  Slope movements (e.g. landslides) are dynamic systems that are complex in
time and space and closely linked to both inherited and current preparatory and
triggering controls. It is not yet possible to assess in all cases conditions
for failure, reactivation and rapid surges and successfully simulate their
transient and multi-dimensional behaviour and development, although
considerable progress has been made in isolating many of the key variables and
elementary mechanisms and to include them in physically-based models for
landslide hazard assessments. Therefore, the objective of this paper is to
review the state-of-the-art in the understanding of landslide processes and to
identify some pressing challenges for the development of our modelling
capabilities in the forthcoming years for hazard assessment. This paper focuses
on the special nature of slope movements and the difficulties related to
simulating their complex time-dependent behaviour in mathematical,
physically-based models. It analyses successively the research frontiers in the
recognition of first-time failures (pre-failure and failure stages),
reactivation and the catastrophic transition to rapid gravitational processes
(post-failure stage). Subsequently, the paper discusses avenues to transfer
local knowledge on landslide activity to landslide hazard forecasts on regional
scales and ends with an outline how geomorphological investigations and
supporting monitoring techniques could be applied to improve the theoretical
concepts and the modelling performance of physically-based landslide models at
different spatial and temporal scales.
