COMPUTATIONAL INVESTIGATION OF THE ORIGINS OF NON-LINEAR SEDIMENT PULSES

Temporally homogeneous or near-homogeneous forcing conditions can result in temporally variable sediment yields, typically characterized through the irregular occurrence of sediment pulses. This has been observed in natural catchments, in laboratory experiments, and in numerical simulations. However, the causes of this phenomenon are poorly understood. Although some hypotheses have been proposed –most notably floodplain sediment storage, in-channel sediment storage, bed armouring, and river bank failure– these have not been tested or proved.

In this study, the CAESAR landscape evolution model is applied to investigate the mechanisms that give rise to sediment pulses in a simple idealized catchment. Significant temporal fluctuations in sediment yield are observed in the simulations, and are related to the spatial dynamics in the catchment's interior. Working backwards from observed peaks in sediment yield at the outlet, sediment pulses are traced upstream to identify their origins. This spatio-temporal backtracking gives additional insight in the mechanisms that give rise to the observed non-linear sediment delivery. Results indicate that, at least for the idealized catchment studied in these simulations, in-channel storage and bed armouring are the dominant influences in regulating the sediment pulses. The observation that sediment pulses can arise from autogenic factors rather than external forcing may have significant implications for the interpretation of sedimentary records.