What is sediment connectivity?
Sediment connectivity describes the connected transfer between sediment sources, sink areas and the outlet in a river network, in terms of transfer rates and time.
Connectivity is a complex property of river networks. Many different sources supply sediment to the river network at specific locations, with a specific supply rate, and a specific distribution of input grain sizes. Morphologic properties of the network (i.e., river gradient and width) and water dischage determine the capacity of the river to transport different grain sizes, leading to different patterns of connectivity. At the same time, the presence of sediment from other sources and the resulting mixture of sediment in a river segment might also significantly impact sediment transfers. (Bracken, 2015)
In free-flowing rivers, sediments are supplied to the river from the river bed material, and from the watershed through either continuous process of weathering on the hillslopes or singular events like landslides. Lithology, tectonics, and climate together with biotic factors and human disturbances (landuse changes, mining, sediment extraction), influence the physical and chemical processes of weathering and delivery of sediment from the landscape to the river. (Sklar et al., 2017).
The transport of sediment within the river is controlled by the amount of energy available for transport in a section of the river (called transport capacity) and the availability and type of sediment to be entrained and transported. In return, the sediment transport alters the type of grains in the river bed, and potentially the geomorphology of the bed.
Sediment flux within river systems has been described as a ‘jerky conveyor belt’ from upstream to downstream areas (Ferguson, 1981), with an upstream area mostly characterized by erosion processes, a middle zone where sediment are transported and the downstream areas where deposition dominates.
Why is it important?
Sediment connectivity plays a fundamental role in the processes of fluvial geomorphology, ecosystem integrity, habitat provision, nutrient transport and generation and stability of the river bed and the related risks. Processes of entraining, transporting or depositing of sediment shape the river, creating new paths for the water and dynamic fluvial forms, and are strongly linked to the health of the river ecosystem itself and the availability of ecosystem services for human use. (Brismar, 2002).
Anthropic disturbances on rivers, e.g. , e.g. dam constructions or sediment delivery from agriculture, mining or deforestation, often lead to mayor cumulative impacts on network sediment connectivity and consequently major shifts in river processes (Kondolf et al., 2014; Richter et al., 2010; Vörösmarty et al., 2003).
Sediment starvation due to the combined sediment trapping effect of multiple dams on the same network can lead to mayor impacts. The water released from the dam, deprived of sediment, erodes the river beds and banks in the areas downstream the reservoir (Kondolf, 1997). This leads to habitat degradation and the destruction of fish spawning grounds, causing drops in biodiversity and fish catch. At the river mouth, the lack of sediment delivery can increase coastal degradation and shrinking of the river delta (Syvitski et al., 2009) Since sediment transport is linked with nutrient delivery, floodplains can experience loss in fertility due to lack of nutrient-rich sediment deposited from flooding events. (James, 2015).
However, the alteration of sediment connectivity is often underestimated or ignored while planning human interventions around or on river systems, leading to unforeseen impacts on the whole river system and costs for the people depending on its resources.
To avoid or reduce these impacts, the preservation of the natural sediment transport processes should be taken into consideration while planning these alterations. Models like CASCADE are born to with the purpose to provide a flexible tool for estimating sediment transport patterns and quantifying sediment connectivity alteration, in context where the lack of data or the large scale of the river basin prevent the use of empirical sediment transport modelling techniques. (…More)
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