2. Materials and Methods
2.1. Study Area
- The study site must be selected in order that the flood discharge increases result in large increases in the stage. Thus, the confined channel is more preferred than the floodplains. Indeed, an increase in the water discharge from 1400 to 3000 m3s−1 implies an increase in depth of 4 m in the main channel of the confined valley analysed herein (see Section 3).
- It is important to find key control reaches with stable cross-sections and well-preserved high-water indicators. Subsequently, a channel with exposed slate substrate and bedrock gorge was selected in this study, allowing the use of two-dimensional shallow water equations over fixed bed in the hydraulic reconstruction [3,8].
2.2. Gauging Records
2.3. Imagery and Documentary Evidence
2.4. Sedimentary Sequences and Botanical High Watermarks
2.5. Two-Dimensional Shallow Water Modelling, Computational Mesh and Boundary Conditions
3.1. Systematic, Historical and Paleostage Flood Records
- The lack of bedforms and fine sediment deposits in the first river reach indicates a transport capacity larger than sediment supply. Only woody debris and a siltline are preserved at elevated bedrock terraces (9 m above the thalweg) or anthropogenic platforms (i.e., roads) below the dam. Surface gravel structures were formed by pebble to cobble grain-sized angular clasts. Downstream imbricated clasts support the argument of low sediment supply to sediment capacity ratio . The high sediment transport capacity below the dam could be associated with the extreme turbulent intensity of the water down the spillways which was able to resuspend fine sediments .
- A field of sandy mud bumps developed on a bedrock bench further downstream. Bumps of cohesive mixtures of mud and sand are preserved downstream of flow obstacles (i.e., downstream flexible-wood trees such as Tamarix sp. and Fraxinus sp.). Shrubs on the bumps could initiate the accumulation of sediments as happens in obstacle dune. The bedforms resemble linear or long straight dunes. They are elongated along the longitudinal axis, being the width and height of the bumps much shorter than its longitudinal length.
- Overbank flood-deposited sediments downstream of the bridge ( m) was identified as a sedimentary evidence of flood level with the depth of 11 m. The documentary rating curve in Figure 3a indicates that the corresponding water discharge should be m3s−1.
- Bank erosion also developed further downstream, on the inner side of the natural river bend ( m). The photograph and the 3D representation of the bend shown in Figure 5a and Figure 7b, respectively, illustrate the lateral sediment bar that formed at the bottom of the inner bank. The elevation of the sediment bar measured during fieldworks (≈178 m) is in good agreement with the LiDAR data shown in Figure 7a. The top of the riverbanks at the bend (≈182 m) is elevated 8 m above the thalweg.
- Laterally-attached sandy siltlines were deposited from the end of the bend up to the flow expansion area ( m) during the April 2013 flood. Their locations could be readily identified in the most recent orthophoto (see the solid line in black in Figure 4a) and the elevations could be inferred based on the LiDAR data. For the sake of the brevity, the elevations are shown in Figure 6 only at the beginning and at the end of the sediment deposits.
- Botanical HWMs are depicted with green right triangles in Figure 6. Figure 5b shows an illustrative example which was identified in the flow contraction area of the outlet channel (see location in Figure 4a). These botanical records are consistent with other flow stage indicators as the top level of lateral bars and imagery for the flood occurred in April 2013.
3.2. Calibration of the Numerical Model Pre- and Post-Vegetation Encroachment
3.3. Verification of the Simulated Floods with PSIs
- Following Blocken and Gualtieri , high-quality experimental data are indispensable for the validation of any computational river dynamics model. Jarrett and England  showed that the elevation at the flood-deposited sediments nearest to channel margins provides a reliable and accurate indication of the maximum height of the flood. Hence, sedimentological HWMs can be used to check the accuracy and reliability of the 2D Saint-Venant equations in paleohydrology.
- Botanical HWMs provide valuable data for understanding recent floods and reconstructing their spatial patterns but botanical PSIs are perishable, might be washed out by more extreme floods and need to be preserved, as recently pointed out by Koenig et al. .
- Comparison with documentary data and imagery demonstrates that botanical and sedimentological HWMs, as well as mud-lines on man-made structures, are reliable data to indicate flood stages. Furthermore, the rating curve obtained from the combined use of 2D numerical simulations and imagery was nearly as accurate as gauging measurements.
- Overbank flood-deposited sediments are the only PSIs that record the most extreme flood. Interestingly, the simulated water depth of the largest flood in this study is much higher than in Jarrett and England’s  work (12 m vs. 4.5 m), and closely matches the observed flood stage, confirming previous findings on the reliability of PSI-HWMs for paleoflood studies.
- Dunes, imbricated pebbles and lateral bars are indirect indicators of the hydraulic conditions. Usually, flow depth and velocity magnitude for a given bedform dimension are deduced from dimensionless correlations under uniform and steady flow conditions [13,60]. However, PSIs are uncorrelated with these universal laws in non-uniform flows.
Conflicts of Interest
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