technical reportforest research - british columbiascuzzy creek chilliwack l idfww/cwhms1/mhmm2...

Technical Report TR-011 March 2000 Research Section, Vancouver Forest Region, BCMOF

Research Disciplines: Ecology ~ Geology ~ Geomorphology ~ Hydrology ~ Pedology ~ Silviculture ~ Wildlife

TR-011 Geomorphology March 2001

Predicting Post-Logging LandslideActivity Using Terrain Attributes: Coast

Mountains, British Columbia

byT. Rollerson, T. Millard, C. Jones, K.Trainor, and B. Thomson

Technical ReportForest Research

Vancouver Forest Region2100 Labieux Road, Nanaimo, BC, Canada, V9T 6E9, 250-751-7001




Page

Summary................................................................................................................................................... 2Keywords.................................................................................................................................................. 2Author Information............................................................................................................................... 2Regional Contact.................................................................................................................................... 2Acknowledgements................................................................................................................................ 21 Introduction......................................................................................................................................... 2

1.1 Objectives....................................................................................................................................... 22 Study Areas........................................................................................................................................... 2

2.1 Physical Setting.............................................................................................................................. 42.1.1 Biogeoclimatic zones and climate................................................................................... 42.1.2 Physiography..................................................................................................................... 52.1.3 Bedrock............................................................................................................................. 5

3 Methodology........................................................................................................................................ 53.1 Sampling Design........................................................................................................................... 53.2 Data Collection............................................................................................................................. 63.3 Data Analysis................................................................................................................................. 6

4 Results.................................................................................................................................................... 74.1 Windward Zone............................................................................................................................. 7

4.1.1 Windward Zone clearcut landslides - univariate analysis.............................................. 84.1.2 Windward Zone roadfill landslides and instability - univariate analysis...................... 94.1.3 Windward Zone clearcut landslides - multivariate analysis........................................... 94.1.4 Windward Zone roadfill landslides and instability - multivariate analysis................... 10

4.2 Leeward Zone................................................................................................................................. 114.2.1 Leeward Zone clearcut landslides - univariate analysis.................................................. 144.2.2 Leeward Zone roadfill instability - univariate analysis................................................... 144.2.3 Leeward Zone clearcut landslides - multivariate analysis.............................................. 154.2.4 Leeward Zone roadfill landslides - multivariate analysis............................................... 16

5 Summary Discussion.......................................................................................................................... 186 References............................................................................................................................................. 20

TABLE OF CONTENTS

Cover Photo: Study slopes in Rogers Creek watershed,Southern Coast Mountains, BC.

Tables:1 Distribution of BEC subzones by watershed.........................42a. Terrain attributes and post-logging landslidepresence, Windward Zone.......................................................72b. Terrain attributes and post-logging landslidedensities, Windward Zone....................................................73. Terrain attributes - clearcut landslides summary statistics,Windward Zone .............................................................84a. Terrain attributes and roadfill landslides orinstability presence, Windward Zone................................94b. Ter rain attributes and roadfill landslide densities,Windward Zone...................................................................95. Terrain attributes - roadfill landslides and stability summarystatistics, Windward Zone ............................................106a. Terrain attributes and post-logging landslidepresence, Leeward Zone..................................................................146b. Terrain attributes and post-logging landslidedensities, Leeward Zone.................................................................147. Terrain attributes - clearcut landslides summarystatistics, Leeward Zone ...............................................................15

8a. Terrain attributes and roadfill landslides orinstability presence, Leeward Zone.....................................168b. Terrain attributes and roadfill landslide densities,Leeward Zone..........................................................169. Terrain attributes - roadfill landslides and stability summarystatistics, Leeward Zone..........................................17Figures:1 Location map of watersheds studied.........................................32 CHAID tree, all clearcut landslide presence, Windward ....113 CHAID tree, >500 m2 roadfill landslide densities, Windward...124 CHAID tree, >500 m2 roadfill landslide densities, Windward...125 CHAID tree, >500 m2 roadfill landslide presence, Windward..136 CHAID tree, all roadfill instability presence, Windward...137 CHAID tree, all clearcut landslide presence, Leeward ....188 CHAID tree, >500 m2 roadfill landslide densities, Leeward...199 CHAID tree, >500 m2 roadfill landslide presence, Leeward .19

10 CHAID tree, all roadfill instability presence, Leeward .....19



SUMMARY

This paper identifies terrain types within the Coast Mountainsof British Columbia that were subject to landslides followinglogging or logging road construction. The study collected fielddata from 2364 terrain polygons from watersheds primarily lo-cated in the southern Coast Mountains. The watersheds weredivided into a wetter “Windward Zone” and a drier “LeewardZone” on the basis of biogeoclimatic zones. Statistical tests wereapplied to the data set to identify relationships between terrainattributes and landslide occurrence following logging or roadconstruction. Landslides were classified as occurring either in aclearcut or within a road-prism.

Overall, the rates of both clearcut- and road-related landslidestend to be much lower in the Coast Mountains study water-sheds than have been observed in other areas on the coast. Forexample, 3.7% of the terrain polygons in the Windward Zoneand 1.3% of the terrain polygons in the Leeward Zone experi-enced clearcut landslides >500 m2 in size. By contrast, 17% ofthe polygons in study areas on the West Coast of VancouverIsland (Rollerson, Thomson and Millard, 1997) and 22% of theterrain polygons in a study in the Skidegate Plateau in the QueenCharlotte Islands (Rollerson, 1992,) experienced clearcut land-slides >500 m2 after logging. Mean clearcut landslide frequen-cies of 0.012 ls/ha (landslides per hectare) in the Coast Moun-tains Windward Zone, and 0.008 ls/ha in the Coast MountainsLeeward Zone, are an order of magnitude lower than theVancouver Island and Queen Charlotte Islands studies at 0.08ls/ha and 0.17 ls/ha respectively.

In common with other studies of post-logging landslides in theVancouver Forest Region, specific terrain attributes were asso-ciated with higher landslide rates. Landslide rates increase asslope steepness increases, except for the very steepest slopesthat likely are dominated by bedrock exposures. Increasing land-slide rates with increasing slope angle is particularly true for roadfilllandslides compared with clearcut landslides. Naturally unstableareas had much higher landslide rates than other areas. Gullieswere generally identified as areas subject to high landslide rates,and larger gullies were usually less stable than smaller gullies.

The results of these tests can be used to develop classificationsystems suitable for identifying vulnerable sites before loggingand road building occur.

1.0 INTRODUCTION

In this paper we focus on identifying landscapes vulnerable tolandslides following logging and road building in the CoastMountains. The study examines relationships between post-log-ging landslide incidence and terrain attributes that can be usedto identify terrain likely to experience landslides if logging op-erations occur. Equally important to identifying terrain typesthat are likely to have landslides, is identifying terrain that is notlikely to have landslides. This study examines terrain that hasbeen subject to conventional road-building techniques andclearcut timber harvesting using cable systems. Landslide fre-quencies for roads and clearcut areas are determined.

Because of differences in climate that may affect landslide fre-quencies, we split the Coast Mountains into two arbitrary cli-matic zones: a Windward Zone (wetter) and a Leeward Zone(drier) on the basis of existing ecosystem mapping. The climatein the Leeward Zone was assumed to be transitional betweencoastal and interior (continental) conditions.

1.1 OBJECTIVES

The objectives of this study are:

• to characterize steepland terrain types that were susceptibleto landslides following forest harvesting (clearcutting) and roadbuilding in the Coast Mountains;

• to develop terrain-based stability classifications that estimatethe likelihood of landslide activity following forest harvestingand road building.

2.0 STUDY AREAS

The study encompasses a number of watersheds located withinthe Coast Mountains of British Columbia. Most of the studywatersheds occur in the southern half of the Coast Mountains(Figure 1). This region varies between a wet climate and denseforests in the west to a drier climate and more open forests inthe east.

This data set is not fully representative of the Coast Mountainsbut it should provide a reasonable indication of relationshipsbetween post-logging landslide activity and terrain attributes thatcan be extended to other areas in these mountain ranges withsimilar geologic and climatic conditions.

KEY WORDS

landslides, terrain, Coast Mountains.

