Reservoir Sedimentation

Reservoir sedimentation is the accumulation of sand, silt and clay carried by inflowing rivers after their velocity and sediment-transport capacity decline inside a reservoir. The deposits occupy space originally available for water and progressively reduce useful storage, meaning the storage that can be withdrawn or operated for purposes such as irrigation, water supply and power generation.

How storage capacity is lost

Coarser sediment generally forms a delta near the reservoir head, while finer particles may travel farther before settling. Deposition raises the reservoir bed and alters its elevation-capacity relationship, so the reservoir stores less water at the same water level.

• Some sediment may initially occupy the lower dead-storage zone provided below the minimum drawdown level.
• Sediment can also accumulate within live storage, depending on reservoir shape, inflow conditions and operating practices.
• As deposits build up, the difference between full-reservoir storage and unusable lower storage contracts, directly reducing usable water volume.

Consequences for dam performance

Loss of live capacity weakens the reservoir's ability to store seasonal inflows and release water during dry periods. Thus, the dependable yield and operational flexibility of the project decline even though the dam structure remains intact.

• Reduced conservation storage can affect irrigation, drinking-water supply and hydropower regulation.
• Encroachment into reserved flood-storage capacity can reduce the space available for moderating flood peaks.
• Deposits may obstruct low-level outlets or intakes, while sediment passing through machinery can cause abrasion.
• Upstream delta growth can raise local bed levels and produce backwater effects.

Assessment and management

The rate of capacity loss depends on catchment sediment yield, reservoir geometry, operating pattern and trap efficiency, which is the proportion of incoming sediment retained. Periodic hydrographic or bathymetric surveys are used to revise capacity estimates.

• Catchment-area treatment and erosion control reduce sediment inflow at its source.
• Sluicing, flushing and sediment bypassing seek to move sediment through or around the reservoir.
• Dredging removes deposited material but is generally constrained by cost, scale and disposal requirements.

How UPSC asks this