The relation between geometry, hydrology and stability of complex hillslopes examined using low-dimensional hydrological models

Key words: Hillslope geometry, Hillslope hydrology, Hillslope stability, Complex hillslopes, Modeling shallow landslides, HSB model, HSB-SM model.

The hydrologic response of a hillslope to rainfall involves a complex, transient saturated-unsaturated interaction that usually leads to a water table rise.

An increase of saturated groundwater flow can act as the triggering mechanism for slope failure.

To account for the three-dimensional hillslope shape in which the groundwater flow and storage processes take place, simple (low-dimensional) but physically realistic models that represent hydrological processes at the hillslope scales are needed for reliable simulation of hillslope stability at the landscape scale.

In this thesis the focus is on investigating the relation between hillslope geometry, hillslope hydrology and slope stability in complex hillslopes and hollows.

Several models have been presented in this thesis which examine the stability of nine characteristic hillslope types (landform elements) with three different profile curvatures (concave, straight and convex) and three different plan shapes (convergent, parallel and divergent).

In addition to testing our models for nine characteristic hillslope types, a general relationship between plan shape and profile curvature of landform elements and the factor of safety is derived for a predefined hillslope length scale.

Our results show that slope stability increases when profile curvature changes from concave to convex.

In terms of plan shapes, changing from convergent to divergent, slope stability increases for all length profiles.

Our analyses also show that the minimum safety factor occurs when the rate of subsurface flow is maximum.

In fact, by increasing the subsurface flow, stability decreases for all hillslope shapes.

Moreover, after a certain period of rainfall, the convergent hillslopes with concave and straight profiles become unstable faster than others whilst divergent convex hillslopes remain stable (even after intense rainfall).

We also demonstrate that in hillslopes with non-constant soil depth (possible deep landslides), the ones with convex profiles and convergent plan shapes have slip surfaces with the minimum safety factor near the outlet region.

Finally, we demonstrate that, in addition to bedrock slope, hillslope shape as represented by plan shape and profile curvature is an important control on hillslope stability.

With respect to the relation between rainfall occurrence and slope instability, a probabilistic model of rainfall-induced shallow landslides in complex hollows is also presented to investigate the relation between return period of rainfall, deposit thickness and landslide occurrence.

A long term analysis of shallow landslides by the presented model illustrates that all hollows show a quite different behavior from the stability view point.

Finally, we conclude that incorporating a more realistic description of hollow hydrology (the hillslope-storage Boussinesq model instead of the kinematic wave model) in landslide probability models is necessary, especially for hollows with a high convergence degree, which are more susceptible to landsliding.

This model helps to theoretically investigate the relationship between return period of rainfall and landslide occurrence related to soil production (deposit thickness) in complex hollows.

In summary this thesis aims to understand theoretically how hydrological processes (subsurface flow and water table dynamics) affect slope stability in complex hillslopes and hollows.

The presented models can widely be applied in many investigations of hillslope stability analysis because of their relative simplicity (low-dimensional).