Debris flow–derived sediment routing and storage in a mountain river network: a D-CASCADE application in the Trebbia-Aveto River (Italy)

Debris flows can dominate sediment budgets in mountain basins, yet their basin-scale routing, residence times, and outlet export remain difficult to quantify. We analyze the Trebbia-Aveto River (TAR) network in northern Italy following the September 2015 event, which produced localized, high-intensity rainfall and multiple debris flows. Using D-CASCADE, a dynamic sediment (dis)connectivity model, we compare two scenarios — with and without debris-flow inputs — to quantify the downstream routing, storage, and basin-scale export of the debris-flow sediment pulse (DFSP) through the TAR river network over nearly a decade. Simulations with five sediment transport equations indicate that less than 1% of the DFSP was exported at the basin outlet in most cases, with in-basin storage dominating over a decadal timescale. Storage hotspots concentrate downstream of debris-flow source zones in the Trebbia and Aveto rivers, at locations of riverbed slope reduction, and downstream of their confluence. Reach-scale time series reveal three characteristic behaviors — monotonic increase, monotonic decrease, and pulse-driven peaks within the study period — consistent with slow, intermittent DFSP remobilization driven by successive rainfall events. Although results are subject to uncertainties inherent to model-based approaches, the spatial and temporal patterns of DFSP storage are broadly consistent with theoretical understanding of sediment dynamics in armored gravel-bed mountain rivers. This work demonstrates that network-scale sediment connectivity modeling offers a tractable and reproducible framework for reconstructing debris flow-derived sediment dynamics at the basin scale.