Tutorial: Computation of Live and Dead Storage Capacity of a Reservoir

Objective

To compute the live and dead storage capacities of a reservoir using the Elevation-Area-Capacity method.

Introduction

Reservoir capacity is divided into three major components:

1. Live Storage – The volume of water between the Full Reservoir Level (FRL) and the Minimum Drawdown Level (MDDL).

2. Dead Storage – The volume of water below the MDDL, which is not available for regular usage.

3. Total Storage – The sum of live and dead storage capacities.

The Elevation-Area-Capacity curve or table provides the necessary data for computing the storage capacity.

Fixing FRL and MDDL

The Full Reservoir Level (FRL) and Minimum Drawdown Level (MDDL) are determined based on several key factors:

Full Reservoir Level (FRL)

FRL is the highest level to which a reservoir is allowed to fill under normal operating conditions. It is determined based on:

1. Catchment Yield Analysis – The assessment of annual and seasonal inflow into the reservoir.

2. Flood Routing Studies – To ensure the dam can safely handle inflow without overtopping.

3. Reservoir Design Constraints – Structural safety limits and spillway capacity.

4. Water Demand Analysis – Ensuring adequate water supply for irrigation, domestic, and industrial use.

Minimum Drawdown Level (MDDL)

MDDL is the lowest level to which the reservoir can be drawn for operational purposes. It is determined based on:

1. Sedimentation Studies – The level below which sediment accumulation is significant.

2. Operational Requirements – Ensuring minimum water levels for power generation, irrigation, and ecological balance.

3. Intake Structure Elevation – The elevation of outlet works to maintain a minimum pressure head.

Given Data (Assumed for Demonstration)

Elevation (m)	Surface Area (km²)	Capacity (MCM)
160	5	10
165	8	30
170	12	70
175 (MDDL)	18	130
180	25	220
185 (FRL)	32	350

Steps for Computation

1. Identify Critical Elevations:

   • FRL = 185 m (Based on maximum designed storage and flood safety considerations)

   • MDDL = 175 m (Based on sedimentation and operational constraints)

   • Lowest bed elevation = 160 m

2. Determine Dead Storage Capacity:

   • Storage below MDDL (Elevation 175 m)

   • From table, at 175 m, capacity = 130 MCM

3. Determine Live Storage Capacity:

   • Storage between MDDL and FRL

   • Live Storage = Total Storage at FRL - Storage at MDDL

   • Live Storage = 350 MCM - 130 MCM = 220 MCM

4. Total Storage Capacity:

   • Total Storage = Live Storage + Dead Storage = 220 MCM + 130 MCM = 350 MCM

Conclusion

• Dead Storage Capacity = 130 MCM

• Live Storage Capacity = 220 MCM

• Total Storage Capacity = 350 MCM

This method helps in effective reservoir planning and management, ensuring sustainable water supply and sedimentation analysis.

Further Considerations

• In real-world applications, interpolation methods (such as the trapezoidal rule) can be used to compute storage at intermediate levels.

• Sedimentation over time may reduce storage capacity, requiring periodic surveys to update capacity curves.

By following this tutorial, one can efficiently compute reservoir storage capacities for water resources management.