AUTHOR INFORMATION

• Terry Rollerson, Golder Associates Ltd., Burnaby, BC• Craig Jones and Kristy Trainor, JM Ryder and Associates

Terrain Analysis Inc., Vancouver, BC• Bruce Thomson, BC Ministry of Environment, Surrey, BC

REGIONAL CONTACTFor further information contact: Tom Millard, ResearchGeomorphologist, Vancouver Forest Region, BCMOF,(250) 751-7115; email [email protected]. Thisreport is available at the Vancouver Forest Region website:http://www.for.gov.bc.ca/vancouvr/research/research_index.htm

ACKNOWLEDGMENTS

We would like to thank Jason Barraclough, Deborah Flemming,Chris Loewen, Tedd Robertson, Graham Shivers, and ClaireTweeddale for collecting data for this study, and JeremyGoodwin, Tom Kanniainen, Karen Kranabetter, and Eric Lottfor their assistance with the field work. We also thank Sid Tsangfor his advice and guidance, and Liz Steele who assisted withthe preparation and editing of various manuscripts. Fundingwas provided by Forest Renewal British Columbia.

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Figure 1. Location map of watersheds studied

3



2.1 PHYSICAL SETTING

2.1.1 BIOGEOCLIMATIC ZONES AND CLIMATE

The Biogeoclimatic Ecosystem Classification (BEC) is an eco-logical classification system used in British Columbia to groupsimilar segments of the forest landscape into categories of ahierarchical classification system (Green and Klinka, 1994). Cli-mate is considered to be the most important factor influencingthe development and distribution of forest ecosystems in Brit-ish Columbia. Because climate stations are rare in remote wa-tersheds in the Coast Mountains we used BEC maps1 to groupor differentiate among watershed areas with respect to climate.

Three biogeoclimatic zones are found in the study areas: theCoastal Western Hemlock (CWH) zone and the Mountain Hem-lock (MH) zone are dominant, but limited areas of the InteriorDouglas Fir (IDF) zone are also present. The CWH zone oc-curs at elevations ranging from sea level to 900 m on windwardslopes, and is much more prevalent in the study areas than theMH zone, which usually forms the subalpine zone above theCWH (Meidinger and Pojar, 1991). The IDF zone has limiteddistribution, but is found on some lower elevation slopes and val-ley bottoms in the central and eastern portions of the southern

Watershed Forest District Zone1 BEC Subzones

Nootum River Mid Coast W CWHvm2/MHvh1Sheemahant River Mid Coast W CWHms2/CWHws2/MHmm2Machmell and Neechanz Rivers Mid Coast W CWHms2/CWHvm3/MHmm1

(CWHws2/MHmm2)Security Bay Mid Coast W CWHvh2/CWHvm2/MHvh1Phillips River Campbell River W CWHvm1/CWHvm2/MHmm1Eldred River Sunshine

CoastW CWHvm1/CWHvm2/MHmm1

Lois River SunshineCoast

W CWHdm/CWHvm2/MHmm1

Clowhom River SunshineCoast

W CWHvm1/CWHvm2/MHmm1

McNair Creek SunshineCoast

W CWHvm1/CWHvm2/MHmm1

Mamquam River Squamish W CWHvm1/CWHvm2/MHmm1Shannon Creek Kalum W CWHws1/CWHws2MacKay Creek Kalum W CWHws1/CWHws2Bolton Creek Kalum W CWHws1/CWHws2

Squamish, Elaho, and Ashlu Rivers Squamish L CWHds1/CWHms1/MHmm2Meager Creek Squamish L CWHds1/CWHms1/MHmm2Rogers Creek Squamish L CWHds1/CWHms1/MHmm2Scuzzy Creek Chilliwack L IDFww/CWHms1/MHmm2American Creek Chilliwack L CWHds1/CWHms1/MHmm2

Table 1. Distribution of BEC subzones by watershed2

1 W = Windward Zone, L = Leeward Zone

Coast Mountains (e.g., in the Lillooet River and the Fraser River).

The Coastal Western Hemlock zone is typically the wettest andmost productive forest zone in British Columbia. The CWHzone typically experiences cool summers and mild winters. Meanannual temperatures range from 5 to 11 degrees Celsius, andmean annual precipitation ranges from 1000 to 4400 mm. Fourmain CWH subzones are represented within the study water-sheds: the Dry Submaritime (ds), the Moist Submaritime (mm),the Very Wet Maritime (vm), and in limited areas the Very WetHypermaritime (vh). The Very Wet Maritime is the most exten-sive subzone and occurs on windward slopes throughout theCoast Mountains. The Very Wet Hypermaritime is restricted tolower elevations near the outer coast. The Submaritime subzoneis restricted to the leeward side of the Coast Mountains. TheDry Submaritime is found only in the central and southern por-tion of the CWH zone and has a climate that is transitionalbetween the coast and interior (Green and Klinka, 1994).

The Mountain Hemlock zone occupies elevations of 800 to 1400m in the study areas, and experiences short, cool, moist sum-mers and long, cold, wet winters (including heavy snow coverthat can persist into July). Mean annual temperature ranges from0 to 5 degrees Celsius. Mean annual precipitation ranges fromapproximately 1700 to 5000 mm, of which up to 70% is snow(Meidinger and Pojar, 1991). The two variants of the MH oc-curring in the study area are the Windward Moist Maritime vari-

2 See Green and Klinka, 1994, for complete descriptions of subzones and subzone abbreviations.

1 The BEC maps for the Vancouver Forest Region used for thiswork were revised in 1994 by F. Nuszdorfer and R. Boettger,Vancouver Forest Region, Research Section.

4



The softer metamorphic and volcanic rocks tend to be finegrained. These rocks are more fractured or jointed than intru-sive rocks and can be weaker and more susceptible to failure.

2.1.4 SURFICIAL MATERIALS

Typically, deposits of unconsolidated materials composed oftill, colluvium, fluvial (alluvial), glaciofluvial and occasionalglaciolacustrine materials are found throughout the study area.In much of the study area steep-sided rock-controlled slopesform valley walls enclosing gentle gradient flood plains. On thelower slopes and valley floors, thick deposits of till, glaciofluvial,fluvial, and to a lesser extent colluvial and glaciolacustrine ma-terials, are found. These materials form a landscape of gentleslopes and terraces cut by steep scarps and gullies. On valleywalls, bedrock outcrops with thin discontinuous coverings oftill and colluvium are found in upper and midslope positions.

The intrusive rocks found in the study area break down into apredominantly coarse combination of rubble, gravel, and sandyresidue, which has been incorporated into the local surficialmaterials. Blocky talus slopes, bouldery glacial deposits espe-cially till, and bouldery stream gravels are characteristic of thisarea. Metamorphic and volcanic rocks, being finer grained, of-ten weather to a combination of gravel, sand, and silt. There-fore till derived from these rocks tends to have a matrix that isfiner than that of till associated with the intrusive rocks.

Dominant mass movement processes in the study areas includesnow avalanches and/or debris flows confined to steep gullies,gully wall debris slides, rockfall from sheer valley sidewalls, anddebris slides and debris flows on open bedrock-controlled slopesor on steep scarps.

3.0 METHODOLOGY

3.1 SAMPLING DESIGN

A number of watersheds were selected for study mainly withinthe southern Coast Mountains. Within each study watershed, allaccessible logged areas, generally ranging in age from five to 15years following harvesting, were sampled. The lower age limitwas set to ensure that the study areas had experienced a numberof large storms and to give time for loss in root strength tooccur. The upper age limit was set because crown closure andincreasing tree height in the regenerating plantations tend tomask local terrain features including small landslides, makingcollection of accurate data difficult. We observed a number ofsituations where rapidly growing conifers and alder completelymasked the presence of individual landslides in clearcut areaswithin 10 to 15 years. As well, young alder growing on the roadsmade both walking and accurate observations very difficult inolder areas.

Each selected area was mapped at a scale of 1:20,000 using theBC Terrain Classification System (Howes and Kenk 1997), and1:15,000 to 1:20,000 scale aerial photography. Each terrain poly-gon was verified in the field. For the sake of efficiency, mostterrain polygons with slopes less than 20 degrees were excludedfrom the study, because they rarely show evidence of post-log-ging failure. Each terrain polygon constituted a single sample.

ant (MHmm1) and the Leeward Moist Maritime variant(MHmm2). The latter has a climate transitional between the coastand the interior, so is somewhat drier and colder than the former(Green and Klinka, 1994).

The Wet Warm Interior Douglas Fir subzone (IDFww) is theonly portion of the IDF that occurs within the study water-sheds, and ranges in elevation from 100 to 1200 m. It has acontinental climate that is transitional to a maritime climate.Summers are warm and dry. Winters are cool and relatively moist,with moderate snowfall (Green and Klinka, 1994).

