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Figure 8. The change in cumulative volume for different widths of deposition and scour at Blueberry Creek. Inspection shows the cumulative flow volume obtained with UBCDFLOW to demonstrate a clear sensitivity to width of entrainment and deposition. Although the curves appear to follow the same general trend, they do not vary in the same manner throughout all reaches of the event path. Entrainment was noted to dominate over the initial portion of the event path. With increasing slope (or travel) distance to Reach #8, the percent variation in cumulative flow volume increased to a typical value of +40% (for a +50% change in reach width) and -40% (for a -50% change in reach width). It remained essentially unchanged thereafter until Reach 19, where the mode of flow changed from one of primarily entrainment to one of deposition. At this point along the event path, the cumulative flow volume either diminished very significantly (for the case of increased width) or relatively modestly (for the case of decreased width). A consequence of these changes, upon the onset of major deposition, was the curves for the three scenarios intersected (see Figure 8). As a result, the scenario with +50% widths terminated earlier than the base case, and the scenario with – 50% widths extended further than the travel distance of the base case for the “observed” event. This implies that, in the final reaches of the event path, the travel distance is moderately sensitive to the width to which the debris spreads out upon reaching the unconfined fan. This zone of terminal deposition is often the most challenging to understand, in any prediction of debris flow impacts and provision of defense measures. The findings imply that cumulative flow volumes generated by UBCDFLOW at Blueberry Creek are quite sensitive to the width of entrainment and deposition, with reaches involving deposition being particularly sensitive. This is attributed to the fact that, as the depositional behaviour starts to dominate, the reaches tend to significantly increase in width given the location on the hillslope and, thus, the percent change in reach width imposes a relatively larger change in magnitude compared to the narrower reaches of entrainment. In effect, the much wider reaches yielded a greater area

available for deposition and, therefore, a greater reduction in cumulative flow volume. As was done for the sensitivity analysis to initial failure volume, the travel distance was determined for each of the three scenarios of reach width, by again assuming that debris would continue to be deposited at the same rate found in the penultimate five reaches of the surveyed event. An estimate was not required for the curve generated for the +50% scenario, because the calculated travel distance did not exceed the base case. In contrast, an estimate was required for the -50% scenario. The minimum travel distance was 1510 m (7% less than the actual value), for the +50% scenario, and the maximum travel distance was estimated at 1810 m (12% more than the base value) for the -50% scenario. While the travel distance was more sensitive to changes in reach width than changes in initial volume, the travel distance calculated by UBCDFLOW for the Blueberry Creek scenario is nonetheless deemed relatively insensitive to changes in reach width. 5. CONCLUSIONS A consideration of findings arising from the sensitivity analysis conducted on travel distance of the Blueberry Creek event leads to the following conclusions: (1) UBCDFLOW, although subject to limited verification

in engineering practice, appears to have significant promise for analysis of debris flow travel distance in steep forested terrain. A back analysis of the Blueberry Creek event reveals excellent agreement with the actual travel distance. A sensitivity analysis to input of the initial failure volume suggests the calculated travel distance, and the cumulative flow volume, are not significantly influenced by a change of ±50% in the size of the event at the point of origin. This is a promising finding because there is considerable uncertainty in estimating the initial volume of a potential slide on an open slope, where most landslides seem to initiate.

(2) Further analysis has shown the travel distance to be

moderately sensitive to change in the input value of reach width. It appears a change in reach width exerts a relatively greater influence on volumes of deposition than entrainment. In the entrainment zone, an increase in width implies a greater channel debris yield rate. In the deposition zone, a greater width implies a more rapid rate of lateral spreading on the fan.

(3) In contrast, the UBCDFLOW model exhibits a

significant sensitivity of cumulative flow volume to the value of reach width. This could have implications for planners and managers wishing to determine the likely impact of a potential debris flow, and consider necessary protective measures. While it appears reasonable to predict the travel distance with reasonable confidence, it is considerably more difficult to ascertain the volume of material at any

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given location along the event path. In this regard, another perceived limitation of the model is the sensitivity of the flow behaviour to two key slope angles, specifically those of 19o for unconfined slopes and 10o for confined slopes, because of the ranges over which the UBCDFLOW regression equations are defined. Accordingly, for applications in engineering practice, we suggest the model be run several times with slope angles both less and greater than those critical angles in order to assess the potential consequences.

