Morphological processes and potential for lacustrine delta restoration

Most lacustrine deltas are nowadays highly anthropized systems. River training works during past centuries considered rivers mainly as vectors of water and sediments from a point source to a sink. However, numerous problems have been identified by residents and the competent authorities, such as lakeshore erosion near river deltas, land-use conflicts, and loss of biodiversity without efficiently managing flood risks. These human-induced changes on the deltaic environment led to the deterioration of ecosystem services. This paper focuses on the lacustrine deltas as a complex biotope at the interface of aquatic and terrestrial environments. It proposes a restoration strategy based on two simple geometrical parameters. This methodological approach defines the potential to recover a quasi-natural state of the delta using the slope of the upstream water course and the opening angle of the delta. The result of this work is the creation of a diagram like the ones classifying rivers according to their morphology. This chart will allow the design engineer to determine, given the actual slope of the water course, which shape a delta would ideally adopt without accounting for the downstream lacustrine morphogenic parameters. It will also help in determining the space required for the watercourse to fulfill its natural functions. This methodological approach is applied to a case study: The Reuss Delta, where a restoration project took place. This approach can be used as a pre-study to determine, according to the available space, if it will be possible to improve the morphological dynamics of the delta and to recover its non-anthropized state.