The watersheds were separated on the basis of thesebiogeoclimatic zones into a wetter Windward Zone and a drierLeeward Zone (Table 1). Watersheds in the Windward Zone aremore exposed to storms sweeping in from the Pacific, and there-fore are likely to experience greater rainfall intensities.

2.1.2 PHYSIOGRAPHY

The study areas lie predominantly within the Pacific RangesPhysiographic Region, with small areas of the Security Bay andNootum River watersheds located within the Hecate LowlandsPhysiographic Region. The Pacific Ranges extend southeast fromBurke Channel to the Fraser River, and contain the highest peaksin the Coast Mountains (Holland 1976). The Hecate Lowland isa strip of low-lying country along the eastern side of the CoastalTrough that extends from Prince Rupert to Vancouver. An ar-bitrary line along the 600 m contour was used by Holland toseparate the Hecate Lowland from the Pacific Ranges to the east.

The landforms and surficial materials of the area are for themost part a legacy of repeated glaciation during the PleistoceneEpoch, which occurred between two million and 12,000 yearsago. Large erosional features, such as valleys and cirques, areubiquitous in the Pacific Ranges region. Valleys with a typicalU-shaped cross-section (glacial troughs) and steep side slopesare common. Topography is rugged, with elevations rangingfrom sea level to almost 4000 m above sea level.

2.1.3 BEDROCK

In both the Windward Zone and the Leeward Zone, bedrockexposures are dominated by intrusive rocks. Over 80% of thebedrock exposures recorded during the study were intrusive.These intrusive rocks include quartz diorite, granodiorite, gran-ite, diorite and quartz monzonite, but are dominated by grano-diorite and quartz diorite. Metamorphic rocks are the next mostcommon type of bedrock and account for slightly less than 17%of the recorded exposures. Metamorphic rocks included: gneiss,granitiod gness, greenstone and schist. Volcanic rocks, includ-ing limited exposures of andesitic and dacitic tuffs and occa-sional occurrences of basalt, are rare. Sedimentary rocks areonly represented by a few exposures of graywacke. Volcanicand sedimentary rocks combined account for less than two per-cent of the recorded bedrock exposures.

The intrusive and harder metamorphic rocks are typically coarsegrained and sparsely jointed. This latter characteristic, combinedwith a high resistance to weathering and erosion, allows the de-velopment of very steep, but relatively stable valley sides.

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The data set consists of 2,364 terrain polygons. The mean ter-rain polygon area is 3.6 hectares with a standard deviation of3.7 hectares. The study areas represent about 8,490 hectares ofmountainous terrain. Updating of landslide incidence may oc-cur periodically, usually after major storm events or when newaerial photography is flown. At any point in time, the data setwill likely slightly under-represent the total number of land-slides that have occurred in the study areas.

With terrain mapping it is common knowledge that no twomappers will produce identical map polygons for a given land-scape, or describe an area in exactly the same manner. Becausethis study involved several mappers, the terrain mapping ap-proach will have introduced some unknown amount of bias orvariability. This was limited by providing clear definitions anddescriptions of the terrain data to be collected and by havingthe mappers work together with a mapping co-ordinator to en-sure a consistent approach. The analysis of categorical terraindata generally involves the grouping of a large number of spe-cific terrain types into a smaller set of more generalised terraincategories. This consolidation reduces the effect of individualdifferences in terrain polygon delineation and classification.

3.2 DATA COLLECTION

For each map polygon, terrain attributes such as landscape po-sition, slope gradient, aspect, slope morphology, slope curva-ture, soil drainage class, surficial material, bedrock type, and thepresence or absence of natural and post-logging landslides wererecorded. Landslides identified in the field or on aerial photo-graphs that were approximately 500 m2 or larger were recordedindividually. The presence of any smaller natural or post-log-ging landslides was tabulated as a terrain characteristic but theiractual numbers were not documented because they could notbe reliably identified in the field or on air photos.

3.3 DATA ANALYSIS

Two measures of landslide rate or activity were used in the analy-sis, landslide presence and landslide density.

Landslide presence is the presence of the initiation point ofone or more clearcut landslides within a terrain polygon, or froma road within a polygon. The analysis reports the percentage ofpolygons with landslides present. This analysis indicates whethera polygon has experienced landslide activity or not, but doesnot indicate whether one landslide or several landslides werepresent within the polygon. Landslide presence was evaluatedusing three criteria:

1. Only those landslides >500 m2 were used as the dependentvariable. This criterion applies to both clearcut landslides androadfill landslides.

2. For clearcuts, landslides of any size were used as a seconddependent variable.

3. For roads, any evidence of roadfill instability was used as asecond dependent variable. This measure included landslidesof any size, tension cracks, fill slope settlement, or other signsof roadfill instability.

Landslide density is the total number of landslides >500 m2,divided by the polygon size. Landslide density was evaluatedusing two criteria:

1. For clearcut areas, landslide density was measured as landslidesper hectare (ls/ha), based on slope area, not planimetric area.

2. For roadfills, the measurement was landslides per 100 m ofroad (ls/100 m).

This approach gave six measurement terms:

• >500 m2 clearcut landslide presence

• >500 m2 roadfill landslide presence

• All clearcut landslide presence

• All roadfill instability presence

• >500 m2 clearcut landslide density

• >500 m2 roadfill landslide density

Cross tabulation analysis and Chi-squared tests were applied toinvestigate relationships between landslide presence and indi-vidual terrain variables in the data set. Kruskal-Wallis tests wereused to test for differences in landslide density for differentterrain variables. The Kruskal-Wallis tests were supplementedwith analysis of variance. We used non-parametric tests in pref-erence to parametric tests because many geographic variables(e.g., landslide density) are often not normally distributed and/or are categorical, and because sample sizes tend to be unequaland sometimes quite small for certain categories of some variables.

Because landslide densities were generally quite low in the studyarea, the use of the presence of all sizes of landslides ratherthan just those >500 m2 provided a more sensitive measure ofpost-logging landslide activity. However, it is likely that not alloccurrences of landslides smaller than 500 m2 were detecteddue to the difficulty of consistently identifying these smallerfeatures both on air photos and in the field.

A limited number of the samples represent deactivated roadswhich have undergone significant fill slope pull back (i.e., re-contouring to eliminate the road and to re-establish the originalground slope). This work generally obliterates evidence ofsmaller road instability features (e.g., tension cracks). With lessaggressive remedial treatments (e.g. partial fill pull back) on steepslopes, indications of instability can reappear within a few years.

In our opinion, assessment of all signs of instability associatedwith roadfills provides better long-term prediction of the likeli-hood of significant road related landslide activity. Due to thenature of much of the older logging road construction (e.g. fillssupported by buried logs and stumps which are slowly rotting)more large landslides may occur as these roads age. There werea few road cut-slope landslides larger than 500 m2 in the dataset. However, because these and other indications of cut slopeinstability were rare, we did not carry out any analysis of stabil-ity problems associated with road cuts.

We carried out limited multivariate analyses for both landslidedensity and landslide presence to determine which combina-tions of terrain variables would most usefully predict landslide

6



activity levels. We used a multivariate method known as CHAID(Chi-squared Automatic Interaction Detector) to group in-dividual terrain polygons into a limited number of multi-fac-tor terrain categories having a similar likelihood or densityof post-logging failure.

CHAID uses a non-parametric, multivariate procedure knownas segmentation modeling (Magidson J. /SPSS 1993). The pro-cedure divides a sample population into two or more distinctgroups based on the best predictors of a dependent variable.Segments defined by the analysis do not overlap. Dependentand predictor variables can be either categorical or continuousvariables. The procedure merges categories of a predictor vari-able that are not significantly different at each segmentation leveland splits those that are different. The analysis produces a treediagram that identifies predictor variables and presents statis-tics for each separate group or segment of the dependent vari-able. These categories can form the basis of terrain stabilityclassifications that estimate the likely presence or density of post-harvest landslides.

For this particular set of analyses, the probability values for split-ting and merging categories within the CHAID segmentationtree were set at alpha = 0.05. We also set stopping rules of aminimum of 20 samples to split a parent node and a minimumof 10 for merging samples to form a child node. The maximumtree depth was set at four.

