Sediment supply affects uncertainties and memory in alpine geomorphic systems

Geomorphic systems are affected by climate forcing and sediment supply. Due to non-linear relationships of forcings and sediment mobilization, it is debated whether environmental signals are preserved in such systems, or if they are rather dampened or shredded in the sediment output. Tracing the cause and effect in such systems is commonly impossible to do from observations alone. Therefore, numerical models are interesting to study geomorphic system behavior. We use a modeling chain consisting of the SedCas sediment cascade model (Bennett et al., 2014; Hirschberg et al., 2021) and the AWE-GEN stochastic weather generator (Fatichi et al., 2011), which has been calibrated for a debris-flow catchment in the Swiss Alps, the Illgraben, and used for climate change impact assessment (Hirschberg et al., 2021). Here we use this modeling setup to study the long-term behavior of such a system under consideration of different mean erosion rates and sediment production mechanisms. This numerical experiment is unique because we conducted simulations at high temporal resolution (hourly) while also spanning geological time scales (10k years).

We show that the analysis of short sediment records is characterized by high uncertainties and that especially supply-limited systems are at risk to have underestimated mean sediment. This is in concert with field observations on short- and long-term erosion rates from other basins, and can be attributed to transient hillslope sediment supply to the channel. Furthermore, we demonstrate how large hillslope landslides, or the absence of sediment supply, introduce long-term memory effects which can be quantified in the sediment yield. This long-term memory increases uncertainty and reduces interannual variability in annual sediment yields. Interestingly, details of the actual timing of sediment supply events are shredded and have no discernible impact on sediment yields at the outlet. The study highlights the need of characterizing variability in erosional events with regard to their stochastic nature. Furthermore, these results will corroborate the analysis of erosion rates, support decision making and decrease the risk of misinterpretation both in natural hazard and climate change impact assessment, especially if they are based on short records.

 

REFERENCES

Bennett, G. L., P. Molnar, B. W. McArdell, and P. Burlando (2014), A probabilistic sediment cascade model of sediment transfer in the Illgraben, Water Resour. Res., 50, 1225– 1244, doi:10.1002/2013WR013806.

Fatichi, S., Ivanov, V. Y., & Caporali, E. (2011). Simulation of future climate scenarios with a weather generator. Advances in Water Resources, 34(4), 448-467.

Hirschberg, J., Fatichi, S., Bennett, G. L., McArdell, B. W., Peleg, N., Lane, S. N., et al. (2021). Climate change impacts on sediment yield and debris- flow activity in an Alpine catchment. Journal of Geophysical Research: Earth Surface, 126, e2020JF005739. https:// doi.org/10.1029/2020JF005739