Chute channels in the Holocene high-sinuosity river deposits of the Firenze plain, Tuscany, Italy

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Abstract

This paper focuses on Holocene deposits of the Firenze alluvial plain (Northern Apennines, Italy) and deals with the sedimentary features of chute channels draining the down-river edges of the meander neck formed by 70 to 100 m wide and 1 to 1·5 m deep sinuous channels. Two main types of chute channels have been recognized. Type 1 is represented by 3 to 6 m wide and 0·5 to 1 m deep straight channels filled with mud aggregates overlying a basal gravel lag made of reworked caliches. These channels drained the point bar top during floods, and are thought to have been initiated as small rills when a shallow flow overpassed the downstream side of the point bar. Type 2 channels, 3 to 6 m wide and 1 to 1·5 m deep, are moderately to highly sinuous and filled with well-stratified sand and gravels sourced from nearby rocky highlands. Type 2 channels were connected to the main river channel also during the base flow stage. The transition from Type 1 to Type 2 channels is documented and is interpreted as the result of the meander cut-off process. Type 1 chute channels represent the early stage of the cut-off phase, when a headcut is incised on the down-river edges of the meander neck. The headcut migrates up-river across the meander neck during floods, when fast currents shape the chute channels into a straight route. The transition from Type 1 into Type 2 channels is linked to the connection of the up-river migrating headcut with the main channel and the termination of the cut-off process. At this stage, the cut-off channel is drained permanently and receives bedload from the main channel. The progressive shaping of the newly formed channel will convert it into the main channel and lead to the formation of an oxbow lake in the abandoned meander branch. Development of chute channels in the Firenze alluvial plain is thought to have heralded a decrease in sinuosity of the main channels, triggered by a climate-driven increase in water discharge.

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