Relationships Between Landscape Attributes and Landslide ...

Relationships Between Landscape Attributes and Landslide Frequencies After Logging: Skidegate Plateau, Queen Charlotte Islands bY Terrence P. Rollerson1

Environmental SciencesGroup MacMillan Bloedel Ltd. . 65 Front Street Nanaimo, B.C. V9R 5H9

1

The author is now with the B.C. Ministry of Forests, Vancouver Forest Region, 4595 Canada Way, Burnaby, 0.C. V5G 4L9.

March 1992

REFERENCE COPY NOJ’EtEkJOVE

/-q.

Ministry of Forests

Canadian Cataloguing in Publication Data Rollerson, Terrence Paul, 1946Relationships between landscape attributes and landslide frequencies after logging, Skidegate Plateau, Queen Charlotte Islands (Landmanagementreport,ISSN0702-9861

; no.76)

Includes bibliographical references: p. ISBN 0-771 8-91 70-9 1. Landslides - British Columbia- Queen Charlotte Islands. 2. Clearcutting - Environmental aspects British Columbia - Queen Charlotte Islands. 3. Forest r o d s Environmental as@& - British Columbia Ween' Charlotte Islands. 1. British Columbia.. Ministry of Forests. II. Title. 111. Series.

-

-

QE599.CXR64 1992 551

3'07

C92-092115-9

0:1992 Provinceof British Columbia Published by the Forest Science Research Branch Ministry of Forests 31 Bastion Square Victoria, B.C. V8W 3E7

Copies of this and other Ministry of Forests titles are available from Crown Publications Inc., 546 Yates Street, Victoria, B.C.V8W 1K8.

ACKNOWLEDGEMENTS This study was supported by the British Columbia Ministry of Forests and MacMillan Bbedel Ltd. Thanks go to Steve Chatwin, David Dunkley and Gray Switzer for unstinting inthehelp field, and to Kim lles and Steven Northway for statistical advice.

iii

ABSTRACT Terrain within the Skidegate Plateau of the Queen Charlotte Islands was studied to determine if landslide frequencies could be correlated to specific landscape attributes or combinationsof attributes. The analysis indicates that roadfill landslide frequencyis associated with slope angle, natural landslide presence, slope position, and slope morphology, whereasclearart landslide frequency is related to these factorsas well as to surficial material,bedrock formation, horizontalslope curvature, and soil type. Combinations of these variables are used to develop landslide risk classifications.

Key Words: Landslide frequency, terrain, logging, road building.

iv

TABLE OF CONTENTS ACKNOWLEDGEMENTS

....................................................................

iii

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

iv

...........................................................................

1

..............................................................................

1

METHODOLOGY ...........................................................................

1

........................................................................

3

INTRODUCTION STUDY AREA

DATA COLLECTION

................................................................ Roadfill Landslides ...................................................................... Clearcut Landslides ..................................................................... MultifactorLandslideRiskClassification ....................................................

RESULTSANDDISCUSSION

4 4 6 8

CONCLUSIONS ............................................................................

10

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

FIGURES 1 . Physiographic regions of the Queen Charlotte Islands

2. Samplearealocations

.........................................

....................................................................

2

3

TABLES ...........................

4

....................................................... 3. Comparison of landscape attributes with clearcut landslide frequency ........................... 4 . Average dearcutlandslidefrequencies ...................................................... 5. Multifactor landslide risk classification for roadfill landslide occurrence ........................... 6. Multifactor landslide risk classification for clearcut landslide occurrence .........................

5

1. Comparison of landscape attributes and roadfill landslide frequencies

2 . Averageroadfilllandslidefrequencies

6

7 8 9

7. Multifactor landslide risk classification for roadfill landslides, significance values, and landslide frequencies .............................................................................. 9 8 . Multifactor landslide risk classification for clearcut landslides. significance values. and landslide

frequencies

..............................................................................

