Slope–channel coupling in steepland terrain: A field-based conceptual model from the Tarndale gully and fan, Waipaoa catchment, New Zealand

Connectivity between an actively eroding gully complex and upper Waipaoa catchment via a steep, 1-km-long alluvial fan is assessed using digital elevation models (DEMs) of the Tarndale fan derived from 10 high resolution GPS surveys between 2004 and 2008. Repeat DEM analysis provides a means of assessing transfer of sediment supplied from the Tarndale gully through the fan to the Te Weraroa Stream and thus inferring connectivity between these landscape components from which a conceptual coupling model is derived. Analysis indicates the operation of two critical junction switches at the (i) gully–fan and (ii) fan–stream nexus, which contribute to considerable complexity in patterns of erosion and deposition on the fan as it responds to sediment supply or starvation from the Tarndale gully and evacuation to the Te Weraroa Stream (described in Fuller and Marden, 2010). The fan does not respond as a coherent unit. Each of six fan feeder tributaries behaves independently according to sediment supplied from six discrete source areas within the gully complex. This reflects sediment production within specific activity zones of Tarndale Gully. Mass movements, triggered by single rainstorm events and/or wet periods contingent upon sediment availability and intrinsic slope mechanics, frequently deliver sediment to the upper fan tributaries. This enhanced gully–fan coupling during storms or wetter periods promotes rapid fan aggradation, with each feeder tributary responding to its discrete source zone. Infilling may propagate down fan in response to upper fan incision once the gully–fan switch deactivates, transferring sediment as far as the Te Weraroa Stream, coupling the fan with the stream system. Floods generated in the Te Weraroa Stream may also activate the junction switch at the fan–stream nexus by trimming the lower fan, prompting up-fan incision via head-cutting in response to changed local base level. Slope–channel coupling in this environment is a function of a range of slope and channel processes responding to discrete storms and intrinsic gully dynamics. The high rates of geomorphic activity in this environment mean the Tarndale system provides a natural laboratory in which to assess key slope–channel coupling processes beyond the plot or discrete event scale.