Geomorphic Work during a “150-Year” Storm: Contrasting Behaviors of River Channels in a New Zealand Catchment

Abstract

The geomorphic work accomplished during an extreme storm event by three primary tributaries in the western Manawatu river catchment (New Zealand) is assessed using sequential aerial photographs. Areas of floodplain and channel erosion were quantified as a measure of the work accomplished during floods triggered by a “150-year” storm in February 2004. Of the three rivers studied, the smallest (Kiwitea, 225 km2) accomplished the most geomorphic work, eroding 1.1 km2 of floodplain along a 30 km long reach. Here spatially discontinuous channel transformation was associated with large-scale bank erosion in response to a flood estimated to be more than five times bigger than the mean annual flood (annual recurrence interval [ARI] ∼100 years). Total energy expenditure in the Kiwitea flood was ∼14,900 × 103 joules. The larger Pohangina (547 km2) and Oroua (329 km2) rivers were less effective, expending ∼14,400 × 103 joules and ∼5,300 × 103 joules and eroding 0.36 and 0.6 km2 of floodplain, respectively. The contrasting amount of geomorphic work between these tributaries relates to valley floor and channel configurations, which prime discrete reaches for instability, sensitizing them to perturbation by this flood event. In the Kiwitea, greatest erosion occurred where floodwaters were confined by terrace bluffs at bends that locally enhance stream powers. The wider channels of the Pohangina and Oroua Rivers wander across broader gravelly floodplains, permitting widespread dissipation of flood flows across the wider active channel and valley floor. Hydrologic, hydraulic, and geomorphic variables thus explain the variability in geomorphic work accomplished during the event. Ultimately, diverse channel behaviors reflect varied catchment and reach sensitivity to the flood event.