A COMPARISON OF EARTHFLOW MOVEMENT MECHANISMS ON FORESTED AND GRASSED SLOPES, RAUKUMARA PENINSULA, NORTH ISLAND, NEW-ZEALAND

Surface movement rates on forested earthflows are 2-3 orders of magnitude less than those on grassed earthflows. Subsurface deformation results largely from extension flow on grassed earthflows and from compression flow on forested earthflows. A rafting mechanism in which blocks of roots from individual trees interact with those of neighbouring trees to retard surface movement is used to explain deformation profiles of forested earthflows. The rheology of earthflow materials is changed by the presence of tree roots.