step 5: Fit the valve and the overflow. The overflow must be at least as large as the outlet, discharging to rock so it does not cut the bank.
Step 6: Protect against floods. Face the bank with stone or riprap, and set the structure’s top above the highest flood mark you can find (Historical Flood Marks).
Step 7: Cover and fence. A lockable lid and a fence keep out animals and people.
Worked Example: Sizing A Screen
Inputs: design flow 0.5 L/s = 0.0005 m³/s; through-speed 0.1 m/s.
- Open area = 0.0005 ÷ 0.1 = 0.005 m², which is 50 cm².
- If half the mesh area is open: gross area = 50 ÷ 0.5 = 100 cm².
- Allow for half the screen being blocked between cleanings: 100 ÷ 0.5 = 200 cm², about 20 cm × 10 cm.
On a clean 200 cm² screen with half its area open, the open area is 0.01 m², so the water crosses at 0.0005 ÷ 0.01 = 0.05 m/s; if half the screen is blocked the speed rises back to 0.1 m/s. The margin is the point: a screen sized to the minimum starts failing at the first leaf fall.
Failure Modes And Their Signs
- Flow drops slowly: screen or rack clogging. Clean it, then add area.
- Flow drops suddenly after a storm: sump filled with silt, or the inlet was undercut or buried. Flush the sump.
- Muddy water after rain: surface runoff entering above the intake. Add a diversion ditch and a cover.
- Water cannot enter at low flow: the inlet is above the dry-season water level. Lower it or build a small weir to hold the level.
- Intake destroyed in a flood: no bank protection or too low a top.
Common Mistakes
- Taking all of a small stream. Leave water in the channel for the people and ecosystem downstream.
- Putting the outlet at the floor. Sand and silt then go straight into the pipe.
- No overflow. A closed valve then pressurises the box.
- Treating the intake as the end of the job. Water from a stream must still be treated (Water Treatment at Scale).
Safety
Flowing water and closed chambers both kill
Do not work in a stream in spate, and never wade in it alone. A sump with a lid is a confined space: its air can be low in oxygen or carry hydrogen sulphide, so no one enters without ventilation at two openings, a harness on a line held by a person outside, and a rule that the person outside never goes in after a collapsed entrant. Do not test the air with a flame.
What’s Next
- Learn next: Settling Basins — taking silt out before it reaches the pipe
- Combine with: Erosion Prevention — protecting the bank around the structure