A sustainable approach to Sediment management

Continuity of sediment flows is an essential element ensuring dynamic stability of rivers and river basins. Deposition of sediment in reservoirs disturbs the continuity of sediment flows in rivers while concurrently reducing the amount of reservoir storage available for flow regulation. Reducing net available reservoir storage reduces the reliability of water supply, power and flood control services offered by dams and reservoirs. Concurrently it disturbs the continuity of sediment flows resulting in the degradation of rivers and aquatic habitat, causing increased harm to the environment, and increasing potential impact on human life and property.

Sustainable management of sediment in reservoirs enables the creation of intergenerational equity, the cornerstone of sustainable development, while concurrently limiting harm to the environment. Preserving reservoir storage volume through sediment management makes it possible for future generations to rely on the services offered by existing infrastructure and not be burdened by decommissioning infrastructure from which they may not have received any benefit but was made redundant through storage loss due to reservoir sedimentation.

Effective reservoir sedimentation management technology should aim at preserving the services provided by existing water resource infrastructure, thereby creating intergenerational equity while concurrently mitigating environmental impacts caused by dams. Existing dams may require modifications to allow the implementation of reservoir sediment management technology.

To sustainably develop new dams, feasible sediment management strategies should be identified in advance and elements for its future implementation incorporated in the original design.

Sediment Monitoring

Historical sedimentation data is necessary to determine the rate of storage loss, identify management alternatives and calibrate sediment transport models. To select and develop design and operational sediment management strategies, it is necessary to evaluate the different management alternatives including sediment transport modelling to evaluate its costs and environmental impacts. Once the strategy is in place, both short- and long-term monitoring should be in place to assess the effectiveness of the implemented strategy, allowing mechanisms to adapt and adjust the strategies as required. 

Sediment management techniques

Management activities to address reservoir sedimentation may be classified into four broad categories:

Reducing sediment yield from upstream

Reducing the amount of sediment entering reservoirs at source stabilizes the upstream catchment while concurrently reducing the amount of sediment flowing into reservoirs. Typical methods used to stabilize upstream catchments and reduce sediment yield are reforestation and sediment retention structures. Adequate numbers of sediment retention structures have an almost immediate effect in reducing sediment yield. Used in conjunction with revegetation such reduction can be maintained over the long term.

Managing flows during periods of high sediment yield

Sediment routing refers to a family of techniques, including sluicing, bypassing and density current venting, aimed at passing sediment flows either past or through reservoirs during flood periods when the sediment loads of rivers are at their highest. The objective is to maintain sediment flow continuity while concurrently minimizing the amount of sediment depositing in reservoirs.

Redistribution or removing sediment deposits

Deposited sediment may be removed from or redistributed within reservoirs. Removal of deposited sediment increases total reservoir storage. Redistribution of deposited sediment moves deposited sediment from active storage zones in reservoirs to dead storage zones. Redistribution of deposited sediment can temporarily increase the active storage capacity of reservoirs without removal of sediment. However, in the long term, the redistribution approach will require removal as the volume of deposited sediment increases beyond available dead storage capacity.

Sediment removal can be accomplished by using techniques such as flushing, and hydraulic or mechanical dredging. Redistribution of deposited sediment is accomplished by employing flow within a reservoir accomplished through fluctuating water levels or by simply moving it from one location to another within reservoirs using hydraulic or mechanical dredging techniques.

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.

references

Annandale, George W., Gregory L. Morris, and Pravin Karki. 2016. Extending the Life of Reservoirs: Sustainable Sediment Management for Dams and Run-of-River Hydropower. Directions in Development. Washington, DC: World Bank. doi: 10.1596/978-1-4648-0836-8. License: Creative Commons Attribution CC BY 3.0 IGO