6.2 Intake and Distribution Systems

6.2 Intake and Distribution Systems

Introduction to Conveyance and Distribution

Once a water source is selected and its quality understood, the engineering challenge shifts to its reliable abstraction and efficient delivery to the consumer. This involves two critical subsystems: the intake works, which safely withdraw raw water from the source, and the distribution network, which conveys treated water under adequate pressure to every user. This unit examines the design, components, and hydraulic principles governing these systems, focusing on practical choices regarding intakes, pipes, valves, storage, and network configuration. The goal is to ensure a continuous, pressurized, and equitable supply of water throughout the service area.

---

1. Intake Structures

1.1 Definition and Purpose

An intake structure is a civil engineering construction placed at a water source to permit the withdrawal of water and its initial conveyance (usually via an intake conduit) to the treatment plant or pumping station.

Primary Functions:

1. To draw water from the source during all seasonal fluctuations in water level.

2. To exclude large floating and suspended debris (trash, ice, fish).

3. To provide a point for measuring flow and sometimes for initial chemical dosing (pre-chlorination).

4. To house necessary screens and sometimes low-lift pumps.

1.2 Types of Intakes

The type is dictated by the source (river, lake, reservoir) and its characteristics (depth, level variation, sediment load).

1. River Intake:

   • Constructed on the bank of a river, usually projecting into the flow.

   • Includes an inlet with coarse trash racks, a conduit leading to a sump well on the bank, and then to the pump house.

   • Suitable for: Rivers with stable banks and sufficient depth.

2. Reservoir or Lake Intake:

   • Located within the impounded water body.

   • Tower Intake: A concrete/masonry tower built inside the reservoir, connected to the dam or shore by a footbridge. Intake ports at multiple levels allow water withdrawal from different depths to select the best quality water (e.g., avoiding surface algae or bottom silt).

   • Submerged Intake: A simple intake conduit placed below the water level, covered with a screen. Used for small supplies from ponds or lakes.

3. Canal Intake:

   • A simple structure to divert water from an irrigation or power canal.

   • Consists of an inlet opening with a trash rack and a sluice gate to regulate flow.

4. Infiltration Gallery:

   • A horizontal, perforated conduit laid underground near a river or lake to collect filtered groundwater from the banks.

   • Produces water of better quality (pre-filtered) and more constant temperature than direct surface water.

1.3 Factors Affecting Location Selection

Choosing the intake site is a critical decision involving hydrological, environmental, and construction considerations.

1. Hydrological Factors:

   • Adequate Depth: Must ensure the inlet is always submerged, even during the dry season's lowest water level.

   • Quality of Water: Site should be upstream of any major pollution sources (settlements, industries, agricultural runoff).

   • Low Turbidity/Silt: Avoid locations with high sediment load or scouring action.

   • Stable Bed and Banks: To prevent structural failure due to erosion.

2. Topographical and Geotechnical Factors:

   • Foundation Conditions: Must be capable of supporting the structure.

   • Accessibility: For construction and maintenance.

   • Proximity to Treatment Plant: To minimize the length and cost of the intake conduit.

3. Navigational and Environmental Factors:

   • Should not obstruct navigation (in major rivers).

   • Minimal environmental impact on aquatic life. Screens should be fish-friendly.

4. Future Considerations:

   • Anticipated changes in land use upstream.

   • Long-term impacts of climate change on water levels and flow patterns.

---

2. Pipe Materials, Joints, Valves and Fittings

2.1 Pipe Materials

Selection depends on pressure, soil conditions, water quality, cost, and lifespan.

Material	Key Properties	Common Uses	Advantages	Disadvantages
Ductile Iron (DI)	High strength, good impact resistance, corrosion-resistant (cement-mortar lined).	Main transmission & distribution lines.	Durable, withstands high pressure & external loads, easily tapped.	Heavy, requires external protection in corrosive soils, relatively high cost.
Polyvinyl Chloride (PVC - uPVC)	Lightweight, smooth interior (low friction), corrosion-proof, easy to install.	Mainly for distribution lines, service lines.	Low cost, easy jointing, chemically inert.	Lower strength, can become brittle in UV/sunlight, not suitable for high temps/pressure.
High-Density Polyethylene (HDPE)	Flexible, high impact strength, excellent chemical resistance, can be welded (butt/electrofusion).	Service connections, trenchless installations, river crossings.	Leak-free joints, good for seismic zones, long lengths reduce joints.	Requires specialized welding equipment, susceptible to damage from sharp rocks.
Mild Steel (MS)	Very high strength, can be fabricated for large diameters.	High-pressure transmission mains, penstocks.	Can withstand very high internal pressure.	Highly susceptible to corrosion, requires internal lining and external wrapping/cathodic protection.
Reinforced Concrete (RCC/PCC)	High compressive strength, durable, can be cast in-situ for large diameters.	Large-diameter low-pressure transmission lines.	Economical for very large sizes, good corrosion resistance.	Heavy, requires careful jointing, brittle (low tensile strength).
Asbestos Cement (AC)	Lightweight, corrosion-resistant, smooth bore. (Note: Use is declining due to health hazards)	Older distribution systems.	Was economical, easy to cut and joint.	Brittle, health risks from asbestos fibers if broken, low impact strength.
Galvanized Iron (GI)	Steel pipe coated with zinc for corrosion protection.	Small-diameter indoor plumbing, service lines.	Good corrosion resistance for small sizes.	Clogging due to scale formation over time, limited to small diameters.

