## 6 Discussion
### Contrasting Responses of Bank Erosion after Riprap Removal between Site 1 and Site 2  
The eroded volumes at each site, five and three years after the removal of the bank protections at Sites 1 and 2, respectively, revealed two significantly contrasting situations. At Site 1, where the riprap was removed at the end of summer 2017, little bank erosion occurred. Between 2017 and 2022, only a small notch was created in the middle part of the restored bank, following a flood with a return period of 2 years (November 2019), and, one month later (December 2019), a flood of lower magnitude (P2\({}_{\text{B}}\); Figure 4). In contrast, the same two floods caused significant bank retreat at Site 2, corresponding to a volume of  
Figure 9: Stratigraphic and granulometric composition of the restored bank at Site 2. 9647 m\({}^{3}\) of eroded sediment (P1\({}_{\rm C}\)). In addition, at Site 2, another 6152 m\({}^{3}\) was eroded from the restored bank during the following study period (P2\({}_{\rm C}\)), caused by another 2-year flood (June 2020) followed eight months later (February 2021) by a lower magnitude flood. In total, at Site 2, almost 16,000 m\({}^{3}\) of sediment was eroded and reinjected into the channel during the first two monitoring periods (P1\({}_{\rm C}\) and P2\({}_{\rm C}\)). For these two periods, the recorded erosion was 14 times and 10 times more important, respectively, at Site 2 than at Site 1 (for emerged surfaces), attesting to two contrasting site characteristics. The primary factors controlling bank erosion are parameters comprising bank geometry (height and slope), bank material properties (grain size and porosity), and vegetation (roots, weight, and soil moisture modification) [50; 51; 52; 53; 54]. The restored bank at Site 2 was mainly composed of coarse sediments, i.e., sand and gravel, with only a few thin layers of fine cohesive sediments (Figure 9). The lack of cohesive sediment layers leads to high bank erosion potential because the undermining of the bank toe is not hindered by any cohesive slump blocks. This was different in the case of Site 1, where auger analyses carried out during the feasibility study by CEN Auvergne revealed a cohesive layer of variable depth (between 30 and 150 cm) composed of silts [36]. The presence of this layer, once destabilised through bank failures and the formation of slump blocks at the toe of the bank, will temporarily protect it from bank undercutting and consequently decrease lateral channel erosion rates [55; 56]. Beyond the shear stress necessary for the movement of cohesive slump blocks, they also alter the shear stress near the bank [57; 58]. Furthermore, the apparent cohesion at the substrate-slump block interface may delay the incipient motion of these cohesive slump blocks [59]. Another determining factor in the decrease in bank undercutting is the presence of marl [36]. The presence of marl bedrock at the base of the bank can strongly reduce bank undercutting due to its higher mechanical resistance to erosion than alluvium. This low erodibility potential at Site 1 was reinforced by two additional factors. First, the presence of abundant ligneous vegetation covering the bank slope increased the roughness and bank cohesion by the root network [60]. Second, the presence of rubble within the bank and the presence of immobile blocks, depending on their size but also on their prevalence, also contributed to the bank's resistance to bank erosion [61]. In addition to these intrinsic bank erodibility factors, bank erosion is guided by the local geomorphological context, which was more conducive to bank erosion at Site 2 than at Site 1. First, the restored bank at Site 2 was more exposed to direct flow than was the restored bank at Site 1, which was positioned at the apex of a low sinuosity (Figure 1).