Sediment connectivity is a fundamental property of river network, which directly influences thegeomorphological processes regulating the formation and development of the different in-channelgeomorphic units and leading to different river types. Alterations of sediment connectivity, e.g.caused by human disturbances such as dam construction or bed mining, are often followed bychanges in channel patterns resulting in potential radical shifts in river types, e.g., from braidedsystems to sinuous single channel, with consequent loss of river ecosystems associated withspecific river types.In this work, we analyze the connections between basin-scale sediment connectivity indices andriver types with the aim of advancing our quantitative ability to inter-relate channel forms andprocesses with type and amount of sediment fluxes available to the river channel. Our studyfocuses on the Vjosa river, Albania, which due to the limited anthropogenic bias still showcases alarge variety of fluvial forms, including ample sections of braided channels, some of the fewremaining in Europe and well renewed as ecological hotspots. The Vjosa river is now interested bylarge scale hydropower development plans, which may threaten the river unique ecological andmorphological value. We estimate sediment transport using the CASCADE model, a modellingframework for basin-scale sediment transport simulation, which generates spatially distributedinformation on sediment movement and connectivity in river networks. The model has beenvalidated using available data on bed load transport in a braided section close to the basin outletand surficial grain size distributions collected across the river network.By integrating CASCADE outputs (i.e., sediment fluxes and size distributions) with availablegeomorphic information at the network scale (e.g., channel slope and water discharge), wesuccessfully tested an empirical formula proposed in literature based on sediment concentration,median grain size, channel slope and bankfull discharge, to disentangle the drivers of braided orsingle channel patterns. We then tested the same threshold for different dam developmentportfolios, showing how even few new dams would alter current conditions in terms of type andamount of sediment availability, leading to multiple channel type shifts from braided to sinuoussingle channel across the network.For the first time, the incorporation of the CASCADE model with more traditional geomorphicanalysis of river system demonstrate how CASCADE sediment connectivity information advances our ability to interpret existing river system processes, to assess stability of the different channelforms and to evaluate resilience and identify tipping points of fragile system like the Vjosa basin.

Network scale sediment connectivity to explore stability andresilience of channel forms and river types in the Vjosa basin

Tangi, Marco;Bizzi, Simone;Castelletti, Andrea
2020-01-01

Abstract

Sediment connectivity is a fundamental property of river network, which directly influences thegeomorphological processes regulating the formation and development of the different in-channelgeomorphic units and leading to different river types. Alterations of sediment connectivity, e.g.caused by human disturbances such as dam construction or bed mining, are often followed bychanges in channel patterns resulting in potential radical shifts in river types, e.g., from braidedsystems to sinuous single channel, with consequent loss of river ecosystems associated withspecific river types.In this work, we analyze the connections between basin-scale sediment connectivity indices andriver types with the aim of advancing our quantitative ability to inter-relate channel forms andprocesses with type and amount of sediment fluxes available to the river channel. Our studyfocuses on the Vjosa river, Albania, which due to the limited anthropogenic bias still showcases alarge variety of fluvial forms, including ample sections of braided channels, some of the fewremaining in Europe and well renewed as ecological hotspots. The Vjosa river is now interested bylarge scale hydropower development plans, which may threaten the river unique ecological andmorphological value. We estimate sediment transport using the CASCADE model, a modellingframework for basin-scale sediment transport simulation, which generates spatially distributedinformation on sediment movement and connectivity in river networks. The model has beenvalidated using available data on bed load transport in a braided section close to the basin outletand surficial grain size distributions collected across the river network.By integrating CASCADE outputs (i.e., sediment fluxes and size distributions) with availablegeomorphic information at the network scale (e.g., channel slope and water discharge), wesuccessfully tested an empirical formula proposed in literature based on sediment concentration,median grain size, channel slope and bankfull discharge, to disentangle the drivers of braided orsingle channel patterns. We then tested the same threshold for different dam developmentportfolios, showing how even few new dams would alter current conditions in terms of type andamount of sediment availability, leading to multiple channel type shifts from braided to sinuoussingle channel across the network.For the first time, the incorporation of the CASCADE model with more traditional geomorphicanalysis of river system demonstrate how CASCADE sediment connectivity information advances our ability to interpret existing river system processes, to assess stability of the different channelforms and to evaluate resilience and identify tipping points of fragile system like the Vjosa basin.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/1171186
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