Landslide inventory in northern Calabria, southern Italy
A. CARRARA 1 Consiglio Nazionale delle Ricerche, Istituto di Ricerca per la Protezione Idrogeologica, 87030 Castiglione Scalo-Cosenza, Italy L. MERENDA J
ABSTRACT were attempted. These studies have shown Mezzogiorno, 1968; Burton, 1970; Meli- the serious slope instability conditions of doro, 1971). In Calabria, the toe of the Italian boot, many regions and have given a general out- Because of its geological history, climatic landsliding and slope instability, produced line of the distribution of stable and unsta- conditions, and extensive human activity, by soft rocks, rapid tectonic uplift, earth- ble areas (Nicotera, 1959; Cassa per il Calabria is extremely susceptible to land- quakes, and seasonally heavy precipitation, constitute a major geologic hazard. For this reason a detailed inventory of landslide and accelerated erosion events has been in- itiated in northern Calabria. To carry out this project, a methodology for the collec- tion and mapping of slope instability phenomena has been established. The method is based on the use of a standardized form for the collection of field data. The data form and related map sym- bols reflect the purpose of the project, which is the collection of information that is as quantitative and objective as possible on a large number of slope instability phenomena by surveyors with varying de- grees of field and laboratory experience. The morphometric and typologic data gathered in this way constitute a convenient basis for statistical analysis of factors re- lated to the slope stability and for the prep- aration of landslide susceptibility maps. The maps also can be used readily for soil conservation measures. This inventory method has been tested in two sample areas and is at present being applied to an area of about 1,000 km2.
INTRODUCTION
In the past decade, regional slope stabil- ity studies have been undertaken by scientific institutions of various countries. In Czechoslovakia, for instance, detailed and general engineering-geological maps have been prepared for most of the country; these maps are based on a large number of field and laboratory data and show the locations of landslides and erosional phenomena and many of their features (Zaruba and Mencl, 1969; Demek, 1972; Rybar and others, 1965; Rybar, 1973). Similar investigations, which are providing basic information needed for land use plan- ning, have been initiated in California, par- ticularly in the more densely populated areas (Nilsen and Brabb, 1973). In Italy, mostly in the southern part of the country, investigations and inventories of landslides Figure 1. Geological sketch map of Calabria and location of study areas.
Geological Society of America Bulletin, v. 87, p. 1153-1162, 9 figs., August 1976, Doc. no. 60809.
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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1153/3444071/i0016-7606-87-8-1153.pdf by guest on 28 September 2021 Figure 2. Example of a damaged village (A) due to landslides (B) at the Calabna-Lucama border.
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1153/3444071/i0016-7606-87-8-1153.pdf by guest on 28 September 2021 LANDSLIDE INVENTORY IN ITALY 1155 sliding and slope instability. The Paleozoic terpretation (particularly for old, fossil, example, for a flysch sequence with alter- and Mesozoic metamorphic and igneous dormant, or exhausted landslides), training nating sandstone and marls, S-C will be en- rocks that underlie most of the territory ex- of the surveyors is needed as well as the tered. perienced intense folding, faulting, crush- standardization of the working criteria. Structural data (sec. 5) are reduced to es- ing, and dislocation during the Hercynian To achieve standardization and to in- sentials to avoid overly complex and time- and Alpine orogenies. These rocks and the itiate a quantitative investigation of slope consuming field investigations. The slope overlying poorly consolidated sedimentary instability phenomena, a field data form has morphology (sec. 8) is that preceding the sequences of Tertiary-Quaternary age (Fig. been prepared (Fig. 3A, B). The aim of this event. The terms "rectilinear," "concave," 1) are deeply dissected by a dense stream form is the collection of quantitative and and "convex" refer to a vertical (first col- network resulting from the rapid tectonic objective information on a large number of umn) and to a horizontal (second column) uplift of the region (about 1,000 m in the slope instability events by surveyors with profile of the hillslope. In the case of a land- past 1 m.y. [compare Ogniben, 1973]). A varying degrees of field and laboratory ex- slide that occurs on a previous landslide Mediterranean climate with abundant and perience. Therefore, geotechnical informa- zone or on terraced slope, the item seasonally intense rainfall has produced a tion