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Environ Manage (2007) 40:493–503 DOI 10.1007/s00267-006-0288-5

ENVIRONMENTAL ASSESSMENT

Procedures for the Documentation of Historical Debris Flows: Application to the Chieppena Torrent (Italian Alps) Lorenzo Marchi Æ Marco Cavalli

Received: 11 August 2006 / Accepted: 22 February 2007 Springer Science+Business Media, LLC 2007

Abstract The reconstruction of triggering conditions, geomorphic effects, and damage produced by historical floods and debris flows significantly contributes to hazard assessment, allowing improved risk mitigation measures to be defined. Methods for the analysis of historical floods and debris flows vary greatly according to the type and quality of available data, which in turn are influenced by the time the events occurred. For floods and debris flows occurring in the Alps a few decades ago (between about 1950 and 1980), the documentation is usually better than for previous periods but, unlike events of most recent years, quantitative data are usually scanty and the description of the events does not aim to identify processes according to current terminology and classifications. The potential, and also the limitations of historical information available for the reconstruction of historical debris flows in the Alps have been explored by analyzing a high-magnitude debris flow that occurred on November 4, 1966 in the Chieppena Torrent (northeastern Italy). Reconstruction of the event was based on the use of written documentation, terrestrial and aerial photographs, and geomorphological maps. The analysis aimed to define the temporal development of phenomena, recognizing the type of flow processes and assessing some basic flow variables, such as volume, channel-debris yield rate, erosion depth, total distance traveled, and runout distance on the alluvial fan. The historical development of torrent hydraulic works, both before and after the debris flow of November 1966, was also analyzed with regard to the technical solutions adopted and their performance.

L. Marchi (&) M. Cavalli CNR IRPI Padova, Corso Stati Uniti 4, 35127 Padova, Italy e-mail: [email protected]

Keywords Debris flow Historical documents Alluvial fan Channel erosion Torrent Control Works Alps

Introduction Debris flows and flash floods in mountainous headwaters are common in Italy, as well as in many other countries, and cause the loss of lives and economic damage. Knowledge of past floods and debris flows may have a major role in hazard assessment and in the definition of mitigation measures. Among the most important outcomes of reconstruction of historical floods in headwater basins are the following: • • •

•

identification of areas affected and damage caused; assessment of the frequency of events; recognition of the type of flow processes, with particular attention to discriminating water floods with sediment transport from debris flows; evaluation of the effectiveness of mitigation measures.

The use of documentary evidence, both published and unpublished, gives an important contribution to our knowledge of historical floods and debris flows (Eisbacher and Clague 1984, Tropeano 1989, Bra´zdil and others 1999, Barnikel and Becht 2003, Barnikel 2004, Cœur and others 2002, Tropeano and Turconi 2004). Documents from historical archives can be integrated with information from geomorphological, sedimentological, and paleo-hydrological studies. The relative importance of data from historical documents compared to other methods for the reconstruction of past floods and debris flows depends on the quality of available information, which, in turn, is affected by several

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Environ Manage (2007) 40:493–503

Table 1 Space and time scales in the historical analysis of torrent hazards Space scale

