Intakes, weirs & forebays

The forebay: the quiet heart of the works

A forebay is a small tank placed between the source and the drive pipe. It is deliberately boring — no moving parts, no valves — and it does four jobs at once: it lets grit settle instead of travelling, it gives the drive pipe a steady vortex-free supply, it stores a few minutes of flow to smooth surges, and it provides a place for floods to bypass the pump rather than find it.

• Settling depth: deep enough that the water moves slowly enough for grit to drop. Around a minute of drive flow in quiet volume is ample for sand; very fine silt will pass regardless and is better handled by choosing your intake height (see what water a ram tolerates).
• Submergence: the drive-pipe mouth enters 2–3 pipe diameters below the surface — deep enough to avoid drawing a surface vortex, off the floor so it doesn't hoover the settled grit. If the tank can't give that depth, fit a simple anti-vortex hood: a plate or upside-down box over the mouth.
• Overflow: the forebay wall must pass everything the stream can deliver, with margin. When the ram stops — and it will, for cleaning — the whole flow arrives at the forebay. A forebay that overtops into the drive-pipe trench undermines its own foundations.
• Access: you will clean this screen in the rain, in winter, possibly at night. A firm standing edge, a lifting handle on the screen, and a rake fixed nearby are not luxuries.

Screens & racks

Two stages, cheap and standard:

• Coarse rack (20–50 mm bars, inclined) at the forebay entrance catches the logs, leaves and fish before they reach the fine screen. Inclination lets the flow's own motion carry floating debris up and over — self-cleaning by slope.
• Fine screen (1–2 mm mesh) immediately before the drive-pipe outlet. Wire mesh corrodes; perforated plate or heavy plastic mesh lasts. Slope it, or clean it will hold a beard of leaves within weeks.

Taking water from a river: the weir

Where the source is a moving stream rather than a spring or pond, the intake needs three things a pond doesn't demand: a stable water level, a way to keep bedload out, and permission. A low weir provides the first two — and the paperwork provides the third.

• A low rubble or concrete weir raises the water surface enough to give the intake its submergence and a quiet pool for settling. It needs a spillway that passes flood flows — a weir that doesn't spill is a dam, with everything that implies.
• Bedload gate or sluice: a gravel-bed stream delivers stones with every flood. A small gated opening at the weir's base lets you flush accumulated bedload downstream without climbing in.
• Off-take angle: take water from the side of the pool, not the middle of the current, and keep the intake mouth facing downstream or sideways — a mouth facing upstream rams debris into its own screen.
• Fish screening is a condition of many abstraction permits. Mesh of a size and a low approach velocity keep fish out; the numbers come from the regulator, and the design conversation lives on Permits & Environment.

Flood behaviour

Design the intake for the flood, not the drought — the drought starves the pump slowly, the flood destroys it quickly.

• High-level flood bypass: a spillway channel around the forebay that carries floodwater past the works without overtopping them.
• Salvageable screens: assume the flood takes anything not tied down. Hinged or liftable racks survive what fixed ones don't.
• Scour protection at the weir toe and around the forebay base — rip-rap or gabions on the downstream face.
• Assume post-flood work: after any real flood, expect to dig the intake out, flush the forebay, and re-seat the screen. Build so this is an hour's work, not a rebuild.

Sizing the works

Everything follows from the drive flow Q and the drive-pipe bore D. Take a worked example — the site used throughout this knowledge base: a 10 L/s drive flow through a 100 mm pipe:

• Screen area. Pipe area = π × (0.05 m)² ≈ 0.008 m², so the fine screen wants about 0.08 m² of open mesh; allowing that mesh is roughly half open area, fit about 0.16 m² of panel — a 400 × 400 mm screen is comfortable.
• Settling volume. One minute of drive flow is 10 L/s × 60 s = 600 L — say a 1 m × 1 m × 0.7 m tank. A trickle scheme of 1 L/s gets away with a wheelbarrow-sized forebay.
• Overflow capacity. Pass the peak the stream is known to deliver, not the drive flow. If in doubt, make the spillway as wide as the forebay wall — it is the cheapest concrete on the job.
• Submergence. 2–3 × D = 200–300 mm below the surface for a 100 mm pipe, with the mouth a similar distance off the floor.

These are sizing floors, not optimisations: enlarging any of them only costs a little concrete, while under-sizing costs the stoppages this page exists to prevent.

Upkeep

• Weekly: glance at the screen; clear the floating debris raft before it sinks.
• Quarterly: lift the screen and hose it; flush the forebay by opening any flush gate or briefly pulling the screen while flow runs through.
• After every flood: inspect, dig out, flush, re-seat, and check the weir spillway still spills freely.

The intake and the beat are connected: air ingested here becomes "won't start", grit ingested here becomes worn valve rubbers (see the silt limits), and a drowned forebay becomes a drowned pump house. Feed the machine well and the machine runs.