Author(s): David Florian Vetsch; Claudia Leuch

Linked Author(s):

Keywords: Bed load; Suspended load; Delta formation; Sedimentation process; Numerical long-term modelling

Abstract: The Alpine Rhine is the main tributary of Lake Constance. Just upstream of the river mouth, its mean annual load amounts to ca. 2×10⁶ tons, consisting mainly of sand and silt (90%) and a minor portion of gravel (10%). This considerable sediment load led to the formation of a river delta at the south-eastern part of the lake after the last glacial period. At the end of the 19th century, river training of the Alpine Rhine included the cut-off of former distributary channels to achieve a straightening of the river. Delta formation thus continued at a bay not affected by sedimentation for centuries. As a consequence, the channel slope decreased upstream of the river mouth and reduced the intended flood protection. As a mitigation measure, the river channel was extended into the lake by construction of levees built on existing delta deposits, starting in the 1970s. The extension of the stream course into the lake conveys fine sediments into deeper waters and avoids further sedimentation of the bay. In the last decade, the extension of the channel was finished for the time being. However, deposition of sediments at the newly created river mouth will continue. A numerical 1D model based on river cross sections and the depth-averaged shallow water equations considering bed load and suspended sediment transport as separate modes is used to study the long-term development of river-mouth structures of the Alpine Rhine in Lake Constance. The computational domain comprises the 3.55 km long river extension and part of the upstream channel and the lake. At the upstream boundary, a unified bed- and suspended load relation is defined based on measured sediment load at the upstream gauging station. The model is calibrated and validated by simulation of bed level changes of two well-documented flood events with different sediment volume balance. In a first step, the model is used to simulate basic delta sedimentation processes over several decades for various scenarios with varying inflow hydrograph, sediment feed and lake level, what enables the estimation of mean progression rates. Subsequently, the potential and limitations of future channel extension are discussed considering continuous rise of bed level and dredging capacity. The findings may raise the discussion about the scope and development of new river-mouth structures and may support the development towards a delta with natural look.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022292

Year: 2022