Morphotectonics as a landscape template for assessing sediment transfers and flood vulnerability

Poster presented at the 2017 annual conference of the Geoscience Society of New Zealand.Cite as: Conley, W., Fuller, I. C., McColl, S. T., Tunnicliffe, J., and Macklin, M. G., Morphotectonics as a landscape template for assessing sediment transfers and flood vulnerability, in Proceedings Geoscience Society of New Zealand, Auckland, 2017, https://doi.org/10.6084/m9.figshare.19210098.v1Our review of high-resolution digital topography in the upper Ruamahanga catchment, Wairarapa, implicates tectonism as a template for structuring flood risk assessment. This runs counter to standard flood risk assessments which are founded on hydraulically modelled inundation extents. Paleofluvial signatures indicate contemporary river paths on inhabited floodplains differ significantly from prevailing prehistoric river alignments and suggest changes may have been abrupt (i.e. avulsion). Overfit and underfit valleys suggest coseismic land deformation may selectively predispose certain communities to avulsion or break-out flooding. Several fluvial process zones are identified where reach-scale form (e.g. confinement and channel pattern), dominant processes (e.g. erosion or deposition), and behavioural tendencies (e.g. lateral movement or incision) can be expected to differ. Subreach fluviotectonic signatures in proximity to mapped faults are also observed away from mapped geologic structures, but along lineaments, implicating a potentially more extensive network of geologic structural control than previously published. We propose differentiating tectonically-forced features from landforms that are purely fluvial in origin will help estimate sediment transfer dynamics and predict channel behaviour, particularly at reach and sub-reach scales.