## 2 Proposed Methodology
### The Metric Analysis Grid Approach  
The analysis grid approach integrates the geographical variations of the active channel in the two-dimensional Euclidean space. After digitalization and georeferencing of the images, delimitation of the channel leads to manually digitize the riverbank boundaries as lines. The lines should be easily digitized when the riverbanks are not covered by tree vegetation. If the riparian forest or alluvial forest covers the riverbank, this creates a mask that does not allow us to see the edge of the river bank. After checking on the field (here, the Clamoux River), it turns out that the riverbanks can be delimited by passing by half of the first row of trees (semi-circles) bordering the active channel. Then, the study reach is rotated in order to obtain a rectangular area of investigation with a flow parallel to the X-axis as well as possible. A grid is created around the area of investigation with GIS software (for example, QGIS). The grid could be generated by vector calculation (with the QGIS'vector grid' toolbox) or raster conversion (with the QGIS'vector to raster' toolbox) with a sufficient resolution in comparison to the width/length of the studied reach. The resolution of the analysis grid is therefore the choice of the operator (Figure 6A). Here, we have chosen a metric analysis grid to follow the evolution of the two banks of the river between Year A and Year B in a 20 m \(\times\) 40 m rectangular area of investigation. Each square (1 m \(\times\) 1 m) occupied by the riverbank has been colored in red for the left bank and green for the right bank. Whatever the resolution choice, we recommend using an identical resolution for each survey of the monitoring. Figure 6: Riverbank retreat/accretion quantification with the metric grid approach (**A**). Graphical output of the riverbank retreat/accretion with the metric grid approach (**B**). 1: Left-bank evolution; 2: Right-bank evolution; 3: Longitudinal alluvial-bar scours and fills. Flow from left to right. The previous step of active-channel digitalization is used to characterize each grid cell by a specific code: 1 for the left bank, 2 for the right bank, 3 for the alluvial bar and 0 for all other objects (additional codes are possible to improve or complete the floodplain analysis by biological, hydrological, sediment or anthropogenic indicators). A classical selection by localization between the grid and geomorphological object (different riverbanks and alluvial bars) allows attributing the right code for each cell grid. The GIS database is then exported in a spreadsheet for different posttreatment as graphical output and indicator calculation. In order to avoid the case with more than one cell by column for the riverbank, the mean Y coordinate of the column is chosen to represent the unique Y coordinate of the column. The bank retreat/accretion is then calculated by the Y coordinate difference between Year B and Year A at each X unit. Careful attention is necessary to characterize the erosion or accretion in the function of the bank side and the flow direction. In our example, erosion of the left bank occurs for positive evolution, whereas accretion occurs for negative evolution and inversely for the right bank. Sometimes, the bank boundary passes through two superimposed squares. In this case, we take the average of the two squares for a diachronic comparison. In the end, the evolution of the bank lines (or edge of the active channel) between Year A and Year B is reported on a histogram (Figure 6B) on which the measured bank sites (\(n=40\)) are placed on the \(X\)-axis, and on the \(Y\)-axis, the values of erosion (bank retreat) or accumulation (bank accretion) for each of the two banks of the river.