CHAPTER 6

Intake Structures for Water Supply

Intake Structures

Specifies design of intake structures for abstracting water from surface sources — rivers, lakes, reservoirs, and canals. Covers submerged, exposed, floating, movable, and bank intakes. Includes screen design, approach velocity, flood/drought provisions.

🏞 Sources & IntakeManual on Water Supply and Treatment3rd Edition (1999) with 2024 revision updates

Key formulas

Screen (bar rack) head loss: h_L = β × (s/b)^(4/3) × (v²/2g) × sin(θ), where β = shape factor (2.42 for rectangular bar), s = bar thickness, b = clear spacing, v = approach velocity, θ = angle from horizontal.
Intake pipe velocity (gravity flow): V = Q / A ≤ 0.6 m/s (to minimize head loss and debris drag-in).
Minimum submergence depth for intake orifice: 2 × D (pipe diameter) above invert to prevent air entrainment vortex.

Key values & thresholds

intake screen approach velocity max mps: 0.3
intake pipe velocity range mps: 0.6 - 1.2
screen bar spacing coarse mm: 50 - 100
screen bar spacing fine mm: 5 - 25
minimum submergence pipe diameters: 2
intake level above river bed m: 1.5 - 2.0
intake level below low water m: 1.0 - 1.5
maximum flood allowance m above HFL: 0.5

Clause-level requirements

Intake type selection: submerged (turbulent rivers), exposed (lakes/reservoirs with stable levels), floating (deep reservoirs with varying levels), movable (seasonal rivers), bank (flat rivers with minimal seasonal variation).
Screen design: coarse screen (bar rack) at entrance removes logs, debris (50-100 mm spacing); fine screen removes smaller material (5-25 mm spacing). Approach velocity < 0.3 m/s to prevent clogging and fish impingement.
Intake pipe: velocity 0.6-1.2 m/s (low end minimizes head loss, high end self-scouring for silt). Pipe fully submerged at all times — minimum 2 × pipe diameter below low water level.
Intake level: minimum 1.5 m above river bed (prevent sediment ingress), 1.0 m below lowest water level (continuous abstraction), 0.5 m above highest flood level (prevent overtopping).
Provide stop logs, penstock gates, and bypass at intake for maintenance, flood protection, and silt flushing.
For rivers with high silt load (Ganga, Yamuna, Brahmaputra) provide pre-settling tank adjacent to intake; design retention 30-60 minutes to settle coarse silt.

Practitioner notes — what goes wrong in the field

River intake on silty Indian rivers (Ganga, Yamuna) requires pre-settling. Without it, sand-laden water wrecks pumps in 6-12 months.
Delhi's Yamuna intakes (Wazirabad, Chandrawal, Haiderpur) have extensive silt problems during monsoon — temporary shutdowns common.
Lake intake (Mumbai's Vihar, Tulsi, Tansa): stable water quality but hyper-eutrophication causes algae issues requiring copper sulphate dosing.
Floating intake (for deep reservoirs with varying level): pump barge with flexible suction hose. Used at Bhadbhut (Narmada), Bhakra.
Movable intake on seasonal rivers (Rajasthan): cart/barge intake moves with water level; simpler for low-flow seasonal sources.
Screen cleaning: manual raking for small intakes (< 50 MLD); mechanical rotating screens for larger. Debris volume 1-5% of water flow.
Approach velocity > 0.5 m/s drags fish against screen (impingement) and sucks small fish through (entrainment) — CPCB and fishery regulations restrict to 0.3 m/s.
Intake pipe material: DI (ductile iron) for durability; MS (mild steel) cement-lined; HDPE for corrosion resistance in saline environments.
Pre-settling tank at intake: retention time 30-60 minutes, settles silt > 0.2 mm; desludged every 1-3 months depending on silt load.
Saltwater intrusion in coastal intakes: Kolkata's Hooghly intake salinity exceeds 500 mg/L in dry season; monitored and blended with sweet water sources.
Power supply reliability: intakes should have dual power feed + diesel generator backup. Power failure = 2-hour outage = supply disruption downstream.

FAQs

What is maximum approach velocity at screen?

0.3 m/s maximum — above this fish are impinged against screen and debris clogs rapidly. Also prevents excessive head loss. Screen area sized to keep velocity below this limit at maximum abstraction.

What is the minimum submergence for intake pipe?

2 × pipe diameter below lowest water level. Prevents air entrainment vortex which would reduce intake capacity and damage pump impellers. Example: 600 mm pipe → minimum 1.2 m submergence.

What intake level for river abstraction?

Per CPHEEO Chapter 6: minimum 1.5 m above river bed (prevent sediment ingress), 1.0-1.5 m below lowest water level (continuous abstraction), 0.5 m above HFL (prevent flood overtopping).

What intake pipe velocity?

0.6-1.2 m/s. Lower end minimizes head loss (for gravity intake); higher end self-scours silt (for pumped intake). Below 0.6 m/s silt deposits in pipe; above 1.2 m/s high head loss.

Do I need a pre-settling tank at intake?

Yes for silty rivers — Ganga, Yamuna, Brahmaputra, monsoon-fed rivers. Retention 30-60 minutes settles coarse silt. Without pre-settling, pump wear is severe (impeller erosion) and treatment plant overloaded.

Cross-references

CPHEEO WS Chapter 5IS 11625 Intake Structures