Jerky conveyor belts under stress: sediment connectivity and routing during an extreme flood event in New Zealand

The routing of sediment from source to sink is commonly described as a “jerky” conveyor belt, in which sediment flux is strongly controlled by connectivity between source areas and transfer zones. However, the extent to which extreme flood events increase source connectivity and modify sediment transfer through river reaches via combined hillslope and fluvial processes remains poorly understood. The increasing availability of multitemporal, high-resolution lidar now enables the development of event-scale topographic sediment budgets, providing new insights into sediment flux during extreme events.

In February 2023, the landfall of Cyclone Gabrielle caused catastrophic flooding along the eastern coast of the North Island of Aotearoa New Zealand. Repeat lidar surveys were acquired for three catchments—Esk (264 km²), Aropauanui (158 km²), and Tangoio (71 km²)—to quantify landscape change and develop catchment-scale sediment budgets. Sediment delivery ratios were estimated to be approximately 0.4 across all three rivers, with sediment volumes delivered to the marine environment of 9.6 × 10⁶ m³ for the Esk, 5.1 × 10⁶ m³ for the Aropauanui, and 2.4 × 10⁶ m³ for the Tangoio.

Sediment budgets were further refined through geomorphic mapping and two-dimensional flood modelling to partition sediment sources and sinks into geomorphic process zones. The sediment routing model D-Cascade was used to route upstream sediment supply, combined with hillslope-derived inputs along the reach, through individual river sections. Results identify river reaches where observed sediment fluxes exceed modelled fluvial transport capacity, indicating locations where debris-flow-dominated transport processes likely governed sediment routing during the cyclone. These findings demonstrate the potential importance of non-fluvial processes in shaping sediment transfer during extreme floods and highlight the value of lidar-based sediment budgets for resolving sediment dynamics at the event scale.