Characteristics of Some Rainfall-Induced Landslides on Natural Slopes, Lantau Island, Hong Kong_...

Quarterly Journal of Engineering Geology, 32, 247-259. 0481-2085/99 $15.00 1999 The Geological Society of London

Characteristics of some rainfall-induced landslides on natural slopes, Lantau Island, Hong Kong

C. A. M. Franks Geotechnical Engineering Office, Civil Engineering Department, 101 Princess Margaret Road, Homantin, Kowloon, Hong Kong

Abstract

The north part of Lantau Island is undergoing rapid development following construction of the new airport at Chek Lap Kok. This development, primarily on reclaimed land, is adjacent to a range of steeply sloping hills that experience levels of annual rain in excess of 2500 mm. Rainfall induced landslides on this steeply sloping natural terrain are potential hazards to developments down slope. Landslides on these natural slopes resulting from two major rainstorms in 1992 and 1993 were studied to determine their characteristics and define the geomorphological factors influencing their debris trails. These data were compared with the aerial photographic record of landslide scars within the same catchment over the period 1945-1995 to determine the potential hazard from natural terrain landslides to downslope development.

The limited data set indicates that channelized debris flows which produce long debris trails pose the greatest hazard potential to downslope development. Factors influencing debris flow paths and deposition zones were assessed from a geomorphological study of the catchment to define criteria for preliminary hazard zoning of downslope developments.

Keywords." case studies, erosion, geological hazards', geomorphology, landslides

Introduction

In October 1989 the Hong Kong Government announced its intention to construct a new international airport at Chek Lap Kok. As part of the associated infrastructure development, a new town is being built at Tung Chung, Lantau Island (Fig. 1). Parts of the development are adjacent to steep natural slopes and future landslides on these slopes may be of consequence in terms of the encroachment of landslide debris into areas of development.

A previous study (Franks & Woods 1993) indicated that much of the area covered by North Lantau is subject to high to extreme geotechnical constraints to development. The Tung Chung area is no exception as most of the hillsides are steep (>30), the peaks are amongst the highest in the Territory and the slopes are covered with colluvium, or soils derived from weathered bedrock. These soils are often prone to instability during or following periods of heavy rainfall. Franks & Woods

(1993) highlighted several areas where landslides from natural slopes were deemed to pose a significant constraint to downslope development.

Following heavy rainfall on 18 July 1992 and 5 November 1993, widespread landsliding occurred in North Lantau (Fig. 2). The largest of these rainfall- induced landslides resulted in a channelized debris flow in 1993 (Fig. 3). The total volume of this channelized debris flow was estimated to be 2300 m 3 and the debris comprised colluvium and weathered bedrock. The debris trail had a total length of about 450 m from a scarp elevation of about 180 m above sea level with the final debris deposition fan forming on an 8 slope encroach- ing onto a site formation platform at a height of 24 m above sea level.

An assessment of the consequences of these potential landslide hazards to downslope development requires knowledge of the failure mechanism, probability and magnitude of the landslide (Soeters & VanWesten 1996; Wu et al. 1996) and the resulting length and path of debris trail run-out in relation to downslope development. Both emprical (Mark & Ellen 1995; Corominas 1996) and quantitative (Sassa 1985; Hutchinson 1986; Hungr 1995) methods for the prediction of debris trail run-out length require a knowledge of the landslide character such as debris transport mechanism and debris volume. These factors in turn are influenced by the geomorphology of the catchment. This study was car- ried out to characterize landslides on natural terrain resulting from some recent rainstorms and determine those factors influencing debris trail run-out length and deposition. The results of this study were used to formu- late baseline criteria that can be used for preliminary hazard zoning of terrain with similar geomorphological and hydrological characteristics.

Study methodology

Aerial photograph interpretation was used to determine the spatial and temporal distribution of past landslides as well as some limited geometric data (i.e. trail length and width) for those sites which could not be accessed in the field. However, aerial photographs covering the

248 C. A. M. FRANKS

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Fig. 1. Location of the Tung Chung landslide study area.

