Rapid channel response to variability in sediment supply: Cutting and filling of the Tarndale Fan, Waipaoa catchment, New Zealand

In the headwaters of the Waipaoa catchment, fluvio-mass movement gully complexes are currently significant contributors of sediment, delivery of which is buffered by fans which have developed in the small tributaries that lead from the gully complexes. Biannual channel cross-section surveys made since 1983 show seasonal aggradation / incision cycles; channels generally incise in summer and infill during the winter months. This seasonal pattern of cutting and filling is driven by variations in sediment supply from the gully complexes. In this paper, to better understand patterns and processes of sediment delivery at the top of the sediment cascade, we examine cut and fill cycles on the ~ 11 ha Tarndale Fan over a two and a half year period. Analysis of sequential DEMs compiled from survey data obtained between December 2004 and August 2007 indicates cutting and filling may involve up to 47,000 tonnes of sediment over as little as three months. During discrete severe rainstorm and/or wet weather periods, mass movements in the gully complex are enhanced and contribute large quantities of sediment to the fan, via debris flows and landslides, which infill channels. During drier periods, mass movement activity is inhibited and runoff incises the feeder channels.