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.

Key Words:

• flood
• floodplain erosion
• geomorphic work
• river channel change

Notes

Source: Based on Fuller and Heerdegen (2005).

^aGEV (Generalized Extreme Value) or Gumbel distribution (2-parameter or EV2).

^bBased on EV2 and including the 15–16 February flood event: Details of these distributions are available in Fuller and Heerdegen (2005).

Source: After Fuller and Heerdegen (2005).

^aMinimum flood channel widths used: Kiwitea: 55 m, Oroua: 125 m, Pohangina: 150 m.

^bAssumes maximum stream power.

^cVegetated bar erosion 0.8 km².

^dVegetated bar erosion 1.4 km².