Importance of storage
Reservoir storage is required to regulate fluctuating, the natural river flows thereby ensuring a reliable supply of water, power, and flood control services. As hydrologic variability and sediment yield increase due to climate change, the need for adequate reservoir storage will increase apart from the demands imposed by a growing world population. Development and management of dams and reservoirs have failed to meet the increased demand for reservoir storage. Estimates indicate that the net global storage volume of reservoirs is reducing at a faster rate than what is added through the construction of new dams. Per capita storage is now estimated to resemble conditions last experienced in the 1960s and continues to trend downward. Increased sediment yields due to climate change will further exacerbate conditions if reservoir sedimentation is not effectively managed. This negative trend can be reversed by implementing sustainable reservoir sedimentation technology at both existing and new dams and reservoirs, thereby satisfying future demands for a reliable supply of water, power, and flood control services and mitigating environmental impact.
The International Hydropower Association (IHA), together with the World Bank hosted South Asia Water Initiative (SAWI), launched the Hydropower Sediment Management Knowledge Hub to promote strategies and case studies for effective sediment management. View the case studies here
Impacts and challenges
for water supply
The increased future need for reservoir storage ensuring a reliable supply of water for domestic, industrial, and agricultural purposes demanded by the effects of climate change and a growing world population is not currently satisfied. The amount of reservoir storage globally lost to reservoir sedimentation is greater than the amount of storage added by the construction of new dams. Failing to implement reservoir sedimentation management technology to reverse this trend will lead to reduced freshwater supplies. Importantly, the agriculture industry, i.e., food production, consumes more than two-thirds of the freshwater supply on earth. Recognizing this far-reaching impact affecting the entire world emphasizes the importance of preserving reservoir storage through the implementation of reservoir sediment management technology implemented at both existing and new projects.
For hydropower
Hydropower energy production reduces annually at a global rate of 1 per cent due to loss of reservoir storage capacity and damage to electromechanical equipment caused by sedimentation. Extreme rainfall and erosion due to climate change will increase the rate of sedimentation and increase the global rate of production loss.
Sediment abrades hydraulic machinery causing efficiency declines in turbines and pumps, also affecting valve and gate seals. Sediment also abrades spillways, aprons, and outlets that pass coarse bed material load, which may range from sand to boulders. Solutions to these problems by implementing sustainable sediment management technology continually advances.
for flood control
Reservoirs can be used to manage floods, as they often are, by temporarily storing inflowing floodwaters and then slowly releasing it downstream. The maximum discharge of released flows is smaller than that flowing into a reservoir, thereby reducing its impact and potential threats to human life and property.
The ability of a reservoir to temporarily store floodwaters depends on the size of its storage volume. Larger reservoir storage volumes can more readily be used to manage floods than smaller ones. As storage volumes decrease due to reservoir sedimentation the ability of dams and reservoirs to manage floods is severely compromised.
This is a concern as flood magnitudes are expected to increase in the future because of climate change. The need to preserve reservoir storage volumes to manage floods now and in the future through the implementation of reservoir sedimentation management technology is obvious.
For the environment
While sediment is needed to build aquatic habitats and reintroduce nutrients, a careful balance between the amount of sediment transported by the water and the amount of flow in a river is necessary to ensure dynamic stability. On the one hand, too much sediment can cause ecosystem decline, while, on the other hand, too little sediment can result in undesirable aquatic conditions harmful to the environment.
Removal of deposited sediment from reservoirs, which after removal may be discharged into a downstream river, can lead to sediment loads higher than what naturally existed in the river prior to construction of a dam. Large sediment loads can cause poor water quality, algal blooms, and deposition-build up. High turbidity associated large sediment loads can cause water temperatures to rise with consequent decreases in levels of dissolved oxygen in the water. High suspended sediment loads can block sunlight and decrease photosynthesis rates of aquatic vegetation.
Failure to remove deposited sediment from reservoirs leads to reduced sediment loads in downstream rivers, lower than what naturally occurred prior to the existence of the reservoirs. The resulting imbalance between sediment transport and deposition means that existing aquatic habitats cannot be maintained, and news ones cannot be formed. In addition to the effects on aquatic life, the reduced sediment loads lead to the flowing water entraining greater amounts of new sediment from the riverbed and -banks, causing physical changes to the surrounding terrain. Notably, far-reaching impacts occur when reduced sediment loads in rivers lead to enhanced and unwanted coastal erosion.
Implementation of sustainable reservoir sedimentation management technology can promote sediment continuity mitigating environmental concerns.
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
Annandale, G. 2013. Quenching the Thirst: Sustainable Water Supply and Climate Change. CreateSpace Independent Publishing Platform. ISBN: 1480265152, 9781480265158
Morris, G. L., and J. Fan. 1998. Reservoir Sedimentation Handbook: Design and Management of Dams, Reservoirs and Watersheds for Sustainable Use. New York: McGraw-Hill