EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 20,231--242 (1995)

MODELLING THE ROLE OF CLIMATE, VEGETATION AND PEDOGENESIS IN SHALLOW TRANSLATIONAL

HILLSLOPE FAILURE

S. M. BROOKS, M. G. ANDERSON

Department of Geography, University of Bristol, University Road, Bristol BS8 ISS. U.K.

AND

A. J . C. COLLISON

Dcpartment of Geography, King'h College, University of London, Strand, London WC2R 2LS. U.K.

Received 7 June 1993 Accepted 28 Janitarv I994

ABSTRACT

Recent research in geomorphology has considered the significance of progressive pedogenesis and climatic change to slope failure initiation for the Holocene, using physically based models. To date, the significance of vegetation change to slope stability has been largely unexplored through modelling, since available physically based models cannot consider vegetation effects directly. T o address the existing deficiency this paper adapts, parameterizes and applies a physically based model of slope hydrology and stability to the combined effect of vegetation change and progessive pedogenesis on slope failure initiation. There is considerable debate in the literature concerning the relative significance of climatic change and vegetation modification to slope failure initiation in the Holocene. This paper uses the model to provide additional evidence for situations in which either climatic or vegetation change is significant to slope failure, depending on the prevailing degree of soil development. The results indicate that young podsols appear to be stable under all the climatic and vegetation conditions considered, but mature podsols may be susceptible to failure. Both climate and vege- tation influence slope stability, but their relative significance depends on the stage of soil development. In particular, the stability of young soils is influenced considerably by vegetation, while climate assumes greater significance in mature soils. It is recognized that this conclusion is limited to freely draining podsol profiles, and that more research is needed to consider other soil type and vegetation combinations.

KEY WORDS slopc stability; modelling; vegetation; climate; holocene

INTRODUCTION

Recent research in geomorphology has extended the application of physically based models to the role of Holocene pedogenesis in determining the temporal occurrence of shallow, translational slope failures (Brooks et al., 1993a). Changing soil profile morphology, horizon differentiation and textural changes taking place within podsol profiles (Brooks and Richards, 1993) create hydrological conditions which lead to profile instability on slopes of around 35". This recent research has also highlighted the significance of negative pore water pressures, which provide additional shear resistance in relatively freely draining material (Brooks et al., 1993b); this factor has been largely overlooked in previous explanations of slope instability in the humid temperate zone, but is particularly relevant in young or freely draining soils.

Also significant to slope stability are the climatic conditions, which determine the rate of moisture receipt. Storms vary considerably in terms of their frequency, maximum o r average intensity, and total rainfall sup- plied. Of particular importance are the differences that exist between storms of different synoptic origin. Convective storms generated under anticyclonic conditions tend to consist of short duration, high intensity

CCC 0197-9337/95/030231-12 0 1995 by John Wiley & Sons, Ltd.

232 S. M. BROOKS AND A. J . C. COLLISON

events, while frontal storms associated with depressions are more protracted and of lower average intensity. Such rainfall differences have a crucial role in determining the range of limiting stable slope angles in differ- ent types of slope-forming material (Brooks and Richards, 1994). The significance of climatic deterioration in initiating slope failure has been examined (Starkel, 1966; Grove, 1972; Brazier and Ballantyne, 1989), and periods such as the Little Ice Age, which involve marked shifts in storm-generating mechanisms (Lamb, 1982), have been found to be associated with widespread slope instability in some areas.

A third major influence on slope stability is the type of vegetation cover. Vegetation exerts hydrological and mechanical controls on slope stability, which can act either to promote slope failure or to increase stability (Gray, 1978; Greenway 1987). Modification of rainfall character, increase in regolith permeability and root reinforcement of regolith shear resistance are among the most significant influences on slope sta- bility. Although the presence of vegetation is generally thought to encourage stability, it can have a negative effect depending on the position of roots in relation to the potential shear surface. Much research has emphasized the role of anthropogenic changes to vegetation as a major triggering factor in hillslope fail- ures (Harvey et al., 1981; Innes, 1983; Harvey and Renwick, 1987; Brazier et al., 1988).

