Simulation of the Parma River blockage by the Corniglio landslide (Northern Italy)

Geomorphology 33 Ž2000. 1–23

Simulation of the Parma River blockage by the Corniglio landslide žNorthern Italy/ Aldo Clerici ) , Susanna Perego Dipartimento di Scienze della Terra, UniÕersita` degli Studi di Parma, 43100 Parma, Italy Received 11 February 1999; received in revised form 11 August 1999; accepted 14 August 1999

Abstract Near the village of Corniglio, in the northern Apennines about 40 km SW of Parma, a big landslide was recently Ž1994–1996. subjected to reactivation, damaging many buildings and partially obstructing the Parma River. In this work, we have carried out a quantitative analysis of the recent evolution of the landslide with the help of a Geographic Information System ŽGIS., in order to predict the most plausible future developments. Then, various simulations of landslide descent were carried out, and above all, to evaluate the possibility of blockage formation. To this purpose, two computer programs that simulate the material descent through a cellular automata model and define blockage characteristics were written. Many different simulations were carried out, 16 of which are presented here. For each of them, the computer output consists of a table defining the quantity of displaced material, area and volume of the dam and lake and of a map showing the areas involved in the landslide movement and upstream flooding. In order to assess the probability of blockage formation and dam stability, we applied the simulation results to some empirical rules defined by preceding authors through statistical analysis carried out on a high number of analogous phenomena. These rules suggest that it is very unlikely that blockage takes place as the result of very slow landslide velocity, riverbed width and high erodibility of the material. If it were to occur, the size and shape of the dam seem sufficient to guarantee a considerable degree of stability, making a sudden failure improbable. q 2000 Elsevier Science B.V. All rights reserved. Keywords: landslide simulation; landslide dam; cellular automata; geographic information system; northern Italy

1. Introduction Landslide dams are a worldwide phenomenon with an associated high risk of submergence of the area upstream and of potential dam failure leading to catastrophic downstream flooding. For this reason, landslide damming has been, and continues to be, the object of numerous studies attempting to define pro)

Corresponding author. Fax: q0039-0521-905326. E-mail address: [email protected] ŽA. Clerici..

cesses, risks and mitigative measures. The subject has been considered in general terms by Swanson et al. Ž1985, 1986., Schuster Ž1993a,b, 1994., Schuster and Costa Ž1986., Costa and Schuster Ž1988., and, for Italy by the work of Pirocchi Ž1992. who discusses 97 cases of landslide dams in the Alps, and Casagli and Ermini Ž1999. who analyzed the characteristics of 68 landslide dams in the Tuscan and Emilian part of the northern Apennines. The upper and middle valley of the Parma River, in the northern Apennines in Italy, is situated in an

0169-555Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X Ž 9 9 . 0 0 0 9 5 - 1

2

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

area of recent uplift characterised by narrow fluvial valleys. The high pelitic component of the outcropping rocks leads to the presence of numerous dormant and active landslides of notable sizes, which reach the riverbed and therefore create conditions favouring a blockage. Over the last century, three episodes of damming and lake formation have been recorded in the Parma valley. For this reason, we embarked on a study to verify the possibility of damming and lake formation in association with the main landslides in the Parma River basin. One of these big landslides is situated on the right side of the upper Parma valley, immediately west of the village of Corniglio, approx. 40 km SW of Parma ŽFig. 1.. It is made up of a main body stretching

from an altitude of 1150 m to the bed of the Parma River at 550 m, with an overall length of approx. 3 km and numerous smaller lateral landslides. Its area of 1.7 km2 makes it one of the largest landslides in Europe. The landslide, inactive since 1902, was recently subject to a reactivation in 1994–1996, which led to the destruction of many buildings, the cutting of an important road and the partial occupation of the bed of the Parma River. Because of the economic importance of the event, numerous studies and photogrammetric and cartographic surveys have been carried out to identify the best remedial measures to be taken. The availability of this material and the instability of the current situation make the Corniglio landslide particularly suitable for analyzing the pos-

Fig. 1. Geological map of the Corniglio landslide area Žfrom Larini et al., 1997, modified..

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

sibility of the formation of a damming lake following eventual further reactivation. In this work, we initially analyzed the recent evolution of the landslide to identify the most likely mechanisms of future evolution and then, we simulated reactivation to assess the conditions that would favour blockage. The analysis was carried out with the help of a Geographic Information System ŽGIS., which facilitated quantitative assessment of the recent evolution and subsequently allowed the simulation of a hypothetic future reactivation. To this purpose, two ‘‘shell programs’’ Žalso called ‘‘shell scripts’’ or ‘‘shell procedures’’. with GIS commands were created, which make it possible to simulate the landslide movement by a cellular automata model and to define the characteristics of an eventual damming lake. In this paper, after a synthetic description of the geological and geomorphological features of the Corniglio landslide, there is a description of the quantitative analysis of the recent landslide dynamics. After a description of the two shell programs, the results of the simulation are illustrated. Lastly, there is a discussion of the likelihood of formation and stability of blockage using indices and statistical reports by previous authors.

2. Geological context The Parma valley is situated in the Emilian segment of the northern Apennines, characterised by the overthrust of three successions corresponding to distinct paleogeographical domains ŽElter, 1973.. The autochthonous basal succession ŽTuscan–Umbrian succession. is made up mainly of Oligo–Miocene silty-sandstone turbidites ŽMacigno, Pracchiola Sandstones and Marra Marls of Zanzucchi, 1963.. The intermediate succession ŽSubligurian Nappe. is made up of a Cretaceous–Eocene succession of clays and limestones overlain by Oligocene sandstone turbidites Žthe Canetolo Complex of Barbieri and Zanzucchi, 1963. and the uppermost succession ŽLigurian Nappe. is made up of a Cretaceous Helminthoid flysch ŽMonte Caio Flysch of Ghelardoni, 1961.. The slope on which the Corniglio landslide originated ŽFig. 1. is made up of tectonic units of the

3

intermediate Subligurian Nappe represented, in the immediate area of the landslide by the Lago Melange ´ and the Ponte Bratica Sandstones, both Oligocene in age. The Ponte Bratica Sandstones, thinly layered and locally highly tectonized arenaceous–pelitic sequences, outcrop east of the main body of the landslide. They are lowered by faults toward the Parma River giving the slope a stepped profile. The Lago Melange unit outcrops in vast areas around the land´ slide. This chaotic unit was formed during the Upper Oligocene by tectonic mixing of sedimentary breccias, bodies of Eocene limestone turbidites and bodies of Oligocene arenaceous turbidites ŽLarini et al., 1997; Vescovi, 1998.. The overlying Cretaceous Ligurian Nappe is represented by the Monte Caio Flysch ŽCampanian– Maastrichtian. outcropping on the top of the slope and characterised by very fractured, thick-bedded marly limestone overlying the Subligurian Units in mainly clayey facies. The landslide scarp develops partially in this formation. Widespread detrital cover of diverse origin is present. In the upper part of the slope, old and thick landslide accumulations are present, which originate from the marly–limestone of the Monte Caio Flysch. They extend over the underlying clays and are made up of minute detrital material associated with large rock blocks. The backbone of the village of Corniglio is constituted by the Ponte Bratica Sandstones, which, in correspondence with the built-up area, are strongly fractured, probably due to involvement in an ancient landslide ŽTellini and Vernia, 1996.. The landslides on the east of the main body, illustrated in the right sector of Fig. 1, represent ancient roto-translational movements that upset the original arrangement of the Ponte Bratica Sandstones. In the depletion zone, these movements determined the formation of clearly distinguishable trenches and back-tilting slope facets and a frontal accumulation of over 24 m directly above the Lago Melange. ´

3. Geomorphological context The northern Apennines have been, for some time, well-known for the presence of landslides that are periodically reactivated, involving entire slopes

