## 6 Discussion
### Lessons to Be Learned from Process-Based Bank Restoration  
The application of a freedom space by removing riprap must consider two sets of criteria: (1) the level of the urban and human pressures in the floodplain and (2) the capacity of the river to erode [27]. The level of urban and human pressure is the first factor concerning the establishment of a freedom space. It should also be noted that in the case where the implementation of a freedom space does not conflict with human issues, it may still be questioned by local stakeholders. The acceptability of such a restoration measure can be achieved only through a good understanding of the river system and interest in restoring river systems [29; 30]. The different criteria to be considered in the assessment of the second point, i.e., the capacity of the river to start eroding its banks again, are the degree of erodibility of the bank and the morphodynamic context. In accordance with observations at Site 1 on the river Allier, the reactivation of lateral channel erosion in a reach of the Old Rhine (France/Germany) was incomplete due to intrinsic bank parameters, such as the possibility that old bank protection underlying the removed bank still exists, the presence of bedrock outcrops at the toe of the bank, and the protective role of vegetation [24]. However, even if the erodibility parameters of the bank favour the restoration of lateral channel erosion, sediment supply and its balance with the transport capacity play an important role. Flume experiments have shown that a low sediment supply does not favour channel widening [70]. Additionally, in the case of the Upper Drau, the authors point out that channel aggradation after widening was possible only because of the upstream sediment supply, not only the local supply [66]. Consequently, for reaches such as Site 1, more interventionist measures may have to be considered to further initiate lateral channel dynamics. Indeed, to initiate the re-establishment of bank erosion, other restoration projects should accompany the removal of riprap with complementary measures, such as the artificial addition of coarse sediment to the channel and/or the installation of groynes, which, by modifying flow hydraulics close to the bank, may increase lateral erosion [21; 25]. The combination of riprap removal and the artificial reinjection of gravel into the channel, the latter, however, being expensive, is another option highlighted considering some sections of the Cher River, France, another tributary of the Loire, where the potential to restore lateral channel erosion is restricted due to the limited sediment supply and the low coarse sediment transport rates [71]. However, even with the restoration of sediment transport in sediment-deficient reaches, the reactivation of lateral erosion may take several years or even decades in contrast to a dynamic reach fed by a high sediment supply [70]. In addition to the sedimentary and geomorphological characteristics of the bank, the reactivation of lateral erosion is controlled by the flood regime. In the case of the two restorations on the middle Allier River, the time elapsed between the restoration works and the first morphogenic flood influenced the effectiveness of subsequent floods. At Site 1, no hydrologic event greater than or equivalent to a 2-year flood occurred after riprap removal in September 2017, which promoted the re-sprouting of stumps left in place (Figure 2). In contrast, this time lapse between riprap removal and efficient hydrologic conditions was only two months at Site 2. Consequently, the remaining stumps did not have time to re-sprout and were washed away during the first period. This clearly favoured subsequent bank erosion on Site 2 in comparison to Site 1 where the stumps remained and re-sprouted, in addition to the low cohesiveness of the sediment and the channel planform at Site 2.