(4) Finally, it should be noted that the UBCDFLOW

model was developed based on empirical data collected from logged areas in the Queen Charlotte Islands. Therefore, care and judgment should be exercised in using it for sites on unlogged terrain, or in different geographic areas.

ACKNOWLEDGEMENTS This study was undertaken as part of the requirements of an undergraduate thesis in the Environmental Engineering Program at the University of British Columbia, dept. of Civil Engineering. REFERENCES Bobicki, E.R. (2006) Landslide Risk Assessment in Steep

Forested Terrain. B.A.Sc. Thesis. Faculty of Applied Science. Department of Chemical Engineering. University of British Columbia, Vancouver, BC.

Eliadorani, A., Fannin, R.J. and Jordan, P. (2003) Debris flows: back analysis of travel distance at Blueberry Creek using UBCDFLOW. 3rd Canadian Conference

on Geotechnique and Natural Hazards, Edmonton, Alberta, 349-356.

Fannin, R.J. and Wise, M.P.W. (2001) An empirical-statistical model for debris flow travel distance, Canadian Geotechnical Journal, 38(5): 982-994.

Holland, S. (1964) Landforms of British Columbia: A

Physiographic Outline. Bulletin 48, British Columbia Department of Mines and Petroleum Resources, pp.138.

Jordan, P. (1993) Memorandum. Re: Blueberry Creek landslide of May 1993. British Columbia, Ministry of Forests, Nelson Forest Region, unpublished report. 7 pp.

Jordan, P. and Nicol, D. (2002) Causes of gentle-over-steep landslides in Arrow and Kootenay Lake Forest Districts. Jordan, P. and Orban, J. (eds.). Terrain

Stability and Forest Management in the Interior of

British Columbia: Workshop Proceedings, May 23–25,

2001, Nelson, B.C. British Columbia, Ministry of Forests Research Branch, Technical Report 3 ,Victoria, B.C. Appendix 2, 7 pp.

Meidinger, D. and Pojar, J. (1991) Ecosystems of British

Columbia. British Columbia, Ministry of Forests, Special Report Series 6. 330 pp.

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RUNOUT CHARACTERISTICS OF OPEN SLOPE LANDSLIDES IN HARVESTED TERRAIN ON VANCOUVER ISLAND: PRELIMINARY RESULTS OF AN INTEGRATED FIELD-DYNAMIC MODELLING BASED STUDY Marc-André Brideau, Earth Science Department, Simon Fraser University, Burnaby, BC. Doug Stead, Earth Science Department, Simon Fraser University, Burnaby, BC. Tom Millard, Research Geomorphologist, BC Ministry of Forest, Nanaimo, BC. George Patton, Earth Science Department, Simon Fraser University, Burnaby, BC. ABSTRACT

Field investigation and dynamic analysis of open slope landslide runout in harvested terrain was undertaken in two field areas on Vancouver Island in 2005. Fourteen landslides in the Sarita-Klanawa watersheds and seven in the Tsitika-Eve watersheds were investigated. Terrain attributes (including slope aspect, slope gradient, slope position, terrain polygon type, soil texture, soil consistency, drainage class, contributing factors) were recorded at each site. The landslide paths were divided into reaches for which the trend, gradient, length, width and depth of deposition or erosion were recorded.

The second component of the study consisted of back-analysis using the dynamic analysis code DAN-W, to simulate the observed runout distances for landslides studied in both field areas. The volumes and cross-sectional profiles obtained from field data collection were incorporated in the runout models.

The final component of this study involved a detailed sensitivity analysis assuming varied input parameters within the dynamic analysis code DAN-W. Three-dimensional surface plots were used to evaluate the sensitivity of runout distance to the pore pressure coefficient and friction angle in the case of the Frictional rheology criterion and to turbulence and frictional coefficients in the Voellmy rheology. RÉSUMÉ

Une étude de terrain et une analyse dynamique de glissement de terrain furent initiées dans deux régions sur l’Ile de Vancouver en 2005. Quatorze glissements dans les basins hydrographiques Sarita-Klanawa et sept dans les basins hydrographiques Tistiak-Eve furent étudiés. Les attributs de terrain (incluant l’aspect de la pente, le gradient de la pente, position sur la pente, classe de drainage, la texture des sols, la consistance des sols, les facteurs qui contribuent à l’instabilité) ont été documentés à chaque site. La trajectoire de chaque glissement de terrain fut divisée en sections pour lesquelles la tendance, le gradient, la longueur, largeur, et profondeur de la déposition et l’érosion furent enregistrés.

