Intro Research - Landslides

8/18/2019 Intro Research - Landslides

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Abstract

A landslide potentiality assessment method that is based on a factorial indexing system has

been created and tested for Mauritius Island. The assessment aims at finding lands that can

potentially slide by making use of an equation that has as input five parameters decisive for

potential landslide namely; rainfall amount, terrain slope angle, permeability of geology

strata, soil and land cover type. The equation output are indexes ranging from to ! suchthat is associated "ith the lo"est risk and ! "ith the highest risk of potential landslide. A

region that has not been assigned an index have no risk of land sliding there even though that

region might be vulnerable to sliding lands from uphill. #esults sho" that the method

succeeds in mapping land that can potentiality slide and in evaluating the corresponding

regions that are at risk from the latter. $ey"ords% &otentiality mapping, factorial indexing,

'andslide, (I), Mauritius

1. Introduction

Mountain and foothill slopes occupy a significant percentage of the surface relief of

Mauritius Island and according to )addul *++-, the landslide prone areas are those that have

terrain slope angle exceeding !/, "here specifically areas "ith slope 0 1!/ are definitely prone to landslide "hile areas "ith slope in the range !21!/ are prone depending on the soil

and land cover type. Mauritius has a total surface area of 343 km and regions "ith slope 0

!/ make up !3.31 km, "hich is .31 5 of the Island67s total surface area *8igure -.

Figure 1 - Terrain slope angle in degrees.

Although the landslide prone areas are kno"n, "hat is unkno"n so far is the potentiality of

land sliding for these prone areas. The ob9ective of this "ork is to map the landslide

potentiality for such landslide prone areas. An initial methodology based on slope and rainfall

amount "as devised to create a preliminary landslide risk map for Mauritius *#ughooputh et

al., !!3-. The methodology used in "ork is based on the "ork accomplished so far by

)addul *++-. )pecifically the latter has concluded that in addition to slope 0 !/, the factorscrucial for landslide are% "hen the soil layer is "eathered, unconsolidated, clayley, tuffaceous


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and consists of colluvial deposits; "hen the geology strata is hard, compact and consolidated;

"hen rainfall amount received is high and:or frequent; "hen the land cover is barren and is

not protecting the soil and finally; "hen land use practices, such as, construction, roads traffic

vibration and agricultural practices have disturbed the soil.

Thus, by taking into consideration the latter information, a 'andslide &otentiality Index *'&I-map has been created based on a factorial indexing equation "ith five geospatial input

parameters% rainfall amount, terrain slope angle, permeability of geology strata, soil land

cover type. The equation "as run in a (I), "hich enable the handling, manipulation, analysis

and combination of the input geospatial data. The '&I equation output is an index ranging

from to ! such that is associated "ith the lo"est risk and ! "ith the highest risk of

landslide. A region that has not been assigned an index has no risk of land sliding. As such,

the indices on the '&I map sho" lands that can potentially slide and do not specifically sho"

all lands that are at risk to sliding lands from uphill.

2. Study area description

2.1 Location, geology and geomorphology

Mainland Mauritius is situated south"est in the Indian 1 $m and its surface area 343 $m.

Figure 2 - Upper right inset shos location map o! "auritius in the Indian #cean. "ain!igure shos a $%" hillshade

@arious authors *e.g., illaime, +3>; )addul, ++- have reported that the island is entirely

of volcanic origin "ith a four stage geochronology consisting of% The Breccia )eries *! 2 =.3

Million Cears *MC- ago-; The


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follo"ed by 'ate 'avas "hich occupy >!5 of the island. 'ate 'avas are different from the

Intermediate 'avas in that they are highly vesiculated and are characterised by many rocky

areas "ith an almost complete absence of surface drainage. The only materials that are not of

volcanic origin are coral reefs, sandy beaches, and some consolidated coral and shell debris in

isolated remnant raised beaches *)addul, ++-.

2.2 &limate, soils, sur!ace drainage and land use

The climate of mainland Mauritius is of the humid tropical type and is under the influence of

atmospheric circulations and static factors such as altitude, exposure to the )outh ?ast Trade

inds and distance from the sea *illaime, +3>-. In the summer period from Dovember to

May, tropical cyclones are the most important climatologic features and during the "inter

period, from April to ; &adya, +3+-.