I Scale 1:100 000

Fig. 2. Typical non-channelized debris flow landslides typical of those generated on the natural terrain above the Tung Chung New Town Development as a result of the rainstorm of 5 November 1993.

RAINFALL-INDUCED LANDSLIDES 249

hydrological features were also visually assessed and recorded. For ease of comparison each landslide trail was divided into sections along which the slope angle was approximately equal and the trail width generally uniform. The predominant characteristic of the debris transport or deposition along any section of the trail was determined and the associated site specific factors assessed.

The length of eight landslide trails (3% of the total length of trails in the study) were significantly affected by obstacles in the flow path leading to an abrupt termination of the trail as the resulting rapid drainage of the debris flow promoted deposition. These landslide trails were discounted from the data analysis. The data from each of the remaining forty-four landslides were analysed to determine the range of typical geometric and volume characteristics in relation to slope angle, past activity, superficial geology, vegetation and geomorphology.

Generalized geological description of the study area

Fig. 3. Channelized debris flow landslide on natural terrain above the Tung Chung New Town Development Area, Phase 1 resulting from the 5 November 1993.

study area were not available for all years from 1945 and hence inferences relating specific rainstorms to landslide occurences could not always be rigorously determined. Terrain classification mapping data from earlier mapping at a scale of 1:5000 were also examined (Emery & Houghton 1991).

Fifty-two landslide scars resulting from the intense 1992 and 1993 rainstorms were inspected in the field during 1994. These were selected on the basis of field accessibility from the more than 80 scars identified from interpretation of aerial photographs covering this period. The detailed description and statistical analysis of the landslide characteristics are based on the data obtained from the field inspections. The terminology used to classify and describe the landslides in this study is largely based on Cruden & Varnes (1996) and as modified by King (1997).

During the fieldwork, observations were made of the landslide type, scarp and trail morphology (including the nature of the materials involved), hydrogeology and vegetation. The steepness of the terrain was estimated (with an accuracy of about -4-1 ) using a hand-held clinometer. Evidence of past instability, erosion and

Bedrock geology

The bedrock in the area (Fig. 4) is dominated by Mesozoic volcanic rocks (Langford et al. 1995) which have been intruded by a dense swarm of younger feldsparphyric rhyolite dykes. These dykes generally trend in an ENE or NE direction. The volcanic rocks consist mainly of fine and coarse ash tuff and lava which are commonly banded. Narrow dykes of lamprophyre and basalt are common within the feldsparphyric rhyolite, although rare within the volcanics. Quartz veins are common within all rock types.

Residual soils and saprolite

The bedrock in this area, as elsewhere in Hong Kong, has been subject to weathering over a very long period of time, and this process continues today. The typical depth of saprolite across the study area is between 5 and 10 m, with a thin covering (generally < 1.0 m) of residual soil. However, there are a number of localities where the weathering depth is much greater (Franks & Woods 1993) such as the slopes to the south of Tung Chung where saprolite depths in the range 20 to 30 m are indicated. The northwest facing slopes to the south of Tai Po also appear to be deeply weathered at a number of locations. This may be associated with the inferred northeast-southwest trending fault that truncates the

250 C. A. M. FRANKS

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Fig. 4. Generalized geological map of the Study Area showing the Tung Chung Development Area boundary and locations of landslides with debris trails longer than 150 m.

predominantly volcanic sequences to the south from the feldsparphyric rhyolite outcrops along the coastline (Campbell & Sewell 1998).

Colluvium

Much of the colluvium in the study area was deposited from the late-Pleistocene through to the Holocene (Langford et al. 1995). The extent of colluvium mapped in the area is based largely on aerial photo interpretation supplemented by limited field checking. Colluvium occurs most commonly as relatively thin (


Hydrology and groundwater

The northwest facing slopes in the study area are drained by numerous small streams most of which only flow during or after heavy or prolonged rainfall. The hillsides are often deeply gullied as a result of erosion caused by ephemeral streams. The catchment boundaries reveal a complex system of narrow, long catchments associated with these gullies within a much larger primary catchment defined by a hillcrest. Mean annual rainfall for the catchment over the period 1961-1991 is in the range of 2000 to 2400 mm (Lam & Leung 1994). This is similar to that experienced by most of the developed area of Hong Kong Island, Kowloon and the New Territories (excluding Ma On Shan central uplands and Sha Tin).

