The influence of large, chronic landslides on the fluvial system

Most rivers draining the Raukumara Range of New Zealand's East Cape are subject to episodic sediment deliveries by transient landsliding and debris flows. The landscape is underlain by relatively weak lithologies, and has been subject to forest clearing in the past. There are a few notably large (>40 ha) and long-lived (chronic) gully complexes that significantly impact trunk channels with varying but persistent additions of gravel and sand at the decadal scale. This leads to channel widening, discontinuities in grain-size trends, and accelerated valley-filling downstream: the transport equilibrium within the trunk channel is typically reset beyond this confluence zone. The study area thus offers good scope for improving our understanding of the nature of channel coupling with major lateral sediment sources, and the balance of possible morphologic and textural responses to sediment loading over decades.

Using three case studies from the Waiapu and Raukokore valleys, we use photogrammetry and field surveys to assess geomorphic coupling between chronic landslides and rivers, and consider the explanatory power of variables such as relative sediment loads, relative grain size differential, and the geometry of the tributary confluence, including channel slopes and the confinement of valley walls. The addition of sand and friable gravel material from landslides and gullies leads to augmented transport capacity and pronounced fining. Trunk channel slope moderates the buildup or evacuation of sediment storage in confluence fans and bars. The major valleys downstream are the ultimate buffers for these large sediment loads before delivery to the Pacific, changing the configuration of their storage zones over decades. By studying this exceptionally active landscape, we derive important insights into changes in channel sediment regime.