Recent shallow translational hillslope failures in Scotland have been variously attributed to climatic change or to anthropogenic vegetation modification (Ballantyne, 1991). It has also been argued that the pro- gressive deepening and increasing internal differentiation of freely draining soils promote slope instability (Brooks et al., 1993a). Once failed, a slope with thin, undifferentiated soil profiles is unlikely to fail again, whatever the state of the vegetation or climate. Post-failure modification involves progressive pedogenesis and vegetation colonization on the freshly exposed surface. In order to investigate the relative significance of climate and vegetation change on slope failure in mature soils, and to investigate the post-failure stability changes which result from vegetation colonization and pedogenesis on exposed slip surfaces, a modelling approach can be employed. Research into the controls of changing landslide frequency has generally centred around correlating events which are thought to be causally linked, without direct consideration of landslide generating mechanisms (Ballantyne, 199 1). Understanding landslide occurrence is therefore lim- ited. One major difficulty involves investigating the relative significance of climatic, vegetation and pedo- genic factors influencing failure mechanisms where all factors are varying concurrently.

This paper employs a modelling approach to elucidate the relative significance of the above controls on hillslope failures. It raises important issues related to the use of physically based models in geomorphol- ogy, especially in relation to uncalibrated models used in assessing the impact of Holocene climatic change. Given the significance of spatial variability in many important model parameters, this paper attempts to investigate whether it is possible to establish sensible values for model inputs, then to investigate the major climate-vegetation-soil processes which lead to slope instability, and finally to use the model out- puts to evaluate the relative roles of each of these controlling variables. Such an approach has been siiccess- fully employed to consider the roles of pedogenesis and climatic variation independently (Brooks et al., 1993a; Brooks and Richards, 1993b). This paper will take this further by considering scenarios in which the effect of vegetation change is also included.

MODEL STRUCTURE AND PARAMETERISATION

The model employed has been described elsewhere for both its one- and two-dimensional applications (Anderson and Pope, 1984; Anderson and Howes, 1985; Anderson et al., 1988). For the research described here, the model was used in its one-dimensional form, as this is more appropriate for translational failures where the soil is made up of several layers with distinct hydrological and mechanical properties, and where the failure edge effects are negligible. It has been selected previously for use in modelling soil hydrology and slope stability in freely draining podsols (Brooks et al., 1993a; Brooks and Richards, 1993b). For the current application, the model was updated to include routines for vegetation effects, both in terms of the influence on rainstorm characteristics and on soil properties (Collison, 1993). The updated version of the one-dimen- sional coupled hydrology-stability model is shown in Figure 1.

The model is a finite-difference scheme, splitting the soil column into a series of vertical cells which can be of different thickness. Three hydrologically distinct layers are permitted in the scheme, enabling separate

MODELLING HILLSLOPE FAILURE

Equations (iterati\,e) Int(tl=rain(t) dcrown aredarea) Storaee(t)=storage(r-l) + int(t) - evnp(t). stemflow(t) Stemflow(t)=stemrate

Rainfall

233

Canopy store

1 Effective rainfall

Runoff Surface I

Computational 1 ,; 1 points

lnitial water

Model base

) I . I

1 Rain’(t)=stemflow(t) + storage excesr(t)

Equation

Evup = F,, sin[2n(t)] ~ ~

ROOT ZONE Equations ’ Increased k, = RAR x k, Increased cohesion = c’ + 1 2 (Tr x RAR)

UNSATURATEDZONE Functional relationships : k = k(0); I+I = I@) (no hysteresis)

SATURATED ZONE Functional relationships : k = ky, = consrant: e = By, = constant

Equation :

time = time (secs) F- =maximurn evapotranspiration at midday k = hydraulic conductivity ( d s ) k, = saturated hydraulic conductivity ( d r ) RAR =root area ratio c = cohesion (kPa) Tr = roof tensile strength (kPa) e = volumetric moisture content (crn’icrn’) By, = saturated moisture content (cm’lcm’) w = suction fin)