4.0 RESULTS

In order to simplify the analysis and presentation of the studyresults, we separated the data into two zones, as described ear-lier in this paper: a Windward Zone and a Leeward Zone. Therecan be significant differences in landslide frequencies between

the watersheds that lie near the coast and those that are furtherinland. For example, the watersheds included in the WindwardZone data set had a greater percentage of terrain polygons withclearcut landslides >500 m2, than the Leeward Zone watersheds(i.e., 3.7% versus 1.3%). The watersheds located in the Terracearea are included in the Windward Zone data set.

4.1 WINDWARD ZONE

Clearcut landslides >500 m2 were present in 53 (3.7%) of the1,446 terrain polygons in the Windward Zone, leaving 96.3%of the samples without landslides. The percentage of polygonswith post-logging landslides increased to 9.0% when clearcutlandslides smaller than 500 m2 were considered. Roadfill land-slides >500 m2 were present in 42 or 4.3% of 983 sample ter-rain polygons containing roads. If all roadfill landslides and othersigns of instability recorded were considered, then 166 or 16.9%of the sample of polygons with recorded road lengths exhib-ited some form of roadfill instability.

There were significant differences for landslide frequenciesamong the various sample watersheds in the Windward Zone(Tables 2a, 2b, 3, 4a, 4b and 5). These values ranged from 0.8%to 8.7% for terrain polygons with clearcut landslides >500 m2

present, and from 2.3% to 20.7% if all the smaller clearcut land-slides recorded were included. Similarly, the range among dif-ferent watersheds for terrain polygons with roadfill landslideswas 0.0 to 25.0% but increased to 3.7 to 61.1% when all in-stances of roadfill instability were considered. These differenceslikely resulted from local climatic differences between water-sheds and differences in the distribution of surficial materials,slope morphology, slope angle and the like. Except for one wa-tershed with steeper average slopes, a means plot did not show

Table 2a. Terrain Attributes and Post-Logging LandslidePresence, Windward Zone

Table 2b. Terrain Attributes and Post-Logging LandslideDensities, Windward Zone

Significance Level Pearson Chi-Square

Variable Landslides>500 m2

all landslides

Slope class .291 .000Natural landslides present .052 .015Landscape position .425 .747Slope morphology .021 .000Terrain category .344 .133Dominant surficial material .862 .772Horizontal curvature .927 .000Vertical curvature .005 .000Soil drainage .477 .000Slope aspect (by octant) .754 .634Elevation .394 .763Bedrock lithology .783 .372Bedrock structure .637 .592Bedrock hardness .338 .012Age of logging .548 .565Watershed .032 .000

Significance LevelsVariable Kruskal

-WallisAnova


7



1proportion symbols: / = dominant/subdominant; + = either component may be dominant or they may be equivalent.2a dash (-) is used to indicate situations where small sample sizes preclude analysis

Variable Code n Slides>500 m2

(ls/ha)

Slides>500 m2

% unitsfailing

Allslides

% unitsfailing

Vertical curvatureconcaveconvexstraightcomplex

1234

6660

13182

.022

.003

.012-

4.51.73.6-

18.28.38.4-

Soil drainagerapidlywellmoderately well

123

1571255

34

.011

.013

.000

3.23.80.0

17.27.7

17.6Slope aspect

NNEENEESESSSSSWWSWWNWNNW

12345678

19726516817420823299

103

.007

.019

.017

.007

.018

.008

.004

.011

2.54.55.42.34.33.03.03.9

8.610.211.96.99.16.5

10.19.7

Elevation (m)0-100101-200201-300301-400401-500501-600601-700701-800801-900901-10001001-1100

123456789

1011

9810114320622812710410394

127115

.003

.015

.008

.008

.012

.017

.011

.018

.009

.027

.008

2.02.02.82.43.53.92.97.84.36.33.5

10.17.88.46.88.89.46.79.78.5

14.29.6

Bedrockintrusive (mainly quartz dioriteand granodiorite)volcanic (andesite/basalt)metamorphic (granite gneissand gneiss)Greywacke

1

23

4

728

12190

10

.012

.000.17

.000

4.3

0.03.7

0.0

9.6

0.07.4

0.0Bedrock structure

massivefracturedsheared

123

26682

8

.019

.015-

7.74.7-

11.58.5-

Bedrock hardnessvery softsoftaveragehardvery hard

12345

19

77229572

--

.000

.010

.017

--

0.05.24.2

--

0.012.79.1

Variable Code n Slides>500m2

(ls/ha)

Slides>500 m2

% unitsfailing

Allslides

% unitsfailing

Slope class (°°°°)15-1920-2626-3031-3536-4041-46>46

1234567

113143634241988749

.000

.004

.015

.013

.017

.021

.004

0.01.64.44.05.63.42.0

9.13.58.38.512.621.816.3

Natural Landslidesabsentpresent

01

13895

.012-

3.6-

8.9-

Landscape positionupper slopemid slopelower slopestream escarpment

2345

91156278

3

.00.014.006

-

-4.12.2-

-9.09.3-

Slope morphologyuniformbenchydissected (gullied)irregularsingle gullies

12356

91911121230165

.012

.062

.021

.008

.011

3.39.19.13.51.8

5.318.223.16.921.8

Terrain categoryMorainal (till)ColluvialGlaciofluvialRockMorainal+colluvialMorainal+glaciofluvialMorainal/rockColluvial/rockRock/colluvial (1)Volcanic (unconsolidated)Glaciofluvial/glaciolacustrine

12356789101112

4931343413212

5238120190

34

.015

.005

.010

.000

.016-

.016

.002

.009--

3.80.72.90

6.1-

5.01.72.6--

9.12.211.80.011.820.011.36.78.9--

Dominant surficial materialColluviumGlaciofluvialMoraine (till)BedrockVolcanic (unconsolidated)

1581011

34635860204

1

.009

.010

.014

.009-

3.22.94.22.4-

7.211.49.88.3-

Horizontal curvatureconcaveconvexstraightcomplex

1234

2251781038

5

.012

.017

.012-

3.63.43.8-

15.16.77.9-

Table 3. Terrain Attributes - Clearcut Landslides Summary Statistics, Windward Zone

dramatic differences in mean slope angle among the various samplewatersheds.

4.1.1 WINDWARD ZONE CLEARCUT LANDSLIDES –UNIVARIATE ANALYSIS

The percentage of terrain polygons with clearcut landslidespresent and clearcut landslide density do not vary significantlywith time elapsed since logging within the Windward Zone(Tables 2a and 2b). The sample set has a range of 4 to 21 yearssince logging. These results indicate that most post-logging land-slides occur within 5 years of harvest.

The presence of post-logging clearcut landslides showed a sta-tistically significant relationship for only a few of the variablestested (Tables 2a, 2b and 3).

Slope morphology was important for all measures of clearcut

landslide activity, with gullied terrain tending to a have higherincidence of landslide activity than non-gullied terrain. Slopeangle was statistically significant when all landslides were con-sidered, but not when landslide incidence was restricted to land-slides >500 m2. Landslide incidence tended to increase as slopeangle increased to the mid-40 degree range and then tended todrop off, likely because bedrock outcrops were beginning todominate the landscape and there was less soil available to fail.

Slopes that were concave in the vertical direction tended to bemore failure prone than convex or straight slopes. There was ahigher incidence of smaller landslides on slopes that were con-cave along the horizontal, but the relationship was not signifi-cant when only landslides >500 m2 were considered.

Similarly, soil drainage was significant when all sizes of land-slides were considered, with rapidly drained and moderately well

8



drained slopes being more prone to landslides than well-drainedslopes. The most obvious explanation for this relationship isthat shallow, rapidly drained soils tend to be associated withsteeper slopes, and moderately well drained areas can containlocal zones of imperfectly or poorly drained soils.

Elevation and aspect showed no relationship to landslide activ-ity. The variable terrain category expresses in a general way thedifferent combinations of surficial materials present in the land-scape. Colluvial and bedrock dominated landforms appeared tobe more stable, whereas landforms dominated by morainal,glaciolacustrine and some glaciofluvial materials appeared toexperience higher landslide frequencies (Table 3). However, thestatistical tests showed no significant differences among thesedifferent terrain categories.

4.1.2 WINDWARD ZONE ROADFILL LANDSLIDESAND INSTABILITY – UNIVARIATE ANALYSIS

The incidence of instability related to roadfills within the Wind-ward Zone of the Coast Mountains showed similar trends tothe incidence of clearcut landslides (Tables 4 and 5), with roadfilllandslide activity being most closely associated with changes inslope angle and slope morphology.