was reduced to the minimum, and "nonuniform slope" will be checked. thick weathering mantle over most of the more space was given to morphometric lithologic units. In addition, Calabria is the data (Institute of British Geographers, Erosion site of frequent strong to catastrophic seis- 1971; Brunsden, 1973; Blong, 1973b). mic activity. All these factors, when as- The study and mapping of erosional sociated with extensive human activity (in- Form for Data Collection phenomena are carried out partly in the cluding systematic deforestation), have field and partly by means of air photo- played an important role in producing For each landslide or accelerated erosion graphs, which are particularly useful for steep, irregular, and highly unstable slopes. event (mapped on a scale of 1:10,000), the this kind of investigation. Sections 12 The costs of landslide damage are ex- form is compiled partly in the field and through 13 can be readily filled in the tremely high in Calabria; expenditures for partly in the laboratory (for more details, laboratory. road, railroad, aqueduct, and housing re- see Carrara and Merenda, 1974). Answers pairs have been estimated, for 1972 to 1973 are given by checking one or more boxes of Landslide alone, to be more than $200 million (com- each section and, where possible or neces- pare also Commissione Interministeriale sary, numbering them according to the The distinction between landslide and per lo Studio della Sistemazione Idraulica e order of importance (Fig. 3 shows a form landslide zone (sec. 18) arises from practi- della Difesa del Suolo, 1974). The human compiled for a landslide located in sample cal necessities; areas of small but wide- and social implications are even more seri- area 2). The form is divided into three main spread landslides affecting large parts of a ous; landsliding is forcing the abandonment parts plus a space for geographic location slope, together with unmappable and (or) of almost 100 villages with a total popula- and file data: (1) general ( sees. 3 through potential landslides and creep phenomena, tion of about 200,000, thus contributing to 10), to be compiled both for landslide and are classified as "landslide zones." The the further impoverishment of this already for erosion; (2) erosion (sees. 11 through landslide classification adopted is substan- poorly developed part of Italy (Fig. 2A, B). 17), reduced to essential information be- tially derived from that of Varnes (Eckel, Because of the urgent need for soil con- cause erosional phenomena are not the 1958), which at present is used by Czechos- servation measures, slope stabilization, and main purpose of the project; and (3) lovakian authors and by the International regional planning, a long-term project in- landslide (sees. 18 through 26), discussed in Geographical Union (Nemcok and others, volving a detailed inventory of landslide more detail because it is the main object of 1972). However, the present classification and erosional phenomena has been initiated the investigation. is considered to be more detailed and for a large part of northern Calabria. This flexible than that of Varnes. Two boxes are will be followed by a statistical analysis of General beside the terms "sliding," "fall," and the data collected for the preparation of "flow"; the first is for indicating the charac- landslide susceptibility maps. This part is filled out in the field using ter of the event in its initial evolutionary Taking into consideration the methodol- geological and topographic maps. Besides stage, the second for its present character. ogy and analytical techniques of various au- marking the presence and the thickness of For example, for the landslide M-8 (Fig. thors (Eckel, 1958; Jones and others, 1961; soil (sec. 3), the lithologic units are listed in 3B) the mass movement was initiated as Rybar and others, 1965; Desio, 1968; stratigraphic or tectonostratigraphic order sliding; at present the phenomenon pro- Brunsden, 1973; Nilsen and Brabb, 1973; (a,b,c) and the formations affected by the ceeds with similar characteristics. There- Blong, 1973b), we have worked out a event are checked. Where possible, the fore, the form has been compiled as in Fig- method for the collection and mapping of thickness and nature of the contact between ure 3B. If, instead, the landslide were now slope instability phenomena. This method, the lithologic units are marked (see also nearly stabilized with the depositional area which is briefly illustrated in this paper, has sketch of sec. 27). Geological maps are affected just by creep phenomena, the form been tested in two sample areas (Fig. 1; see available on a 1:25,000 scale for all of should be filled in as follows: sliding CDD also Carrara and Merenda, 1974) and at Calabria, and their formational symbols and creep •m. a present is being applied to an area of about (Sfe, P !