Monitoring and effects assessment of landslide-induced aggradation in the Poerua Valley, Westland

Abstract

The devastating fanhead aggradation that followed the 1999 Mt Adams landslide into the Poerua River catchment, Westland, is a type of hazard that has been previously overlooked. Landslides of this size and greater are expected to result from the anticipated (currently - 1%-2% annual occurrence probability) Alpine fault earthquake, so there is an urgent need to delineate the land areas at risk from this type of aggradation hazard.

Consideration of the impacts of the Mt Adams landslide led to the hypothesis that such events cause anomalously steep, entrenched fanheads to develop at the range front, the young surfaces of which should overlie soils buried between layers of river gravels. This hypothesis was tested using map data, information from a microscale laboratory model and soil stratigraphic investigations of a number of West Coast fanheads; all with positive results. It is concluded that fans with these characteristics reflect the occurrence of large sediment inputs into river catchments. A hazard zone delineation methodology was developed that allows areas likely to be susceptible to fanhead aggradation to be identified. Independent testing of this methodology remains to be carried out.

Monitoring of Poerua River bed elevations suggests that the aggradation episode may be passing its peak, 5-6 years after the event. If this is the case, then about 10% of the volume of the Mt Adams landslide has contributed to fanhead aggradation, and the aggradation event is equivalent to about one-twelfth of the total aggraded fanhead volume, suggesting that this is about a 500-year aggradation event. Similar aggradation events are expected to result from the next Alpine fault earthquake.