Foncerning intra2annual variability, on average 3!5 of mean annual rainfall is received

during the summer period. 8ebruary is the "ettest month and has the highest probability of

cyclone formation and =5 of the island67s surface. The AKonal soil groups are located mostly on


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about >5 of indigenous vegetation remains. )crubs cover about 5 of the island, forest

occupies =5 *including indigenous vegetation-, built2up areas occupy 45 and the rest are

"ater bodies, "etlands and sandy beaches *Ministry of Agriculture Datural #esources,

+++; @enkatasamy, ++-.

'. "aterials and methods

'.1 The !actorial inde(ing system

The factorial indexing system takes the form of equation belo" and has as input slope, rain,

geology, soil and land cover and produces 'andslide &otentiality Index as output.

The factorial indexing system "orks in a "ay such that, a higher index is given to a class that

has a higher potentiality and a lo"er index is given to a class that has a lo"er potentiality of

landsliding. ?ach indexed input map is then multiplied "ith a factor "eight. Applying afactor "eight has the effect of giving more importance to certain maps than others. 8or

example, if slope is the most determinant factor for '&I follo"ed by soil, then slope is given

say >!5 "eight, compared to a lo"er value given to soil *say 5 "eight-. Thus, the slope

map "ill be multiplied by !.> *>!5- prior to addition. The values chosen for factor "eights

are such that their sum is one *-. The minimum and maximum indexes are set to and !

respectively. These t"o conditions allo" the creation of an '&I map having indexes ranging

from to !, "hich enable an easy interpretation.

'.2 Factor eights and inde(es

)lope being the most dominant factor is given a high factor "eight, namely >!5. The indexes

assigned to slope classes are sho"n in table .

Table 1) Inde(es !or slope classes

)lopeFlass

*degress-

'andslide

vulnerabilityIndex

L 1! / @ery high !

! 1! / Gigh 3


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! ! / Do #isk Do

Hata

A value of DoHata is thus given to all slopes less than !/ such that these regions are givenno risk "hen running equation . According to )addul *++- the usual mechanism that

triggers landslide is "ater that run bet"een an unconsolidated soil layer and an impermeable

geology strata. As such, the indexes for rainfall *table - are increased "ith an increase in

amount of rainfall received. The factor "eight assigned is !..

Table 2) Inde(es !or rain!all

#ainfall *mm- Index

>!! 2 !! !

!! 2 >!! +

2 !! 3

According to (iorgi et al. *++3-, the impermeability of the geology strata increases "ith an

increase in the age of the strata. The indexes chosen for geology *table 1- are thus increased

"ith an increase in the age of the geology strata and the factor "eight assigned is !..

Table ') *eology strata and inde(es assigned

(eological Flasses *Fhronological order- Index


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Flayey and tuffaceous soils s"ell during rainfall *)addul, ++- and then slide do"nhill. By

considering the percentage of clay content and infiltration rate from Balaghee *!!- and

$remer *!!!- respectively, the indexes applied to each of the soil group are as sho"n in

table > and the factor "eight assigned is !..

Table +) Inde(es assigned !or soil groups

)oil (roup Index

'o" Gumic 'atosols, ' =

Gumic 'atosols, G =

Gumic 8erruginous 'atosols, 8 4

'athosolic #ed &rairie )oils, & 1

'athosolic Bro"n 8orest )oils, B =

Hark Magnesium Flay, M !

(round ater 'aterite,

(rey Gydromorphic )oils, H +

'o" Gumic (leys, (

#egosols, F

'ithosols, T 1

Mountain )lope Fomplexes, ) +

Alluvial )oils, A

'andslide potentiality is increased "hen the land cover is barren and is not protecting the soil

or "hen land use practices, such as, urban:built2up, roads traffic2vibration and agricultural practices have disturbed the soil. As such, the indexes applied to each of the land cover class

are as sho"n in table and the factor "eight assigned is !..

Table ) Inde(es assigned !or land coer classes

'and Fover Flass Index

Orban:Built2Op:)ettlements !