Figure 1. Structure of the one-dimensional coupled hydrology-stability model incorporating vegetation

specification of hydrological and strength properties for the root zone, the eluvial and illuvial horizons of the soil profiles being modelled. In this way different soil morphologies can be considered, representing different stages of pedogenesis, with or without different types of vegetation. For these simulations, three soil profiles were selected to correspond to different stages of development of a podsol profile. Initially, undifferentiated regolith is considered, but field evidence indicates that through time the profile develops three hydrologically distinct horizons (Brooks et a[., 1993a). These include a surface organic-rich horizon (which reaches a maxi- mum depth of 15 cm), an eluvial horizon (also reaching a thickness of around 15 cm) and a Bs horizon which increases to around 70 cm in thickness. Changes occur in hydraulic properties, such as in saturated hydraulic conductivity and soil moisture characteristic curves, and the profile shows a progressive increase in both individual horizon thicknesses and total depth (Brooks and Richards, 1993).

For the simulations, the model specifies rainstorm properties as rainfall totals falling over a particular time period. In most cases this is hourly, although shorter time intervals can be considered. When slopes are tree- covered, a proportion of the rain is intercepted by the canopy according to the crown area ratio. From the canopy store, rain can undergo evaporation or pass to the ground by stemflow and throughfail, depending on whether the storage capacity of the canopy is exceeded. Rainfall reaching the ground can be detained on the surface or infiltrate into the soil. The latter occurs depending on the hydraulic potential between the detention store and the top cell. Runoff occurs once the detention store is exceeded, and evaporation occurs during the interstorm period. Within the soil column, water is routed vertically from cell to cell provided the column remains unsaturated, according to Richard’s equation solved with hydraulic conductivity calculated using the Millington and Quirk (1959, 1960) procedure. Saturated flux is calculated using D’Arcy’s law with

234 S. M. BROOKS AND A. J. C. COLLISON

the saturated hydraulic conductivity specified. The model simulates changes in soil moisture status and pore water pressures (positive or negative) for each iteration, and at the end of each hour uses the pore pressures in a stability analysis. By calculating the ratio between material shear resistance (according to the Mohr- Coulomb failure criterion) and downslope shear stress, the model calculates the factor of safety against fail- ure for each cell in the vertical stack. It is possible to examine the way in which the probability of failure varies with depth at different stages of the storm, in order to establish the most likely position for the shear surface and the time at which failure will occur. This is important when trying to evaluate the full significance of rainstorm character to hillslope failure. Different rainfall intensities can produce different depths to the failure surface. In the case of this one-dimensional soil column, the infinite planar slope stability analysis method is used. For this, the model requires values for material unit weight, cohesion and angle of internal friction. A slope angle is also specified, and in the following simulations a range of angles from 20 to 40" was employed to examine thresholds of stability in different soils with different vegetation.

To evaluate the effects of vegetation two basic scenarios were considered. The first involved a grass cover with no interception or canopy store, relatively shallow roots providing additional cohesion, and enhanced permeability in the rooting zone. The second scenario involved a cover of pine trees with canopy storage, root reinforcement of shear resistance to greater depths than for grass, and permeability enhancement twice that of grass. Values for canopy effects in pine are taken from Aldrich and Jackson (1968), permeability enhancement data are given by Arend (1942) and Johnson (1940), and root strengths are quantified by both Schiechtl(l980) and Selby (1993). Enhanced shear resistance for the soil is found using the equation of Wu et al. (1979), which relates it to root tensile strength and the proportion of the shear surface inter- sected by roots (see Figure 1). On this basis, the value for the additional cohesion provided by pine roots was set at 4 kPa and that provided by grass was specified as 1 kPa. Pine trees add around 2 kPa to soil unit weights, which adds to stability on low-angled slopes but promotes failure on steeper slopes (Selby, 1993). This was also included in the modelling. Given the greater rooting depth of pine, modelling the young