There were statistically significant differences in the frequencyof roadfill instability over time (Table 4a and 4b) but inspectionof the data showed no obvious trend of increasing instability asthe roads age. We speculate that the differences in the frequencyof fill slope instability with differing ages of logging were dueto local variations in terrain, road construction and other con-ditions specific to the location and age of roads.

Roadfill instability was significantly related to slope angle, in-creasing as slope angle increased. Gullied terrain had the high-est incidence of road instability. Uniform slopes were interme-diate between irregular or benchy slopes and gullied terrain.

Terrain category was significantly related to roadfill instabilityonly when all types of roadfill instability were considered. Inthis case, terrain dominated by morainal materials was morefrequently associated with evidence of roadfill instability. Therewas some indication that roadfills on hill slopes facing towardsthe northeast and southwest were less stable. Similarly, hard in-trusive bedrock may be associated with a slightly higher inci-dence of fill slope instability than other rock types.

Landscape position, slope curvature, and soil drainage condi-tions appeared to have no or only limited influence on roadfillstability.

4.1.3 WINDWARD ZONE CLEARCUT LANDSLIDES –MULTIVARIATE ANALYSIS

Analysis of the Windward Zone data using CHAID was moresuccessful when all sizes of landslides were considered thanwhen only landslides >500 m2 were considered.

For clearcut landslides >500 m2, CHAID found only slope angleto be a useful predictor of landslide density. Slope angles greaterand less than 35° were associated with mean landslide densitiesof 0.008 and 0.024 ls/ha respectively, not a particularly useful

separation. In the case of the presence or absence of clearcutlandslides >500 m2 CHAID did not detect any significant pre-dictor variables.

When the presence of all sizes of clearcut landslides was con-sidered, then CHAID was able to identify several significantpredictor variables (Figure 2). In this case, slope morphology,maximum slope angle, gully depth, dominant surficial material,and soil drainage class were used to define a series of dichoto-mous splits in the data. Gullies deeper than about four meters,and benchy areas and gullies with either rapidly drained or mod-erately well drained soils, were associated with the highest per-centage of terrain units experiencing post-logging clearcut land-slide activity.

Table 4a. Terrain Attributes and Roadfill Landslides or Insta-bility Presence, Windward Zone

Table 4b. Terrain Attributes and Roadfill Landslide Densities,Windward Zone

Significance Level Pearson Chi-SquareVariable landslides >500 m2 roadfill instability

Slope class .000 .000Natural landslides present .714 .021Landscape position .816 .465Slope morphology .012 .079Terrain category .203 .001Dominant surficial material .940 .262Horizontal curvature .460 .142Vertical curvature .932 .009Soil drainage .996 .410Slope aspect (by octant) .441 .003Elevation .728 .239Bedrock lithology .208 .020Bedrock structure .171 .296Bedrock hardness .038 .029Age of logging (roadbuilding)

.019 .001

Watershed .007 .000

Significance Levels

Variable Kruskal-Wallis

Anova


9



4.1.4 WINDWARD ZONE ROADFILL LANDSLIDESAND INSTABILITY – MULTIVARIATE ANALYSIS

In the case of roadfill landslide densities for landslides >500m2, CHAID used two predictor variables – the minimum slopeangle recorded for each terrain polygon, and terrain category –to split the data (Figure 3). Roadfill landslide densities were sepa-rated into three groups on the basis of increasing minimumslope angle. The steepest slope category, those areas where theminimum slope angle was >34°, were separated into two groups,one dominated by morainal, glaciofluvial and glaciolacustrine

materials and the other dominated by colluvial materials andbedrock or morainal materials associated with either colluviumor bedrock. These two groups had mean landslide frequenciesof 0.945 and 0.077 ls/100 meters of road respectively. If thealpha for category separation was set at 0.1 rather than 0.05, afurther split of the colluvial and bedrock dominated categorywas made on the basis of bedrock type, or in this case, the pres-ence or absence of bedrock3 in the terrain unit (Figure 4).

CHAID produced a similar set of segmentations for the pres-ence of landslides >500 m2, using minimum slope angle, and

1proportion symbols: / = dominant/subdominant; + = either component may be dominant or they may be equivalent.

Table 5. Terrain Attributes - Roadfill Landslides and Stability Summary Statistics, Windward Zone


(ls/100mof road)

Slides>500 m2

% unitsfailing

% unitswith

roadfillinstability

Slope class (°°°°)15-1920-2626-3031-3536-4041-46>46

1234567

52262502961245527

-.001.013.059.156.049.243

-0.42.04.4

12.17.3

14.8

-4.9

17.219.328.227.318.5


01

9803

.051-

4.3-

16.7-


2345

37961831

-.038.108

-

-4.05.5-

-17.713.7

-Slope morphologyuniformbenchydissected (gullied)irregularsingle gullies

12356

6378

80134124

.058-

.088

.000

.045

4.4-

100

4.8

16.8-

25.010.419.4

Terrain categoryMorainal (till)ColluvialGlaciofluvialRockMorainal+colluvialMorainal+glacio-fluvialMorainal/rockColluvial/rockRock/colluvial (1)Volcanic(unconsolidated)Glaciofluvial/glaciolacustrine

123567

891011

12

34899276

1533

160731101

3

.078

.011

.043-

.034-

.027

.042

.049-

-

4.92.03.7-

5.9-

1.35.55.5-

-

16.46.1

11.1-

26.1-

15.611.022.7

-

-

Dominant surficialmaterialColluviumGlaciofluvialMoraine (till)BedrockVolcanic(unconsolidated)

1581011

235286021171

.026

.042

.062

.046-

3.43.64.55.1-

13.610.717.422.2

-

Horizontalcurvatureconcaveconvexstraightcomplex

1234

1511217092

.119

.018

.042-

6.63.33.9-

23.216.515.7

-


(ls/100mof road)

Slides>500 m2

% unitsfailing

% unitswith

roadfillinstability


1234

4342

8971

.035

.009

.053-

4.72.44.3-

18.635.715.9

-Soil drainagerapidlywellmoderately well

123

9086924

.034

.052

.049

444.34.2

14.417.48.3

Slope aspectNNEENEESESSSSSWWSWWNWNNW

12345678

1451761041281451546863

.049

.078

.039

.021

.032

.035

.179

.000

5.56.32.93.14.83.25.90.0

15.925.017.310.921.416.95.99.5

Elevation (m)0-100101-200201-300301-400401-500501-600601-700701-800801-900901-10001001-1100

123456789

1011

7062

101133150768073669181

.071

.054

.083

.019

.032

.027

.033

.081

.017

.098

.062

5.71.62.03.83.33.95.06.83.05.57.4

8.66.5

14.918.019.319.717.515.118.218.723.5

Bedrockintrusive (mainly quartzdiorite andgranodiorite)volcanic(andesite/basalt)metamorphic (granitegneiss and gneiss)Greywacke

1

2

3

4

485

10

122

9

.045

.000

.011

.000

4.5

0.0

.8

0.0

19.8

0.0

10.7

0.0Bedrock structuremassivefracturedsheared

123

18472

5

.000

.049-

0.04.7-

5.620.6

-Bedrock hardnesssoftaveragehardvery hard

2345

450

166379

-.013.091.031

-2.08.43.2

-4.0

22.919.3

10



slope morphology with a final separation of terrain units withand without bedrock exposed in the terrain unit or in the roadcut (Figure 5).

When all instances of roadfill instability were considered (e.g.,roadfill landslides of all sizes, tension cracks and settlement ofthe roadfill), CHAID was able to separate a greater number ofcategories. In this case CHAID used, in sequence, minimumslope angle, terrain category, the presence of gullies deeper thanthree meters, landscape position, and bedrock type to separate dif-fering categories of fill slope instability (Figure 6). In this particularcase, we also included as a variable road length within the terrainpolygons, and found it was an effective predictor for one split.

4.2 LEEWARD ZONE

Only 12 (1.3%) of the 918 samples in the Leeward Zone hadclearcut landslides >500 m2, leaving 906 or 98.7% of the sampleswithout landslides. The percentage of polygons with landslidesincreased to 5.6 % if all observations of clearcut landslidessmaller than 500 m2 were included. A total of 20 or 2.9% of the692 terrain units with road lengths recorded had roadfill land-slides that were >500 m2. When all signs of roadfill instability

were included, then 216 or 31.2% of the polygons with roadsexhibited some form of fill slope instability.

