Combined Gravel Augmentation and Flood Control Area to Rehabilitate Riverbed Diversification: Lesson From Unexpected Effects of a 30 Years Flood (Upper Rhine)

Le résumé fourni par la source

ABSTRACT Gravel augmentation measures are common practices in the context of large, bypassing river reaches, with the aim of mitigating the adverse effects of sediment starvation conditions that frequently occur below dams. However, the impacts of these structures on bedload transport reactivation and habitat diversification are typically transient due to the rapid downstream transfer of sedimentary material. The objective of this study is to provide feedback on the management framework for the Old Rhine River, downstream of the Kembs diversion dam, which combines artificial channel widening by the implementation of a flood retention area and gravel augmentations located upstream. These two operations were carried out in two different contexts with different objectives by two different managers. A field survey was conducted from 2019 to 2024 with the following objectives: (i) to assess planimetric and topobathymetric changes in the flood control area from two airborne topobathymetric LiDAR surveys and three UAV surveys, (ii) to identify the possible contribution of upstream gravel augmentations to topobathymetric changes in the flood retention area from a bedload tracking survey (PIT tags) and (iii) to evaluate the effects of the flood control area on local hydraulic conditions and bedload transport during the July 2021 flood events. Following the 30‐year flood, a substantial gravel bar (18,000 m 3 ) was deposited in the main channel along the flood control area. The bar persisted during the whole monitoring period. The bedload deposition can be attributed to two factors: a decrease in local water flow energy as a result of the implementation of the flood control area enhanced by an unexpected widening of the upstream connection by bank/dike erosion (over a length of 25–30 m). The bedload is thought to have originated from the remobilization of the riverbed of upstream reaches and gravel augmentations. The findings of this study illustrate the efficiency of a channel widening downstream of gravel augmentations to diversify riverbed morphology.

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