-2) are used in the form. In section 4 Most of the morphometric data (sec. 21) 2 1,000 km . the geotechnical characteristics of the rocks are derived from Eckel (1958), Brunsden in the area of slope instability are given (1973), and Blong (1973a, 1973b). Meas- INVENTORY METHOD qualitatively. For example, the formation urements (in metres) can be taken on the Sfe (Cretaceous epidote schist) may be solid ground or from the map according to the Systematic mapping of landslides re- or partly solid according to the degree of precision or the aim of the study. Most of quires a familiarity with slope and landslide tectonization. If the formation has alternat- the morphometric information comes from morphology in the field and on aerial ing rock types, the appropriate section is sliding-type events. In any case, the data photographs. To avoid discrepancies in in- marked by the symbols S, P, L, and C. For that are more difficult to obtain concern the
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1153/3444071/i0016-7606-87-8-1153.pdf by guest on 28 September 2021 c.n.r. ir pi IINVENTORY BHD STATISTICAL ANALYSIS DF LANDSLIDES
Siïëet 229 z>uè\teyoè: © init.ana. kiOZm M & LANDSLIDE a numbe"* v Date 20/4/74 • LANDSLIDE ZONE lítame. L ATT fiRICO STREAM EROSION • cCownship T)Zainage~T3cisin • T. FINWA EROSIDNAL ZONE
MONDLITHOLOGIC LITHOLOGY ALTERNATED
a ll C • 0D solid
• •• partly solid
• •• loose
• •0 cohesive c
© DIPPING VERT. I HO RIZ. © • town, village etc. [houses 0? ) downstream I upstream I oDììque 0 country houses ( ias.2 _ . _ . j 0LAYERING • S • 0 Hmain roads [ to_s.M«R.T I I over 100.000 k [• lateral erosion ©SLOPE ANGIE © MAP AREA • STREAM EROSION !• incision © © CAUSES © CORRECTIVE MEASURES YETI NOI • initial lithology •.structures • groios • afforestation • matore climate-exposure • deforestation • dams • trellis-works morphology • cultivation • old • draioage ditches • walls-terraces • laodslides • Figure 3. Form for data collection. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1153/3444071/i0016-7606-87-8-1153.pdf by guest on 28 September 2021 LANOSI DE © IANDSL DE ZONE © MOVEMENT SUPINE [TU' mi I 11 HQIHI- CREEP hi Occurlgl 0 • widespread slides • 0 rotational • rock • debris • debris Probable • oomappable slides very rapid • • L • predisposed • earth • earth • bedding rapid m/<..„ • d • sheet slide • mod slow •% • a. • potential slides very slow m,% M O H P HO M E T BY TI first movement 00 veay ols original thick c.(. length total ground area CROWN 0 ' ® flt4 9 s 50 a single "[last mnvement ¿H 7 -' * 2 0 present tw. c.t. distance total map area • multiple © STflBIHTV CONDITIONS 20 3 Cr gO,ooo [•single recent nld fossil ! 0 • • c.l. length c.f. distance map area SHEAR PLANE s multiple 0 [•successive 500 300 ^ 65 ooo CREST-CROWN DIST.' volume degree active stabilized - 2,800,000' kïù debris overlap of 0 — m • LUNAR CRACKS » • stabil. dormant 0 present repose LfOO 85 % • reduced UE • ¡iOO degree • absent angle iMjjj. exhansted minor scarps 0 of 0 • with instab. MH 8 AA develop. area> par s a ii.° width advanced @ IIOIIIf[RODS CONDITIOTS © POSSIBLE CAUSES ©CDRHECTIIÍE MEASURES-VES I INOL SURFACE WATER [I] eros. M /-/"[ •climate 0 into landslide ^¡•unloading at toe • slope • afforestation SLOPE DBAINAGE • nff landslide • loading at head 41 angle •Ldrainage • nnt estimable S litholngv Gmnrphology • surface-• underground 4^weatliering •earthquake • soil removal • present SHOE DBAINAGE structures • human activities 0 gabions - • walls • absent • stagnant waters LQlayeringHfaultsTpouts • loading at toe - 0 dams • soil hardening - • anchorage GROUND WATER | •permeability grad. Qabandon •impeded drainage Qdeforestatioo • geotechnical investigations ground water seeps • springs • • ncoltivation • 7 « 9 10 14 cm. Figure 3. (Continued). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1153/3444071/i0016-7606-87-8-1153.pdf by guest on 28 September 2021 1158 CARRARA AND MERENDA landslide thickness (Fig. 4), for which, al photographs were used first to locate un- nant both in the initial and present stages of where possible, two values are given: the stable areas, then to map them in more de- the phenomena, but flows and falls are sig- original and that of the debris affected by tail. By using these base maps and corre- nificant in the present stage only (Fig. 9). movement at the present time. The presence sponding data forms, synoptic maps at a In sample area 2, more than 20 percent of of a crown is noted, and, where possible, scale of 1:25,000 were prepared (compare the area is covered by landslides and ero- the height and depth of the scarp are indi- Carrara and Merenda, 1974). A few obser- sional event sites (Fig. 7). Some 45 percent cated in the triangle with dotted lines. vations are presented here; a statistical of the mapped landslides (227, Fig. 8) are "Crest-crown distance" is the map distance treatment of the data will be carried out recent; moreover, active