A stream intake for a gravity system is a small weir or dam across the stream, a screened intake pipe upstream of it set above the stream bed to avoid sediment, a settling basin between the intake and the main pipe, and an overflow that carries flood water past the system. Take only a share of the stream’s dry-season flow, build it to survive floods or fail safely, and always treat the water, because stream water is surface water that carries disease organisms.
A stream intake diverts part of a stream into a pipe while keeping out debris and sediment. Unlike a spring, a stream carries whatever its catchment puts in it, so the intake design and the water treatment matter equally.
Choose The Site Before You Build
- Upstream of everything: settlements, grazing, latrines and fields. River Assessment lists the checks before trusting a stream, and River & Stream Assessment gives the flow estimate.
- High enough above the settlement to give the head you need (Water Source & Elevation).
- A stable reach: firm banks, not a bend that is eroding. On a bend, as I recall from river engineering, the outside is deeper and the inside collects bed sediment; an intake on the outside bend collects less bed load but is exposed to bank scour.
- Away from the flood path of any building, and where a flood will not cut a new channel around the weir.
How Much Of The Stream To Take
Measure the stream in the driest part of the year with the bucket, float or weir method (Flow Measurement). Take only a share of the dry-season flow, so the stream below keeps water for its life and for other users (Water Rights). I have no sourced percentage; the share is a decision for the community. A household needs little: a 0.33 L/s pipe flow is 28,800 L a day, while 4 people use 120-240 L a day on the parent page’s per-person figures (30-60 L each including laundry).
The Parts, From The Parent Page
- A small dam or weir across the stream of rocks, logs or sandbags.
- A screened intake pipe upstream of the dam, positioned above the stream bottom to avoid sucking in sediment.
- A settling basin between the intake and the main pipe, a small pond or tank, to let sediment drop out.
- An overflow to handle high water without pressurising or damaging your system.
Worked Example: How Big A Screen?
Open screen area = flow ÷ (open fraction × speed through the openings). Take the design flow 2 L/s = 0.002 m³/s, half the screen area open (an assumed value for a mesh), and a through-speed of 0.1 m/s, a deliberately low design choice I make so that debris is not pinned to the screen, not a sourced figure.
- Open area needed = 0.002 ÷ 0.1 = 0.02 m².
- Total screen area = 0.02 ÷ 0.5 = 0.04 m², about 20 cm × 20 cm.
A larger screen blocks later and is easier to clear. Place the screen where you can reach it to rake it daily during leaf fall. The size of the settling basin follows from the flow and the particle size to be removed; Settling Basins shows the method, and Sediment Management the clean-out.
What You Need
- Rock, logs, sandbags or timber for a low weir
- A pipe and a fine screen for the intake, and a sluice or valve to isolate the pipe
- A settling basin with an inlet, outlet and drain
- An overflow channel lined against erosion
- Tools for clearing, a rake and a brush
Method: Build Low, Build To Fail Safely
Step 1: Mark the intake position and measure the dry-season flow.
Step 2: Build a low weir that raises the water only enough to cover the intake pipe. A tall dam makes a pond, breeds mosquitoes and fails in a flood.
Step 3: Fit the screened intake above the bed and upstream of the weir crest.
Step 4: Lay the pipe to the settling basin, with a valve at the intake so the line can be shut during floods or maintenance.
Step 5: Build the overflow and the flood spill so water above the design level goes over the weir or round the basin, not into the pipe.
Step 6: Clear the screen and basin on a routine (daily in leaf fall, weekly otherwise), and after every flood.
Treat The Water Every Time
A stream carries bacteria, viruses and parasites from animals and upstream settlements. A settling basin removes silt, not disease. Always filter and disinfect before drinking (Water Purification).
Common Mistakes
- A tall dam. It becomes a pond and a flood hazard.
- The intake on the stream bed. It takes in sand and gravel.
- No isolation valve at the intake.
- Taking all the dry-season flow.
- Skipping treatment because the stream “looks clean”.
Safety
Floods, drowning and untreated water are the hazards of a stream intake
Work in the stream only at low flow, with a second person on the bank and a rope, and leave when the water rises or turns brown. Do not drink from the intake or settling basin without treatment. A settling basin that holds still water breeds mosquitoes; cover or drain it. Keep the intake line separate from any non-drinking line (Plumbing Separation).
What’s Next
- Learn next: Pipe Materials & Construction — the pipe that follows the basin
- Combine with: Settling Basins — size the basin to the flow