Grainsize Patterns and Bed Evolution of the Rhone River (France): A Present-day Snapshot Following a Century and a Half of Human Modifications

Abstract:

Over the past 150 years the Rhône River has been heavily altered by human infrastructures. The first wave (1860 - 1930) of modifications consisted of dikes and groynes designed to narrow the channel and promote incision in order to facilitate navigation. A second period (1948 - 1986) involved the construction of a series of canals and dams for hydroelectricity production. These works bypass multiple reaches of the original channel and drastically reduce the discharge and sediment load reaching them. A comprehensive study underway is aimed at describing the present-day morphology of the Rhone along its 512 km length from its source at Lake Geneva to its sink at the Mediterranean Sea and quantifying the role of management works in the evolution to its current state.

Grainsize distributions and armour ratios were determined using a combination of Wolman counts on bars and in shallow channels and dredge samples collected from a boat in navigable reaches. Long profiles were constructed from historical bathymetric maps and bathymetric data collected between 1950 – 2010. Differential long profiles highlighting changes in bed elevation due to sediment storage and erosion were analyzed for three different periods: post-channelization, post-dam construction, and a recent period of major floods.

Results show a complex discontinuous pattern in grainsize associated with hydraulic discontinuities imposed by dams. The D₅₀ for bypass reaches is 45 mm compared to a D₅₀ of 34 mm in the non-bypass reaches. The lower D₅₀ as well as a finer tailed distribution in non-bypass reaches reflects fining associated with storage upstream of dams. Armour ratios are on average around 2 but are notably higher for reaches in the middle section of the Rhone.

The average incision rate was 1.8 cm/yr for the period of post-channelization and 1.2 cm/yr following dam construction, suggesting the post-dam Rhone was already partially armoured due to incision associated with channelization preceding dam construction. A period marked by large floods between 2001 – 2003 had an average incision rate of approximately 3 cm/yr. Changes in bed elevation for this period highlight destocking and restocking of fine sediments in reservoirs upstream of dams. Only a few downstream reaches where floods were most intense and grainsize is finer were truly active.