events are equal in between the landslide crown and the hill- when a wider area has been mapped and a percentage to dormant-stabilized events. slope crest; "debris overlap" is a field esti- greater number of slope instability events is Conversely, the degree of development is mate, in percent, of the degree of overlap available. similar to that of sample area 1. Erosion, between erosional and depositional areas In sample area 1, the percentage of area which is areally limited (about 2 percent), (see Blong, 1973a, 1973b). covered by landslide and accelerated ero- affects both sandy and clayey terrains. In Section 23, "stability conditions," is par- sion is relatively low (about 10 percent; Fig. this area also, sliding phenomena prevail ticularly important because of the need to 7), and the percentages for mapped land- (Fig. 9); however, due to the larger diffu- classify landslides according to age, degree slides (188; Fig. 8) are as follows: (1) "old- sion of clay-rich sediments (Fig. 7), of stability, and degree of development fossil," more than 70 percent and (2) flow-type landslides become numerically (compare Zaruba and Mencl, 1969). In this "dormant-stabilized," about 65 percent, important in the present stage of evolution way it is possible to investigate the evolu- with a generally advanced degree of de- of the slope stability of the area. tion in time and space of the slope instabil- velopment. Erosional phenomena, although In summary, sample area 1 can be ity of an area. areally limited (about 3 percent), affect defined as a relatively unstable area mostly most the stream valleys formed in sandy characterized by old, dormant-stabilized Map Legend of Slope terrains. Sliding-type events are predomi- sliding-type phenomena; conversely, area 2 Instability Phenomena For the present project, a map legend of instability events was prepared with charac- ters partly complementary to those of data form. The symbols adopted are generally simple and already used in geomorphologi- cal literature (Fairbridge, 1968; Demek, 1972); however, they have been combined in a new fashion to provide readily usable maps for soil conservation measures. In the legend, stability conditions and landslide typology are shown; in addition, colors (red and black for landslide, yellow and green for accelerated erosion) give an index of danger of the mapped event (Fig. 5). Red symbols represent active phe- nomena, while black indicates dormant or stabilized events; red is also used to delin- eate recent scarp and depositional area lim- its, and black is used for limits of old or fos- sil landslides. By combining all these colors and symbols, one can represent various situations as illustrated in Figures 5 and 6 (where, to avoid printing difficulties, red and yellow colors are indicated by thick lines and symbols in boldface). In the latter figure, a portion of sample area 2 is shown that indicates the use of the symbols and mapping scheme (see also Carrara and Merenda, 1974). SAMPLE AREAS The outlined inventory method was tested in two sample areas that are geologi- cally and morphologically representative of most of northern Calabria (Fig. 1). For these areas a systematic mapping (scale 1:10,000) of landslides and accelerated ero- sion phenomena was carried out and for Figure 4. Sliding morphometry, section, and plan. Terms and sketch modified after Eckel (1958) each event a field data form compiled. Aeri- and Brunsden (1973). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/8/1153/3444071/i0016-7606-87-8-1153.pdf by guest on 28 September 2021 LANDSLIDE GENERAL LEGEND IDE DETAILED LEGEND EROSION ACCELERATED EROSION LANDSLIDE UNMAPPABLE MAPPABLE DEBRIS I TYPE I SYMBOL UNMAP. MAPPAB. I TYfE 1 |ACT-R£G| iPO-OUal |ACTIVE 1 |DORMANT] |STABIL. ] [ RECENT | | OLD | |tm-*t| ¡INTENSEI IFOSSIL | | T Y P E SLIDING V V V SHEET V V SLIDING V EROSION o 1 0 FALL °° FALL T áí. RILL U JU T EROSION FLOW u u flOW u u GULLY CREEP EROSION LANDSLIDE ZONE -Symbols, oriented according .to dominant direction of movement., BAD coloured in red indicate active phenomena,coloured in black LANDS dormant phenomena. I ACTIVE I IDORMAN.I I FOSSIL I -For landslide zones discontinous lines indicate superficial phenomena, continous lines deep phenomena. ¿"i EROSION NICHES SLUING VERTICAL LANDSLIDE ELEMENTS STREAM EROSION I»ECENT I I O L D | | FOSSIL I [STABILIZED! LATERAL SCARP STREAM SUPERFICIAL EROSION DEBRIS LIMIT «... MIL -Continous line idicates defined mapping discontinous line approximate mapping. Jf;Ä Sti PC OFICIAL • • yî, 3 9 FLOW •jo A » POTENTIAL • » AMACI D SCARP jC»ACKS root MW CMOS COMSTI- SKIING POND SICTIOM -potential landslide scarp, in red indicates phenomenon with a probable rapid aitd dangerous evolution, in black in- dicates phenonsen-o-n with probable slow and not danferoas evolution. fUPI OFICIAL -in red indicate a>ctive, in black dormant phenomena. 4—landslide sca