La deuxième composante de cette étude consiste d’une analyse à rebours utilisant le code d’analyse dynamique DAN-W, pour simuler la longueur de déposition observée sur le terrain pour les glissements de terrain étudiés. Le volume de matériel et le profile en section obtenu a partir des données de terrain furent incorporés des les models de longueur de déposition. La composante finale de cette étude consiste en une analyse détaillée de la sensitivité des paramètres d’entrée pour le code d’analyse dynamique DAN-W. Des surfaces tridimensionnelles furent utilisées pour évaluer la sensitivité de la longueur de déposition au coefficient pression interstitielle et angle de friction pour la rhéologie Frictionnelle et des coefficients de turbulence et de friction pour la rhéologie Voellmy. 1. INTRODUCTION

The occurrence of debris flows has been documented in all parts of the province of British Columbia. The relation between landslides and logging in coastal British Columbia has been the subject of recent research (Jakob, 2000; Guthrie, 2002; Rollerson et al., 2001; 2002, and Millard et al, 2002). These workers found that while the increase in activity varied between different watersheds, logged areas were more prone to landsliding. Terrain attributes correlated with slopes susceptible to failure were identified and terrain stability maps produced (Howes and Kenk, 1997; Rollerson et al., 2001; 2002). While landscapes prone to landslides can be identified, estimating landslide runout distance still poses a significant challenge. This is particularly

problematic for open slope landslides as the simple empirical methods developed for confined debris flows (Benda and Cundy, 1990) are not applicable and both probabilistic (Fannin and Wise, 2001) and dynamic (Hungr, 1995) models have yet to be thoroughy calibrated for theses types of event. 2. FIELD STUDY AREAS

Two main field study areas were visited during the 2005 field season. The first area was the Sarita and Klanawa watersheds on southwestern Vancouver Island and the second the Tsitika-Eve watersheds in northeastern Vancouver Island. These areas were chosen as initial locations for the research since both had recently had landslide inventories completed. (Denny

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Maynard & Associates Ltd. and Golder Associates 2002; 2004a). Both study areas lie within the Vancouver Island Ranges physiographic region, and are characterized by deeply dissected and moderately rugged terrain. Elevation varies from sea-level to 1400 meter ridges in the Klanawa-Sarita watersheds and 1600 meter peaks in the Tsitika-Eve watersheds. The areas were glaciated during the late Wisconsin glaciation (Alley and Chatwin, 1979; Howes, 1981a, b). Surficial deposits in both areas are predominantly composed of till and colluvial material with minor amounts of outwash, glacio-lacustrine deposits with in addition glacio-marine deposits in the Tsitika-Eve watersheds (Alley and Chatwin, 1979; Howes, 1981a,b).

Figure 1: Location map of the field study areas

The bedrock geology of the Sarita and Klanawa watersheds is dominated by volcanic rocks from the Bonanza Formation and intrusive rocks of the Island Intrusions (Muller 1976, 1982). The bedrock geology of the Tsitika-Eve watersheds is dominated by volcanic rocks from the Karmutsen Formation and intrusive rocks of the Island Intrusions. The variation in climate between the Sarita-Klanawa watersheds (Bamfield station) and the Tsitika-Eve watersheds (Alert Bay station) is summarised in Table 1. The Sarita-Klanawa watersheds experience, on average, warmer and wetter weather than the Tsitika-Eve watersheds. The Sarita-Klanawa watersheds also experience more extreme daily precipitation. Research by Caine (1980), suggests that the higher rainfall intensity in the Sarita-Klanawa watersheds would make it more susceptible to landsliding than the Tsitiak-Eve watersheds.

