Last reviewed 16 Sept 2026 · 9 min read
Requirements of a good distribution system
- Supply water to all consumers at adequate pressure (and fire flow when needed).
- Deliver the required quantity at all times, including peak hours.
- Maintain water quality — watertight, no contamination, adequate residual chlorine.
- Be reliable — a break should affect as few consumers as possible (loops, valves).
- Allow easy maintenance, repairs and extensions; economical.
- Minimise leakage and unauthorised use.
Methods of distribution
| Method | Description | Suitability |
|---|---|---|
| Gravity system | Source (reservoir) at sufficient elevation; water flows by gravity | Hill sources above the town — cheapest and most reliable |
| Pumping without storage | Pumps deliver directly into mains | Not preferred — pumps must follow demand, power failure stops supply |
| Combined (pumping with storage) | Water pumped at a uniform rate to elevated/service reservoirs; excess stored during low demand and supplied during peaks | Most common — pumps run at uniform rate, storage covers peaks, fire and breakdowns |
Systems of supply
- Continuous (24 × 7) supply — water available at all times; best for quality (positive pressure keeps contamination out) and convenience; needs good leak control and metering.
- Intermittent supply — water supplied for a few hours; pipes empty and contaminated water can be sucked in through leaks; consumers store water wastefully; used where sources are inadequate.
Layouts of distribution systems
| Layout | Description | Merits | Demerits |
|---|---|---|---|
| Dead-end (tree) system | Main → submains → branches ending in dead ends | Simple, cheap, easy design, fewer valves | Stagnant water at dead ends; a break cuts off all downstream consumers; pressure falls at remote ends |
| Grid-iron (reticulation) system | Interconnected mains and branches forming loops | No dead ends; water reaches points from several directions; small area affected by repairs; good for fire flow | More pipes and valves; complex design (Hardy Cross); costlier |
| Ring (circular) system | A main ring around a district with branches inside | Every point supplied from two directions; good for well-planned towns | More pipe length |
| Radial system | Area divided into zones, each with a central reservoir supplying radially outward | High pressure and quick service; efficient | Needs several reservoirs |
Planned cities with rectangular street patterns suit the grid-iron system; old irregular towns often have dead-end systems.
Service reservoirs
Service (distribution) reservoirs store treated water within the distribution area.
Functions: balance the difference between uniform supply (pumping) and fluctuating demand; maintain pressure; store water for fire fighting and emergencies/breakdowns; allow pumps to run at uniform rate and during off-peak power periods; reduce pipe sizes.
Types:
- Surface (ground-level) reservoirs — at high ground; masonry/RCC; large capacity.
- Elevated reservoirs (overhead tanks) — on staging (columns or shafts) where no high ground exists; provide the pressure head.
- Standpipes — tall cylindrical tanks resting on the ground.
Fittings: inlet and outlet pipes, overflow pipe, washout (scour) pipe, ventilators, water level indicator, manholes, ladder, lightning conductor.
Capacity
Total capacity = balancing (equalising) storage + breakdown storage + fire storage.
Balancing storage is found by the mass curve method (or the analytical/hydrograph method):
- Tabulate the hourly (or 2-hourly) supply and demand.
- Compute the cumulative (supply − demand).
- Balancing storage = maximum cumulative surplus + maximum cumulative deficit (i.e. the range of the cumulative difference).
Pipes
Pipe materials
| Material | Features |
|---|---|
| Cast iron (CI) | Durable, corrosion-resistant; heavy and brittle; largely replaced by DI |
| Ductile iron (DI) | Strong, tough, less brittle than CI; cement-mortar lined; widely used for mains |
| Mild steel (MS) | High strength, large diameters, lighter than CI; needs corrosion protection (lining and coating); used for trunk mains |
| Prestressed concrete (PSC) | Large diameters, high pressures, long life; heavy |
| Reinforced cement concrete (RCC) | Low-pressure mains, gravity conduits |
| Asbestos cement (AC) | Formerly common; light, corrosion-resistant; brittle; health concerns over asbestos have restricted its use |
| uPVC / PVC | Light, cheap, smooth, corrosion-free; for small diameters and moderate pressures; affected by sunlight and heat |
| HDPE | Flexible, light, joints by fusion welding; corrosion-free; good for house connections and difficult terrain |
| GRP / FRP | Light, corrosion-resistant, large diameters |
Joints
Spigot and socket (lead or rubber ring), flanged joints (pumping stations, valves), mechanical joints, flexible joints (settlement, river crossings), expansion joints (steel pipes exposed to temperature), push-on (rubber gasket) joints, welded joints (steel), solvent cement joints (PVC), butt or electrofusion joints (HDPE).
Hydraulic design
Hazen–Williams (SI):
≈ 140–150 for PVC/HDPE; about 130–140 for new DI/CI and steel with smooth lining; lower for old or tuberculated pipes.
Darcy–Weisbach:
Manning: (gravity conduits, sewers)
- Residual pressure at the ferrule point commonly recommended (CPHEEO): 7 m for single-storey, 12 m for two-storey and 17 m for three-storey buildings. Higher buildings use their own sumps and pumps.
- Velocities are kept moderate (roughly 0.6–2 m/s in mains) to avoid deposits at low velocities and excessive head loss and water hammer at high velocities.
- Equivalent pipe method — replaces a series/parallel combination with a single pipe of equal head loss.
- Hardy Cross method — iterative balancing of flows in loops: ( = 1.852 for Hazen–Williams, 2 for Darcy).
- Other methods: circle method, Newton–Raphson; software such as EPANET, LOOP and similar hydraulic models.