2.2 Pipe Joints

Joints must be leak-proof, strong, and flexible (to accommodate minor settlement).

1. Flexible Joints (Allow movement):

   • Spigot and Socket (Push-on) Joint: Rubber gasket in socket provides seal. Common for DI, PVC.

   • Flanged Joint: Bolted flanges with a gasket. Used inside pump houses, valve chambers. Rigid.

   • Mechanical Joint (MJ): A gland compresses a gasket against the pipe. Used for DI.

2. Rigid Joints (No movement):

   • Welded Joint: For steel pipes. Provides a continuous, strong, leak-proof connection.

   • Screwed/Threaded Joint: For GI and small steel pipes.

   • Solvent Cement/Weld Joint: For PVC pipes; chemical fusion forms a permanent bond.

   • Butt Fusion Joint: For HDPE pipes using a fusion welding machine.

2.3 Valves

Valves control flow, pressure, and direction in a pipeline network.

1. Sluice Valve (Gate Valve)**:

   • Purpose: To start or stop flow in a pipeline (fully open or fully closed).

   • Operation: A wedge-shaped gate is raised or lowered.

   • Location: At every major junction and at intervals (every 500m) on long pipelines for isolation during repairs.

2. Check Valve (Non-Return Valve):

   • Purpose: To allow flow in one direction only, preventing backflow.

   • Operation: Closes automatically if flow reverses.

   • Location: On the discharge side of pumps to prevent backflow when the pump stops.

3. Butterfly Valve:

   • Purpose: To regulate (throttle) flow as well as isolate.

   • Operation: A disc rotates 90° within the pipe.

   • Advantage: Compact, lighter, and cheaper than sluice valves for large diameters.

4. Air Valve:

   • Purpose: To release air that accumulates at high points (which can block flow) and to admit air when the pipe is emptied (to prevent vacuum collapse).

   • Types: Single Air Release (small air pockets), Air & Vacuum (large air volumes).

5. Pressure Reducing Valve (PRV):

   • Purpose: To reduce and maintain a constant lower downstream pressure in zones with excessive head.

6. Scour Valve (Blow-off Valve, Drain Valve):

   • Purpose: To empty or flush a pipeline section by draining out water and sediments.

   • Location: At all low points in the pipeline.

2.4 Pipe Fittings

Used to change direction, branch off, or change pipe size.

1. Bends/Elbows: 90°, 45°, 22.5°.

2. Tees: For making a branch connection.

3. Reducers: To connect pipes of different diameters.

4. Couplings/Sockets: To connect two straight pipes.

5. Nipples: Short pipe lengths with threads.

6. Hydrants: Outdoor water outlets for firefighting and street washing.

---

3. Break Pressure Tanks

1. Purpose: To dissipate excess pressure in long gravity-fed pipelines. When a pipeline follows a steep downhill slope, the static head (due to elevation difference) can create dangerously high pressures at the bottom. A Break Pressure Tank (BPT) interrupts this continuous head, resetting the hydraulic grade line (HGL).

2. Location: Placed at an intermediate elevation along the pipeline route.

3. How it Works:

   • Water from the upstream pipe enters the tank, discharging freely into the atmosphere.

   • The water level in the tank establishes a new, lower HGL for the downstream pipe.

   • The downstream pipe takes off from near the bottom of the tank.

4. Design Considerations:

   • Must have adequate capacity to handle inflow variations.

   • Must be vented to the atmosphere.

   • Inlet and outlet pipes are separated to avoid short-circuiting.

   • Equipped with overflow and drain pipes.

---

4. Service Reservoirs and Capacity Determination

4.1 Purpose and Types

Also called Clear Water Storage or Balancing Reservoirs.

Primary Functions:

1. Balancing Storage: To store water during low-demand hours (night) to meet demand during peak hours (morning/evening). This allows treatment plants and pumps to operate at a more uniform rate.

2. Emergency Storage: To supply water during source/pump/treatment plant failure or during fires.

3. Pressure Stabilization: Elevating the reservoir provides the necessary head (pressure) for distribution.

Types Based on Location:

• Ground Level Reservoir (GLR): Constructed at ground level. Requires pumping to supply the network (used for balancing only).

• Elevated Storage Reservoir (ESR): Supported on towers. Provides gravity pressure for distribution.

• Standpipes: Tall cylindrical tanks resting on the ground; height provides pressure. Economical for moderate heights.

4.2 Capacity Determination

The total storage capacity is the sum of three components:

$\text{Total Capacity} = \text{Balancing Storage} + \text{Emergency Storage} + \text{Fire Storage}$

1. Balancing Storage (or Equalizing Storage):

   • Calculated by analyzing the hourly demand curve over 24 hours.