Time scale

Single catchment

Region or large river basin

Single event

1

2

Multiple events— Time series

3

4

factors, in particular by the time of occurrence. In many cases, the documents concerning ancient events report just the date of occurrence and, sometimes, summary information on the damage caused, whereas detailed information on temporal development and characteristics of the events occurred in the last decades often exists, together with some quantitative data. Benito and others (2004) present a review of the methods that can be used, in an integrated way, in reconstructing river floods in order to improve risk assessment. These authors point out the extra value represented by documents that allow the assessment of social and economic impact caused by past floods, which had a much greater intensity than events reported in the instrumental series. From a space scale viewpoint, two approaches can be distinguished: 1) the analysis of historical events on a regional scale, where the vastness of the collection offers samples that can be used for statistical elaborations, even if single cases cannot be studied in depth; 2) the detailed study of a single basin or set of adjacent basins. The temporal scale of the study can depict either a single event or the reconstruction of a complete historical series of floods in a certain area (Table 1). Obviously, there are also intermediate situations in which detailed studies on particularly interesting basins are accompanied by a regionalscale analysis, or studies dedicated to a single flooding event that are subsequently integrated with data, albeit a summary, of previous events. One of the earliest investigations on collection and analysis of archive data for the study of flash floods and debris flows in the Italian Alps was carried out by Govi (1975). In the following decades, this topic was considerably developed, leading to the setting up of a national database of documents on floods and landslides (Guzzetti and others 1994, Guzzetti and Tonelli 2004). The reconstruction of time series of flash floods and debris flows in single catchments (class 3 in Table 1) and in wider regions (class 4) provides an important integration to the detailed analysis of specific flow events and to studies for hazard assessment. There are fewer studies dedicated to the reconstruction of a particular event in a single catchment (class 1) or in relatively limited areas. Among studies that integrate different methods for the reconstruction of a historical flood in a medium-sized area in

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Italy, a study that is worthy of note is a recent monograph by Esposito and others (2004) about the flood occurring in October 1954 in a coastal area near Salerno (southern Italy). Particularly severe events (for a historical flood from a large river, Turitto (2004)) are sometimes the object of studies on a regional scale (class 2). This paper aims to illustrate the most important methodological aspects concerning the reconstruction of a debris flow on a single basin scale by taking into account the Chieppena Torrent (northeastern Italy). The event studied is the debris flow of November 4, 1966, which was the most recent and one of the most serious disasters that occurred in this basin. The collection of basic information on past flooding events in this basin complements the reconstruction of this event. Methods Bibliographic sources and historical documents and maps were collected and analyzed to identify the frequency of past floods and debris flows and their relevant characteristics. The evidence of the 1966 event consisted of accounts of witnesses reported in local newspapers, monographs, scientific papers, and photographs. These were used to reconstruct the temporal development of the flow processes, their typology, and the determination of some quantitative parameters. Some empirical equations were used to analyze basic quantitative parameters of the November 1966 debris flow. Basin Studied The Chieppena Torrent is a stream of the Eastern Italian Alps and a tributary of the Brenta River (Fig. 1 and Table 2). The Cinaga Torrent joins the Chieppena Torrent on the alluvial fan, so that sediment dynamics in its basin are independent from those of Chieppena Torrent basin. The Chieppena Torrent follows a fault line, which separates magmatites and metamorphites in the north from sedimentary rocks in the south (Fig. 2). The northern part of the basin consists mostly of granite. In the central part of the basin, metamorphic rocks (phyllites and cornubianites) crop out. Dolomites and limestones occupy most of the southern part of the basin where other sedimentary rocks are also found, in particular the arenaceous complex of Val Gardena sandstones and the Bellerophon Formation (an alternation of silty marls or sandstones with gypsum nodules). Quaternary deposits are widespread in the basin and are the main source of sediments for the debris flows of the Chieppena Torrent. They consist of moraines, partly reworked as fluvioglacial deposits, scree, and alluvial deposits.


495

Fig. 1 Map of the study area, showing source areas of the debris flow of November 4, 1966, extent of deposits, and sites described in the text

Table 2 Principal morphometric parameters of the drainage basin and alluvial fan Drainage basin above the fan apex Area (km2)

27.0

Average slope (%)

54

Average elevation (m)

1333

Maximum elevation (m)

2482

Minimum elevation (m)

459

Length of the main channel (km)

8.9

Average slope of the main channel (%)

18.6

Alluvial fan Area (km2)

2.84

Average slope (%)

6.8

Average channel slope on the alluvial fan (%)

5.1

Coniferous forests cover large areas between 900 and 1600–1700 m a.s.l. Deciduous forests are located along the middle and lower parts of the streams. Meadows and farming areas are present near urban settlements. Alpine grasslands and bare ground (tussock, scree, and outcropping rocks) occupy vast areas at the highest elevations in

Fig. 2 Geological settings of the Chieppena Torrent basin

the northern part of the basin. Agricultural areas (meadows, orchards) mixed with urban settlements occupy most of the alluvial fan. A railway and a state road cross the lower part of the alluvial fan. The study area is characterized by an alpine climate with a Mediterranean influence. Precipitation is relatively abundant throughout all the year, with maxima in May– June and October–November. Mean annual precipitation amounts to approximately 1250 mm.

