Non-uniform flow conditions are often encountered in gravel-bed streams due to changes in river planimetric and altimetric configuration (i.e. a non-cylindrical geometry) and boundary conditions. In the present work, the implications of non-uniformity on sediment transport are investigated during the propagation of flashy floods (duration of no more than few hours), which are typical of rivers characterized by small catchments. A combined field sampling– numerical modeling methodology is here proposed with the aim to develop reliable and accurate sediment rating curves at gauging stations accounting both for bed and suspended load transport. The developed methodology is here applied to the case of a monitoring station located in the Versilia river (located in Tuscany, Central Italy), which is a rather wide gravel bed stream (the average width being about 35 m) where flashy flood events occur. Field measurements (i.e. water and bed levels, free surface width, flow velocities, samples of bed load and suspended load) were carried out during various flood events. The measurements were taken along 5 verticals distributed along the monitoring cross- section using classical USGS instruments and techniques; this number of verticals allowed us to capture the lateral variability of the flow and the sediment transport while reducing the errors in the estimation of the flow discharge due to the unsteadiness of a rapidly varying hydrograph. To overcome the limitations imposed by the small number of available monitored events, field measurements were carried out in combination with a 1D hydro-morphodynamic numerical model for the propagation of flood waves on the mobile bed in the river reach under investigation. In this way, we obtained physically-based reliable flow and sediment transport relations extrapolating field measurements to a wider range of conditions. Results show that the field data are interpreted by the numerical model only when the main physical ingredients of the problem (i.e. non-uniform flow conditions due to unsteadiness and lateral variability, bed mobility, fractional sediment transport) are included. Moreover, a simple interpolation of the field data would lead to unrealistic results.
Sediment transport during flash floods in a gravel-bed stream / Solari L.; Francalanci S.; Paris E.. - ELETTRONICO. - (2013), pp. 1-1. (Intervento presentato al convegno 2013 AGU Fall Meeting tenutosi a San Francisco).
Sediment transport during flash floods in a gravel-bed stream
SOLARI, LUCA;FRANCALANCI, SIMONA;PARIS, ENIO
2013
Abstract
Non-uniform flow conditions are often encountered in gravel-bed streams due to changes in river planimetric and altimetric configuration (i.e. a non-cylindrical geometry) and boundary conditions. In the present work, the implications of non-uniformity on sediment transport are investigated during the propagation of flashy floods (duration of no more than few hours), which are typical of rivers characterized by small catchments. A combined field sampling– numerical modeling methodology is here proposed with the aim to develop reliable and accurate sediment rating curves at gauging stations accounting both for bed and suspended load transport. The developed methodology is here applied to the case of a monitoring station located in the Versilia river (located in Tuscany, Central Italy), which is a rather wide gravel bed stream (the average width being about 35 m) where flashy flood events occur. Field measurements (i.e. water and bed levels, free surface width, flow velocities, samples of bed load and suspended load) were carried out during various flood events. The measurements were taken along 5 verticals distributed along the monitoring cross- section using classical USGS instruments and techniques; this number of verticals allowed us to capture the lateral variability of the flow and the sediment transport while reducing the errors in the estimation of the flow discharge due to the unsteadiness of a rapidly varying hydrograph. To overcome the limitations imposed by the small number of available monitored events, field measurements were carried out in combination with a 1D hydro-morphodynamic numerical model for the propagation of flood waves on the mobile bed in the river reach under investigation. In this way, we obtained physically-based reliable flow and sediment transport relations extrapolating field measurements to a wider range of conditions. Results show that the field data are interpreted by the numerical model only when the main physical ingredients of the problem (i.e. non-uniform flow conditions due to unsteadiness and lateral variability, bed mobility, fractional sediment transport) are included. Moreover, a simple interpolation of the field data would lead to unrealistic results.
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