In general, piezometric records from previous site investigations indicate that the regional groundwater table lies either just within the slightly to moderately weathered bedrock or within the overlying saprolite (Franks & Woods 1993). The relative permeability of the colluvium deposits when compared with underlying saprolite allows for the development of transient perched water tables at the interface during or following periods of intense rainfall.

Factors influencing the distribution of landslides in the study area

Previous rainstorms Aerial photo interpretation indicates a number of periods during which significant numbers of new landslide scarps are visible in the study area. The earliest photographs from 1945 reveal widespread relatively fresh and also degraded landslide scars on a terrain with thin vegetation cover. Examination of the photographs between 1963 and 1973 reveals many new landslide scars on the hills to the northeast and east of Tai Po. Conversely, relatively few new landslide scars are observed from photographs between 1979 and 1982. The 1993 photographs show most of the new scarps on the hills to the south of Tai Po whilst the 1994 photographs show new landslides to be widespread in the study area, consistent with the expected consequences of the intensity and duration of rainfall experienced during 5 November 1993 which was much heavier and longer than that of 18 July 1992.

Although detailed rain gauge data are only available for the study area from 1991, the following rainstorms are considered to be likely trigger events for the widespread landsliding observed from examination of the subsequent aerial photographs:

14/15 June 1959 (24-hr rolling rainfall of 308.9 mm, 1 in 5 year event), recorded at raingauge R01 (see Fig. 1 for location of raingauge)

18/19 August 1972 (24-hr rolling rainfall of 186.8 mm, 1 in 1 year event, 2-day rainfall 446.4 mm 1 in 10 year event), recorded at raingauge R01

18 July 1992 (24-hr rolling rainfall of 454 mm, 1 in 28 year event), recorded at raingauge N17 (see Fig. 1 for location of raingauge)

5 November 1993 (24-hr rolling rainfall of 742 mm, 1 in 796 year event), recorded at raingauge N17

Although other major rainstorms have been recorded at R01 over the period 1945 to 1995, they have been discounted from further study due to the poor corre- lation between rainfall at the R01 gauge and rainfall in the study area and the lack of aerial photographic evidence indicating widespread landsliding in the Territory of Hong Kong.

Bedrock geology

The slopes where the underlying bedrock is mapped as volcanic appear generally more susceptible to landsliding than the slopes underlain by feldsparphyric rocks, but this may be because the topographic relief is greater where the bedrock is volcanic. Although Vandine (1985), who studied debris flow landslides in British Columbia, discounted underlying bedrock geology as a predisposing factor, recent studies of natural terrain landslides in Hong Kong have concluded that geology together with slope angle are significant predisposing factors (Evans et al. 1997; Evans & King 1998).

Vegetation

Another factor which may affect the susceptibility to landsliding appears to be the presence or absence of thick vegetation, but there is much conflicting evidence in the literature concerning this (Johnson & Rodine 1984; Collinson & Anderson 1996). Irigaray et al. (1996) have stated that apparent correlations of landsliding susceptibility with vegetation have to be considered with care, as vegetation is very strongly interdependent on factors such as geology and hydrology. Within the study area, the vast majority of landslides resulting from the 1992 and 1993 rainstorms occurred in terrain with low scrub and grass.

Surface drainage

Studies in Canada (Vandine 1985) and California (Reneau & Dietrich 1987) indicate that surface drainage is an important factor controlling debris flow susceptibility and this is confirmed within the present

252 C. A. M. FRANKS

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Fig. 5. Typical natural terrain landslide scar showing features common to landslides in the Tung Chung study area.

study where most landslides occur within or adjacent to significant drainage lines or hollows.

Landslide characteristics in the study area

All the landslides had the following common features (Fig. 5); a source area (defined by a surface of rupture which comprises the main scarp and the scarp floor), a landslide trail downslope of the source (where debris transport predominates, although erosion and outwash may also occur), and a deposition fan (where the majority of the debris is deposited).