V

10

The identificationof terrain that will be subject to high frequenciesof landslides following clearcut logging and road building is a high priority for forest management in coastal British Columbia, and the Queen Charlotte Islands in particular. This paper addresses post-logging landslide frequencies as they relate to individual landscape attributes and combinations of landscape attributes. The relationships described are directed toward the development of multifactor classifications for slope hazard mapping.

STUDY AREA The study area comprises a series of selected clearcut areas within the Skidegate Plateau in the Queen Charlotte Islands. The Skidegate Plateau is a long, narrow, northwest-trending surface which separates the Queen Charlotte Rangesin the west from the Queen Charlotte Lowlands to the east (Figure 1). The plateau surface slopes gently towards the east. Althoughit is well dissected, it exhibits a topography composed of subdued, rounded hills and ridges separated by moderately wide, low-gradient valleys. Elevation of the plateau surface ranges from a few metresin the east to 750 metres in the west. Bedrock formations are composed primarily of volcanics, sediments, and metasediments; intrusive rocks are rare. The plateau was overridden by ice during the Pleistocene. As a consequence, all but the steepest slopes are dominated by morainal materials.

The Queen Charlotte Islands have a temperate coastal maritime climate. Mean monthly temperatures nea sea level range from1.2-3.6"C in January to 13.2-145°C in July. Seventyto 80% of the precipitation occurs between October and April; most of the high-intensity rainfall events occur during this period, frequentlyin October and November. Annual precipitation is highest on the west side of the Queen Charlotte Ranges (4500 mm), decreasing gradually across the Skidegate Plateauto the eastern edge of the Queen Charlotte Lowlands (1 200 mm). Maximumrainfall intensities follow a similar pattern (Karanka 1986).

METHODOLOGY The study wasrestrictedto logged areas that were from to 156 years old. The lower age limit was imposed to allow time forroot strength to deteriorate andto provide a reasonable opportunity for large stormsto act on the logged terrain. The upper age limit was set because well-advancedconifer regeneration often masks the presence of smaller landslides. Clearcut areas of the appropriateage range were stratified by underlying bedrock formation and numbered in a systematic fashion.A subset of areas from eachformation was then randomly selected for study. For frequency estimation, landslides that could not be clearly distinguished on 1 :20000 scale aerial be identified at this scale of photography were excluded from the study.A landslide 0.05 ha in area can usually photography; landslides smaller than this were excluded even though they may have been visible in the field. Landslides identified in the field, which were 0.05 ha or larger and which apparently occurred after the photography, were included in the data set.

Theterrainwithineachselectedareawasmappedatascale of 1:20000 according to aterrain classification systemin common usein British Columbia (Ryder and Howes 1984). With this system, each map polygon is labelled with a series ofcodesdescribingthegeologicmaterials,materialtextures,surface morphology, and any active or inactive geomorphic processes present. For efficiency, terrain units with slopes less than 15" were generally excludedfrom the study becauseshow they evidence rarely of failure. Each terrain unit mapped was considered to be a single sample. As no two terrain mappers will produce identical maps, some unknown amount of bias will have been introduced by this procedure.

Graham Island

Hecate Strait Pacific Ocean

-”

0

Physiographic boundary

--

50

I 75 I

KILOMETRES

FIGURE 1. Physiographic regions of the Queen Charlotte Islands (from Sutherland-Brown1968).

2

DATA COLLECTION Twenty-eight randomly selectedlogged areas were studied (Figure 2), making up a total of 768 terrain polygons. The aggregate area mapped was approximately 3380 ha. Every terrain unit mapped was verified in the field, and a set of data describing the morphology and composition of the unit was recorded. The data included surficial materials present,slope angle, aspect, elevation, bedrock formation, soil type and texture, landscape position, drainageclass,number of landslides, slope morphology,slopecurvaturealongthe horizontal, and the presence absence or of gullies.

Graham Island

4 Pacific Ocean

0

50

75 1

KILOMETRES

FIGURE 2. Sample area locations.