Historical Events Table 3 presents basic information on past events reported or estimated from historical records. For most historical events, available descriptions of the phenomena made it possible to classify the flow processes as debris flows or water floods. The historical events have been classified into two classes of intensity (high and moderate) on the basis of the description of the phenomena and reported damage. The lack of detailed information does not permit assessment of the intensity of the 1564 and 1655 events. It is important to note that the occurrence of casualties is not necessarily linked to the severity of an event. As an example, occasional circumstances caused seven fatalities

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Table 3 Historical floods and debris flows in the basin of Chieppena Torrent Date

Notes

1564 1649

Destruction of Gallina bridge (ponte Gallina, Figure 1)

1655

Type of flow event

Estimated intensity

Reference

Not known

Not known

Eisbacher and Clague (1984); Cerato (1999)

Not known

High


Not known

Not known


August 18–19, 1748

Seven fatalities, damage to agricultural areas and settlements

Not known

High

Cerato (1999)

August 30–31, 1757

Debris flow of the Cinaga Torrent, destruction of houses and four casualties in Samone

Debris flow

High

Eisbacher and Clague (1984)

1823

Flooding of the middle and lower part of the alluvial fan

Water flood

Moderate

National Archive of Trento

1825


Water flood

Moderate


1839


Water flood

Moderate


1843

Flooding of the lower part of the alluvial fan

Water flood

Moderate


August 3, 1851

Strigno village damaged by Chieppena and Cinaga and torrents, deposition of large boulders in the hamlet of Villa

Debris flow

High


September 17–18, 1882

The failure of a landslide dam on the Rio Gallina triggered a major surge that caused severe damage on the alluvial fan

Debris flow

High

Cerato (1999)

September 24–25, 1924

Loss of lives (five on the Chieppena Torrent, two on the Cinaga Torrent)

Water flood

Moderate

Local newspaper article reported by Pedenzini (2001)

November 1, 1928

Erosion of channel bed and banks in the lower part of the stream. Peak discharge estimated to about 130 m3s–1

Water flood

Moderate

Archive of Autonomous Province of Trento

during the flood of September 24–25, 1924, for which available documents depict a relatively moderate intensity. Historical data show a rather low frequency of highintensity events in the Chieppena Torrent basin, with only three events (August 1851, September 1882, and November 1966) in the last two centuries. The availability of data regarding minor events in the 19th and 20th centuries should be ascribed to greater attention for document processing and better conservation of the documents themselves.

The Event of November 4, 1966 General Situation The most recent flood in the Chieppena Torrent occurred on November 4, 1966. The flood of November 1966 in

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northeastern Italy was caused by widespread precipitation, which coupled long duration with the persistence of high intensity. The most intense phase of the rainstorm lasted about 40 hours, from the morning of November 3 to the evening of November 4 (Dorigo 1969). Tonini (1968) suggested that the principal factor for the severe floods of November 1966 was the occurrence of a large amount of precipitation, corresponding to the average monthly total, in a short time, affecting soils that were already in critical saturation condition. A further non-negligible contribution, although not decisive, came from the melting of the snow cover at elevations above 850–1200 m. In the same days, extreme rainfall caused widespread flooding of many rivers in Tuscany, with the infamous inundation of Florence. Only one rain gauge (Bieno, Figure 1) was located in the Chieppena basin. Rainfall recorded in Bieno within


497

Table 4 Time evolution of the event from accounts of witnesses collected by Gorfer (1967, 1977) Approximate time

Observed phenomenon

Notes

2.15–2.30 PM

Strigno village flooded by the Cinaga Torrent

Sudden water flood; damage increased by stream culverting

3.30 PM

First surge of the Chieppena Torrent

Initiation caused by the failure of a channel blockage (debris and log dam) near the confluence of Gallina and Fierollo streams (Figure 1)

