Path Length and Sediment Flux Validation in Braided River Systems: Application of the VMD Method and D-CASCADE Model

The movement of sediment through river networks is crucial for the health and functionality of river ecosystems, flood control, and water availability. Network-scale sediment connectivity models have emerged in recent decades but lack robust validation with field measurements. Here, we perform a path length-based application of the morphological method, the Variational Mode Decomposition (VMD) method, to the Tagliamento River, a large braided river in northeastern Italy, to validate the sediment flux estimates generated by the network scale sediment connectivity model D-CASCADE.

The results indicate that D-CASCADE can generate sediment flux estimates that align with those derived from the VMD method and with values documented in literature. Furthermore, we observe that the generated path length estimates align with the expected path length based on the spacing of confluence-diffluence couplets which has been previously proposed as a proxy for path length. These results underscore the need for careful calibration of grain size distributions for specific rivers to improve model accuracy. Additionally, we identify the importance of estimating a fundamentally unknown input parameter, the active transport width (the part of the river channel where bedload is moving for a specific discharge), and its impact on the modeled sediment transport estimates. Finally, we see from the field acquisitions that even during small flood events on the Tagliamento, there is significant compensation when comparing the erosion and deposition volumes during each flood event.

These results demonstrate that the VMD method provides reasonable estimates of path length and sediment flux, thereby serving as a valuable validation tool for network-scale sediment connectivity models and increasing the robustness of the D-CASCADE model in large, complex river systems. The presented field data also help clarify when topographic changes are not a reliable representation of bedload fluxes due to high flow events or confined planform morphology, which then limits the applicability of the VMD method. Overall, the present study is a step forward in validating and refining our understanding of sediment transport processes in braided river environments and provides practical implications for the sustainable management of riverine ecosystems.