Marshy :etlands

'akes, #eservoirs


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Agricultural fields 4

Barren 'ands, )hrubs

8orested areas

)andy areas

'.' Sources o! input !actor maps

The slope map used for the study *8igure - "as derived from a =P= m Higital ?levation

Model *H?M-, the latter that "as produced by interpolating


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Figure + ain!all map !or February

The soil map *8igure - has been produced by digitiKing the %!!,!!! soil map of H


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Figure / Land coer map

'.+ 0rocessing o! the inputs and running the euation

After the paper maps "ere digitiKed into vector maps, the classes of each digitiKed map "ere

given their indexes and the ne" indexed maps "ere converted into raster maps to be used for

map algebra *of equation - in Arc(I) soft"are. Huring conversion of the vector maps into

raster maps, a cell siKe is chosen based on the scale of the original paper map. 8or example a

%!!,!!! map is given a cell siKe of !!P!! during rasteriKation. Maps already in raster

format *such as slope and rainfall- are simply reclassified to their corresponding indices, forexample the slope class !21!/ is directly converted to its index of nine *+-. After all the

rasters have been produced, equation is run using the map algebra function of the Arc(I)

soft"are *version +.-.

+. esults and discussion

8igure = sho"s the '&I map produced. Most values of landslide potentiality are above 4..

km of settlements are found directly on these landslide prone areas and

significant portion of settlements are also found on the path of sliding lands. In addition to

that, some landslide prone areas are found close to dams "hich in turn are found close to

settlements. In case of landsliding, overtopping of these dams can be disastrous for these

nearby settlements. 8inally another significant portion of land transportation net"orks are

found on the path of sliding land or even found directly on the landslide prone areas.


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Figure L0I map


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map can be more accurately produced using methods of classification for the most recent

optical and:or radar polarimateric remote sensing images *Rensen, ++4; 'illesand and

$ieffer, !!!; Bamler and Gartl, ++3; Floude and &apathanassiou, ++3-.

. &onclusion

'&I is a method for computing the potentiality of a piece of land to slide. It has the form of anequation that is based on a factorial indexing system and "hich relates landslide potentiality

index to slope, rain, geology, soil and land cover. The tool required to run the equation is (I)

that have map algebra *raster- capabilities. The output 'andslide &otentiality Index map

indicate areas that can potentially slide through interpretation of the indices represented on

the map. The lo"est and highest indexes * !- correspond to the lo"est and highest

potentiality of land to slide and lands "ith no indexes have no risk of sliding even though

such lands can still be vulnerable to sliding lands from uphill. An '&I map has been

successfully produced for Mauritius and it is enrichment for the already defined landslide

prone areas "hich are those lands that have slope 0 !/. The produced '&I map as is, does

not explicitly define regions that are vulnerable to landsliding. Indeed, lands that have no

potentiality of sliding are still at risk from uphill sliding lands. But "ith the '&I methodcoupled "ith its supporting geospatial platform, it is possible to at least visualise those

do"nhill areas and land features that are at risk from uphill sliding lands.

Ac3noledgements

e "ant to thank the Oniversity of Mauritius for providing the necessary facilities for

carrying this research. The Fartographic )ection of the Ministry of Gousing and 'ands and

Meteorological )ervices are gratefully ackno"ledged for providing the digital ! m contours

data and the rainfall data respectively. This "ork "as given support under a scholarship at the

Oniversity of Mauritius from the Tertiary ?ducation Fommission *T?F-.

2 )ee more at% http%::geospatial"orld.net:&aper:Application:Article@ie".aspxS

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Among the natural haKards, "hich cause much loss of life and property around the "orld,

landsliding is perhaps the most under recogniKed and under estimated. In recent decades, man

has "itnessed great devastations caused by catastrophic failures of mountain slopes involving

the sudden and dramatic collapse of large volumes of rocks and soils along mountain slopes.

This problem is very common in volcanic island domes located in tropical areas. Mauritius is

a volcanic island dome "here mountains and mountain slopes *altitude 1!!S3!! m- and

slopes ranging from L ! U to 4! U cover an area of more than 1!,!!! hectares i.e 4 5 of our

total surface area. As far as land suitability and land capability index are concerned, all slopes

exceeding 1! U along a mountain becomes dangerous for human activities "ithout appropriate

engineering "ork.