7.30 PM

Second surge of the Chieppena Torrent

Probable failure of a channel blockage

November 3–4 amounted to 213.7 mm, i.e., about 17% of mean annual rainfall. In other rain gauges installed in the vicinity, total rainfalls ranged from 177.2 to 217.2 mm. The return period of 2 days’ rainfall (November 3–4) has been estimated to be approximately 100 years. Time Evolution and Effects Produced The event that struck the Chieppena basin and alluvial fan is quoted in various articles dealing with the effects of the November 1966 floods in northeastern Italy. Some studies cite the Chieppena Torrent as one of the streams in which the most serious damage was produced, especially in inhabited areas located on its alluvial fan (Dorigo 1969, Castiglioni and others 1971, Croce and others 1971). Moreover, an in-depth study by Venzo and Largaiolli (1968) was specifically dedicated to the Chieppena basin. These authors paid particular attention to the basin’s geological setting and mass wasting processes, which are illustrated also by photographs and related captions. Their monograph is accompanied by a geological map and a geomorphological map, both at a 1:10,000 scale. Temporal information available for the event is reported in Table 4. Gorfer (1967) reports a particularly effective description of the event: ‘‘… The second surge followed at 19:30 hours. In the streets, devastated and plunged in darkness, people cried out that there was an earthquake. The second ‘‘wall of water’’ came rushing down, skimming past the village of Strigno. The landslide broke out on the right flank towards Villa whereas the water flowed down on the left towards Agnedo.’’ The flood produced damage to roads, farming areas, urban and production settlements on the alluvial fan, and also caused the loss of three lives. In addition, most of the torrent control works were destroyed. Typology of Flow Processes Most reports and papers, which mention the Chieppena Torrent as one of the streams that produced severe damage in the November 1966 flood, do not provide a classification of the flow process that inundated the alluvial fan. In particular, the basic distinction between debris flows and

water floods with sediment transport (Costa 1988, Pierson 2005) was not taken into account. However, some descriptions of the event provide us with useful clues leading to better classification. In particular, Gorfer (1967) uses the term frana (landslide) to designate the flooding of the hamlet of Villa, whereas the early occurrence of the surge is described as a muraglia d’acqua (wall of water). These two contrasting terms seem to be typical of debris flows, which are processes intermediate between landslides and sediment transport by water floods. The description by Gorfer, reported in the previous paragraph, could be interpreted as follows: the debris-flow front, particularly rich in large boulders, moved towards Villa, whereas Agnedo, located slightly downstream on the opposite side of the channel (Fig. 1), was affected by more fluid materials, probably belonging to the debris-flow body following the front, and to the recession phase. It should be noted that at the time of the 1966 flood and in the years immediately afterwards, most debris flows were not recognized and analyzed as such in Italy. The poor level of attention to and interest in debris flows and their specific features had already been stressed by Castiglioni in 1971. This scientist, when quoting an Austrian publication where ‘‘Muren-type phenomena’’ (i.e., debris flows) were often described, says: ‘‘It might be due to problems of terminology, since in Italian there is not a specific word to describe this phenomenon, but I have the feeling that it is not sufficiently understood or correctly illustrated, although it is quite widespread also in our Alps with characteristics that distinguish it both from purely stream-related processes and mass wasting processes.’’ In more recent years, the event of November 1966 on the alluvial fan of the Chieppena Torrent has been classified as a debris flow by Eisbacher and Clague (1984), D’Agostino and others (1996), Cerato (1999), and Moscariello and others (2002). The sources available, which essentially consist of descriptions of the event, which have been reported in the papers quoted above and related pictorial material, confirm this interpretation. In particular, it is possible to state that the second surge, which was characterized by the transport of huge amounts of boulders, had the characteristics of a debris flow. The debris flow that occurred in the Chieppena Torrent can be

