ESTABLISHMENT OF HILLSLOPE FOREST CHARACTERISTICS IN BANKS PENINSULA: IMPLICATIONS FOR ROCKFALL MODELING AND RUNOUT

Hillslope protection forests have been established as an effective method for mitigating rockfall hazard. However, the majority of research on this subject has been performed in alpine European forests, and as a result, their findings and developed models may not adequately reflect the attributes and effects of forests on rockfall in other parts of the world. Here, we characterize slope vegetation at two locations in Banks Peninsula, New Zealand to evaluate the potential effect a temperate hillslope regenerating forest would have on local rockfall hazard. Diameter at breast height (DBH), tree height, nearest neighbor (ANN), and percentage cover of vegetation were measured in the field and then compared to equivalent characteristics and attributed drag coefficients in alpine European forests. Our results indicate that temperate regenerating forests of Banks Peninsula are dense (84.88 m²/ha) with a random distribution (1.35 ANN) of thin (7.09 cm DBH), short (6.1 m) trees. Many of the Banks Peninsula trees consist of multiple stems which could act as a net to decrease rockfall runout despite the small DBH of each stem. Attributed drag coefficients ranged from 884.47 kg/s to 1905.33 kg/s which are greater than RAMMS rockfall model’s dense forest preset (750 kg/s). Our study results and field observations of arrested rockfalls in native BP forest suggest regenerating forest would effectively mitigate rockfall hazard.

We propose that rockfall numerical models such as RAMMS need to be updated to include a variety of forest types and distributions (random, regular, or clustered) to account for variable forest and tree structure characteristics and to effectively model the interplay between multiple variables (i.e. density, DBH, height, and structure) during rockfall runout.