Table 1: Summary climatic data for weather stations near each of the field study areas. (data from Environment Canada, 2005) Weather Station

Average Daily Temperature (oC)

Average Yearly Rainfall (mm)

Extreme Daily Rainfall (mm)

Average Yearly Snowfall (cm)

Bamfield 9.4 2869 185.91 20 Alert Bay

8.5 1591 116.12 63

1 For the 1959-1998 period; 2 For the 1913-1994 period; 3. RESULTS 3.1 Landslide Volumes

The volume distribution for the landslides investigated displayed a weakly bi-modal distribution with a large concentration of small events (<500m3) and a minor concentration of large events (>2000m3) (Figure 2). Such a distribution was only previously encountered in databases with a small number of events (e.g. supplementary data in Wise, 1997). In larger databases an approximately exponential decrease in landslide volume is expected.

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Figure 3 shows the recorded runout distances landslides visited during the 2005 field season. A well developed bimodal distribution is observed. One mode is centered on a runout distance of about 100m while the second mode is at approximately 500m. Such a bimodal distribution was not observed in previous studies (Wise, 1997; Johnson et al., 2000) and similar to the landslide volume in Fig. 1 and is probably related to the small sample size (N=21) at this preliminary stage in the research.

The elevation difference (∆H) divided by the travel distance (L) is a frequently quoted parameter for large landslides (Scheidegger, 1973). It is generally accepted that the larger the volume of a landslide the smaller the ∆H/L ratio (Scheidegger, 1973; Corominas, 1996). In Vancouver Island, a similar relationship was observed for landslides with a volume greater than 500 m3 while no relationship was observed for smaller landslides (Figure 4). A similar dependence on volume was noted by Finlay et al., (1999) for small landslides in Hong Kong.

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A moderate drainage class was predominant at the visited sites (Figure 5). Seepage on the failure surface was observed at all but two sites (Figure 6). Seepage in the headscarp and on the ground surface surrounding the landslide was also commonly observed. The presence of seepage in the headscarp and along the failure plane has frequently been reported by other workers (e.g. Johnson et al., 2000).

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While the importance of pore water pressure in

reducing the shear strength of soils is well recognised the spatial distribution of zones of high pore water pressure is often very complex and difficult to characterize. (Pierson, 1980; Wilkinson, 1996; Fannin and Jaakkola, 1999; Fannin et al., 2000). The water retention capacity of soil and related factors such as the variation in soil depth and the grain size distribution have been suggested as some of the important controlling factors in pore water pressure distribution (Pierson, 1980). Iverson and Major (1986), Johnson and Sitar (1990), and Fannin et al. (2000) all suggest that flow interception by macropores, flow from soil into bedrock fractures and bedrock topography might also influence pore water pressure distribution.

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There was a marked concentration of landslides occurring in the lower section of the slope (Figure 7). This is possibly a consequence of topography controlling the hydrology/groundwater conditions at the sites. In classical hillslope hydrology, the higher positions on the slope are generally considered to be zones of recharge while lower positions are zones of discharge. This would lead to the development of the highest pore water pressure and seepage forces in downhill portions of the slopes. Fieldwork from Pierson (1980) however has shown that highest piezometric values are not always localized in the lower sections of hollows. Analytical solutions, numerical modelling and fieldwork from Iverson and Major (1986; 1987) and Johnson and Sitar (1990) demonstrate that groundwater conditions in the bedrock and small scale slope topography can have a strong influence on pore-pressure distribution and shallow landslide initiation.

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The headscarp stratigraphy of the 21 landslides consisted of 60-70cm of weathered till or colluvium overlying unweathered till (up to 100cm) and bedrock. Soils of the podzolic order are developed in the weathered till or colluvium horizons while the unweathered till represent the parent material (C-horizon). The soil strength (S1-S5) was estimated in the field for the landslides in the Tsitika-Eve watersheds. The weathered till was found to be a very loose to loose soil while the unweathered till could only be excavated with a pick and was a compact to dense soil (Figure 8 and Table 2). Samples of weathered and unweathered till were collected from both field sites for particle size analysis. Table 2: Soil Strength (Density) Classification used. (after BC Ministry of Forest (2002). Class BC Ministry of Forest

description Field parameters used in this study

S1 Very Loose - Easily excavated with spade

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Grain size distribution curves were determined for fifteen samples from the Sarita-Klanawa watersheds and nine samples from the Tsitika-Eve watersheds. The samples were found to be characteristic of well graded sand or gravel (GW, SW) soils. Figure 9 shows the results for five representative samples collected in the Sarita-Klanawa watersheds. From those examples, it can be seen that the material in the headscarp of example C is overall finer than the material in the deposit. This is due to preferential removal of fine material by rain in soil exposed to the elements (no vegetative cover). Similarly the unweathered till at example B contains a higher percentage of fine material than the weathered till at the same site. This is due to eluviation processes in soil development.