In general, the trails resulting from the downslope displacement of disaggregated displaced material were complex, comprising a number of sections within which either deposition or channelization of debris or erosion of the underlying materials (substrate)was the predominant characteristic. Minor deposition of debris occurred along the lateral boundaries (as levees) of many of the landslide trails, however deposition of most of the debris generally occurred at the end of the trail. Examination of the character (matrix and composition) and form of the debris indicated that at some locations secondary mobilization of the previously deposited landslide debris resulted in further deposition downslope as an alluvial deposit. This is considered to


Table 1. Geometry of landslide sources

Length of Main scarp Source Natural No. of Material surface of width volume slope angle slides composition rupture (m) (m) (m 3) () (as % of of source (Range) Range (Range) (Range) all scarps) (as % of

no. of slides)

9 (5-25) 5-9 76 (13-400) 37 (28-80t) 39 (45%) Colluvium 70% Residual Soil 30%



20 (20) 20-24 400 (400) 40 (40) 1 (1%) Colluvium 18 (12-30) 25-30 450 (450-1500) 37 (38-40) 4 (5%) Colluvium (50%)

Weathered Rock (50%)

Notes: Average of range in bold. tThe very steep slope angles are from scarps on the sides of steep drainage channels.

be the result of intense rainfall following the initial failure.

The landslides can be recognized in terms of the character and complexity of their associated landslide trails as follows (Tables 2 and 3):

landslides which have unconstrained landslide trails which include non-channelized debris flows and debris slides. Most of this group are debris flows with the predominant characteristic within the complete trail length being deposition.

landslides which have constrained landslide trails which are complex and include gully erosion and channelized debris flows (viz. debris torrents of Hungr et al. 1984). These landslides often include erosion, deposition, outwash (secondary alluviation subsequent to initial debris deposition) and channelization within different sections of the complete trail.

Landslide sources

Typically the landslides were initiated from sources with shallow scarps (

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Table 3. Range of values for trail length and erosion volume for constrained and unconstrained landslides

Landslide model

Type

Typical main source volume400 m 3 Trail No. of Erosion range No. of Erosion range length landslides (m 3) Trail landslides (m 3) (m) in study (average) length (m) in study (average)

Unconstrained trail Debris slide (i) 2-10 2 0 Debris flow--predominantly (ii) 15-100 16 0-30 depositional (15) Debris flow~deposition and (iii) 20-85 6 12-80 erosion (46) Debris flow--deposition and (iv) 5-100 9 0-20 outwash (10)

Constrained trail Gully erosion

Channelized debris flow-- includes deposition, erosion and channelised flow

(v) 25-95 3 50-330 (190)

(vi) no occurrence 0 no occurrence

no occurrence 0 no occurrence 35-70 4 0-20

(10) no occurrence 0 no occurrence

no occurrence 0 no occurrence

no occurrence 0 no occurrence

70450 4 90-860 (470)

where erosion predominated was 25 m with a range from 5 to 52 m. The average length of trail section where deposition predominated was 28 m with a range from 2 to 100 m.

The width of deposition within the trail was generally controlled by the topography, either leading to unconstrained or constrained deposition depending on the absence of otherwise of drainage lines and hollows along the trail. Typically the width of the trail averaged 9 m, and ranged from 1 m to 30 m for unconstrained debris flow, where the topographic controls are relatively sub- tle. For channelized debris flows the width of the trail was less than 8 m.

Table 3 summarizes the observed range of values for run-out length for those landslides with unconstrained and constrained trails in the study area. Deposition was the predominant characteristic observed along the complete length of many of the trails, especially from those landslides with a small source volume400 m 3, were more complex and comprised a number of sections that included some or all of the following; erosion, deposition, outwash, or channelized debris flow. The run-out length of these trails was typically in the range of 35 to 450 m.

