Experimental characterization of root-reinforced riparian sediment deposits in a restored and widened river course

The Arbogne River is located mainly in the Swiss canton of Fribourg. The River was meandering through the plane at a very low gradient and several exceptional historic floods with large sediment accumulation are mentioned. After important river training works with straight alignment and riprap bank protection in the 70s, the river has been locally widened and revitalized between 2007 and 2009. Soon after completion, again due to large sediment accumu-lation and abundant vegetation growth, the level of the riverbed has risen and frequent flooding occurred once again. Fine sediments are being rapidly colonized by plants, which reinforced them via their root structure, thus limiting their erodibility and the river is no longer able to remobilize riparian deposits during floods. The mechanical properties of the freshly formed root reinforced "naturally stabilized" sediment deposits in the main river course have been investigated with large size undisturbed sediment deposit samples. Among other parameters, shear resistance with and without roots has been determined. The large size shear test showed a continuous displacement without shear failure up to four times larger compared to the unreinforced soil. Small-size soil samples are not able to reproduce the same behavior, as their size is of the same order or smaller than the root length.