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The factors which contribute to the reduction of the stability of the slope are presented in Fig. 10. Two dominant contributing factors were observed on Vancouver Island. The first was the loss of material strength by root degradation. Root strength degradation via rotting after logging has been recognised as a potentially important factor in forest soil strength (O'Loughlin, 1972; Ziemer and Swanston, 1977; Buchanan and Savigny, 1990). It should be noted that

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since the investigated landslides were all located in logged area root degradation as an important contributing factor is not unexpected.

The second dominant contributing factor was evidence of localized high water pressures due to seepage in surficial material and through bedrock fractures. Localized high pore water pressure through fractured bedrock was observed in several cases in the field. Denny Maynard and Associates and Golder Associates (2004b,c) had previously noted the association between highly fractured rock and the spatial distribution of landslides in the Klanawa watershed. Water flow discharging from fractured bedrock into the overlying surficial deposits was also noted to play an important role in the spatial distribution of debris flows in (e.g. Johnson and Sitar, 1990).

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Figure 10: Contributing factors to investigated landslides. 4. DYNAMIC MODELLING

DAN-W is a one-dimensional dynamic analysis numerical modelling code developed by Hungr (1995) to estimate the runout of rapid flow slides, debris flows and avalanches. DAN-W represents rapid landslide movement as a continuum equivalent fluid and uses the Lagrangian solution to the St-Venant equations of motion. The heterogeneity in the landslide is represented by an equivalent material which yields the same dynamic component (velocity) and deposit geometry (runout distance, thickness of the deposit) as observed in the field (Hungr, 1995). The rheology of the flow is defined by the user and can be varied for different reaches of the debris flow path (DAN-W, 2004). The width of the channel can be specified thereby controlling the volume involved and accounting for lateral confinement (Hungr, 1995).

Ayotte et al. (1999) applied DAN-W to small volume open slope debris flows (100-1500m3) in Hong Kong. For very small volume events (<800m3) they found that unrealistically high friction angles were necessary to model the runout distance and debris distribution satisfactorily. The energy of the small volume events was suggested to be rapidly dissipated on obstacles such as cobbles, boulders, shrubs, small trees or woody debris. Ayotte et al., (1999) also suggested that the frictional model provided better results for partly saturated material while the Voellmy model was better at modelling more saturated flows. Bertolo and Wieczorek (2005) recently applied DAN to small debris flow in the Yosemite Valley, California. They used the Voellmy, Bingham and a

combination of Frictional (source) and Voellmy (transport) rheological models.

A series of preliminary back-analyses were performed on 12 of the 21 landslides investigated. The landslide geometry and volume as measured in the field were used as input in the DAN-W models. For the first series of analyses a frictional rheology was adopted and the material was assumed to by dry. Figure 11 compares the back-calculated friction angles from the frictional rheology models of selected landslides investigated on Vancouver Island with results from Ayotte et al. (1999) for small landslides in Hong Kong. Both set of values have similar ranges. While a good fit between the observed and modelled runouts was achieved the velocities in the frictional models were consistently in the order of 20 to 30 m/s. Similar velocity overestimation was also encountered by Bertolo and Wieczorek (2005) in preliminary runs of models of debris flows using only a frictional rheology. Based on the 12 models from the Vancouver Island sites the landslide volume (> or < 500 m3) does not appear to have a marked effect on the back-calculated friction angles of a dry frictional material.

Figure 11: Back-analysed friction angles for 12 of the Vancouver Island landslides compared with results from Hong Kong (Ayotte et al., 1999).

A graph of the input values used in the runout modelling of the Vancouver Island landslides using the Voellmy rheology is presented in Fig. 12. Input values used by Ayotte et al., (1999), Revellino et al. (2004) and Bertolo and Wieczorek (2005) for small landslides are also plotted for comparison. For the Voellmy rheology both the friction and turbulence coefficient were varied. An assumed value of 15 m/s was chosen for the maximum velocity as an additional modelling constraint. A maximum velocity of 15m/s could not be attained for landslides with a volume less than 500 m3, the modelled maximum velocity not exceeding 6-10 m/s. Compared to previously published model results, the Vancouver Island landslides (regardless of their volume) had a higher friction coefficient. This difference may possibly be attributed to the degree of channel confinement (Yosemite Valley), high material water content (Hong Kong) and/or difference in source material (volcanic soil, Campania). Further study will be required to confirm those preliminary findings.