Lau & Woods (1997) have examined the relationship between the run-out lengths of the landslides in the study area, as defined by the angle of reach method (Corominas 1996), and landslide source volumes. The relationship is shown in Fig. 6 and confirms that many of the landslides fall within the bounds typical of debris flows and translational slides. However, some have a

much shorter run-out length than would be predicted using Corominas' method and this is considered to be the result of local obstructions to downslope transport due to gross morphological changes or obstructions along the debris flow path.

Hazard zones for downslope development

The primary hazard zone for any downslope development lies within the debris transport and deposition areas as debris impact will result in injury, fatality or economic losses. The landslide trail defines this zone and extends from the landslide main scarp to the end of the deposition fan. About 96% of all landslide trails associated with both unconstrained and constrained landslides were found within slope classes that correspond to 15 to 40 and 71% were found on slopes within the range of 15 to 30 .

Debris deposition fan

Although minor deposits of debris were formed along many of the lateral boundaries of the landslide trails, in the absence of any obstacles to the debris flow path, much of the mobilized debris was transported downslope until deposition occurred as the slope angle reduced to some critical value. At this critical value of slope angle (deposition point), a deposition fan of material started to form in the trail. The limited data from the study area indicates that a lower bound for deposition is 10 without channel confinement and 8 for channelized debris flows landslides.

256 C .A .M. FRANKS

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20

. . . . . . . . . . . . . . . A, -~ . . . . . L , ** . . . .

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Rockfalis (Corominas, 1996).

. . . . . . Translational landslides (Corominas, 1996).

. . . . Debris flow landslides (Corominas, 1996).

~ Earthflows and mudflows (Corominas, 1996).

1 1.5 2 2.5 3 3.5

Log Landslide Source Volume (m)

Fig. 6. The lower bound envelope of angle of reach for landslide source volumes in the Tung Chung study area (after Lau & Woods 1997).

Hungr et al. (1984) in his study of debris avalanches in Canada, reported a limiting angle of 8 to 12 with channel confinement and 10 to 14 in its absence, for the deposition point. Furthermore Hungr et al. (1987) concluded that the presence of channel confinement is crucial to the maintenance of flow for slopes less than 18 and that a width to depth ratio of less than 5 is required for confined flow. Within the Tung Chung study area, channel width to depth ratios typically ranged from 10 to less than 3.

In general, most unconstrained landslides formed deposition fans on slope angles in the range of 20 to 32 , typically with more than 30% of the main scarp volume being deposited in the debris fan.

For channelized debris flows, erosion along some sections of the landslide trail provided additional debris to that from the original main scarp. In some cases this additional debris was estimated to be close to 50% of the original scarp volume.

Preliminary hazard zoning

A preliminary hazard zoning for the study area was developed based on slope angle classification and inden- tification of drainage gullies with the potential for channelizing debris flows. Bearing in mind the accuracy

of the field observations the following four slope classes have been adopted; 30 . Figure 7 shows the hazard zone map produced from a digital terrain model of the study area, together with the identified drainage gullies with the potential for channelizing debris flows.

Most of the landslide sources will originate from the >30 slope angle zone and those debris flows that are unconfined will typically form debris fans on slope angles in the range of 20 to 30 . However, a few unconstrained debris flows with a source volume greater than 400 m 3 resulted in debris fan deposition on lower slope angles in the range of 10 to 20 . At those locations where drainage gullies with the potential for channelizing debris flows intersect slope classes < 10 then the formation of a debris fan is likely, the dimensions of which can be estimated by applying the appropriate values for deposition in Table 2 to a suitable source volume.

For example assuming a maximum source volume of 1500 m 3 and additional entrainment of debris of say 2000 m 3 (assuming erosion along the complete length of the longest drainage line), this will give a maximum likely channelized debris flow of 3500 m 3. If 30% of this combined volume deposits as a debris fan, then the debris fan will be 70 m long from the deposition point, if an average deposition thickness of 0.7m and a maximum 30 m wide debris fan is assumed.