4

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

and often human works in their movement. The Corniglio landslide has been mentioned by various authors who have described its evolution, demonstrating the recurrent nature of this event. The first mention in history dates back to 1612 ŽAlmagia, ` 1907; Martelli, 1916., on which occasion much damage was done to the houses in the hamlet of Linari and the surrounding areas, with the formation of a damming lake. A subsequent movement in 1740 is recorded by the same authors and by Boccia Ž1804.. The best described historical event is, however, that of May–June 1902 ŽAlmagia, ` 1907; Martelli, 1916., when a detachment near the ridge, linked with the abundant rainfall of the period, provoked an advancement along a front of approx. 1 km in the Parma River, which narrowed the riverbed. Also on that occasion, damage was done to the buildings in Linari and surrounding areas and a new lake was formed, although depleted within a few days by the gradual erosion of the blockage. Unfortunately, none of the authors quoted above supply quantitative assessments of movement velocity or damming lake size. From the 1920s, much hydraulic work was carried out on the streams that delimit the sides of the main body, as well as on the Parma River along the landslide toe. Check dams were constructed and partial bed reshaping was done, repeated in the 1970s together with reforestation on the upper parts of the affected slope. The more recent landslide reactivation of 1994–1996 has once more attracted the interest of experts, which has led to the publication of several papers ŽPellegrini, 1996; Pellegrini et al., 1996, 1998; Tellini and Vernia, 1996; Larini et al., 1997; Gottardi et al., 1998.. In mid-November 1994, after a period of heavy rainfall Ž1077 mm from 1st September to 15th November, of which 700 m fell in September alone, compared to an average annual rainfall of 1500 mm. a reactivation with falls and roto–translational movements along the main scarp occurred. This provoked the retreat of the scarp and the descent of material which came to a halt in the middle portion of the landslide. The resulting damage was limited to the destruction of the conifer wood planted in the 1930s and the only house situated in the middle portion of the landslide. The landslide did not register further movements from mid-1995 until 1st January 1996,

when, following a seismic shock recorded the night before, other detachments along rotational surfaces occurred. The material moved for the whole month of January until it also reached the middle portion of the landslide. On 29th January 1996, following another mild seismic shock, there was a movement of the entire lower part of the landslide body as far as the Parma River, causing the narrowing of the riverbed and breaking some check dams. This movement lasted until the end of April and produced considerable damage. Approximately 70 houses and five ham seasoning plants in the hamlet of Linari, all built on the landslide accumulation area, underwent so much damage that they were declared uninhabitable, jeopardising the local economy. From April 1996, there followed a long period of inactivity until Autumn 1998 when modest movements occurred in the middle and lower portions of the landslide, which continue until today ŽDecember 1998.. The Corniglio landslide is thus a good example of an intermittent landslide, with slow kinematics and a reactivation time of about one century. This landslide can be considered complex, because it evolves with falls and roto-translational slides causing scarp retreat with planar slides in the lower-middle portion. Locally, shallow earth flows are present, which involve material thicknesses of 10–20 m ŽPellegrini, 1996.. The recent reactivation of the landslide appears to be connected to different factors, some of which are preparatory and others triggering, linked to the lithological and structural conditions of the substratum, to the heavy rainfall in the area, to the decay of geomechanical characteristics of the material and, to a lesser degree, to the mild seismic shocks which affect this sector of the Apennines from time to time. On the other hand, the undermining of the toe by the Parma River appears to be of little relevance ŽTellini and Vernia, 1996.. 4. Quantitative analysis of morphological evolution in 1994–1996 4.1. Data acquisition The analysis was carried out by Geographical Resources Analysis Support System ŽGRASS., a pub-

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

lic domain GIS originally developed by the US Army Construction Engineering Research Laboratory ŽShapiro et al., 1993; USACERL, 1993. and running on different platforms under one of the UNIX family operating systems. Before the 1994–1996 reactivation, only an aerial photographic survey of 1976 and the derived map at 1:10,000 scale with a 5 m contour interval, were available for the study area ŽRegione EmiliaRomagna, 1976.. Following the reactivation, further aerial photographic surveys were made, from which topographic maps of the immediate landslide zone were obtained on a scale of 1:5000 and a 2-m contour interval. After the 1996 event, 23 transverse and longitudinal seismic profiles of the main body of the landslide were carried out, making it possible to assess the depth of the bedrock and, therefore, the thickness of the displaced material which appears very variable, between 30 and 120 m. With GRASS, in an early phase of the current work, the altitude data, contour lines and spot heights of the 1976 map and two later maps, produced on 16th January 1996 and 13th December 1997, respectively, were digitized and turned into raster format with 5-m side cells. Three Digital Elevation Models ŽDEM. were thus obtained, reproducing the situation in three different stages of the landslide evolution. The first DEM, referring to 1976, reproduces the situation after the 1902 event and before the 1994 reactivation. The second, dated 16th January 1996, reproduces the situation after the events of November 1994 and January 1996 which provoked detachments and movements in the upper and middle part of the landslide. The third, dated 13th December 1997, reproduces the final situation with the descent of the lower part of the landslide as far as the bed of the Parma River. From the same topographic maps, the following were digitized: the landslide boundary, hydrography, perimeters of most of the buildings on the landslide and close-by, and the main road which crosses the body of the landslide in its lower part. The digitized elements are illustrated in simplified form in Fig. 2. The elevation values along the seismic profiles drawn-up in 1996 were digitized and the basal surface of the displaced material was reconstructed by interpolation. A longitudinal profile of this surface is reproduced in Fig. 3. In order to underline the mor-

5

phological features of the landslide in the various phases of its evolution, three slope maps were generated from the DEMs. The analysis of the aerial photos, the three DEMs and the three slope maps, make it possible to singleout portions of the landslide with different morphological features, which are distinguishable on the basis of slope values and the position on the body of the landslide. These five Morphological Units ŽMU., numbered from the top to the bottom ŽFig. 2., are made up of: — An even slope crown area, delimited upslope by a secondary scarp of moderate height ŽMU1.; — The main scarp, subject to falls and roto– translational movements, with high slope values ŽMU2.; — A zone of transition and partial accumulation of the material descending from MU2; this unit ŽMU3. shows a slightly concave profile in the left portion of the landslide Žsee profile in Fig. 3. but a substantial depression in the right portion Žsee Fig. 2.; — An accumulation zone, of which the uppermost part ŽMU4. has a limited slope and a reduced transverse section, while the remainder ŽMU5., of wide extent, is characterised by an alternation of sub horizontal surfaces separated by scarps of moderate size and is delimited downslope by a fluvial erosion scarp. The investigation was carried out on the main body of the landslide as defined in Fig. 1, which is the only one subject to consistent movements. Areas like the wide landslide to the east of the main body and the western sector of the village of Corniglio, subjected to extremely limited shifts, measurable in cm, were not considered. 4.2. Data processing and analysis To highlight the quantity of material underlying the single MUs and the entire landslide at the three stages under study, we subtracted the basal surface of the displaced material from each of the three DEMs. The results are shown in Table 1, where the areal extension and the average slope value of each MU are also reported. To quantify material movement, we also subtracted from each DEM the earlier

6 A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23 Fig. 2. Simplified topographic maps of the three phases considered in the analysis. Map A shows the situation in 1976, map B refers to the 16th Jan. 1996, map C to the 13th Dec. 1997. The 5 Morphologic Units are showed. Contour interval is 5 m. The cross-section A–B is shown in Fig. 3. See Table 1 for characteristics of the various MUs.

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

7

Fig. 3. Cross-section of the Corniglio landslide along the trace A–B in Fig. 2C. The landslide surface was obtained by the Digital Elevation Model, the bedrock surface from interpolation of seismic profiles.