Mature cohesion kS,I

.& grass

pine

a rootzone

a A horizon

Ae horizon

B horizon

GRASS PINE +1.0 kPa +4.0 kPa +2 x10-6m/s +4 Xl0~6rryS

canopy effect none time-lag

r

Sequence of podsolisation

Figure 2. Scenarios for modelling the effect of soil development and vegetation on slope stability

MODELLING HILLSLOPE FAILURE

Table I. Rainstorm properties used in the simulations (based on data from Newson, 1980)

235

Intensity Duration Total (mm h- ) (h) (mm)

Channel group storm 1 80 1 80 storm 2 40 5 200

Slope group Storm 1 20 4 80 Storm 2 10 5 50

soil presented particular difficulties where the pine roots extended to a greater depth than the total soil thick- ness. While it is possible that roots could extend to the parent material and anchor the soil, it is more likely that the roots stop short of the parent material, providing a thin plane of weakness immediately below the root zone along which failure could take place. The basic scenarios and parameter values for soil and vege- tation variation are shown in Figure 2.

Consideration of the effect of Holocene climatic change is considerably complicated by the fact that the best estimates of changing rainfall totals relate to annual averages, while it is short-term variations in intensity during storms which are of greater significance to slope failure. Recent research has indicated significant differences between the frequency, duration, intensity and distribution of rainfall for storms generated under convective and those generated under frontal conditions (Brooks and Richards, 1994). This can be related to Holocene climatic change by considering the way in which areas are dominated alternately by frontal and convective storms, depending on the location of a given area with respect to changing global atmospheric circulation patterns (Lamb, 1982). Under particular climatic conditions it is the ‘extreme’ storms that are likely to promote failure, but storms can be extreme in duration, intensity or total. Newson (1980) recognized this distinction for a number of current day storms, grouping them according to whether they produced channel changes or slope instability. In the former case, storms were of relatively high intensity and short duration (convective) while in the latter case storms were of lower intensity and longer duration (frontal). As the position of the westerly air flow has altered during the Holocene, areas have been subjected alternately to ‘extreme’ frontal storms and ‘extreme’ convective storms. By plotting intensity-duration relationships for the storms identified by Newson (1980), clear relationshps are apparent which vary depending on the synoptic origin of the storm (Figure 3). By selecting a variety of storm types on the basis of these plots, the effect of climatic variation through changing synoptic conditions can be assessed. The characteristics of the storms selected for the model simulations are given in Table I.

SIMULATION RESULTS

Initiul conditions for the simulations The main soil-climate-vegetation conditions employed in the simulations have been described above.

To determine appropriate values for initial moisture contents in each scenario, the profiles were saturated and then allowed to drain for 120 h with no rainfall. There are clear differences in drainage resulting from different degrees of soil development, with mature soils retaining more moisture during interstorm periods. Reasons for this have been discussed by Brooks et al. (1993a). However, the simulations suggest that differences in moisture retention due to vegetation cover are negligible, with both pine- and grass-covered profiles behaving similarly for a given stage of soil development. These results enable selection of appropriate moisture contents for each cell at the start of subsequent simulations.

236 S. M. BROOKS AND A. J. C. COLLISON

100 r 1

Channel Group (anticyclonic) Slope Group (cyclonic)

16,

14

12

10

8 -

6 -

4 -

0

-

~

- 0 0

B

6ol +

80-

I + +

+

401 20 + 2 -

n 0 i I I

0 1 2 3

0

+ +

+

4 5 6 7 8 9 10 0 5 10 15 20 25

time (hours) time (hours)

Figure 3. Characteristics of different geomorphically effective rainstorms (after Newson, 1980)

