Floods, landslides and short-term meandering bedrock river dynamics
Résumé
Actively incising bedrock meanders are an ubiquitous feature of mountain belts, but the mechanisms leading to their formation and evolution are still poorly understood. As for straight bedrock rivers, we expect the combination of stochastic discharge, sediment supply and river transport capacity to play a key role in the partitioning between vertical and lateral incision. But the sinuous planform geometry yields localized high rates of outer bank incision driving localized hillslope mass wasting processes. The resulting deposits may alter patterns of sedimentation and incision leading to a strongly coupled channel-hillslope system. We aim at better understanding this coupling following two approaches : a detailed quantification of channel response to individual floods and mass-wasting events using Terrestrial Laser Scanner surveys and recent historical data; and the integration of this short-term dynamics at longer-timescales through numerical modelling. In particular, we note that many of these rivers exhibits numerous strath terraces abandoned in their inner bend documenting an evolution which is not purely continuous but rather punctuated by rapid changes in the balance between vertical and lateral erosion. Whether these changes can be tied to specific extreme events (floods, landslides, major earthquakes…) or an intrinsic instability is a key question to better understand bedrock meandering dynamics. It also has potentially important implications for the reconstruction of paleo-extremes from dated terraces or for the management of infrastructures located near actively migrating meandering bedrock rivers. This presentation focuses on the use of Terrestrial Laser Scanner to investigate the spatio-temporal patterns of bank erosion in the Rangitikei river (New-Zealand) over 3 years. The Rangitikei river is incising weakly consolidated mudstone at an average rate of 5 mm/yr since 15 kyr and has developed a very sinuous meandering pattern with several cut-off bedrock meanders. Lateral undercutting of 100 m high cliffs generates failures of up to hundred of meters. Alluvial material consists of coarse resistant material (D50 ~ 20 cm) sourced from upstream, and large boulders locally derived from rockfalls. Bedrock exposure is rare on the bed but shows that abrasion, weathering by wetting-drying cycles and plucking are important bank erosion mechanisms. Six TLS surveys were performed at bi-monthly to yearly intervals with a Leica Scanstation 2 in 4 reaches of variable planform curvature. Survey length varies from 300 m to 1200 m and point spacing from 5 mm to 5 cm. We developed field techniques and post-processing tools allowing to detect down to 5 mm change between surveys. These data were augmented by aerial photographs illustrating the decadal dynamics of these rivers, and bathymetric surveys. TLS data show how even cliff failure events of moderate size can significantly alter the pattern of inner and outer bank erosion. We also document complex phases of inner bar deposition and erosion after floods of different magnitude. Overall, these data highlight the importance and episodic nature of the coupling between channel migration and hillslope mass wasting.