Water distribution design – selecting the reservoir type when both gravity and pumping are used In municipal water-supply practice, when a distribution system operates with a combination of gravity flow (from an elevated storage) and pumping (from treatment works or low-level sources), which type of distribution reservoir is generally adopted to balance demand, provide pressure, and offer fire/operational storage?

Introduction / Context:
Cities commonly blend gravity and pumped delivery. Treatment plants or intakes pump into storage, while consumers receive water by gravity from strategically located reservoirs that provide pressure and reliability. Choosing the reservoir type is central to hydraulic performance and service continuity.

Given Data / Assumptions:

• The system uses both pumping (to fill storage) and gravity (to distribute to customers).
• Reservoir must equalize hourly demand, meet fire flow, and stabilize pressures.
• Urban setting with variable topography and zoning.

Concept / Approach:
An elevated service reservoir (ESR) stores treated water above ground on a tower so that flow to the network is driven by static head. Pumps can run at near-constant rate to refill the ESR, while consumers draw by gravity during peaks. An Intze tank is a structural form of ESR, so it is an example of the same concept rather than a distinct function. Ground-level reservoirs and clear-water sumps require booster pumping for distribution and cannot maintain pressure during outages. Stand pipes provide limited volume and are uncommon for large distribution zones.

Step-by-Step Solution:
Identify the need for head and storage → elevated storage provides both.Recognize that the pump fills the reservoir; end users are supplied by gravity from the reservoir.Conclude that an elevated service reservoir (ESR) is the standard choice.

Verification / Alternative check:
Design manuals show typical duty-point operation with pumps refilling ESRs during off-peak hours, while ESRs meet peak-hour demands without additional pumping to the network.

Why Other Options Are Wrong:

• Ground-level reservoir/sump: needs continuous pumping to maintain pressure.
• Stand pipe: limited volume, not suitable for large zones.
• Intze tank: not wrong in function, but it is a type of elevated tank; the general correct answer is ESR.
• Clear-water sump: storage before pumping only, not distribution storage.

Common Pitfalls:
Confusing structural form (Intze) with function (elevated storage). The functional requirement is met by an ESR regardless of its structural shape.

Final Answer:
Elevated service reservoir (overhead tank)