Periodic Reporting for period 1 - POSTCOLD (Understanding the influence of sediment dynamics on postglacial landscape evolution)
Période du rapport: 2022-06-15 au 2023-12-14
Repeated glacial-interglacial cycles during the last 2.6 million years have significantly impacted the topography of many mountain ranges around the world. Yet, the response of landscape evolution to repeated climate oscillations has not been well quantified. In recently deglaciated landscapes, the transition from glacial to fluvial/hillslope processes have induced progressive topographic adjustments, and the large amounts of sediments inherited from glacial periods and generated through landsliding of oversteepened glaciated topography may act as a fundamental control on the incision of postglacial rivers. These sediments can enhance fluvial incision rate by providing more tools for erosion, or inhibit incision by armoring the river bed. Characterizing when and where sediment enhances or inhibits fluvial incision in postglacial landscapes is critical for understanding the response of mountain topography to deglaciation. The main scientific objectives of POSTCOLD were 1) to develop a landscape evolution model to account for the complex impact of sediment dynamics on fluvial incision in postglacial landscapes and 2) to investigate the response of fluvial incision to changes in sediment supply and assess the effects of sediment on postglacial landscape evolution. The results of POSTCOLD provide a quantitative understanding of postglacial fluvial incision histories, which is critical for ecosystem management and natural hazard assessment in recently deglaciated mid-latitude mountain ranges and perhaps in high-latitude mountain ranges where continued climate change may eventually lead to deglaciation.
1. Various landscape evolution models have been developed to account for the coevolution of alluvial cover and sediment-flux-dependent bedrock incision. We compare the two most commonly used models for sediment transport: an Exner-type model and an erosion-deposition model, to see how their predictions of long-term postglacial river incision rates differ. We found that, even though the two models aim to mimic the same scenarios, they predict that rivers will erode at much different speeds when sediment is occasionally dumped into the river. In particular, the erosion-deposition model short sediment transport length can capture the behavior of the Exner-type model and thus has a wider applicability than the Exner-type model.
2. We have combined the erosion-deposition model with the saltation-abrasion model for sediment and built a one-dimensional river profile model. Using this model, we have run a group of simulations to investigate the impact of sediment on knickpoint retreat rate in postglacial rivers. The results indicate that the knickpoint retreat distance is controlled by the accumulated amounts of sediments delivered to the river.
2. We have combined the erosion-deposition model with the saltation-abrasion model for sediment and built a one-dimensional river profile model. Using this model, we have run a group of simulations to investigate the impact of sediment on knickpoint retreat rate in postglacial rivers. The results indicate that the knickpoint retreat distance is controlled by the accumulated amounts of sediments delivered to the river.
This project integrates recent advances in understanding the sediment dynamics and sediment-flux-dependent river incision and opens new possibilities to quantify the complex impact of sediment flux on postglacial river incision rates. Our comparison of the erosion-deposition model and Exner-type model highlights the importance of sediment transport length, suggesting that in order to validate models of sediment-flux-dependent river evolution, future research should focus on constraining the value of sediment transport length. Additionally, our results reveal a testable relationship between the knickpoint retreat distance (thus the response timescales) and the total amount of sediments transported by the river. Future research should test this relationship, and this finding can help us understand the evolution of basins with different sediment regimes in the Alps and other glaciated mountain ranges.