There were significant differences in the frequencies of bothclearcut and roadfill landslides >500 m2 among the varioussample watersheds in the Leeward Zone (Tables 6a, 6b, 7, 8a,8b and 9). When landslides of all sizes were considered, clearcutlandslide incidence did not vary significantly among these wa-tersheds, but the incidence of fill slope instability did. The val-ues ranged from 0.3% to 5.9% for polygons with clearcut land-slides >500 m2 present, and from 5.2% to 7.7% when the pres-ence of smaller clearcut landslides was evaluated. The rangeamong different Leeward Zone watersheds for sample polygonscontaining larger roadfill landslides varied from 0.0 to 7.7% ofall terrain polygons. The incidence of road instability increasedfrom 16.3 to 72.7% of the terrain polygons with roads when allindications of roadfill instability were included.

The percentage of terrain polygons experiencing clearcut land-slides following logging did not vary significantly with the ageof logging within the Leeward Zone (Table 6). The sample sethad an age range of three to 20 years, but only six of the samplesrepresented clearcut areas less than five years old. It appearedthat the initiation of new clearcut failures decreased substan-tively within a few years of logging. There were statistically sig-nificant differences in the frequency of roadfill instability over time(Table 8a and 8b). However, as in the Windward Zone, the datashowed no clear trend of increasing instability as the roads aged.

Figure 2. CHAID tree for all clearcut landslide presence– Windward Coast Mountains

3 When interpreting the CHAID segmentation trees, be aware thatthe term ‘missing’ listed with some bedrock variables (i.e., bedrocktype and bedrock structure) indicates that deeper surficial materi-als are likely present. That is, bedrock was not exposed in the cutslopes of roads traversing these polygons.

11



Figure 3. CHAID tree for >500 m2 roadfill landslide densities (ls/100 meters) – Windward Coast Mountains

Figure 4. CHAID tree for >500 m2 roadfill landslide densities (ls/100 meters) – Windward Coast Mountains

12



Figure 5. CHAID tree for >500 m2 roadfill landslide presence – Windward Coast Mountains

Figure 6. CHAID tree for all roadfill instability presence – Windward Coast Mountains

13



4.2.1 LEEWARD ZONE CLEARCUT LANDSLIDES –UNIVARIATE ANALYSIS

The incidence of larger post-logging clearcut landslides showeda statistically significant relationship for only a limited numberof the variables, but a greater number of the variables weresignificant when the smaller landslides were included in the analy-sis (Tables 6a, 6b and 7).

All measures of clearcut landslide activity varied significantlywith slope angle. Landslides were absent on the small popula-tion of slopes >46° which tends to represent terrain units domi-nated by bedrock outcrops. Slope morphology was importantwhen all clearcut landslides were considered, but not for thegroup of clearcut landslides >500 m2. When all landslides wereconsidered, gullied terrain had a higher incidence of landslideactivity than non-gullied terrain.

The variables terrain category and dominant surficial materialwere both significantly related to landslide incidence.Glaciofluvial materials and complexes of glaciofluvial and mo-rainal or glaciolacustrine materials tended to be most prone tofailure. These relationships were clearer when all landslides wereconsidered. The sample set for glaciolacustrine terrain was verylimited so it was not considered in this analysis.

Slope curvature was not a significant predictor of landslides>500 m2, but was significant when all landslides were consid-ered, in which case curvature along the horizontal was signifi-cant. The highest incidence of post-logging clearcut landslidesoccurred on slopes that exhibited concave or complex curva-ture (i.e., both concave and convex slopes) along the horizontalcontour. Unlike the Windward Zone, soil drainage was not sig-nificantly related to clearcut landslide incidence. Elevation and

aspect showed no relationship to clearcut landslide activity.

Bedrock type, structure, and hardness were all significantly linkedto landslide activity in the Leeward Zone. The limited sampleof terrain underlain by metamorphic rocks exhibited a higherthan average incidence of post-logging clearcut landslide activ-ity when all landslides were considered, compared with areasunderlain by intrusive bedrock.

4.2.2 LEEWARD ZONE ROADFILL INSTABILITY –UNIVARIATE ANALYSIS

Roadfill landslides and instability within the Leeward Zone ofthe Coast Mountains show similar trends to those found in theWindward Zone (Tables 8a, 8b and 9). Roadfill stability wasstrongly related to slope angle, with the frequency of landslidesand evidence of instability increasing as slope angle increased.There was a moderately strong relationship with slope morphol-ogy. Gullied terrain typically exhibited the highest frequency offill slope landslides and other evidence of instability.

There was no significant relationship between terrain categoryor surficial materials and fill slope instability. There was no ap-parent relationship between landscape position, slope curvature orsoil drainage regime and fillslope landslides or fillslope instability.

There was no significant relationship between slope aspect andlandslide activity for roadfill landslides >500 m2, but when allevidence of roadfill instability was considered, slope aspect wassignificant. A higher percentage of terrain units experienced fillslope instability on slopes which face northeast and southeastto southwest than on slopes with other orientations.

There were no significant differences for different elevationbands when roadfill landslides >500 m2 were considered. How-

Table 6a. Terrain Attributes and Post-Logging LandslidePresence, Leeward Zone

Table 6b. Terrain Attributes and Post-Logging LandslideDensities, Leeward Zone

Significance Level Pearson Chi-SquareVariable landslides

>500 m2all landslides

Slope class .001 .000Natural landslides present .000 .000Landscape position .275 .010Slope morphology .482 .000Terrain category .061 .000Dominant surficial material .007 .000Horizontal curvature .164 .000Vertical curvature .758 .122Soil drainage .403 .968Slope aspect (by octant) .539 .403Elevation .232 .100Bedrock .000 .000Bedrock structure .000 .000Bedrock hardness .000 .000Age of logging .768 .818Watershed .013 .633

Significance Levels

Variable Kruskal-Wallis Anova


14



ever, there were significant differences when all fill slope insta-bility was considered. There was a general increase in presenceof roadfill instability with elevation in the Leeward Zone. Slopeangle, which was also linked to fill slope instability, does notincrease with elevation in this area. We speculate that the rela-tionship between elevation and fill slope instability may be linkedto greater snow accumulation at higher elevations and subse-quent higher water contents in roadfills during snow melt orrain-on-snow events.

There were no significant differences between roadfill landslides

>500 m2 and bedrock lithology, or hardness, but there may bewith bedrock structure. When all evidence for fill slope instabil-ity was considered, then there were significant differences amongbedrock lithology and structure. Metamorphic bedrock and platystructure were associated with a higher incidence of fill slopeinstability.

4.2.3 LEEWARD ZONE CLEARCUT LANDSLIDES –MULTIVARIATE ANALYSIS

Multivariate analysis of the Coast Mountain Leeward Zone data


(ls/ha)

Slides>500 m2

% unitsfailing

Allslides

% unitsfailing

Slope class (°°°°)15-1920-2626-3031-3536-4041-46>46

1234567

41862452771732112

.029

.000

.000

.017

.009

.035

.000

-0.00.81.81.74.80.0

-0.02.06.512.723.80.0


01

90513

.006

.1580.9

30.84.576.9


2345

307651230

.017

.006

.014-

3.31.02.4-

3.34.711.4

-Slope morphologyuniformbenchydissected (gullied)irregularsingle gullies

12356

5951594112102

.012

.000

.001

.000

.000

1.90.01.10.00.0

36.79.62.715.7

Terrain categoryMorainal (till)ColluvialGlaciofluvialGlaciolacustrineRockMorainal+colluvialMorainal+glaciofluvialMorainal/rockColluvial/rockRock/colluvial (1)Volcanic(unconsolidated)Glaciofluvial/Glaciolacustrine

1234567891011

12

30584617

17122151004511230

20

.010

.000

.016-

.000

.007

.000

.005

.000

.000

.000

.084

1.30.03.3-

0.02.50.01.00.00.00.0

10.0

3.30.014.8

-0.06.626.75.04.40.910.0

40.0

Dominant surficialmaterialColluviumGlaciofluvialGlaciolacustrineMoraine (till)BedrockVolcanic(unconsolidated)Undifferentiated

1578101114

181709

492131296

.003

.014

.083

.010

.000

.000-

0.62.9-

1.60.00.0-

2.815.7

-5.51.510.3

-

Horizontal curvatureconcaveconvexstraightcomplex

1234

18415556415

.014

.000

.008

.000

2.70.01.20.0

12.50.64.413.3

1 proportion symbols: / = dominant/subdominant; + = either component may be dominant or they may be equivalent.

