The infinite slope method is widely used as the geotechnical component
of geomorphic and landscape evolution models. Its assumption that
shallow landslides are infinitely long (in a downslope direction) is
usually considered valid for natural landslides on the basis that they
are generally long relative to their depth. However, this is rarely
justified, because the critical length/depth (L/H) ratio below which
edge effects become important is unknown. We establish this critical L/H
ratio by benchmarking infinite slope stability predictions against
finite element predictions for a set of synthetic two-dimensional
slopes, assuming that the difference between the predictions is due to
error in the infinite slope method. We test the infinite slope method
for six different L/H ratios to find the critical ratio at which its
predictions fall within 5% of those from the finite element method. We
repeat these tests for 5000 synthetic slopes with a range of failure
plane depths, pore water pressures, friction angles, soil cohesions,
soil unit weights and slope angles characteristic of natural slopes. We
find that: (1) infinite slope stability predictions are consistently too
conservative for small L/H ratios; (2) the predictions always converge
to within 5% of the finite element benchmarks by a L/H ratio of 25
(i.e. the infinite slope assumption is reasonable for landslides 25
times longer than they are deep); but (3) they can converge at much
lower ratios depending on slope properties, particularly for low
cohesion soils. The implication for catchment scale stability models is
that the infinite length assumption is reasonable if their grid
resolution is coarse (e.g. >25?m). However, it may also be valid even at
much finer grid resolutions (e.g. 1?m), because spatial organization in
the predicted pore water pressure field reduces the probability of short
landslides and minimizes the risk that predicted landslides will have
L/H ratios less than 25. Copyright (c) 2012 John Wiley & Sons, Ltd.