   • It is the difference between the cumulative inflow (assumed constant at average rate) and the cumulative variable outflow (demand).

   • Simplified Empirical Rule: Often taken as 25% to 50% of the Average Daily Demand (ADD). A common design value is 30% of ADD.

2. Emergency Storage:

   • To provide water during breakdowns or maintenance.

   • Typically taken as 12 to 24 hours of Average Daily Demand.

   • Common value: 25% of ADD.

3. Fire Storage:

   • Based on firefighting demand calculated using formulas (e.g., Kuichling's) for the design population.

   • Usually expressed as a volume required for a certain duration (e.g., 4-10 hours of fire flow).

   • May be kept separately in underground reservoirs.

Typical Total Capacity: For a medium-sized town, total storage is often designed for one-half to one times the Average Daily Demand (i.e., 0.5 to 1.0 day's average supply).

---

5. Design of Water Distribution Systems

The network of pipes that delivers water from the service reservoir to every consumer.

5.1 Network Configurations

1. Dead-End or Branch System (Tree System):

   • Layout: One main pipeline runs through the center of the area with sub-mains and branches, resembling a tree. Flow is unidirectional towards dead ends.

   • Advantages:

     • Simple layout and design.

     • Pipe diameters decrease progressively towards the ends.

     • Lower initial cost (less pipe length).

   • Disadvantages:

     • Stagnation of water at dead ends, leading to quality deterioration.

     • In case of a pipe break, all downstream consumers are cut off.

     • Lower pressures at the ends.

2. Grid-Iron or Looped System:

   • Layout: Mains, sub-mains, and branches are interconnected, forming loops. Water can reach a point from multiple directions.

   • Advantages:

     • No dead ends, water circulation prevents stagnation.

     • A break can be isolated with minimal disruption; water is rerouted through other pipes.

     • More uniform pressure throughout the system.

   • Disadvantages:

     • More pipe length is required.

     • Design and analysis are more complex.

     • Higher initial cost.

3. Ring System:

   • A main pipe surrounds the area (like a ring road) with branches supplying inward. A special case of a looped system, good for towns/cities.

Modern practice strongly favors the Looped System for reliability and water quality.

5.2 Design Principles and Steps

The goal is to determine the economic diameter for each pipe in the network such that, under peak flow conditions, adequate pressure is available at all points (typically a minimum of 15 m head or 1.5 kg/cm² at the consumer's tap).

1. Data Collection:

   • Detailed map of the area with contours.

   • Population density and projected water demand (Peak Hourly Demand).

   • Location of fire hydrants and their required flow.

2. Layout of Network:

   • Draw the proposed pipe network (preferably looped) on the map, following roads.

   • Divide the area into demand zones and assign the peak hourly demand to each node (junction of pipes).

3. Preliminary Pipe Sizing:

   • Assume a reasonable velocity range (0.6 - 1.2 m/s for economic design, up to 2 m/s for transmission mains).

   • Use the Continuity Equation to estimate diameter: $Q = A \times V = \frac{\pi}{4} D^2 \times V$

   • Round up to the nearest commercially available diameter.

4. Hydraulic Analysis (Most Critical Step):

   • The designed network must be analyzed to compute pressures at all nodes under peak flow conditions.

   • Methods:

     • Hardy Cross Method (Head Balance Method): A manual iterative method for analyzing flow in looped networks. It balances heads around each loop based on the principle that the algebraic sum of head losses around any closed loop is zero. $\sum_{Loop} h_f = 0$ where head loss $h_f$ is given by the Hazen-Williams or Darcy-Weisbach formula.

     • Computer Software: Modern standard practice. EPANET (free from USEPA) is the industry benchmark for modeling water distribution hydraulics and water quality.

5. Head Loss Calculations:

   • Hazen-Williams Formula (Most Common for Water): $h_f = \frac{10.67 \cdot L \cdot Q^{1.852}}{C^{1.852} \cdot D^{4.87}}$ where $h_f$ = head loss (m), $L$ = pipe length (m), $Q$ = flow (m³/s), $D$ = diameter (m), $C$ = Hazen-Williams roughness coefficient (e.g., 130 for new PVC, 100 for old CI).

   • Darcy-Weisbach Formula (more fundamental): $h_f = f \frac{L}{D} \frac{V^2}{2g}$

6. Check Pressure and Iterate:

   • Starting from the known HGL at the reservoir, subtract head losses along pipes to find the HGL and pressure at each node.

   • Pressure Criteria: Minimum residual pressure at peak flow should be 15-20 m. Maximum pressure should be less than pipe/material rating (often 60-70 m).

   • If pressures are inadequate or excessive, adjust pipe diameters and repeat the analysis.

Conclusion: The intake and distribution system forms the circulatory system of a water supply scheme. Its design requires careful integration of civil, mechanical, and hydraulic principles to ensure that water, once made potable, is conveyed reliably and with sufficient pressure to the point of use. The move towards looped networks analyzed with computer models represents the standard for achieving robust, efficient, and safe water distribution.