DEM. We thus obtained three maps ŽFig. 4. showing positive and negative elevation differences between the three stages under study. For each of these last three maps, we then calculated the algebraic sum of the volume differences. This ‘‘mass balance’’ represents the difference between accumulation and removal of material in the various MUs and in the whole landslide in the three stages of its evolution. The results are shown in Table 2. 4.2.1. Discussion of Õolumetric changes Analysis of the data in Table 1 shows that the area, and consequently the volume, of the underlying material of the entire landslide increase progressively from 1976 to 1997, reaching a maximum of 1756.3

m2 = 10 3 and 106.373 m3 = 10 6 respectively. Nevertheless, the mass balance is always negative, as shown in Table 2, highlighting progressive material loss. The deficit of 557.8 m3 = 10 3 between 1976 and 1996 is justified by various factors. First of all, the map of 1976 does not show the situation immediately prior to the reactivation of 1994, nor does the 1996 map show the situation immediately afterwards. In the 18 years between 1976 and 1994, there was removal of material by surface runoff and by linear erosion by the two streams bordering the landslide. After 1976, a certain amount of erosion at the landslide toe should also be attributed to the construction of five check dams along the Parma River which raised the bed, facilitating lateral shift-

Table 1 Characteristics of the Morphological Units ŽMU. and of the whole landslide in the three phases considered in the analysis 1976

MU1 MU2 MU3 MU4 MU5 whole landslide

1996

1997

Average slope Ž8.

Area Žm2 = 10 3 .

Displaced material Žm3 = 10 6 .

Average slope Ž8.

Area Žm2 = 10 3 .

Displaced material Žm3 = 10 6 .

Average slope Ž8.

Area Žm2 = 10 3 .

Displaced material Žm3 = 10 6 .

19.9 30.3 17.4 5.6 10.4 12.6

66.9 67.9 239.2 65.8 1243.2 1683.1

2.737 4.014 10.173 4.540 81.274 102.737

20.9 28.7 16.0 8.0 10.9 12.8

64.3 120.2 166.2 105.4 1248.3 1704.5

2.716 5.296 6.394 6.682 83.202 104.290

21.4 27.9 16.7 8.9 10.6 12.7

66.8 132.0 162.1 104.0 1291.3 1756.3

2.748 5.446 5.140 5.775 87.263 106.373

8 A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23 Fig. 4. Elevation difference in the landslide main body for the time intervals 1996–1976 ŽA., 1997–1996 ŽB. and 1997–1976 ŽC.. See Table 2 for the mass balance of the various Morphological Units and of the whole landslide in the three time intervals.

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23 Table 2 Mass balance of the Morphologic Units and of the whole landslide in the three periods considered in the analysis

MU1 MU2 MU3 MU4 MU5 whole landslide

1976–1996 Žm3 =10 3 .

1996–1997 Žm3 =10 3 .

1976–1997 Žm3 =10 3 .

y68.1 y1734.7 y426.1 482.6 1188.5 y557.8

y108.9 y516.2 y1185.2 y522.1 2104.2 y228.2

y176.6 y2250.2 y1647.3 y26.4 3328.3 y772.2

ing and erosion, as can be seen by the fact that the position of the landslide toe on the 1996 map is slightly further back ŽFig. 4A.. Areal and linear erosion are even more accentuated in the period 1994–1996, i.e., after reactivation, helped by the destruction of the widespread woodland and the availability of loose material with a large clay component. In fact, the deficit should be greater considering the volume increase due to the dilation of landslide material. This quantity is, however, difficult to define because of the shortage of studies on the subject ŽCruden and Varnes, 1996, pp. 42–43.. The deficit of 228.2 m3 = 10 3 between 1996 and 1997 is, on the other hand, due substantially to erosion of the advancing toe by the Parma River, although part of the removal should be attributed to areal and linear erosion on the landslide body. In fact, whereas up to 1996, the shifts almost exclusively concerned the upper and middle parts of the landslide ŽMU1–4., the movement of January 1996 caused MU5 to advance into the bed of the Parma River. To highlight the horizontal component of the shift, we combined on a single map: the perimeters of the buildings, the road crossing the lower part of the landslide and the bed of the Parma River in their 1996 and 1997 positions ŽFig. 5.. Whereas the position of the buildings outside the perimeter of the landslide is unchanged, those within the perimeter undergo visible shifts. Measurement of the displacement provides more of less identical values for all the buildings, varying from 25 to 26 m. The uniformity of the movement is underlined even more clearly by the parallel shift of the road.

9

The advancement of the toe is less accentuated. In fact, from the calculations made by dividing the area between the 1997 and 1996 toe positions by the average toe width, the shift comes out at an average of 12 m. Accepting that a slight diminution in advance can be attributed to the broadening of the landslide foot and, therefore, to the distribution of the material over a wider area, it is, nevertheless, evident that the difference of 13 m between the shift of the buildings and that of the toe is mainly attributable to the erosion the Parma River. It is worth observing that the width of the riverbed in 1996 in a long tract near the left side of the toe, is of approximately 25 m and therefore equivalent to the landslide advancement. Therefore, even in the absence of erosion, the toe would probably have just been able to reach the opposite valley slope.

Fig. 5. Buildings, road and Parma River positions on 16th Jan. 1996 Žlight gray. and on 13th Dec. 1997 Ždark gray. and landslide boundaries. The buildings and road show a uniform displacement Ž25–26 m.. The 1997 river position and the landslide limit in the toe zone Žcontinuous line. show a less marked advancing respect to 1996 position Ždotted line. due to the river erosion.

10

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

Analysis of Fig. 4A, which shows the differences between the 1996 and 1976 situations, and of the data in Tables 1 and 2, clearly shows a notable amount of material removal in the upper part of the landslide. This is particularly the case in MU2, which presents notable broadening both upslope and laterally and a conspicuous deficit of 1734.7 m3 = 10 3, and in the upper part of MU3, where the maximum negative difference of 43 m is reached. Some of this material is accumulated in the lower part of MU3, where there is the maximum positive difference of 27 m, and in the areas immediately below, with no landslide toe advancement. MU1 extends toward SE, where it presents negative differences, testifying the descent of material that partially accumulates in the lower part of the same MU. In the lower part of MU5, presenting a surplus of material of 1188.5 m3 = 10 3, the elevation differences vary little from zero, with positive values in the eastern sector and negative ones in the western sector. This situation reflects the movements that have followed one another since the beginning of reactivation until the 1st of January 1996. In fact, the detachment of new material from the main scarp in 1994 did not, in the same period, cause a toe advancement, but only a material redistribution in the upper-middle part of the landslide. The relatively limited positive values in MU4 and in the upper part of MU5 reflect the beginning of the material descent in the accumulation zone from 1st to 16th of January 1996, the date on which the topographic map was made. Fig. 4B, the difference between the 1997 and 1996 situations, and the relative mass balances in Table 2, highlight the descent of the material previously accumulated at the base of the main scarp, causing the bulging of the accumulation zone with the advancement of the toe into the bed of the Parma River, as can also be seen by the position of the maximum values of y27 and q24 m. The mass balances are significant in as much as they testify the transferral of material from the higher MUs ŽMU1–4. to the lowest ŽMU5.. In particular, from MU3, presenting a deficit of 1185.2 m3 = 10 3 , and from MU4, from which practically all previously accumulated material is removed. In MU5 an overall 2104.2 m3 = 10 3 arrived. As far as MU1 is concerned, the presence of negative differences over the whole area affirms the general lowering of the MU with the

consequent accentuation of the upslope scarp. This also clearly emerges from an analysis of the aerial photos. The overall shifts are highlighted in Fig. 4C, which reproduces the difference between 1997 and 1976 and therefore between the situation before the landslide reactivation and the final one. Maximum material removal took place in the upper part, where there is the greatest elevation difference of y49 m, whereas, the greatest positive difference of q22 m at the foot bears witness to the advancement of the toe into the bed of the Parma River. There is a notable overall increase of MU2 and MU4, which practically double their extension, mainly at the cost of the intermediate MU3. MU2 and MU3 present conspicuous deficits of 2250.2 m3 = 10 3 and 1647.3 m3 = 10 3 respectively. MU5 presents a surplus of 3328.3 m3 = 10 3 corresponding to the overall quantity of material originating from MUs 1–4 minus the quantity of 772.2 m3 = 10 3 lost through erosion caused by the Parma River, as discussed above. This quantity of material is largely responsible for the bulging of MU5, whose average height goes from 670.4 m in 1976 to 673.0 m in 1997, with no significant slope variation, and, only to a small degree, for the average toe advancement of 12 m. It is worth noting how the mass balance only involves a small fraction of the displaced material, as can clearly be seen by comparing the data in Tables 1 and 2. In other words, only a small part of the displaced material contributes to the morphological modifications.

