Research has confirmed that severe storms have been a major cause of erosion and aggradation (the burial of land by sediment deposits) around Mount Taranaki.

The increased volume of detritus being carried in watercourses from Egmont National Park into the intensively occupied and highly productive ring plain is resulting in a series of inter-connected issues. Aggradation reduces channel capacities, and leads to bank erosion, increased flooding through spillage from channels, loss of pasture by erosion and deposition, and damage to public and private access and assets (roading, tracking, culverts, bridges, fords and buildings).

Taranaki Regional Council (TRC) was concerned that erosion on the mountain appeared to be worsening and suspected that storm events were becoming worse and more frequent.  The council commissioned Landcare Research to provide a historical account of erosion and aggradation changes on the mountain to get a sense of how much things had changed, which in turn could have suggested the likely trend into the future. This pilot study focused on the Stony River catchment.

An analysis of historical aerial photography and available rainfall and stream flow data made it clear that there had been a major increase in the amount of erosion and aggradation from around the mid-1990s. This corresponded with an increase in flood magnitude, although no trend was discernable from the rainfall data.

“It appeared that, while there was no change apparent in long term annual average rainfall, individual storm events may be getting more intense, leading to the higher flood peaks and the greater erosion/aggradation evident in the aerial photography,” says Landcare Research scientist, Harley Betts.

1998 was a crucial year during which multiple severe storm events destabilised many catchments on the mountain and left them vulnerable to subsequent storms.  Since 2007 (the end of the period studied in this project), further severe storms have caused more damage on the mountain. 

TRC will use the results to help guide their strategies for dealing with the aggradation problems in rivers arising from increased erosion on the mountain. 

While the council has no intention or opportunity to intervene with the natural processes occurring in park, it is trying to establish the nature, size and extent of the threat to local land.

TRC is initially addressing this issue in its role as a catchment authority with river and flood management responsibilities. It does have the opportunity to develop management strategies that may be applied directly or through planning processes available to councils through the Resource Management Act.