Onfortunately, there are different sectors of mountain slopes in Mauritius along "hich

settlements have escalated higher and higher beyond the Kone of high risk and thereby

maximiKing the risk of landslide haKards. Mauritius is located in a cyclone basin and a fe"

days of excessively high cyclonic rainfall can have dramatic effects along mountain slopes,

"hich are heavily loaded "ith unconsolidated and loose "eathered materials. A look at "hat

is currently happening along the slopes overlooking the Moka #ange near 'e Bocage and

along the slopes of Fandos Gill as "ell as along the slopes of Tourelle du Tamarin "ill a"e

inspire any ?arth )cientistS. and not the Mauritian authorities.

Fhitrakoot is located at an altitude of m on a very dynamic upper mountain slope *above

1 /- of @alle des &rVtres, a completely enclosed amphitheatre valley carved by the 'atanier

river through the 3 million years old &ort 'ouis mountain range. The processes that have been

at "ork since millions of years and "hich are still active indicate that Fhitrakoot is located in

a potentially danger Kone, "here a slo" mass movement of materials do"n the slope from the

vertical (rand (lacis mountain "all to"ards the village and do"n"ards cannot be "ritten off

at any stage. A slo" mass movement can suddenly become a sudden rotational colluvial

*rocks and soils- landslide "ith drastic consequences if appropriate geotechnical "orks and

mitigating measures similar to the one carried out at the 'a Butte Kone are not undertaken to

prevent the risk of loss of life and property.

The danger is that a mass movement of materials *rocks and soils- can at any time happen

along a Kone of discontinuity as sho"n in the diagram. A classic example is "hat happened at

'a Butte located along the 1! U W1 U slopes of the )ignal Mountain in +3= and +33.


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The upper mountain slope of @alle des &rVtres receives annual rainfall varying from !! S

! cm, but torrential outbursts during the passage of a tropical cyclone can cause the loose

unconsolidated materials found higher up of Fhitrakoot to absorb "ater, inflate and slide

along the Kone of discontinuity by a process called solifluction *a technical term used to

denote the sudden flushing do"n of thousands of tonnes of loose materials-.

A close look at the topography bet"een Fhitrakoot and the mountain "all of the (rand

(lacis and &ieter Both sho" the presence of terraces. The most visible terrace is located at an

altitude of 1! meters "ith other minor balconies lo"er do"n at !! meters. It is most

probable that these have been formed by subsidence and slide in steps in the past. Along the

"estern slopes of 'S?chelle #ock, located at the entrance of the valley, a stepped topography

is easily observed. Gence, the name 'S?chelle #ock.

?ven through the eyes of a layman, there are clear2cut indications of mass movements in the

vicinity of the Fhitrakoot primary school. 8issures on the "alls, cracks on the roof and "alls

of buildings, slanting roofs, slanting trees and electric poles, bulging "alls, thick ridges of

loose materials "ith convex slopes are visible. In fact, not only the primary school but, all the

buildings in the area are at riskS at high risk.

In conclusion


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Title % The &ro9ect of 'andslide Management

&eriod % March ! 2 )eptember !>8unded by % RIFA

Mauritius is particularly vulnerable to climate change and has an increasingly serious

problem "ith landslides caused by natural disasters, environmental changes and an increase

of tourism2related structures and land development on steep slopes. Against this background,

various climate change adaptation programs are being conducted in Mauritius mainly by the

Rapanese (overnment and the Africa Adaptation &rogramme *A&&- under the Onited Dations

Hevelopment &rogramme *ODH&-, including this pro9ect. It is closely tied to another pro9ect

by $okusai $ogyo experts to assist Mauritius in developing capacity in coastal protection

and rehabilitation, "hich is also related to A&&.

This coastal protection pro9ect involves five components%

. Hevelopment of a landslide database by collecting and putting in order existing data, and

"hen deemed necessary, to conduct field surveys, geophysical exploration, test drilling

and aerial photo interpretation and to make topographic and geological maps.

. 8ormulation of a landslide management plan to establish a landslide monitoring system.

1. Implementation of a feasibility study and pilot pro9ect to examine, carry out and learn

specific approaches.

>. ?nhancement of the technical capacity of staff in Ministry of &ublic Infrastructure *M&I-

and other administrations.

. Implementation of pilot pro9ects focusing heavily on public participation and involvement

of relevant agencies; the results of "hich are to be reflected in the landslide management plan.


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Intro Research - Landslides

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