4.2.2. Considerations on future eÕolution Any attempt to predict the future evolution of the landslide must consider some important aspects of our analysis. Ž1. The retrogressive evolution of the upper part of the landslide and the deferred homogeneous advancement of the toe portion. Ž2. The presence of a scarp in the crown zone at the uppermost border of MU1 which retreated and grew in 1994–1996 period. In view of the retrogressive evolution of the landslide, it seems plausible that future detachment could take place along the surface delimited by the scarp with the consequent descent of material underlying MU1 and MU2.

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

Ž3. Only a small part of the mobilized material contributed to the advancement of the accumulation zone. Most of the material did not complete its descent and is blocked at the level of MU5. Accumulated material contributed to MU4 upslope extension and slope angle increase, while MU5 underwent an increase of its average height of 2.6 m. The situation, therefore, appears to have an extremely precarious equilibrium and it would seem plausible that any future reactivation would lead to the completion of the descent of this material. We therefore decided to verify the effects of these two movements, assessing if the descent and advancement of the material is able to produce the river blockage and, if so, assess its extension and that of the relative lake. To simulate the displacement of the material various GRASS commands were used, organized in a shell program based on a cellular automata model. A second shell program checks for a complete blockage and computes dam and lake dimensions.

5. Simulation programs In the landslide movement simulation field, cellular automata have been used by a small numbers of authors ŽBarca et al., 1986, 1987; Deangeli et al., 1994; Di Gregorio et al., 1994a,b, 1996; Segre and Deangeli, 1995.. In all these cases, deterministic models were used, requiring the introduction of geotechnical parameters to be processed by empirical geotechnical laws. The extreme variability in threedimensional space and in time of geotechnical characteristics, which hinders their assesment over wide areas, and the complexity of calculation, require considerable computing power and limit the extension of the study area andror the number of cells processed. For example, for the debris flow simulation of Mount Ontake ŽJapan. ŽDi Gregorio et al., 1996. a network of 32 transputers was used to analyse an area of 11 = 3.5 km, subdivided into a matrix of 224 = 70 cells. The same authors point out the problems in defining the geotechnical and environmental parameters used in the simulation, some of which are also subjected to change during the evolution of the landslide. In all cases, specific com-

11

puter programs were implemented. Less attention has been paid to GIS use in landslide movement simulation. In a paper by Wadge Ž1988. the potential of GIS in the study of slope stability and gravity flow kinematics is described. Although the problem is dealt with on a deterministic basis in this case too, mention is made of the possibility of simulating a flow through a series of incremental additions of mass to the flow front. Specifically concerning the creation of shell programs with cellular automata models using GRASS commands, the simulation of a water flow across a landscape is described in a GRASS handbook by Albrecht Ž1992.. In the present work, our aim is to define the characteristics of an eventual river blockage following the reactivation of a pre-existing landslide, in accordance with evolution modes considered to be plausible on the basis of analysis of its most recent evolution. Therefore, the geotechnical characteristics of the involved materials are not directly considered; rather, the importance of the movement and, therefore, the quantity of mobilized material are defined by using geomorphological criteria. In fact, the descent of material and eventual toe advancement are induced by the reduction of the average slope of the material. In other words, starting with a landslide, characterized by different average slope values in its different geomorphological elements and assuming certain evolution modes, we aim to define the final setting of the landslide following slope angle diminution of the geomorphological elements. Material advancing must obviously occur with adaptation to the riverbed and slope morphology and guarantee morphological compatibility with the rear portion of the landslide. The simplicity of the problem allows the use of the functions of GIS GRASS and standard computing resources. Two shell programs were created which, apart from the shell commands, use GRASS commands and the ‘‘gawk’’ language ŽClose et al., 1993. and run on personal computers or workstations under UNIX. The first, AUT16, simulates landslide material descent through a cellular automata model, while the second, DAM10, verifies blockage presence and determines the extension and volume of blockage and damming lake. AUT16, simulates a cellular automata model as the study area is subdivided into square cells and

12

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

each cell is characterized by four quantities, or substates, that are iteratively modified in relation to the substates of the eight neighbouring cells. The substates of each cell are represented by the following. Ž1. Qt, the elevation of the topographic surface. Ž2. Qs, the elevation of the ‘‘limit surface’’, i.e., the surface forming the lower limit of the removable material and which generally coincides with the base of the displaced material. It, therefore, represents the lowest elevation that the cell can reach for material removal. Ž3. Si, the slope angle. Ž4. Sf, the final slope angle, i.e., the slope value the cell must acquire at the end of the simulation. The input data that define the four characteristics consist of three raster maps covering the entire study area: the elevation map ŽDEM., the limit surface map and the final slope angle map. The latter is created by the user who assigns to each portion, or morphological unit, of the landslide the slope value to be reached at the end of the simulation. The fourth characteristic, i.e., the slope angle Si, is computed by the program from the DEM. At each step, the elevation Qt of each cell is modified. More precisely, it increases by an INC thickness for each of the eight neighboring cells with a higher elevation and diminishes by the same quantity for each of the cells with a lower elevation, i.e., Qt s Qt q Nh ) INC y Nl ) INC, where Nh is the number of higher cells and Nl is the number of lower cells in the neighbourhood. The INC value is calculated in the program as 1r4 of the average elevation difference between all the cells in the landslide area. At each step, the cell under study, or central cell, therefore receives a quantity of terrain of volume L2 ) INC Žwhere L is the cell side. from each of the neighbouring higher cells and yields the same volume to each of the lower cells. This transferral only takes place if: Ža. The central cell has an elevation Qt equal to or less the quantity 2 ) INC than higher cells and equal or higher the same quantity than neighboring lower cells, i.e., Qt F Qh y 2 ) INC and Qt G Ql q 2 ) INC where Qh and Ql are the elevations of each of the higher and lower cells, respectively. This rule prevents the cell from reaching an elevation higher than the neighboring higher cell or lower than the neighboring lower cell, following the addition or subtraction of material.

