Dead storage allowance in reservoir capacity Dead storage (reserved for silt deposition over project life) is generally taken as what fraction of the total capacity Q?

Introduction / Context:Reservoir capacity is divided into dead storage (below outlets) and live/active storage. Dead storage often includes allowance for silt deposition during the design life so that active storage is preserved as long as possible.

Given Data / Assumptions:

• General planning guidance; actual projects calibrate with sediment yield studies.
• Design life where significant siltation is expected.

Concept / Approach:Textbook and exam practice frequently take a dead-storage allowance on the order of about one-third of total for preliminary sizing in silt-laden rivers, subject to later refinement. This ensures intake function and maintains live storage for much of the project life.

Step-by-Step Solution:Relate dead storage to projected sediment volume over life.Select the commonly adopted preliminary fraction ≈ 1/3 Q when no site-specific study is given.

Verification / Alternative check:More detailed designs use trap efficiency curves and catchment sediment yield to refine the fraction, but 1/3 Q remains a standard exam value.

Why Other Options Are Wrong:

• Fractions 1/8 to 1/4 may under-allow silt in high-sediment basins for preliminary design.

Common Pitfalls:Using a rule-of-thumb instead of a sedimentation study at DPR stage; omitting dead-storage elevation relative to intakes.

Final Answer:1/3 of Q