Table 7. Terrain Attributes - Clearcut Landslides Summary Statistics, Leeward Zone


(ls/ha)

Slides>500 m2

% unitsfailing

Allslides

% unitsfailing


1234

5474

7882

.014

.000

.008-

1.90.01.4-

11.11.45.6-

Soil drainagerapidlywellmoderately well

123

10377342

.000

.008

.018

0.01.42.4

5.85.64.8

Slope aspectNNEENEESESSSSSWWSWWNWNNW

12345678

3745

11216816623511342

.000

.064

.001

.010

.002

.006

.008

.000

0.04.40.91.80.61.70.90.0

2.713.34.54.24.86.46.24.8

Elevation (m)0-100101-200201-300301-400401-500501-600601-700701-800801-900901-10001001-1100

1234567891011

52

1835424688

11385

113371

.000

.000

.000

.000

.000

.000

.027

.000

.000

.001

.013

0.00.00.00.00.00.04.50.00.00.91.9

0.00.00.00.00.010.911.46.22.46.25.4

Bedrockintrusive (mainly quartzdiorite and granodiorite)volcanic (andesite/basalt)metamorphic (granitegneiss and gneiss)

1

23

413

242

.002

-.001

0.5

-2.4

2.9

-16.7

Bedrock structuremassivefracturedshearedplaty

1237

20391

114

.000

.002-

.002

0.00.5-

7.1

0.03.6-

35.7Bedrock hardnessaveragehardvery hard

345

544

371

-.011.001

-6.80.3

-15.93.0

15



using CHAID produces a number of useful subdivisions andgroupings of the data. However, CHAID was more successfulin splitting the sample population when all clearcut landslides,rather than just clearcut landslides >500 m2, were considered.

CHAID distinguished only two groups when landslide densityfor clearcut landslides >500 m2 was considered. Polygons thathad natural landslides of all sizes present had a mean of 0.206ls/ha, whereas polygons without natural landslides had a meanof 0.007 ls/ha. Similarly, CHAID used the presence or absenceof natural landslides to separate terrain polygons into groupswith higher and lower clearcut landslides >500 m2 presence. Inthis case, polygons without natural landslides had a landslideactivity rate of 0.9%, and polygons with natural landslides had a30.8 % landslide activity rate. Neither of these analyses wereparticularly successful in separating terrain vulnerable to land-slides from terrain not vulnerable to landslides; however, theyclearly showed that areas with existing natural landslides can beexpected to have higher post-logging landslide frequencies thanother areas.

When clearcut landslides of all sizes were considered, CHAIDfirst grouped the data based on the presence or absence of natu-ral landslides of all sizes (Figure 7). It then further subdividedthe group with no visible natural landslides on the basis of maxi-mum polygon slope angle. Further divisions used terrain cat-egory or the presence or absence of gullies greater than threemeters deep. Although the separations on the basis of terraincategory were not entirely consistent, it appears that polygonsdominated by glaciofluvial and glaciolacustrine materials weremore vulnerable to post-logging landslides. Terrain units domi-nated by morainal materials had intermediate landslide frequen-cies, and colluvial and bedrock dominated units the lowest land-

slide frequencies (Figure 7). Terrain polygons in the steepestand gentlest slope categories exhibited the lowest likelihood ofpost-logging landslides. Terrain units with gullies greater thanthree meters deep tended to have higher landslide frequenciesthan terrain units with no gullies or with gullies less than threemeters deep.

This particular CHAID analysis seemed to be reasonably suc-cessful in separating terrain polygons with a high or moderatelikelihood of post-logging landslides from those with a negli-gible or low likelihood of post-logging landslides. As we note inthe summary discussion, it is wise to expect differences in poly-gon size to bias the landslide likelihood estimates to some de-gree. However, we did include polygon area in this particularanalysis and it was not selected as a predictor variable.

4.2.4 LEEWARD ZONE ROADFILL LANDSLIDES –MULTIVARIATE ANALYSIS

CHAID grouped >500 m2 roadfill landslide densities based ongully depth (where ‘0’ indicates an absence of gullies of anydepth), and then average polygon slope angle if gullies wereabsent or less than four meters deep (Figure 8). The highestroad landslide densities occurred in terrain units with gulliesdeeper than 12 meters. In areas with no gullies or where thegullies were less than four meters deep, the higher roadfill land-slide densities occurred on slopes steeper than 36°. Landslidefrequencies in gullies of moderate depth (i.e., 4 to 12 metersdeep) were intermediate. CHAID did not select road length as apredictor variable. CHAID was able to separate more distinctgroups for roadfill landslides >500 m2 than it did for clearcutlandslides >500 m2, and it produced a slightly larger number ofdistinct groups when the presence of any indication of roadfill

Table 8a. Terrain Attributes and Roadfill Landslides orInstability Presence, Leeward Zone

Table 8b. Terrain Attributes and Roadfill Landslide Densities,Leeward Zone

Significance Level Pearson Chi-Square

Variable landslides>500 m2

roadfillinstability

Slope class .005 .001Natural landslides .070 .0021Landscape position .434 .011Slope morphology .000 .046Terrain category .683 .074Dominant surficial material .760 .178Horizontal curvature .111 .624Vertical curvature .931 .053Soil drainage .208 .139Slope aspect (by octant) .095 .000Elevation .048 .000Bedrock .410 .007Bedrock structure .007 .000Bedrock hardness .153 .056Age of logging (roadbuilding)

.000 .004

Watershed .000 .000

Significance Levels

Variable Kruskal-Wallis Anova

Slope class .005 .002Natural landslides .067 .044Landscape position .440 .706Slope morphology .000 .000Terrain category .674 .052Dominant surficial material .758 .553Horizontal curvature .104 .008Vertical curvature .926 .768Soil drainage .201 .287Slope aspect (by octant) .092 .094Elevation .048 .413Bedrock .424 .619Bedrock structure .008 .078Bedrock hardness .178 .597Age of logging (roadbuilding)

.000 .002

Watershed .000 .001

16



instability was considered.

A similar relationship was found when the measure of stabilitytested was the presence or absence of roadfill landslides >500m2 (Figure 9). In this case, CHAID selected gully gradient ratherthan gully depth as the most significant predictor variable, andthen used maximum slope angle to further subdivide the gen-tler gully gradient limb of the segmentation tree. Gullies withchannel gradients steeper than 25° had a higher likelihood ofroadfill landslide activity than gullies with gradients less than25°. In the case of gully gradients gentler than 25°, or in areaswhere gullies were absent (note: a ‘0°’ gradient on the CHAIDtree indicates an absence of gullies), roadfill landslide activity

increased up to a maximum slope of 55° and then decreased(Figure 9). The reason for this decrease was unclear. However,we can speculate that these very steep slopes were dominatedby bedrock, so any fill materials were composed of angular shotrock with a high angle of internal friction, and that in gulliesthese fills were supported by the gentle gradient floor of thegully. A second possibility is that roads were built on these steepslopes using full bench and end haul construction techniques.CHAID did not select road length as a predictor variable in thisparticular analysis.

When all field indicators of roadfill instability were used as themeasure of road stability, CHAID was able to make a slightly


(ls/100mofroad)

Slides>500 m2

% unitsfailing

% unitswith

roadfillinstability

Slope class (°°°°)15-1920-2626-3031-3536-4041-46>46

1234567

4136197210129106

.000

.000

.002

.025

.080

.121-

-0.01.52.97.810.0

-

-19.129.934.841.940.0

-Natural Landslidesabsentpresent

01

6857

.023

.1602.814.3

30.871.4

Landscape positionapexupper slopemid slopelower slope

1234

01358891

-.000.027.025

-0.03.21.1

-53.832.519.8

Slope morphologyuniformbenchydissected (gullied)irregularsingle gullies

12356

43710798482

.010

.000

.048

.000

.109

1.60.08.90.07.3

27.740.044.333.334.1

Terrain categoryMorainal (till)ColluvialGlaciofluvialGlaciolacustrineRockMorainal+colluvialMorainal+glaciofluvialMorainal/rockColluvial/rockRock/colluvial (1)Volcanic(unconsolidated)Glaciofluvial/glaciolacustrine

1234567891011

12

24063434121031380297618

11

.010

.018

.035-

.000

.015

.000

.056

.015

.042

.000

.222

2.11.67.0-

0.01.90.05.03.43.90.0

9.1

31.727.046.5

-16.730.169.230.027.623.727.8

36.4

Dominant surficialmaterialColluviumGlaciofluvialGlaciolacustrineMoraine (till)BedrockVolcanic(unconsolidated)Undifferentiated

15781011

14

132525

3919117

4

.015

.029-

.021

.062

.000

-

2.35.8-

2.64.40.0

-

28.840.460.032.522.029.4

-Horizontal curvatureconcaveconvexstraightcomplex

1234

13412342312

.073

.013

.014

.000

6.01.62.40.0

35.830.929.833.3

1proportion symbols: / = dominant/subdominant; + = either component may be dominant or they may be equivalent.