Žb. The cell stays above, or at least at the same level as, the limit surface, i.e., Qt y INC G Qs. In this way only the material between the topographic surface and the limit surface can be removed. Žc. The slope Si of the neighboring cell is greater than the final slope Sf set for that cell, i.e., Si ) Sf. Thus, the neighboring cells that have reached the preset slope value do not participate in transferral of material and, therefore, do not contribute to the altitude modifications of the central cell, unless other material arrives and causes a slope increase. After each step, therefore, a new DEM is created, by means of which the new slope Si of each cell and the overall volume of the displaced material are recalculated. If the resulting volume is lower than the volume mobilized in the previous step, the INC value is diminished to allow the displacement of material among cells with smaller altitude difference. INC is progressively reduced until it reaches the minimum value INCMIN which is calculated as a fraction of the cell size by the program. Therefore, this dynamic model simulates a progressive downward sliding of material, which initially affects the steeper parts of the landslide characterized by greater elevation differences and, progressively, with the diminution of INC, the less steep parts until the final setting is reached in which the different portions of the landslide achieve the slope angle assigned by the user. This method has some general limitations as well as more specific ones. Among the former, is the absence of the erosive action of the flowing water along the advancing landslide toe. This action could be particularly important, especially where high river discharge is concerned. Also, the possibility that the material runs up the slope on the opposite side of the river is not taken into account.Thus, there is an implicit concept of movement with low kinetic energy. This limitation, however, is not of decisive importance in the case of the Corniglio landslide and the other large landslides of the Parma valley, all of which appear not to be so rapid to run up a slope. As far as specific limitations are concerned, it should be specified that the conditions stated in the previous points a, b and c make it possible to achieve the final setting with a certain degree of approximation. In the areas with an initial slope angle lower than the final one, it is possible that this is never reached, unless

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

the arrival of material from above increases the elevation difference among the cells and therefore the slope angles. Lastly, it is also possible that, in limited areas, counterslopes form, since in the algorhithm, the slope angle is considered but not its direction. The possibility that this circumstance occurs depends on many factors, among which mainly the presence of counterslopes in the initial morphology. Despite these limitations, in practice the program proved to be efficient enough, enabling landslide movement simulation over a wide area in a relatively short time. As for the running time, this obviously depends on the the area extension, the cell size, the

13

number of steps necessary to reach the final result and the computer used. For the area studied in this work, adopting 10 m side square cells, a 344 = 300 matrix is obtained and running time is 7 s for each step using a Pentium Pro 200MHz with a RAM of 64MB under the Linux operating system. Running time is reduced to 3 s for a 20 m cell side. The overall number of step increases as the quantity of material that can be moved increases and the INCMIN value decreases. For the 16 simulations presented in this work, adopting 10 m side cells and an INCMIN value of 10 cm Ži.e., 1r100 of the cell side., the average number of steps was 2350, corresponding to an average running time of 4.6 h.

Fig. 6. Output example of DAM10 shell program referring to the simulation 2.4.

14

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

The second program, DAM10, is much simpler and faster than the preceding one. Its task is to calculate the amount of landslide material in the riverbed, verify the presence of a blockage and, if affirmative, calculate the damming lake size. Its input is the DEM resulting from AUT16 as well as the files containing the water stream and the riverbed boundaries. Once the cell with the highest elevation along the landslide toe has been identified, the DEM is virtually intersected by the horizontal plane corresponding to that elevation. If this plane is interrupted

in correspondence with the cell, it means that the landslide has led to a complete blockage of the riverbed and the part of the horizontal plane located upstream reproduces the surface of the damming lake. If, on the other hand, there are lower elevations beyond the landslide toe, the horizontal plane stretches out without interruption upstream and downstream of the cell, demonstrating the absence of complete blockage. The area, volume and maximum and average thicknesses of the material in the riverbed are then calculated. In case of a blockage, the area,

Fig. 7. Block diagram of the simulation 2.4 obtained from the output of DAM10 shell program.

16 16 16 16 16 8 8 8 8 8 8 15 14 13 12 11

7 6 5 4 3 8 8 8 8 8 8 7 6 5 4 3

9 8 7 6 5 10 9 8 7 6 5 9 8 7 6 5

5

y1.609 y1.479 y1.578 y1.807 y2.612 y2.696 y2.696 y2.693 y2.689 y2.697 y2.698 y0.581 y0.828 y1.068 y2.459 y2.635

1

y0.913 y0.968 y1.089 y1.260 y4.752 y4.108 y4.090 y3.965 y3.801 y4.676 y5.294 y0.687 y1.064 y1.413 y3.744 y4.900

2

1.854 1.490 1.232 1.036 y3.385 2.072 1.961 1.776 1.769 0.063 y4.903 0.539 0.375 0.160 y2.051 y4.684

3

MU mass balance Žm 3 = 10 6 .

0.489 y0.036 y0.131 y0.335 y4.858 2.752 2.395 1.819 1.138 0.490 y4.738 0.156 0.090 0.026 y1.026 y4.967

4

y1.062 y1.035 y0.850 y1.100 y3.647 0.365 0.413 0.521 0.438 1.362 y3.264 y0.711 y0.555 y0.223 2.942 y4.006

5

1.242 2.028 2.416 3.466 19.254 1.615 1.484 2.320 3.069 5.484 20.897 1.285 1.982 2.518 6.338 21.192

115.2 154.0 172.0 224.6 584.0 90.6 121.0 161.8 196.2 272.2 608.1 118.0 150.7 177.1 294.9 609.3

External Dam Žm 3 = 10 6 . Area Žm 2 = 10 3 . 1.177 1.890 2.231 3.188 13.741 0.821 1.262 2.105 2.788 4.555 14.653 1.214 1.838 2.320 5.223 14.701

Volume Žm 3 = 10 6 . 27.77 33.01 33.64 36.96 59.08 25.66 28.09 34.01 37.01 46.05 59.49 28.10 33.81 34.25 48.28 59.56

Max. thickn. Žm . 10.22 12.27 12.97 14.20 23.53 9.07 10.43 13.01 14.21 16.74 24.10 10.29 12.20 13.10 17.71 24.13

Aver. thickn. Žm .

Lake

31.4 53.9 82.1 89.8 390.3 30.3 25.6 62.4 97.5 150.1 440.8 30.9 38.4 81.4 154.3 422.9

Area Žm 2 = 10 3 .

0.098 0.129 0.312 0.384 3.442 0.070 0.067 0.179 0.395 0.833 4.146 0.064 0.125 0.241 0.959 3.998

Volume Žm 3 = 10 6 .

578 580 583 585 605 576 578 581 584 590 608 577 580 582 591 607

Surface elev. Žm a.s.l. .

572 574 574 575 581 571 574 574 574 576 582 573 574 574 577 582

Bottom elev. Žm a.s.l. .

6 6 9 10 24 5 4 7 10 14 26 4 6 8 14 25

Max. depth Žm .

3.11 2.40 3.80 4.27 8.82 2.32 2.61 2.87 4.06 5.55 9.41 2.07 3.24 2.95 6.22 9.45

Aver. depth Žm .

In the ‘‘Displaced material’’ column, the total amount of material moved during simulation is shown. MU slope represents the slope angle adopted for the various Morphological Units. The ‘‘MU mass balance’’ is the quantity of material lost Žnegative values . or gained Žpositive values . by each Morphological Unit. In the next column, ‘‘external’’, the quantity of material advancing along the landslide front and provoking the river damming is reported. The characteristics of the resulting dams and lakes are given in the next columns. Simulations 1.1–1.5 refers to Fig. 8, 2.1–2.6 to Fig. 9, and 3.1–3.5 to Fig. 10.

16 16 16 16 16 8 8 8 8 8 8 26 25 24 23 22

7.189 7.831 7.996 8.960 30.911 10.710 11.003 11.093 11.087 12.808 33.348 5.536 6.773 7.355 13.390 33.392

1.1 1.2 1.3 1.4 1.5 2.1 2.2 2.3 2.4 2.5 2.6 3.1 3.2 3.3 3.4 3.5

16 16 16 16 16 8 8 8 8 8 8 19 18 17 16 15

Displaced MU slope Ž8. material 1 2 3 4 Žm3 = 10 6 .

Sim. N.

Table 3 Results of the three groups of simulations of landslide movement

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23 15

16

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

volume, maximum and minimum elevations, maximum and average depth of the damming lake are also calculated. Lastly, a map is produced showing the damming lake, hydrography and contour lines ŽFig. 6.. The results of the processing can also be used to make block diagrams ŽFig. 7.. Running times are extremely short; the process to produce the map in Fig. 6 took 2 min.