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Figure 12: Back-analysed Voellmy rheology parameters for 12 of the Vancouver Island landslides compared with previously published values

Three conceptual profiles were selected to investigate the sensitivity to values of input parameters in the Frictional and Voellmy rheologic models in DAN-W (Fig. 13). For the three profiles the width (30 metre) and volume (2000 m3) were maintained constant. Only the results for Profile 3 will be presented in this paper. Fig. 14 is a 3-dimensional plot prepared using the software Surfer (Golden, 2002) of the Fahrboschung number (vertical travel distance/horizontal travel distance) using the Voellmy rheology in DAN-W. As expected the Fahrboschung decreases (landslide mass travel greater horizontal distance) as the friction coefficient decreases. Figure 14 also illustrates that there is a relationship (albeit weaker than with the friction coefficient) between the Fahrboshung and the turbulence coefficient. Figure 15 is a maximum velocity surface for the Voellmy rheology. The maximum velocity increases as the turbulence coefficient increases. Figure 15 also illustrates that there exist a relationship between the maximum velocity and the friction coefficient. Figure 14 and 15 therefore imply that to define a specific set of Voellmy parameters (friction and turbulence coefficients) for a given landslide information about its runout distance (Farboschung) and estimates of its maximum velocity are required.

Figure 13: Three conceptual slope profiles used in the sensitivity analyses

Figure14: 3-D surface for Profile 3 of the Fahrboschung as a function of turbulence and friction coefficients

Figure 16 represents a surface for profile 3 of the Fahrboschung number for the Frictional rheology. As expected the Fahrboschung number decreases (longer runout distance) as the friction angle decreases and the pore water pressure increases. The flat portion of the surface for friction angle lower than 20o represent values where the landslide material reached the end of the modelled profile (see figure 13). Figure 17 represents a surface for Profile 3 of the maximum velocity for the frictional rheology. The maximum velocity is seen to increase markedly as the friction angle decreases but also more gently with decrease in pore water pressure. The maximum velocity plateaus for friction angle values below 20o as a result of the landslide material reaching the end of the profile. A sharp decrease in maximum velocity is noted for friction angle values around 40o as the landslide movement barely initiates. Similar to the Voellmy rheology, the frictional rheology model requires information about the runout distance and maximum velocity to define a unique set of rheological parameters.

Figure 15: 3-D surface for Profile 3 of maximum velocity as a function of the turbulence and friction coefficients

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Figure 16: 3-D surface for Profile 3 of Fahrboshung as a function of pore water pressure coefficient and friction angle

Figure 17: 3-D maximum velocity surface for Profile 3 as a function of the friction angle and pore water pressure coefficient. 5. CONCLUSIONS

Preliminary analysis of the summer 2005 Vancouver Island open slope landslide field data show two ranges in landslide volume (0-750 and 2000-2750 m3). It is expected that further observations will produce an exponential decrease in volume. The volume of the landslide appears to have a marked effect on the runout distance. This correlation is more pronounced for landslides with volumes >500m3. The stratigraphy in the headscarp was generally composed of weathered till overlying unweathered till on top of bedrock. The density of the weathered till was estimated in the field to be very loose to loose, while the unweathered till was compact to dense. Preliminary dynamic modelling of selected Vancouver Island landslides using the frictional rheology model compared well with previously published values for small open slope debris flows but the modelled velocity of the failed material was unrealistically high. Preliminary results using the Voellmy rheology consistently produced parameters with larger friction coefficients than the previously published values. Conceptual DANW

sensitivity analyses models using 3-dimensional surface plots, illustrated the expected relationships between input parameters and runout distance (i.e. lower friction angle and high pore-pressure coefficient resulted in long runout in frictional model). While the specific runout values were found to vary with the profile used, the behaviour between the input parameters and the runout distance seemed independent of the profile shape. 6. ACKNOWLEDGEMENTS

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