3

rain

As geographic variables are often not normally distributed, parametric tests were supplemented by nonparametric tests. Analysis of variance and the Kruskal-Wallis test were used to comparelandslide frequencies with selected landscape attributes. In addition, multiple range tests were used to indicate groupings of attribute characteristics that are associated with similar landslide frequencies. Cross-tabulation and Chisquared tests were used to compare landscape attributes with the presence or absence of post-logging landslides. The attributes compared include soil type, slope position, slope morphology, horizontal slope curvature, surficial material or terrain type, aspect, presence or absence of natural failures, soil moisture regime, and underlying bedrock formation. The development of a multifactor slope hazard classification using a series of variable combinations follows the approach of Rollerson and Sondheim (1985) and Howes(1987). The most common types of landslides withinstudy the area are clearcut and roadfill landslides, at 84.5 and 14.0% respectively. Roadcut landslides and landslides occurring at the clearcut boundaries are rare, accounting for only 1.3 and 0.2% of all landslides. Consequently, only clearcut and roadfill landslides were considered for further analysis. Clearcut landslide frequencyis expressed as the number of landslides per hectare. In the case of road failures, frequencyis expressed on the basis of number of landslides 100per m of road within each terrain unit. Roadfill Landslides A number of landscape attributes are related to roadfill landslide frequency in a statistically significant way (Table 1). For this analysis, the average slope angles for each terrain polygon were grouped into four slope clear of increasing landslide frequency with increasing slope classes (Table 2). These four classes show a trend angle. Because of the lack of roadfill failures on the most gentle slope class, the analysis then focused on polygons with average slopes greater than20". Roads locatedin terrain polygons that exhibit larger natural landslides (>500 m 2 ) are associated with relatively high failure frequencies, but this trend is less clearin those cases where minor natural landslides are present. Roadfill failure frequencies varied with landscape position.

TABLE 1. Comparison of landscape attributes and roadfill landslide frequencies Slgniflcance Levelsb Variable Slope group Presence or absence of natural landslides Presence or absence minor natural of failuresd ns Landscape Slope Dominantlsubdominant types Bedrock Horizontal slope curvature Dominant Soil Slope

ANOVA

Kruokal-Wallis

**

t .

. t

tt

*.

. t

..

tt

tt

tl.

*t

tt \It

ns

ns

Chi-square

ns ns ns

All variables exceptslope group referto areas withslope angles greater than20". Significance levels: ** = 0.01; = 0.05; ns = not significant. Based on the presence or absence of post-logging landslide activity. d Landslides witha surface area less than 500 m2.

b

4

tt

**

ns ns

TABLE 2. Average roadfill landslidefrequencies (failures per 100 rn of road) Variable Slope group: 0-20" 21-29" 30-35" 36+O

Natural landdides (SO0 m?): absent present (e500 m*): Minor natural failures absent present Landscape position: lower slope upper dope middle slope headwater basin stream escarpment Slope morphology: benchy single gullies uniform irregular dissected Terrain types": wcv Mbv CVR and/orMVR Mbv and Cbav Cbav Bedrock formation: Honna Haida Longarm Masset Yakoun Kunga Horizontal curvature: convex straight concave complex Dominant s o i l type:

podzolic gleyed podzolic Soil drainage: moderately wellto imperfectly well to moderately well imperfectly to poorly rapidly to well Slope aspect: NE NW

sw

SE

Homogeneous Count Percent

Average frequency groups= unitsbfailing

104 32

0.00 0.09 0.13 0.24

20 25

234 12

0.10 0.56

13 50

223 23

0.1 1 0.25

13 39

1 59 110

0 7

27

0.02

7

47

0.07

9 14

140 27

0.16

.