The role of vegetation in slope stability for soils at diferent stages of development To consider the role of soil profile development and vegetation on slope stability, four storms were

selected. Those occurring under convective conditions were of 1 and 5 h duration, with intensities of 80 and 40mmK' respectively. The frontal storms were of 5 and 20h duration, with intensities of 10 and 4mm h-] respectively. Figure 4 shows the resulting minimum factors of safety developed at each depth in the soil profile during the simulations, for a 30" slope. As the soil becomes better developed the factors of safety approach unity (the point at which failure will occur), which is consistent with the findings of earlier research (Brooks et al., 1993a). However, in the current example the effect of vegetation is also apparent. The most striking feature is the effect of the pine cover on the youngest soils, where root reinforcement can pro- mote stability throughout the whole soil profile under all storms considered. With more mature, deeper soils, pine roots are unlikely to develop through the entire soil profile (Figure 2). In such cases the beneficial effects of root reinforcement are countered by the adverse effects of increased permeability, leading to lowered stability in pine-covered slopes as compared with grass-covered slopes. As the soil matures, this becomes increasingly apparent and is thought to be related to two factors. First, an increasing difference between permeability in the root zone and in the Bs horizon leads to higher pore water pressures at the potential shear surface. Second, model results show that with greater total soil depth, the potential shear surface exists within the B(s) horizon below the rooting zone of both vegetation types, at a similar depth in each case. Whilst the grass roots terminate in the lower permeability Ae horizon, the deeper pine roots provide a higher permeability route through the Ae to the critical B(s) horizon, where percolating water can have a detri- mental effect on slope stability. In young soils this situation does not arise, since both pine and grass pene- trate to within a short distance of the critical shear surface.

The trends identified above are clearly illustrated by Figure 5, which compares soil profile and vegetation effects under two types of storm. Again, simulations are for a 30" slope, with 24 h of soil profile drainage. The minimum factor of safety represents the worst-case conditions which develop during the storm; this normally occurs during the last hour of the storm in the lowest cell. However, in the case of the mature soil, the time- lag is slightly longer, and worst-case conditions develop 2 h after the cessation of rainfall under low intensity storms. Again, the effects of soil type are very clear. Also apparent is the greatly enhanced stability in the

MODELLING HILLSLOPE FAILURE

a) Young soil b) Intermediate soil

6

25 0

0 -

20

40

60

80

100

- anticyclonic storm

_ _ _ _ _ -cyclonic storm

c) Mature soil

-

-

- 0

2 3 4 5 6 1.dw d nimy

237

factor of setev

Figure 4. Minimum factors of safety for various soil-vegetation-rainstorm combinations

youngest soil profile with pine vegetation. Young soil profiles with a grass cover are considerably less stable. Under such conditions, deforestation could promote slope failure under high intensity storms on slopes somewhat steeper than 30". With greater soil development the effects of vegetation are far less clear, and for both intermediate and mature soils under both storms a grass-cover appears to be more stable than a pine-cover. Removal of trees and their replacement with grass might ultimately lead to enhanced stabi- lity, although it needs to be recognized that there are a number of stages of vegetation colonization which are not simulated in this paper. Finally, it is again apparent that the higher intensity rainstorms are more likely to cause failure, whatever the stage of soil development or type of vegetation.

In terms of both the overall minimum factor of safety and the changes during individual storms, the model has indicated clear differences in stability between soils of different ages with different vegetation covers. It has also illustrated possible explanations that might be employed when considering these differences, thereby indicating the complex interaction which exists between vegetation and soil development.

The role of climatic variability in mature podsols with diferent vegetation cover To consider the effect of climatic variability through the changing nature of extreme storms, the four

'representative' storms described in Table I were applied to all the soil-vegetation combinations, using a range of slope angles. Figure 6 plots the minimum factor of safety which develops in each soil profile, for each storm, under the two types of vegetation. Again, this relates to the lowest cell during the last hour of the storm (in the case of some mature profiles, this relates to 2 h after the cessation of rainfall). Apart from the clear differences in stability which result from progressive pedogenesis, the most obvious trend is towards a reduction in stability as rainfall intensity increases from 4 to 80 mm h-' , despite the same total

238 S. M. BROOKS AND A. J . C. COLLISON

Figure 5. Comparison of stability for different soil, vegetation and storm types

amount of rainfall being delivered in each case. This contradicts the results of Newson (1980), who found that these high intensity storms promote runoff and channel changes, with slopes being more likely to fail under the lower intensity storms. In the example specified here, the soils are freely draining with high infil- tration capacities. No runoff is generated, and the higher the intensity, the more detrimental the storm. With other soil types of lower permeability such conclusions may not be reached, and the findings of Newson (1980) might be substantiated.

