Past, present and future behaviour of the Waiho River, Westland, New Zealand: a new perspective

摘要

High-resolution microscale modelling of the response of the Waiho River to width reduction by stopbanks reveals that, unless the river is already aggrading in its prior state, width reduction does not on its own cause bed aggradation. This finding conflicts with and corrects findings of earlier work, due to improvements in data acquisition technology, and has prompted a review of the prehistoric and historical behaviour of the Waiho River. The river has aggraded by about 10 m at the State Highway 6 bridge cross-section since records began in 1907, and since 1980 it has aggraded by about 6 m (an average rate of about 0.17m/year). The reason for this multi-decade aggradation is presently not well understood; other major rivers in the region are not behaving in this way. At the start of the 20th century the Waiho River was incised by about 8 m into its fanhead, but by about 1970-80 aggradation was causing overbank flows during floods, affecting infrastructure, and stopbanks were constructed to confine the river to its previous bed area. The confinement caused aggradation to continue at the previous rate, whereas without stopbanks the river would have aggraded across its entire fanhead (about three times the current bed area) at about one-third the rate, i.e. at about 0.06m/year. If stopbanks are maintained in their present positions, about 17 m of further aggradation can be anticipated over the next century; if, by contrast, the river is allowed free access to the whole of its natural bed the short-term response would be about 2 m of degradation, followed by 6 m of aggradation after 100 years -that is, a bed-level rise of about 4 m over the next century. During this time, however, a severe aggradation episode, probably due to an earthquake, is likely to alter the river behaviour drastically. The response of the river to climate change over the next century is unclear at present.