0.27

37 40

5

0.68

6 14 168 14 44

0.00 0.00 0.10 0.10 0.30

36

1

0.00

0

0 0 12 7

124

0.06

8

16 65 40

0.16 0.18 0.22

19 25 20

0.06 0.06 0.11

13 7 19

0.12

12 19

47

28 16 82 37

0.13 0.30

25

13

0.00 0.13 0.15 0.26

0 15 20 23

227 19

0.1 1 0.21

15 21

91

0.08 0.15

11

139

10 6

0.16 0.19

10

35 78 60 73

0.07 0.07 0.17 0.18

36 32

136

65

17

t

t

. . t

t

.. t

.

.

33 14

12 13

21

Except forthe variable slope group, all comparisons are for slope angles greater than 20°. Cross-tabulation analysis showing the percentage of units with oneor more landslides. C Multiple range analysis, by the method 95% of confidence intervals. Vertical columns indicate variables that not aresignificantly different. d WCv =bedrock dominant over colluvial veneers. hbv =morainal blankets(>1 m deep) and veneers. CVR =coUuvial veneers and bedrockin equal proportions. MvR =morainal veneers and bedrock in equal proportions. Cbav =colluvial blankets, aprons and veneers as homogeneous units or complexes. 8

b

5

.

The highest roadfill landslide frequencies were assoaatedwith first-order (headwater) drainage basins a some relationship to landslide frequency: the highest frequenstream escarpments. Slope morphology shows cies are found in those terrain units dissected by several gullies. Larger individual gullies (large enough to m as individual units) and benchy areas experienced no roadfill failures.No significant differences were detected with differing terrainor surficial material types, bedrock formations, slope curvature, soil type, soil drainage, or slope aspect. In some cases weakbut logical trends were evident in the data.

Of note is the correspondence between roadfill landslide frequencies and the percentage of units within a specific variable group that experience roadfill landslide activity (Table 2). Groups that experience a high percentage offailing units also exhibit higher landslide frequencies. For purposes of ranking for hazard or ris either statistic would seem suitable.

Clearcut Landslides of landClearcut landslide frequenciesshow statistically significant relationships with a greater number scape attributes than do roadfill failures (Tables 3 and 4). As with roadfill failures, frequency increases generally correspond to an increase in thepercentageof units within a given class that experience failure. The percentage ofunits within a group experiencing failure can be taken as anofindication the future likelihood, or probability, of failure following logging on similar terrain (Rollerson and Sondheim 1985).

Groupings of average slope angles show a trend of increasing landslide frequency with increasing slope angle. Both large and minor natural landslides show a positive relationship to clearcut failure frequencies. The highest post-logging landslide frequencies recorded are associated with those terrain polygons containing natural failures. As with road failures, first-order headwater basins and stream escarpments are associated with highclearcutlandslidefrequenaes.Terrainpolygonsdissectedbygulliesagainshowhigherlandslide frequencies than any other slope form; benchy and irregular slopes have very low landslide frequencies. Some clear differences are evident between different terrain or surficial material categories. In part, the differences in failure frequencies between surficial material categories are thought to be a functionof slope steepness. For example, deep morainal materials that have relatively low failure frequencies are typically found on gentler slopes than colluvial materials show that higher failure frequencies.

TABLE 3. Comparison of landscape attributes with clearcut landslide frequency Significance Levelb Variable

Kruokal-Wallis

ANOVA ~

Slope group Presence or absence of natural landslides Presence or absenceof minor natural failures Landscape position Slope morphology Dominantlsubdominant terrain types Bedrock formation Horizontalslope curvature Dominant soil type Soil drainage -slope aspect

Chi-squar8

.... ..

~~

.. **

.. *. ..*. .. **

.*

..

t .

ns ns

* All

variables exceptslope group refer to areas with dope angles greater than20". = 0.01; = 0.05;ns = not significant. Basedon the presence or absence of post-kgging landdide activity.

b Significance levels: *'

c

It

**

t .

ns

6

. t

. t

. t

.

..

*. ..I .* .* ns ns ns

TABLE 4.

Average clearcut landslide frequencies (failures per hectare)

Variable. Percent

Average

Slope group: 0-2oO 21-29" 3035" 36+"

Natural landslides( > S O m*): absent present Minor natural failures (