Geomorphic impacts of a 100-year flood: Kiwitea Stream, Manawatu catchment, New Zealand

The geomorphic impacts of a 100-year flood are assessed in the Kiwitea Stream (254 km2), a tributary within the Manawatu River catchment (New Zealand), using sequential aerial photographs and reach-based morphological sediment budgeting. Channel expansion and avulsion eroded in excess of one million cubic metres of sediment over 1 km2 of floodplain along a 30-km-long reach of Kiwitea Stream. Channel transformation was spatially discontinuous and predominantly associated with large-scale bank erosion in response to a flood over 5 times bigger than the mean annual flood (annual recurrence interval (ARI) ∼ 100 years). Total energy expenditure of this flood in the Kiwitea was ∼ 14,900 × 103 J. The spatial discontinuity of channel transformation relates to valley floor and channel configurations. High stream powers generated in confined channels at bends produced catastrophic channel transformation. Where flood flows dissipated overbank, stream powers and the extent of channel transformation were reduced. Hydrologic, hydraulic and geomorphic variables can be invoked to thus explain the variability of geomorphic impacts encountered during this event.