a) Young soil

b) Intermediate soil

c)Mature soil

Figure 6 . Trends in the minimum factor of safety with rainfall intensity

MODELLING HILLSLOPE FAILURE 239

There are interesting vegetation effects. In young soils, stability is greatest with pine vegetation, as described above, under all four storms. With an intermediate level of development there is a small difference between pine and grass under anticyclonic storms, which is sensitive to rainfall intensity, but as intensity is lowered grass-covered slopes become more stable than pine-covered slopes. This effect is even more apparent in the mature soil, where the lower intensity storms show a greater difference between grass and pine, with the former promoting greater stability.

The application of the above results to natural landscapes can be attempted by considering limiting angles of stability which develop under different soil-vegetation-storm combinations. Figure 7 plots the minimum factor of safety against slope angle for the scenarios described above. Taking the young soil profile, slope failure is unlikely in all cases, although there is a clear difference between low and high intensity storms, as well as between the two types of vegetation. For the intermediate soil, high intensity storms promote failure on all slopes above 36", but this range is reduced to 35" for slopes having a pine-cover. Low intensity storms do not produce failure on any slopes. It is for the mature soil that the differences become most sig- nificant, since both types of vegetation and storm promote failure. However, the range of angles under which failure occurs varies for the different conditions. Grass-covered slopes subjected to low intensity storms really only fail if steeper than about 39". If covered by pine this angle is reduced to 36". With high intensity storms the limiting angle of stability is reduced to 32", with just 0.5" difference between the two vegetation types. In this case the effect of climatic change, involving a switch from predominantly frontal to predomi- nantly convective storms, far outweighs the effect of vegetation change. Conversely, given the inherent stability of slopes with less developed soils, a similar switch in climate will have very little effect on the land- scape in terms of promoting slope instability. However, it is clear that the combined effect of soil development,

a) Young Soil b) Intermediate soil

3 t I

20 25 30 35 40

slope angle (degrees)

1 -

I anmbnrerotrns 1

- --- pine __ grass

c ) Mature soil facior of safety

1 8 ,

20 25 30 35 40

slope angle (degrees)

20 25 30 35 40

slope angle (degrees)

Figure 7. Limiting angles of stability for different soil-rainfall-vegetation combinations

240 S. M. BROOKS AND A. J. C. COLLISON

vegetation colonization and climatic change presents a complicated problem for interpretation of long-term landscape change through shallow translational failure.

DISCUSSION

Although the above modelling approach has considered just a few simple scenarios, it provides some preli- minary results which indicate the potential of physically based models to enhance our understanding of slope stability issues in the light of simultaneous changes in soil profile, vegetation character and climate. Clearly, there are several avenues for future development but the above results can be used to substantiate certain arguments involving the effect of climate and vegetation change on slope stability.

It has been argued that climatic deterioration during the Little Ice Age is responsible for enhanced slope activity in many regions (Grove, 1972; Brazier and Ballantyne, 1989). The above results support this con- tention if applied to slopes covered by relatively mature but freely draining soils. Increased rainfall totals promote failure whether the vegetation consists of a pine or grass cover. During the Little Ice Age, cyclonic storms in eastern Scotland were severe (Lamb, 1982; Whittington, 1985), producing extreme rainfall totals. It is likely that under such conditions slope instability would have increased, but mostly for slopes support- ing mature soil profiles. With poorly developed soils large rainfall totals do not promote failure, especially where such soils are supporting pine vegetation. A factor which remains to be substantiated by field evidence is the possibility that in past Holocene periods, involving greater continentality and higher intensity storms, slope failure became more widespread. Evidence is available for enhanced mass movement earlier in the Holocene (Starkel, 1966), which might have taken place under more continental conditions. However, at that time the soils were possibly at a young stage of development. Continental phases, such as the Mediaeval Warm Epoch, might have had a generally more favourable climate, but a higher incidence of high intensity storms might have produced more widespread mass movement activity. Furthermore, with extension of agri- culture under favourable climatic conditions, the combined effect of vegetation change and climatic change needs to be considered. To date there is little field research to support this, although an increased incidence of mass movement coincident with agricultural expansion has been suggested (Harvey et al., 198 1).