6. Simulations The relatively short running times of the two shell programs made it possible to carry out an extremely high number of simulations. In this work, we have reported only those simulations which appear most plausible in relation to the most recent evolution of the landslide.

As described above, one can hypothesize that eventual reactivation of the landslide would cause the detachment of the highest portion, with the retreat of the main scarp Žas occurred in 1994. and the descent of the material along the accumulation zone Žas occurred in the 1996 event.. An initial simulation was therefore carried out assuming a detachment in correspondence with the scarp forming the upslope limit of MU1 and the descent of material underlying MU1 and MU2. For this purpose in the final slope angle map, the current slope values of these two MUs of 21.48 and 27.98, respectively, were reduced to a common value of 168, the same as the lower MU3 Žrounded down.. MU4 and MU5 were assigned, in the final slope angle map, the current values of 88 and 108. The latter value of 108 was also assigned to all cells outside the landslide area so that the material advancing along the toe should have

Fig. 8. Landslide dam and lake for the first group of simulations Ž1.1–1.5.. Dotted line shows landslide boundary at the 13th Dec. 1997.

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

morphological continuity with MU5. As far as the limit surface map is concerned, the cells inside the landslide area were assigned the elevations of the displaced material basal surface, which was constructed, as described above, by interpolating values along the seismic profiles, whereas, externally, the cell elevations correspond to current topography. Thus, only the material within the landslide can be removed, whereas in the external areas only accumulation can take place. This first simulation, despite the considerable quantity of material originating from MU1 and MU2, did not lead to blockage of the Parma River. In fact the material accumulated mostly on MU3, MU4 and MU5, increasing their altitude and causing a modest toe advancement, in a manner similar to the 1994–1996 reactivation. This suggests that, for the toe to advance consistently, maintaining

17

a slope of 168 in MUs 1–3, the mobilization of the great quantity of material underlying MU4 and MU5 is necessary. This mobilization was simulated by reducing the two MU slopes by 18, i.e., to 78 and 98, respectively. The results, shown in the first line of Table 3 Žsimulation 1.1., demonstrate that the slope reduction leads to the formation of a damming lake, albeit a small one. It clearly results that the material advancing beyond the toe Ž1.242 m3 = 10 6 . originates mostly from MU5 Ž1.062 m3 = 10 6 . and almost entirely constitutes the accumulation in the riverbed, i.e., the dam volume Ž1.177 m3 = 10 6 .. With a further progressive reduction of the slopes of MU4 and MU5 to 38 and 58 respectively, further toe advances were obtained with the progressive increase of the impoundment size Žsimulations 1.2–1.5 in Table 3.. In the simulation which reduced the two

Fig. 9. Landslide dam and lake for the second group of simulations Ž2.1–2.6.. Dam and lake of simulation 2.4 are shown in Figs. 6 and 7.

18

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

slope values further, the material flowed along the bed of the Parma River outside the study area without provoking a significant increase in accumulation thickness. Using the output of DAM10 program, for each of the simulations, a map of the final part of the accumulation was produced highlighting the damming lake. Buildings present near the north-western border of the landslide were also represented, to assess their involvement in the landslide and upstream flooding ŽFig. 8.. To assess the effects of the descent of a greater quantity of material from the higher MUs, a second group of simulations was carried out assigning the slope value of MU4, i.e., 88, to the first three MUs. As indicated in Table 3, the descent of material is sufficient to form a damming lake even when MU5 has the current slope value of 108 Žsimulation 2.1.. The large quantity of material coming from MU1 and MU2 not only contributes to the positive balance of the lower MUs, but also allows a riverbed accu-

mulation of 0.821 m3 = 10 6 . In this series of simulations too, the slope of MU5 was progressively reduced to a minimum value of 58, to assess the effects caused by mobilization of the material underlying the same MU Žsimulations 2.2–2.6.. The final portion of the landslide and the damming lakes are shown in Fig. 9. In this case, too, a further slope reduction below 58 caused the material to flow away along the bed of the Parma River. In a third series of simulations, we considered a more general case of material descent and consequent toe advancement with the progressive reduction of all MU slopes. The results are shown in Table 3 Žsimulations 3.1–3.5. and in Fig. 10. An overall analysis of the simulations carried out clearly indicates that mobilization of the material underlying MU5 is necessary for the formation of a damming lake. The sole exception is simulation 2.1, in which the descent of a large quantity of material from MU1 and MU2 is necessary for the creation of

Fig. 10. Landslide dam and lake for the third group of simulations Ž3.1–3.5..

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

a modest damming lake. In effect, MU5 alone includes more than 80% of the displaced material and, therefore, even small decay of geomechanical parameters entailing slight slope reductions can produce the descent of a substantial quantity of material. Clearly, it should be considered that the greater the slope reduction, the greater must the decay of geomechanical parameters be. Therefore, the probability of the conditions predicted by the simulations occurring, decreases progressively with the decrease of slope angle assigned to MU5. Furthermore, it should be specified that the simulations carried out in this study concern the whole landslide, or most of it. We cannot exclude the possibility that small local movements, like falls along the toe or shallow earth flows, can reach the riverbed. Nevertheless, the material involved would appear presumably to be of a limited quantity and therefore easily eroded even with low river discharge. Even in the unlikely case of the formation of blockage, this would be of limited height and the lake would be relatively small. As far as the effects of flooding upstream are concerned, it is possible to verify, from the surface elevation of damming lakes in Table 3 and from Figs. 8–10, that even the smaller lakes Žwith the sole exception of simulation 2.1. cause the submersion of the bridge across the Parma River at 577 m, immediately upstream of the blockage. With the increase of mobilized material, there ensues not only the submersion of a growing number of buildings, but also many of these come into direct contact with the landslide material.

7. The possibility of blockage formation and its stability In the simulations described above, the occurrence of blockage and its characteristics were simply related to how much material is mobilized, with the only constraint of a general morphological compatibility. Obviously, blockage formation and stability do not exclusively depend on the amount of available material, but also on other factors that can be summerized as follows: 1. Landslide velocity 2. Riverbed width

3. 4. 5. 6.

19

River water discharge Grain size and texture of the blockage material Dam size and geometry Impoundment depth and size.

Despite the large number of landslide blockages that have been identified and studied throughout the world Žmore than 600., at present there are no multivariate statistical analyses for assessing the joint effect of the abovementioned factors on the occurrence of the phenomenon and on the degree of blockage stability. This is probably because of the difficulty, particularly in older landslides, in measuring all the variables, and especially landslide velocity and river water discharge. Studies so far have limited themselves to analysing the influence of each individual factor, or at most, a couple of factors. Sometimes those variables which are particularly hard to determine have been replaced by other variables which are easier to assess and which the authors consider to be closely related to those missing. As far as the possibility of blockage formation is concerned, Swanson et al. Ž1985, 1986. proposed an ‘‘Annual Constriction Ratio’’ ŽACR., the ratio between the speed of movement and the channel width. According to a study carried out in the USA ŽOR., for blockage to form, this ratio should reach a value of at least 100. In the Corniglio landslide, as mentioned above, the maximum velocity recorded is 80 cmrday on the days immediately after the seismic shock of 29r1r96; it then went down to 25 cmrday during February–March 1996. This velocity is equal to 7.5 m per month Ž90 mryr. which means that the landslide would be in the slow movement class on the Cruden and Varnes’ Ž1996. landslide velocity scale. The riverbed width, as stated above, is approximately 15 m along most of the landslide toe and reaches a minimum of 10 m in some sectors. Only further downstream does it widen suddenly, reaching a width of over 100 m. Even if the minimum width of 10 m is used, the resulting ACR would be 9, which is far smaller than the proposed limit of 100. Obviously, blockage results as being even more improbable using the average riverbed width, which is considerably greater. Therefore, according to this parameter, a damming lake should not form. Any eventual reactivation of the Corniglio landslide would