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is shown in Figure 3. Two classes were distinguished: deposits with abundant coarse-grained material and a wider area flooded by water and finer sediments. The first class corresponds to actual debris-flow deposition, encompassing direct and indirect impact zones, according to the definition by Kellerhals and Church (1990), whereas water flooding with finer sediment, as well as the formation of erosion tracks, can be related to the recession phase of the event and to fluvial reworking of debris-flow deposits. Coarsegrained deposits also display relevant thickness, as confirmed by some terrestrial photographs, but available information does not permit quantitative assessment of the depth of the deposits. An isolated area of coarse material (indicated with ‘‘A’’ in Figure 3) can be ascribed to local topographic conditions, including a railway embankment, which favored the deposition of a debris-flow surge that had left only scanty deposits in the upstream path. Elements for Quantitative Analysis

Fig. 3 Map of debris-flow deposits on the alluvial fan

ascribed to granular debris flow, characterized by abundant coarse material and by a sandy matrix, with limited or negligible percentage of the finest material (silt and clay). The occurrence of a granular, cohesionless debris flow agrees with the prevalence of granite rocks in the source area (Moscariello and others 2002). In fact, although channel erosion within the basin has also involved phyllites, the deposits mostly consist of granite boulders and cobbles. The presence of a sandy matrix greatly varied in the deposits; matrix-free accumulation of boulders could be due to the erosion of the sandy fraction in the recessional, more liquid phase of the event. Some uncertainties exist about the first pulse, whose deposits were reworked and partly obliterated by the second surge. The first surge, less rich in coarse material, might not have attained the concentration of a debris flow, and consisted of a hyperconcentrated flow. The limits of the deposits on the alluvial fan were set by using two oblique aerial photographs of the alluvial fan (views from north and from south), taken immediately after the debris flow of 1966. The photographs were georeferenced and rectified by means of a polynomial transformation of control points identified on the photographs and on a georeferenced topographic map. The map of the deposits

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Quantitative data about the November 1966 debris flow are scarce. A first approximation of the solid volume deposited was proposed by Cerato (1999): ‘‘over one million cubic metres of debris on the alluvial fan where the villages of Strigno and Villa Agnedo are located.’’ A slightly smaller amount (950,000 m3) was reported by D’Agostino and others (1996). These evaluations are necessarily approximate and do not consider the material conveyed as far as the Brenta River and removed by the same watercourse. Although with inevitable uncertainties, we can assess the total volume of the material deposited on the fan as approximately 106 m3. A huge boulder, with a size of 10 · 9 · 12 m, was transported for about 900 m and was deposited a little upstream of the fan apex (Fig. 1). Quantitative data from event documentation have been compared with data on other debris flows in Alpine basins and with the results of empirical equations. The total deposit volume, assessed as 106 m3, was divided by the total length of the hydrographic network affected by the most intense erosions, corresponding to parts of the Gallina and Fierollo streams and Chieppena Torrent as far as the starting point of deposition on the alluvial fan. The identification of the parts of the hydrographic network to be taken into account was carried out on the basis of the geomorphological map by Venzo and Largaiolli (1968). It was therefore possible to calculate the rate of yield of debris, which is a particularly significant index in assessing erosion due to debris flows. Figure 4 shows the cumulative frequency distribution of debris-yield rate for unit channel length, for a sample of more than 120 debris flows that have occurred in northeastern Italy since the 19th century. The value obtained for the Chieppena Torrent (120 m3 m–1) lies among the highest historically recorded in this Alpine


499 Table 5 Runout distance on fan and total distance traveled

Fig. 4 Debris yield rate for unit channel length in the Eastern Italian Alps

region. This result is in agreement with high-intensity erosion processes that affected the Chieppena Torrent on November 4, 1966. Available documentation (i.e., a geomorphological map by Venzo and Largaiolli (1968), and both aerial and terrestrial photographs) has enabled the runout distance of the debris flow on the alluvial fan and total distance traveled from the initiation point to the lowest point of deposition to be approximately assessed. The analysis regards the second surge, whose deposits are visible in the photographs taken immediately after the event. Measured values have been compared with the results of the following empirical equations: Lf ¼ 8:6 ðV tan JÞ0:42

