Reservoir Planning

Reservoir planning determines the storage needed to regulate a variable river flow to meet a demand, and allocates the storage among dead, live and flood zones. The mass-curve (Rippl) method is the classic tool for finding the required live storage.

The reservoir is zoned: dead storage below the lowest outlet holds accumulated sediment over the reservoir life, live (conservation) storage between the dead-storage level and the full-supply level meets the demand, and flood storage above provides moderation of flood peaks.

Multi-purpose reservoirs balance competing uses — irrigation, hydropower, water supply, flood control and navigation — each with different level requirements, so operating rules and level fixation (FSL, MDDL) reconcile the demands over the year.

Relation 1 —

Write this relation with symbols exactly as in Irrigation & Water Power Engineering — BC Punmia before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Write this relation with symbols exactly as in Irrigation & Water Power Engineering — BC Punmia before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in Irrigation & Water Power Engineering — BC Punmia before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

The mass curve plots cumulative inflow against time; the demand is a line (or curve) of cumulative draft. The maximum vertical gap between the demand line drawn tangent to the mass-curve peaks and the following trough gives the required storage to meet that demand through the dry period — the essence of the Rippl method.

Dead storage is provided for the sediment expected to deposit over the design life; the trap efficiency (fraction of incoming sediment retained) and the sediment inflow rate determine how much storage is lost each year, so a reservoir’s useful life ends when sediment fills the live storage.

Reliability expresses the percentage of the period during which the demand is fully met; a reservoir sized for 100% reliability would be uneconomically large, so a design reliability (e.g. 75% for irrigation) is accepted.

Level fixation defines operation: the full-supply level (FSL) is the normal maximum conservation level, the minimum drawdown level (MDDL) is the lowest level for the intended use (e.g. minimum head for power), and the maximum water level accounts for flood routing above FSL.

State assumptions explicitly before using any relation for reservoir planning — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Water Resources viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Water Resources papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to reservoir planning.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Reservoir Planning appears in agricultural and municipal water supply. In Indian civil curricula this topic is tested because it connects theory to canals, reservoirs, and irrigation.

GATE and semester exams often combine reservoir planning with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use reservoir planning?" — answer with a lab, mini-project, or plant visit example if possible.

• Ignoring dead storage for sediment when sizing the reservoir.
• Misreading the maximum departure on the mass curve for required storage.
• Designing for 100% reliability, giving an uneconomic size.
• Confusing FSL (conservation) with the maximum water level during floods.

Before attempting reservoir planning problems, confirm you can:

1. Dead storage for sediment; live storage for regulation
2. Fixation of FSL and MDDL for flood moderation
3. Multi-purpose: irrigation, power, flood control, navigation

Revise the solved examples in Irrigation & Water Power Engineering — BC Punmia and one previous-year GATE or university paper for this unit.