River processes: erosion, transportation, deposition
The total sediment load carried by rivers is the sum of washload, suspended load, and bedload. Bedload consists of coarse sediment particles located on a river bed and dragged along by flowing water. Suspended load normally consists of finer sediment particles that are light enough for the turbulence in the water to retain them in suspension. Wash load usually consists of very fine particles, such as clay particles or silt.
Erosion and sediment transport are driven by hydrological processes in a watershed and by the hydraulics of river flow, playing important roles with regards to the evolution of the landscape, loss of agricultural soils, the stability of riverbeds and -banks, natural hazards, coastal processes and dependability of water resources infrastructure.
In the upper course of a river, gradients are steep and river channels narrow. Vertical erosion is greatest in the upper course of a river. As a result of this, typical features include steep valley sides, rapids, gorges, and waterfalls. This is a sediment production area where erosion is dominant.
The middle course of a river has more energy and volume than the upper course. The gradient is gentler and the river channel deeper. This is a sediment transfer zone where both erosion and sediment deposition naturally occur along the river channels.
At the lower course of a river, the volume of water is at its greatest. The river channel is deep and wide and the land around the river is flat. This is a sediment storage area where deposition dominates.
Landscape changes
Under natural conditions, without the influence of man, the natural flow of sediment and water in rivers in addition to tectonic forces cause landscape changes. Large floods can move river channels to different locations, cause riverbank erosion, and change geomorphic and aquatic characteristics. Tectonic forces can lead to landscape changes that can redirect the courses of river channels. However, left to its own devices, nature adjusts over the long term and re-establishes the dynamic equilibrium of rivers and their surroundings.
Human impacts
Demands for a reliable supply of water, power, and flood protection services by a growing world population and expanding economies require the construction of dams and reservoirs to regulate river flows. In the absence of sustainable reservoir sedimentation management, such reservoirs capture large amounts of sediment carried by rivers flowing into them, disturbing sediment flow continuity. Sediment deposition in reservoirs reduces the amount of sediment available in downstream rivers, leading to river degradation as the river water erodes riverbeds and -banks in its yearning to re-establish the ideal sediment-water flow ratio required for dynamic stability.
Successful implementation of reservoir sedimentation management technology can re-establish sediment continuity in rivers regardless of the presence of dams and reservoirs and can lead to the dynamic stability of rivers closely resembling conditions prior to the presence of the dams and reservoirs. Maintaining reservoir storage capacity while concurrently promoting continuity of sediment flow in rivers is a win-win setting ensuring concurrent preservation of the environment and sustainable management of water resource infrastructure required to enhance and maintain life quality.
Climate impacts
Climate change is expected to increase hydrological variability requiring even greater reservoir storage volumes to regulate river flow for a reliable supply of water, power, and flood control services. Moreover, climate change is also expected to increase the amount of sediment carried by rivers, thereby increasing the amount of sediment deposited in reservoirs and reducing their useful storage volumes at a faster pace in the future. The effects of climate change highlight an even greater need for implementing sustainable reservoir sedimentation management technology that will benefit the environment and concurrently preserve reservoir storage space ensuring a reliable supply of services provided by dams and reservoirs.
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
ETH Zürich, 2017. Erosion and Sediment Transport, Hydrology. https://ethz.ch/content/dam/ethz/special-interest/baug/ifu/hydrology-dam/documents/lectures/hydrologie/lectures/HYI_HS17_Transp_10_Erosion.pdf