ð1Þ

Lf ¼ 15 V 1=3

ð2Þ

Lt ¼ 1:9 V 0:16 He0:83

ð3Þ

Lt ¼ 30 ðV He Þ0:25

ð4Þ

where Lf is the runout distance on the alluvial fan (m), Lt is the total distance traveled (m), J is the mean slope of the transportation zone (), V is the volume (m3), and He is the elevation difference between the starting point and the lowest point of deposition (m). Eq. 1, reported by Ikeya (1989) for Japanese alluvial fans, was rearranged by Bathurst and others (1997) in the form presented above. Eqs. 2–4 were proposed by Rickenmann (1999). The exponents of eqs. 2 and 4 satisfy Froude scaling, whereas eq. 3 best fits the experimental data (Rickenmann 1999). It has been assumed that the volume deposited by the second surge amounted to 500,000 m3, i.e., 50% of the total event volume. Because

Lf observed (m)

2170

Lf eq. 1 (m)

1055

Lf eq. 2 (m)

1190

Lt Fierollo observed (m)

8935

Lt Fierollo eq. 3 (m)

6415

Lt Fierollo eq. 4 (m)

4900

Lt confluence Gallina—Fierollo observed (m)

7630

Lt confluence Gallina—Fierollo eq. 3 (m)

3860

Lt confluence Gallina—Fierollo eq. 4 (m)

4200

the starting point of the debris flow is not known for sure, there is considerable uncertainty regarding the total path run by the debris flow. Two hypotheses have been considered: debris flow initiation at the confluence of the Gallina and Fierollo torrents, and from the upstream point of the erosion areas mapped by Venzo and Largaiolli (1968) on the Rio Fierollo (Fig. 1). The results, reported in Table 5, show an underestimation of values for both variables analyzed. In considering the outcome of the comparison between the data observed and those calculated, it is important to remember that such formulae produce only approximate results. Nevertheless, the fact that empirical equations produce values of Lf and Lt shorter than those arising from the documentation of the event is not without meaning. A possible explanation for this can be found in the relatively low solid concentration which, on the basis of the available descriptions, the debris flow seems to have had, as well as in the low viscosity of the solid–liquid mix involved. Both circumstances might have contributed to the high mobility of the flowing mass. On the other hand, eqs. 1–4 lead to the determination of values that reflect the comprehensive conditions of the samples they are based upon and that comprise diverse types of debris flows. Some simple sensitivity tests (Fig. 5) have emphasized that the assumption relative to the volume of the second surge does not influence appreciably on the results: the values resulting from the application of eqs. 1–4 are lower than those observed even by utilizing a volume double of the first reference. The values of channel erosion depth reported in papers and monographs describing the event have been compared with an empirical relationship proposed by KronfellnerKraus (1984) for Austria and used also by Rickenmann and Zimmermann (1993) for analyzing the debris flow that occurred in Switzerland in 1987: D ¼ 1:5 þ 0:125 S

ð5Þ

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500


Fig. 5 Sensitivity analysis of the equations for the runout distance on the fan and the total distance traveled. a Runout distance on fan. b Total travel length—debris-flow initiation in Rio Fierollo. c Total travel length—debris-flow initiation at the confluence of Rio Gallina and Rio Fierollo

where D is the depth of channel bed erosion (m), and S is the channel slope (%). Channel-bed incision in proximity of Ponte Gallina (Fig. 1) was measured at about 10 m (Provincia Autonoma di Trento 1991). More generally, Venzo (1968) reports ‘‘a channel erosion of 6–7 m within a few hours’’ for the Chieppena Torrent upstream of Bieno. Considering that the channel-bed slope at Ponte Gallina is 28%, the value of 10 m is higher than the value resulting from eq. 1. The same outcome results by considering the erosion depth of 6–7 m reported by Venzo (1968) in the Chieppena channel-bed upstream of Bieno, which has an average slope of 21%. The fact that the erosional depths observed are more pronounced than the values resulting from a relationship based on a large sample of high-intensity events further emphasizes the great intensity of the erosional processes occurring along the Chieppena Torrent during the event of November 1966. It should be considered, however, that the depth of erosion analyzed is the cumulative value of the two surges of the November 1966 event. If we assume that each surge caused 50% of the total erosion observed, the resulting value would be close to that obtained by means of eq. 5. Torrent Control Works Since the 19th century, severe damage produced by the Chieppena Torrent to settlements, roads, and agricultural areas has required mitigation measures. Figure 6a presents a sketch, from an 1848 project, showing stone masonry check dams being constructed a little upstream of the alluvial fan. These dams incorporated large boulders left in the channel bed by previous debris

