Monitoring Sediment Transport During Floods in Tuscany
This work describes the monitoring activity of flow and sediment discharges carried out in four river stations on four different rivers in Tuscany: Arno, Serchio, Magra and Versilia characterized by a large difference in the drainage areas ranging from about 100 km2 to about 4,100 km2. From December 2006 to November 2012, 43 floods events were monitored and measured in these stations, with the aim of deriving flow-sediment rating curves. Field measurements include more than 150 samples of bed load and suspended load and flow discharges up to about 700 m3/s. Laboratory analyses of collected samples show that bed load was mostly composed by gravel in the case of the Versilia and Magra rivers, while Arno and Serchio showed a dominant sand transport. The measured data constitute a precious and unique dataset at the regional scale for calibrating sediment budget model and morphodynamic model, and for predicting the sand and gravel volume fractions annually conveyed to the sea.
KeywordsSediment transport Field measurements Rating curve Gravel-bed river
The knowledge of sediment load carried by a river is a fundamental, yet difficult, task.
Estimates of sediment transport rates are required in the field of stream restoration, river engineering, and hydro-geological risk (Ferguson 1986). Problems such as sediment budgets and morphodynamic evolution of the shoreline, relys on verified estimates of sediment fluxes. An example of these problems is given by the shoreline of Tuscany region, located in the central part of Italy, which nowadays is, in some areas, experiencing large erosion phenomena, of the order of several meters per year, and the primary source for summer tourism is disappearing.
In order to have a better understanding of this type of problems, a reliable estimate of the volume of sediments annually delivered by the rivers to the coastal zone is needed. This estimate can be obtained through the development, at given monitoring stations, of accurate flow-sediment rating curves, i.e. relationships between flow and sediment discharge (Jansson 1996; Asselman 2000; Wilcock 2001).
In the framework of the present monitoring project, the rating curves were derived using an integrated approach combining several methods: field measurements, laboratory analyses, numerical modelling, analysis of the accuracy of the estimated flow discharges. A detailed description of the approach is reported in Francalanci et al. (2013), and applied to the case of the Versilia river: it appears to be suitable for the formulation of physically-based flow-sediment rating curves in gravel-bed rivers characterized by rapid and intense flood events. Moreover, it overcomes the limitations related to either purely field investigations, which suffer from a limited amount of measurements, or theoretical studies, which require some field calibration.
In this work, we present the whole set of collected measurements, which were further used to calibrate the sediment rating curves. Despite the large difference in the drainage areas and sediment size, results show that rating curves (total sediment discharge vs. flow discharge) display a similar behaviour thus suggesting the existence of common scaling laws.
70.2 Field work
Summary of monitored events and drainage areas
Drainage area at monitored station (km2)
Basin area (km2)
D50 (mm) at monitored station
For each flood event, the monitored cross-section was divided into a given number of vertical transects, properly chosen to minimize the errors due to flow unsteadiness and to optimize the description of the lateral variation of flow and sediment discharge. In each vertical, the following measurements were collected: water depth; flow velocity at several water depths by means of an USGS type AA current meter; sampling of suspended load and bed load were collected using, respectively, the depth-integrating reel type US D-74 and classical Helley-Smith bed load sampler with a 7.6 cm square intake opening and a sample bag with a 0.25 mm mesh of polyester.
70.3 Analyses and Results
The paper summarizes field sampling in unwadeable and flashy flood events occurred in Tuscany from 2006 to 2012 in four river stations. These data are precious because event based gravel-sand mixed transport data in rapidly changing conditions are largely missing, in particular for gravel-bed rivers in small catchments.
The collected measurements were used to calibrate reliable flow-sediment rating curves for each station, to be useful for river management and restoration practices and for morphodynamic models devoted to predicting the evolution of the river-coastal system.
This work has been funded by Regione Toscana within the projects “Realizzazione del primo stralcio della rete di monitoraggio su tre corsi d’acqua della Regione Toscana”, “Realizzazione del secondo stralcio del monitoraggio del trasporto solido nei corsi d’acqua della Toscana”, “Monitoraggio delle portate liquide e solide sul Fiume magra”. CERAFRI-LAV is also acknowledged for sustaining the publication of this work.
- Turnipseed DP, Sauer VB (2010) Discharge measurements at gaging stations: U.S. geological survey techniques and methods book 3, Chap. A8, 87 p. (Also available at http://pubs.usgs.gov/tm/tm3-a8/)