Suspended Sediment Load and Sediment Yield During Floods and Snowmelt Runoff In The Rio Cordon (northeastern Italy) (original) (raw)

2002

Abstract

Suspended sediment transport in high mountain streams display a grater time-space variability and a shorter duration (normally concentrated during the snowmelt period and the duration time of single floods) than in larger lowland rivers. Suspended sedi- ment load and sediment yield were analysed in a small, high-gradient stream of East- ern Italian Alps which was instrumented to measure in continuous water discharge and sediment transport. The research was conducted in the Rio Cordon, a 5 Km2 small catchment of the Dolomites. The ratio of suspended to total sediment yield and the re- lations between sediment concentration and water discharge were analysed for eleven floods which occurred from 1991 to 2001. Different patterns of hysteresis in the re- lation between suspended sediment and discharge were related to types and locations of active sediment sources. The within-storm variation of particle size of suspended sediment during a mayor flood (September 1994, 30 years<Tr<50 years) indicates a coarsening of transported material for increasing discharge. An analysis of grain size has shown that erosion areas on hill-slopes were the main source of suspended load. The relation between water discharge and S.S.C. for both floods and snowmelt runoff shows larger scatter for both series of data, with snowmelt data less scattered than rain- fall induced floods. This is accounted for by the variable effectiveness of erosion pro- cesses and sediment supply mechanisms during snowmelt and rainfall-induced floods. During snowmelt, erosion processes essentially consist in the removal of loose, fine- grained sediment from slopes by surface runoff; as a consequence, suspended sedi- ment transport takes place also with rather low discharges. Abundant suspended sedi- ment transport was recorded during the snowmelt period of May 2001, that followed a winter characterized by a huge snow cover and late snowfalls. Different sources of sed- iment contribute to suspended load during the May11, 2001, snowmelt induced-small- flood. Therefore, the most important erosion processes was a landslide that originates a debris-mud-flow, occur only as a consequence of infiltration-saturation processes and hypodermic runoff (without associated rainfall and/or snow fall). This situation caused the instability of a large part of a sub-catchment hill-slope, the movement and the transport of a big quantity of fine-medium sediment (4176 m3) as mud flow, part of which deposited on the fan tributary (4100 m3) and afterwards reached the main 1 collector and subsequent the instrumented station (400 m3). Total sediment yield of this snowmelt flood measured at the Rio Cordon station is three orders of magnitude greater than previous one. The main processes by which sediments are transferred from hillslope to the channel outlet is also shown. 2

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