20

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

have to occur with a considerably higher velocity. To obtain an ACR of 100, velocity should be 1500 mryr Ž4.17 mrday., i.e., over 11 times greater than in 1996. This conclusion is also confirmed in a study of damming lakes in the Tuscan–Emilian Apennines, i.e., in the same region as the Corniglio landslide and, therefore, the same lithological and morphoclimatic conditions ŽCasagli and Ermini, 1999.. The 68 damming lakes studied by these authors all formed as a result of movements from moderate Ž13 mrmonth to 1.8 mrhr. to very rapid Žfrom 3 mrmin to 5 mrs.. None of the damming lakes formed as a result of a slow movement. As for grain size, a fine grain size presents high erodibility and is clearly less likely to form a blockage than coarse material. In the case of the Corniglio landslide, the material at the toe is made up mainly of sandstone and limestone blocks measured in centimeters, scattered in a mainly pelitic matrix. In the study by Casagli and Ermini Ž1999., the only work to propose a statistical analysis of damming lake formation on the basis of grain size and texture, only six out of the eighteen landslides with fine grain size and a matrix-supported texture gave rise to damming lakes. In relation to dam lifetime and, more precisely, the probability of dam failure, Swanson et al. Ž1985, 1986. proposed a geomorphic index, applied later to the Italian Alps by Pirocchi Ž1992. and, with some modifications, to the Tuscan–Emilian Apennines by Casagli and Ermini Ž1999.. According to those authors, the ratio between dam volume Ždirectly related to dam size. and the drainage basin area Žstrictly related to river discharge and valley width. provides a criterion for discriminating between failed and not failed dams. This relationship is expressed by the ‘‘Blockage Index’’, Ib: Ib s log Ž VdrA b . where Vd represents the dam volume in cubic meters and A b the basin area in square kilometers. Plotting a bilogarithmic chart of the 68 cases of landslide dams in the Tuscan–Emilian Apennines, Casagli and Ermini Ž1999. identify four regions. The dam stability region has Ib values greater than five, while a dam instability region is defined by values under four. Values between four and five indicate a region

with dams of uncertain evolution, while the region with values below three comprises of those cases where blockage does not occur. Applying the index to the different blockages simulated for the Corniglio landslide gives the results shown in Table 4. The Ib, in almost all cases, gives values between four and five, and therefore in the uncertain evolution region. Only the last simulation in each of the three groups has a value above five. It should, however, be noted that this index does not account for the shape of the blockage, i.e., its dimensions along and across the river. In this respect, useful indications regarding blockage stability can be obtained by comparing the blockage extension along the river with the width of an artificial embankment of the same height ŽCosta and Schuster, 1988; Schuster, 1993b., without forgetting that natural blockages lack those structures of defence and stability that characterize artificial dams and the heterogeneous, unconsolidated nature of their material. Assuming a very conservative ratio between height and width of 1 to 8 in a hypothetical artificial embankment, its width should be 32 m for the shallowest of the simulated lakes Ž4 m. and 208 m for the deepest lake Ž26 m.. The length of the natural blockage, in the two cases referred to above, is over 1 km and 2 km, respectively, and therefore much greater.

Table 4 Blockage Index ŽIb. and Impoundment Index ŽIi. for the 16 simulated landslide dams Sim. N.

Ib

Ii

1.1 1.2 1.3 1.4 1.5 2.1 2.2 2.3 2.4 2.5 2.6 3.1 3.2 3.3 3.4 3.5

4.23 4.44 4.51 4.66 5.30 4.08 4.26 4.48 4.61 4.82 5.33 4.24 4.43 4.53 4.88 5.33

1.08 1.16 0.85 0.92 0.60 1.06 1.28 1.07 0.85 0.74 0.55 1.28 1.17 0.98 0.74 0.57

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

In further regard to blockage stability assessment, all authors attribute particular importance to the impoundment depth. The pressure which the water applies to the blockage is determined by its depth. However, there are no existing statistical analyses of the influence of this parameter. Casagli and Ermini Ž1999. take the impoundment volume into consideration instead, proposing an index for the assessment of the stability of an already formed dam, the ‘‘Impoundment Index’’, Ii defined as:

21

where Vd is the blockage volume and V l is the lake volume. They found that the value of Ii s 0, and, therefore, Vd s Vi , distinguishes between stable and unstable dams very efficiently. In other words stability occurs when blockage volume is greater than impoundment volume, and, therefore, Ii ) 0. In our simulations, in all cases, the Impoundment Index is always greater than 0, with blockage volume values which are far greater than impoundment values, as shown in Table 4. Grain size and texture also influence stability. Six out of the 18 of the abovementioned landslide dams which failed had fine grain size and matrix-supported texture.

blockage formation and its stability. The results indicate that the occurrence of blockage is improbable, unless there is a considerable increase in landslide velocity. Furthermore, the blockage would be of modest size unless there is a drastic deterioration of the geomechanical characteristics of the material in the accumulation zone. If blockage did occur, the formation of a lake, albeit of modest size, would cut off a main road and submerge several buildings. Regarding the stability of the blockage, in all the simulations its size and shape resulted as being sufficient to guarantee a considerable degree of stability, making a sudden failure improbable. Considering the notable solid load of the Parma River, a gradual filling and a retrogressive slow erosion seem more likely. It must be considered, however, that in spite of the low hazard, i.e., the low probability of dam failure, the area presents a high vulnerability, owing to the presence downstream of large inhabited areas close to the river valley floor. Consequently, blockage of the Parma River involves a high risk and ought to be carefully monitored and controlled by taking active measures to prevent blockage formation or mitigative measures to reduce the hazard in case of blockage formation.

8. Conclusions

Acknowledgements

Analysis of the evolution of the Corniglio landslide between 1976 and 1997, carried out with the aid of GIS GRASS, has provided useful indications regarding possible developments following an eventual reactivation, paying particular attention to the possibility of the formation of a damming lake, one of the highest risk events connected to landslide phenomena. Cellular automata enabled us to simulate the descent of landslide material assuming different modes of evolution, defined in terms of the slope reduction of various morphological units identified in the landslide. A second shell program enabled us to define the characteristics of blockage and the consequent lake. Using indices and statistical relationships proposed in the literature, we verified the possibility of

We are very grateful to C. Tellini and P. Vescovi of the Earth Science Department of Parma University for the useful discussions and suggestions on the geological and geomorphological setting of the study area and D. Peis of the Computer Center of the same Department for his kind assistance in solving computer problems. We thank G. Larini of the ‘‘Servizio Provinciale Difesa del Suolo Risorse Idriche e Risorse Forestali’’ of Parma for providing topographic maps, aerial photographs and seismic profiles, and the two anonymous referees for valuable suggestions and constructive comments. This study has been supported by the Italian ‘‘Cofinanziamento MURST 1997. Programma di ricerca: Risposta dei processi geomorfologici alle variazioni ambientali.’’ National co ordinator: Augusto Biancotti, local co ordinator: Aldo Clerici.