Slope angle Hazard Zone Description

< 10 Limit of debris fan deposition for potential channelised debris flows

10-20

20-30

Limit of debris fan deposition for potential non channelised debris flows

>30 Zone of most landslide sources

f J Main drainage The most likely flow path for potential channelised debris flows gullies

Scale

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Discuss ion

The landslide characteristics from the 1992 and 1993 rainstorms have been compared with those landslide characteristics that can be obtained from interpretation of the available aerial photographs over the 50-year period 1945 to 1995. This indicates that intense rain-

storms during this period in Hong Kong have resulted in widespread landsliding in the study area.

The results of the study indicate that channelized debris flows generally result in a greater magnitude of mobilized debris with consequent longer debris run-out than non-channelized debris flows. Consequentially this larger volume of debris will produce a greater deposition fan area.

258 C .A .M. FRANKS

Review of all the aerial photographic evidence and comparison with the field evidence of landslides generated during the 1992 and 1993 rainstorms indicates that a channelized debris flow mobilizing about 2300 m 3 of material, with a debris run-out (trail length) of 450 m is likely to be the maximum debris flow landslide that has occurred during the last 50 years. However, based on the average erosion rate of about 3.7 m3/m (Table 2) established for channelized debris flow in the study area the potential for the entrainment of additional debris of about 2000 m 3 is a possibility for the longest drainage gullies.

For downslope developments at the footslopes (i.e on slopes less than about 20 ) of natural terrain that is drained by gullies with steep slope gradients this type of landslide is potentially the most hazardous. However, developments on slope gradients greater than about 20 will also be at risk to hazards resulting from non-channelized debris flows.

The Tung Chung development area is bounded to the southwest by natural terrain with slope gradients generally greater than 20 . However, a number of gullies with steep downslope gradient intersect this development boundary and provide potential flow paths for channelized debris flows.

This study has examined the characteristics of rainstorm induced landslides on natural terrain above the Tung Chung New Town and defined those criteria influencing the debris flow paths and deposition areas of these landslides and hence the zones of hazard for downslope developments. These criteria are: the maximum likely landslide source volume, whether the likely landslide trail is constrained or non-constrained, and the downslope gradient along the likely debris trail. Although recent regional studies (Evans et al. 1997) have concluded that relative landslide susceptibility at a particular location is also influenced by bedrock geology, this could not be confirmed in this study area where most of the landslide sources were underlain by a single rock type.

The assessed maximum likely channelized debris flow and trial length is based on the assumption that landsliding during the previous 50-year period is an indication of future potential hazards in the catchment. The possibility of larger landslides occurring within the catchment during the next 50 to 100 years cannot be discounted on the basis of the 50-year timescale examined using aerial photographic interpretation. However, detailed ground investigation and topographic survey along all drainage lines and hollows would be required to improve the hazard definition.

Conclusions

Most of the rainfall-induced landslides that have occurred on the natural slopes above the Tung Chung

New Town Development are unconstrained with debris volumes generally less than 400 m 3. These landslides generated relatively short trails, generally less than 50 m in length, with deposition on slope gradients >20 .

The largest magnitude rainfall-induced landslides were those that were constrained resulting in channelized debris flows. These are the events that produced the longest trails and debris run-outs and hence are potentially the most hazardous to downslope developments. Some rainfall induced channelized debris flow landslides have occurred with debris volumes >2000 m 3. These landslides generated long trail lengths some of which have reached the boundary areas of the Tung Chung New Town Development.

Within the limits of the accuracy of the field data, baseline values for defining the zones of hazard for downslope development have been established as follows:

Most landslide sources originate from areas with slope angles greater than 30 .

Typically unconstrained landslides have a source volume of less than 400 m 3 and are likely to form a debris fan on slope angles less than about 20 . A relatively few unconstrained landslides have a source volume greater than 400 m 3 and are likely to form a debris fan on more gentle slope angles having a lower bound of about 10 .

For constrained landslides, the source volume is typically greater than 400 m 3 and the limit of debris fan formation is likely to be on slope angles less than 10 .

The trail lengths typically range from 5 to 100m for unconstrained landslides and from 25 to 450 m for constrained landslides.

Acknowledgements. This paper is published with the permis- sion of the Mr B. M. T. Lain, Director of Civil Engineering of the Hong Kong Government.

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Received 23 June 1997; accepted 20 November 1998

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