The argument for the importance of anthropogenic vegetation modification, generally involving a change from tree to grass cover, appears to be less well supported by the simulation results. In the soils considered here, which are freely draining podsols, mature profiles show little difference in stability for the vegetation types, although with lowered rainfall values the difference is more marked. Grass-covered profiles are slightly more stable than pine-covered soils, so a change from forested to grazed land is unlikely to promote slope instability in itself. On thin, poorly developed soils under exceptional storm totals, slope angles in excess of 38" might fail, but most slopes will remain stable.

It is recognized that many issues have been omitted from the foregoing analysis and that the conclusions reached are tentative. In terms of parameterization, three issues are particularly pertinent. First, the soils considered above are freely draining podsols. Such soils often develop impeded drainage resulting from the presence of fragipan horizons. This may lead to greater instability generally, but may also change the arguments developed above in relation to the effect of climate or vegetation change on slope stability. Other soil types might also produce different results, especially where they are of lower permeability than those considered here. Second, storm characteristics are very complicated, and it is a major simplification to assess the effect of climatic change through a change in contemporary rainfall totals, intensities and durations. Finally, the inclusion of the role of vegetation is also highly simplified. In terms of model structure, a one-dimensional scheme can only consider cases in which lateral throughflow occurs parallel to the slope, with little increase in pore water pressures downslope. In the case of shallow translational failure for long rectilinear slope sections with relatively freely draining material, this is a reasonable representation. For convexo-concave profiles with downslope variation in regolith character, this is less appropriate.

Despite this, the methodology adopted above has provided some supportive evidence for arguments related to the role of vegetation change and climatic variability in slope instability. With the limitations in mind, the results can be interpreted to shed some light on this complicated issue and to elucidate some of the main factors responsible for the initiation of widespread slope failure. Such a methodology, involving

MODELLING HILLSLOPE FAILURE 24 1

the use of physically based coupled hydrology-stability modelling, has not previously been employed in such a context. Future applications can also consider the cyclic nature of slope failures (Butler, 1959; Walker, 1966). Periodicity in slope failure occurrence has been related to the rate of recovery of the slope following failure, and the development of conditions that permit subsequent failure. This includes post- failure pedogenesis on the exposed slip surface, but also vegetation colonization. The above results have provided some preliminary ideas concerning polsolization and vegetation change. Where soil depth increase proceeds at an equivalent rate to root penetration in the soil, the situation is unlikely to promote failure, especially with tree cover. Vegetation establishment and change generally operate on a shorter timescale than pedogenesis. Once the vegetation is established, usually over a period 100-200 years (Fanta, 1986) pedogenesis proceeds under stable conditions until mature soils result. The rate at which this occurs for podsols is debated, but establishment of a ‘mature’ podsol may be of the order of several thousand years (Chandler, 1942; Burgess and Drover, 1953). At each stage in the vegetation-soil maturation sequence the stability criteria change, complicating the interpretation of the effect of Holocene climatic change on slope stability. It is clear that a considerable amount of new field evidence is needed to parameterize physi- cally based models before they can be applied to such issues with confidence. Nevertheless, this preliminary example has indicated the considerable potential that such model development, parameterization and appli- cation can offer to understanding slope failure initiation in the Late Holocene. The development of such models for use with appropriate field data can provide significant insight into the direct causal mechanisms leading to instability.

ACKNOWLEDGEMENTS

The ideas presented here are largely the outcome of research undertaken in the Department of Geography, University of Bristol, under the guidance of Professor M. G. Anderson. Early development of the one- dimensional coupled hydrology-slope stability model was also carried out at Bristol under the supervision of Professor Anderson.

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