Ii s log Ž VdrVl . ,

22

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23

References Albrecht, J., 1992. GTZ-handbook GRASS. Vechta, Germany. Žhttp: r r www.geog.uni-hannover.de r grass r gdpr welcome. html.. Almagia, ` R., 1907. Studi geografici sopra le frane in Italia Ž1.. Mem. Soc. Geogr. Ital. XIII, 95–101. Barbieri, F., Zanzucchi, G., 1963. La stratigrafia della valle di Roccaferrara. Atti Soc. Ital. Sci. Nat. 102, 155–210. Barca, D., Di Gregorio, S., Nicoletta, F.P., Sorriso-Valvo, M., 1986. A cellular space model for flow type landslides. In: Messina, G., Hamzda, M.H. ŽEds.., Proc. of the IASTED Int. Symp. ‘‘Computer and their application for Development’’, Taormina, Italy, pp. 30–32. Barca, D., Di Gregorio, S., Nicoletta, F.P., Sorriso-Valvo, M., 1987. Flow-type landslide modelling by cellular automata. In: Proc. AMS Int. Congress Modelling and Simulation, Cairo, Egypt, pp. 3–7. Boccia, A., 1804. Viaggio ai monti di Parma. Palatina, Parma. Casagli, N., Ermini, L., 1999. Geomorphic analysis of landslide dams in the northern Apennine. Transactions Japanese Geomorphological Union 20 Ž3., 219–249. Close, D., Robbins, A.D., Rubin, P.H., Stallman, R., 1993. The GAWK Manual. 0.15 edn. Free Software Foundation, Cambridge, MA Žhttp:rrwww.cslab.vt.edurmanualsrgawkr gawk-toc.html.. Costa, J.E., Schuster, R.L., 1988. The formation and failure of natural dams. Geol. Soc. Am. Bull. 100, 1054–1068. Cruden, D.M., Varnes, D.J., 1996. Landslide types and processes. In: Landslides: Investigation and Mitigation. Turner, A.K., Schuster, R.L. ŽEds.., Transportation Research Board Special Report 247, pp. 36–75. Deangeli, C., Giani, G.P., Segre, E., 1994. Modellazione numerica di colate detritiche con il metodo degli automi cellulari. In: Proc. Conf.: ‘‘Ruolo dei Fluidi nei Problemi di Ingegneria Geotecnica’’, Mondovı, ` Italia. . Di Gregorio, S., Nicoletta, F.P., Rongo, R., Sorriso-Valvo, M., Spataro, W., 1994a. A two dimensional cellular automata model for landslide simulation. In: Proc. of 6th Joint EPS–APS International Conference on Physics Computing, pp. 523–526. Di Gregorio, S., Nicoletta, F.P., Rongo, R., Sorriso-Valvo, M., Spataro, W., Spezzano, G., Talia, D., 1994b. Landslide simulation by cellular automata in a parallel environment. In: Proc. of 2nd Int. Workshop on Massive Parallelism: Hardware, Software and Application, Capri, Italy, pp. 392–407. Di Gregorio, S., Nicoletta, F., Rongo, R., Siciliano, C., Spezzano, G., Sorriso-Valvo, M., Talia, D., 1996. Simulazione di frane con automi cellulari. Computazione Evolutiva 1, 5–16. Elter, P., 1973. Lineamenti tettonici ed evolutivi dell’Appennino settentrionale. In: Moderne vedute sulla Geologia dell’Appennino. Acc. Naz. Lincei 183, pp. 97–118. Ghelardoni, R., 1961. Serie stratigrafica di M. Caio. Boll. Soc. Geol. Ital. 80, 35–40. Gottardi, G., Malaguti, C., Marchi, G., Pellegrini, M., Tellini, C., Tosatti, G., 1998. Landslide risk management in large, slow slope movements: an example in the Northern Apennines

ŽItaly.. In: Proc. of the II Int. Conf. on Environmental Management ŽICEM 2., Wollongong, Australia, pp. 951–962. Larini, G., Marchi, G., Pellegrini, M., Tellini, C., 1997. La grande frana di Corniglio ŽApp. sett.-Prov. di Parma., riattivata negli anni 1994–1996. In: Proc. Int. Conf. CNR ‘‘La prevenzione delle catastrofi idrogeologiche: il contributo della ricerca scientifica’’, Alba ŽCuneo., pp. 505–516. Martelli, A., 1916. Contributi di geologia Applicata alle sistemazioni idraulico-forestali-La frana di Corniglio. Annali del R. Ist. Sup. Forestale Naz. I, 3–12. Pellegrini, M., 1996. Alcune caratteristiche della grande frana di Corniglio ŽApp. sett., Prov. di Parma.. Acc. Naz. Sci. Lett. Arti di Modena, Collana di Studi 15, 363–376. Pellegrini, M., Larini, G., Marchi, G., 1996. La grande frana di Corniglio ŽApp. Parmense. negli anni 1994–96. Publ. 1526, CNR-GNDCI, pp. 1–5. Pellegrini, M., Tellini, C., Vernia, L., Larini, G., Marchi, G., 1998. Caratteristiche geologiche e geomorfologiche della grande frana di Corniglio ŽApp. sett.-Prov. di Parma.. Mem. Soc. Geol. Ital., Žin press.. Pirocchi, A., 1992. Laghi di sbarramento per frana nelle Alpi: tipologia ed evoluzione. In: Proc. I Nat. Conf. Giovani Ricercatori in Geol. Appl., CUEM. Ric. Sci. Educaz. Perm. Suppl. 93, pp. 128–136. Regione Emilia-Romagna, 1976. Carta Tecnica Regionale, Sezione n. 217100 Corniglio. Schuster, R.L., 1993a. Landslide dams: hazards and mitigation. Geogr. Fis. Dinam. Quat. 16, 17–19. Schuster, R.L., 1993b. Landslide dams: a worldwide phenomenon. In: Proc. Annual Symposium of the Japan Landslide Soc, pp. 1–23. Schuster, R.L., 1994. Hazard mitigation for landslide dams. In: Oliveira, R., Rodrigues, L.F., Coelho, A.G., Cunha, A.P. ŽEds.., Proc. 7th Intern. IAEG Congress. Balkema, Rotterdam, pp. 1441–1450. Schuster, R.L., Costa, J.E., 1986. A perspective on landslide dams. In: Landslide Dams: Processes, Risk and Mitigation. Schuster, R.L. ŽEd.., American Society of Civil Engineers. Geotechnical Special Publication 3, pp. 1–20. Segre, E., Deangeli, C., 1995. Cellular automation for realistic modelling of landslides. Nonlinear Processes in Geophysics 2, 1–15. Shapiro, M., Westervelt, T.J., Gerdes, D., Larson, M., Brownfield, R.K., 1993. GRASS 4.1 Programmer’s Manual. U.S. Army Construction Engineering Research Laboratory, Champaign, IL Ž http:rr w w w .geog.uni-hannover.dergrassrgdpr welcome.html.. Swanson, F.J., Graham, R.L., Grant, G.E., 1985. Some effects of slope movements on river channels. In: Proc. of the Int. Symposium on Erosion, Debris Flow and Disaster Prevention, Tsukuba, Japan, pp. 273–278. Swanson, F.J., Oyagi, N., Tominaga, M., 1986. Landslide dams in Japan. In: Landslide Dams: Processes, Risk and Mitigation. Schuster, R.L. ŽEd.., Amer. Soc. of Engineers Geothec. Special Publ. 3, pp. 131–145. Tellini, C., Vernia, L., 1996. La frana di Corniglio, una catastrofe naturale ricorrente. L’Orsaro, Serie IV 2, 1–15.

A. Clerici, S. Peregor Geomorphology 33 (2000) 1–23 USACERL, 1993. GRASS 4.1 User’s Reference Manual. U.S. Army Construction Engineering Research Laboratory, Champaign, IL Ž http:rrwww.geog.uni-hannover.dergrassr gdprwelcome.html.. Vescovi, P., 1998. Le Unita` Subliguri dell’alta Val Parma ŽProv. di Parma.. Atti Ticinensi di Scienze della Terra 40, 215–231.

23

Wadge, G., 1988. The potential of GIS modelling of gravity flows and slope instabilities. Int. J. Geographical Information Systems 2, 143–152. Zanzucchi, G., 1963. La geologia dell’alta Val Parma. Mem. Soc. Geol. Ital. IV, 131–212.