Sediment management strategies

Management activities to address reservoir sedimentation may be classified into four broad types of methodologies:

1. Reducing sediment yield through erosion control and upstream sediment trapping
2. Managing flows during periods of high sediment yield to minimize trapping in reservoirs
3. Removing sediment already trapped in reservoirs using a variety of techniques
4. Adaptive structural and functional measures

A combination of strategies will usually be used, and the techniques most suitable for implementation will change over time as sediment conditions change.

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Case study classification

The case studies featured on this website are classified according to the following categories.

1. Reduce sediment yield from upstream

The initial stage in sediment management is reducing the amount of sediment entering the reservoir from the contributing catchment. This can be done by minimising sediment production in the catchment through erosion control methods and trapping sediment before it reaches the reservoir.

Reduce sediment production (watershed management)

This involves minimising soil erosion through land management approaches such as reforestation, controlled grazing and terracing.

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Trap sediment upstream of the reservoir

Structures like check dams and detention basins trap sediment by intercepting the flow of water and thereby settling and accumulating the sediment in the trapping structure.

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2. Route Sediments

Sediment routing refers to a family of techniques that take advantage of the timewise variation in sediment discharge, managing flows during periods of highest sediment yield to minimise sediment deposition in the reservoir. Routing techniques include:

By-pass channel/tunnel

A by-pass tunnel can be used to route sediment laden flows around the reservoir directly to the downstream reaches, or by diverting clear water into the reservoir.

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Sluicing

Drawing down the reservoir to pass sediment-laden floods through the impoundment at a high velocity to minimise deposition.

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Venting turbid density currents

Turbid currents are sediment-laden dense flows that flow along the bottom of the reservoir. Turbid currents can be subsequently released through the bottom outlet of the dam as they flow into the reservoir during flood events.

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In all cases the objective is to release sediment-laden water and impound clear water. This ensures the continual supply of sediment downstream reaches of the during high flow events, which mitigates downstream environmental impacts while maintaining the hydropower plant’s production efficiency.

3. Redistribute or remove sediments  

The principal means of redistributing sediment within the reservoir is by fluctuating the water levels. Hydraulic and mechanical techniques are commonly used to remove sediment from reservoirs to recover part, or all the initial storage capacity.

Modifying the operating level.

Raising the reservoir’s minimum operating water level slows the advancement of the reservoir’s delta, while drawing down the water level scours down the sediment deposited in the delta further into the reservoir.

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Pressure Flushing

Opening the dam’s bottom outlets when the water level is high releases water flows at high pressure. This facilitates scouring the deposited sediments in the vicinity of the outlets.

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Empty flushing

By taking advantage of the high pressure within the reservoir, accumulated sediment can be scoured from the bottom of the reservoir and passed through the bottom outlets of the dam by drawing down or completely emptying the reservoir.

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Mechanical excavation/dredging

Mechanical excavation allows for the removal of sediment from the bottom of the reservoir without having to lower the water level.

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4. Adaptive strategies

Adaptive strategies are actions to mitigate the impacts of sedimentation that do not involve handling the sediment. This includes structural modification strategies to increase the reservoir volume by raising the dam or increasing the resistance of equipment from sediment impacts by applying protective coating.

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Glossary

Here you will find definitions for terms mentioned frequently in the case studies.

Large Dam: A dam with a height of 15 metres or greater from lowest foundation to crest or a dam between 5 metres and 15 metres impounding more than 3 million cubic metres. [1]

Long-term: Within the context of sustainable reservoir sedimentation management, the term “long term” refers to multi-generational uses, in the ideal case.  We realize that this is not always possible, but the intent is to create projects that will last multiple generations.

Storage Capacity: Storage capacity refers to the gross volume of water which can be stored in reservoirs that are used for regulation of naturally variable incoming flows. The flow regulation capability of a reservoir depends on:

- The active storage capacity of the reservoir

- The relative hydrologic size of the reservoir (ratio of capacity to Mean Annual Runoff)

- The required level of reliability

- The over-year, seasonal or daily variability of incoming flows

[1] INTERNATIONAL COMMISSION ON LARGE DAMS (ICOLD) website, “Definition of a Large Dam” https://www.icold-cigb.org/GB/dams/definition_of_a_large_dam.asp, last accessed 16/06/2022

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