Table 9. Terrain Attributes - Roadfill Landslides and Stability Summary Statistics, Leeward Zone


(ls/100mof road)

Slides>500 m2

% unitsfailing

% unitswith

roadfillinstability


1234

36615932

.034

.002

.027

.000

2.81.63.0-

44.419.731.7

-Soil drainagerapidlywellmoderately well

123

6859430

.054

.023

.000

5.92.70.0

23.531.543.3

Slope aspectNNEENEESESSESSWWSWWNWNNW

12345678

2327871381311807531

.084

.000

.000

.059

.014

.029

.000

.000

8.70.00.05.13.83.30.00.0

56.648.123.044.930.522.225.329.0

Elevation (m)0-100101-200201-300301-400401-500501-600601-700701-800801-900901-10001001-1100

123456789

1011

421330302762866688284

.000

.000

.000

.029

.000

.000

.000

.000

.000

.031

.048

--

0.03.30.00.00.00.00.02.36.0

--

0.06.713.329.619.417.433.328.444.7

Bedrockintrusive (mainly quartzdiorite and granodiorite)volcanic (andesite/basalt)metamorphic (granitegneiss and gneiss)

1

23

311

235

.037

.000

.077

3.9

-8.6

30.9

-57.1

Bedrock structuremassivefracturedshearedfoliated

1237

17292114

.000

.032

.000

.192

0.03.4-

21.4

70.629.5

-92.9

Bedrock hardnessaveragehardvery hard

345

535284

-.075.033

-5.73.5

-40.032.0

17



higher number of splits in the sample population (Figure 10).The first split was based on maximum slope angle. Intermedi-ate slope categories were then split on the basis of either terraincategory or bedrock structure (Figure 10). As we noted above,the incidence of roadfill instability generally increased with in-creasing slope angle but decreased when the maximum polygonslope angle exceeded 55°. In the 29° to 35° maximum slopeangle category, a higher incidence of roadfill instability was as-sociated with morainal and glaciofluvial materials. In the caseof the 35° to 55° maximum slope angle category, massive,sheared, and foliated bedrock had a higher incidence of insta-bility than areas of fractured bedrock or areas where bedrockwas not exposed. There was no obvious explanation for thisparticular difference in the incidence of roadfill instability.

5.0 SUMMARY DISCUSSION

Several common themes run through this analysis. Landslidesfollowing both clearcutting and road building are more com-mon in gullied terrain than elsewhere. Deeper gullies are morelikely to experience landslide activity than shallow gullies. Steeperslopes tend to be more prone to landslides than gentler slopes,but the relationship is not always significant. There can be situ-ations, more commonly with non-road landslides, where verysteep, bedrock dominated slopes may experience less landslideactivity than moderate and steeply sloping areas. Terrain unitswith deeper surficial materials, and in particular, morainal,

Figure 7. CHAID tree for all clearcut landslide presence – Leeward Coast Mountains

glaciolacustrine and glaciofluvial materials, tend to be more vul-nerable to landslide activity than areas dominated by bedrockand colluvial materials. However, these relationships are not al-ways consistent. The presence of natural landslides, though rarein this particular data set, was associated with higher post-log-ging landslide activity.

Overall, both clearcut and road related landslide rates tend tobe much lower in the Coast Mountains study watersheds thanhave been observed in other areas on the coast. For example,3.7% of the terrain polygons in the Windward Zone and 1.3%of the terrain polygons in the Leeward Zone experienced clearcutlandslides >500 m2. By contrast, 17% of the polygons in studyareas on the West Coast of Vancouver Island (Rollerson,Thomson and Millard, 1997) and 22% of the terrain polygonsin a study in the Skidegate Plateau in the Queen Charlotte Is-lands (Rollerson, 1992,) experienced clearcut landslides >500m2 after logging. Mean clearcut landslide frequencies of 0.012ls/ha in the Coast Mountains Windward Zone, and 0.008 ls/hain the Coast Mountains Leeward Zone, are an order of magni-tude lower than the Vancouver Island and Queen Charlotte Is-lands studies at 0.08 ls/ha, and 0.17 ls/ha respectively.

Finally, as a point of caution, we note that when using the pres-ence of landslides within map polygons as a measure of land-slide activity, for both clearcut landslides and road landslides,there is an expectation that the likelihood of a landslide will

18



Figure 8 CHAID tree for >500 m2 roadfill landslide densities– Leeward Coast Mountains

Figure 9. CHAID tree for >500 m2 roadfill landslides presence – Leeward Coast Mountains

Figure 10. CHAID tree for all roadfill instability presence – Leeward Coast Mountains

19



increase as polygon size or road length increases. This meansthat as a measure of landslide activity, the presence or absenceof landslides in terrain polygons will always be affected by poly-gon size or road length – that is, it is not a true probability. Inthe case of clearcut landslides, we found that the mean area ofterrain polygons with landslides was significantly larger that themean area of polygons without landslides (4.9 ha versus 3.5 harespectively). There is no obvious way to avoid this issue, otherthan to use landslide density (e.g., ls/ha or ls/100m of road) asthe preferred measure of landslide activity. However, landslidedensity values tend to be highly skewed because of the largenumber of terrain polygons with no landslides (i.e. the land-slide density is zero for these samples), therefore the mean land-slide density values documented in this report may not repre-sent true mean values. However, this limitation aside, the re-ported landslide density values do serve as a useful indicator ofexpected landslide activity.

6.0 REFERENCES

Green. R.N., and Klinka. K. 1994. A Field Guide for Site Identifica-tion and Interpretation for the Vancouver Forest Region. Land Man-agement Handbook 28. Ministry of Forests Research Branch.Victoria, BC.

Holland, S.S. 1964. Landforms of British Columbia A PhysiographicOutline. Bulletin 48, British Columbia Mines and PetroleumResources, Victoria, BC.

Howes, D.E. and E. Kenk. 1997. Terrain Classification System ForBritish Columbia (Revised Edition). Ministry of Environment,Ministry of Crown Lands, Victoria, BC.

Howes, D. 1987. A method for predicting terrain susceptible to land-slides following forest harvesting: a case study from the southern coastmountains, British Columbia. Proc., International Associationof Hydrological Sciences, Symposium. XIX General Assem-bly of the International Union of Geodesy and Geophysics,August 19-22, 1987. University of British Columbia,Vancouver, BC.

Magidson, J. / SPSS Inc. 1993. SPSS for Windows CHAID Release6.0. SPSS Inc., Chicago, Illinois, USA 60611.

Meidinger, D.V. and J. Pojar (editors). 1991. Ecosystems of BritishColumbia. BC Special Rep. Series No. 6, BC Ministry of For-ests, Victoria, BC.

Pack, R.T. 1995. Statistically based terrain stability mapping methodol-ogy for the Kamloops Forest Region, BC. Proc., 48th CanadianGeotechnical Conference, September 25-27, 1995, Vancouver,BC.

Roddick, J.A., J. E. Muller, and A.V. Okulitch. 1979. Fraser River(Sheet 92). 1:1,000,000 Geological Atlas, R.J.W Douglas coordi-nator, Geological Survey of Canada. Ottawa, Ont.

Rollerson, T. 1992. Relationships between landscape attributes and land-slide frequencies after logging - Skidegate Plateau, Queen CharlotteIslands. Land Management Report No. 76. Ministry of For-ests, Victoria, BC.

Rollerson, T., B. Thomson, and T. Millard. 1997. Identification ofCoastal British Columbia Terrain Susceptible to Debris Flows. FirstInternational Conference on Debris Flow Hazards Mitiga-tion: Mechanics, Prediction and Assessment. San Francisco,Calif., U.S.A. American Society of Civil Engineers.

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