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flows. Other check dams had been designed for upstream channel stretches. Some control works were carried out after the 1882 flood, and a more systematic project was set up in 1913. The implementation of the project was prevented by World War I (the Chieppena Torrent was close to the Front) but some interventions were carried out in the 1920s, following the guidelines of the 1913 project. A comprehensive project aiming at complete control of torrent erosion was prepared in 1927 and was subsequently implemented. Works continued in the following decades, until the 1960s. The check dams were built in stone masonry, using blocks taken from the channel bed (Fig. 6b). The debris flow of November 4, 1966 almost completely destroyed the control works in the Chieppena Torrent. The failure of the containment works implemented before the event of November 1966 can be ascribed both to inadequacy of design and to poor construction quality. Indeed, the check dams had not been specifically designed to cope with debris flows. Moreover, stone masonry check dams could not provide sufficient resistance to the dynamic impact of the flow. New control works were carried out after the November 1966 event according to design criteria and management policies prevailing in Italy at that time; these consisted essentially of bulky concrete dams, with a design channel slope equal to zero (Fig. 6c). Seventy check dams were built: the aim of these works was to reduce channel slope in order to stabilize channel bed and banks, thus preventing formation and propagation of debris flows. The construction of a number of large, bulky check dams may appear to be in contrast with recent trends in torrent management, which produce control works with less environmental im-


501

Fig. 6 Different types of check dams outlining the historical development of control works in the Chieppena Torrent. a A stone masonry check dam from an 1848 project (National Archive of Trento). b A stone masonry check dam built in the 1930s (archive of Autonomous Province of Trento). c Concrete check dams built after the debris flow of November 1966 (archive of Autonomous Province of Trento). d An open check dam built in the early 1990s

pact, but they are justified by the need to contain highintensity torrent erosion that had caused major damage to settlements on the alluvial fan. In recent years, an open check dam has been added to the traditional check dams. This was built immediately downstream of the confluence of the Gallina and Fierollo torrents, and aims to prevent downstream propagation of debris flows that could be generated in the upper part of these channels (Fig. 6d). The compact structure of the check dams built after the November 1966 debris flow ensures much higher resistance to dynamic pressures than the works destroyed by the 1966 debris flow. Up to now, the works implemented have contributed effectively to the control of erosion and instability phenomena along the channel and on the adjacent side slopes. We should stress, however, that these works have not yet had to withstand events comparable in intensity to the November 1966 debris flow.

Discussion and Conclusions The reconstruction of the November 1966 debris flow in the basin of the Chieppena Torrent offers two considerations: both the particular case studied and methodological considerations useful for reconstructing other historical floods in stream basins of the Alps. With regard to the first consideration, the reconstruction of the time evolution of the event was coupled with the recognition of the type of flow processes occurring in the basin studied. The characteristics of the second surge lead to its classification as a granular debris flow, whereas there is some uncertainty about the first pulse, which could have consisted of a hyperconcentrated flow. Data available for

characterizing the phenomenon quantitatively depict an event in which the mobilization of huge amounts of solid material was matched by a remarkable mobility of the flowing mass. A further comment regards the frequency of past events, assessed on the basis of historical documentation. The frequency of high-intensity flow events in the Chieppena Torrent basin is rather low. This seems to depict a relatively stable system, but one that is prone to large debris flows when particular meteorological conditions trigger widespread landslides and intense erosion on the channel bed and banks. The mere availability of loose material subject to mobilization is not a limiting factor for the occurrence of debris flows in the Chieppena Torrent. Very erodible Quaternary deposits are widespread in the basin, so that the potential debris supply to the channel network can be deemed unlimited and long recharge times between two subsequent events are not necessary. With reference to the classification of debris-flow prone basins into weathering-limited and transport-limited, proposed by Bovis and Jakob (1999), the large availability of erodible material would cause the Chieppena Torrent to be classified as a transport-limited basin. However, the relatively high stability of basin slopes and minor tributaries, which fails only in response to high-intensity meteorological events, causes large debris flows to be less frequent than is commonly observed in Alpine basins with an unlimited sediment supply. The study of the November 1966 event in the Chieppena Torrent has allowed assessment of the potential and the limitations of information commonly available for the reconstruction of debris flows occurring in past decades in the Italian Alps (Table 6). We can therefore consider that

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Table 6 Summary of activities and results in the reconstruction of a historical debris-flow: the experience from the study of the Chieppena Torrent Activity

Results achieved

Drawbacks and limitations

Collection and analysis of data from historical archive

Assessment of the frequency Data on small-intensity events are seldom reported of the events for the most ancient periods

Analysis of bibliographic sources, gray literature and unpublished documents regarding the event studied

Reconstruction of the time evolution of the event Recognition of the type of flow processes

Analysis of aerial photographs

Mapping of the areas flooded

Difficulty in assessing the thickness of the deposits

Application of empirical equations

Quantitative assessment of flow variables

Intrinsic approximation of available equations

The information is essentially qualitative Limited information regarding the upper parts of the basin (debris-flow initiation zones) The information is prevailingly qualitative

Assessment of damage

the devised procedure, based on collection and analysis of published and unpublished documents and aerial and terrestrial photographs, could be applied to the reconstruction of other floods and debris flows occurring in past decades. For floods and debris flows that took place in previous times (approximately before the 1950s), available information is often limited. For older events, aerial photographs are not available and terrestrial photographs are usually scarce. As for the most important flooding events, archive documentation may be sufficient for a reliable reconstruction of the main characteristics. The use of 1-D and 2-D models can provide important contributions to the quantitative analysis of historical debris flows, allowing reconstruction of flow depth and velocity and simulations of flooded areas. On the other hand, empirical evidence of large-magnitude historical events can represent a challenging test for application and validation of numerical models. The application of some empirical equations has been deemed adequate for this study, which aims to document a historical debris flow and to outline its basic quantitative aspects. Numerical models, which couple higher resolution with remarkable data requirement, could represent a further development in the analysis of this event. In this study, a reconstruction of flood discharge by means of rainfall-runoff models has not been carried out because the flood event of November 1966 in the Chieppena Torrent was substantially influenced by the failure of temporary obstructions of the channel. An analysis of rainfall-runoff transformation would have given a limited contribution to the reconstruction of the timing and intensity of the surges that produced major geomorphic effects along the channel and on the alluvial fan. In the Province of Trento, as well as in other Alpine areas, torrent control works have been extensively carried

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out since the 19th century. The study of the historical evolution of such works is thus an important part of studies intended to improve our knowledge of mountain territories. The Chieppena Torrent offers a good example for this kind of analysis, especially because of the great severity of past debris flows, which also outlined the inadequacy of some traditional control works, as demonstrated by the failure of the check dams designed in the 1920s. Acknowledgments This study was funded by the Provincia Autonoma di Trento–Servizio Bacini montani (contract no. 1543 CONV) and the European Commission (Sixth Framework Programme, HYDRATE Project, contract no. GOCE-037024). The authors thank M. Cerato for the useful discussion on historical records of flood events, and F. Tagliavini for the information on the geolithological setting of the basin. The staffs from the Historical Archive of the Autonomous Province of Trento and State Archive are thanked for their collaboration in the collection of historical documents. We also thank R.L. Baum, F. Guzzetti, an anonymous reviewer, and the Editor-in-Chief, V.H. Dale, for valuable comments on the manuscript.

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