Abstract. The estimation of debris flow magnitude is essential for the assessment of debris flow hazard. Historical data are an important basis for evaluating.
Phys. Chem.Earth (C), Vol. 26, No. 9, pp. 657463,200l 0 200 1 Elsevier Science Ltd.
Pergamon
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Debris Flow Magnitude in the Eastern Italian Alps: Data Collection and Analysis V.D’Agostino’
and L. March?
‘Dipartimento Territorio e Sistemi Agro-Forestali, Universita ‘CfiR IRPI, Corso Stati Unifi 4, 35127 Padova, Italy
di Padova, Agripolis,
35020 Legnaro (Padova), Italy
Received 31 July 2000; accepted 23 December 2000
Abstract. The estimation of debris flow magnitude is essential for the assessment of debris flow hazard. Historical data are an important basis for evaluating magnitude and frequency of debris flows in a given geographical region. Data on debris flow magnitude concerning 130 basins of the Eastern Italian Alps have been collected from scientific and technical journals, technical reports and historical documents gathered from local archives and through field surveys carried out in the last few years. The analysis of collected data includes various approaches. Regression techniques were used to correlate debris flow magnitude to morphometric parameters and to geological characteristics of the basins. A comparison of historical magnitudes with geomorphic field estimations carried out in recent years is presented and the relations between debris flow magnitude and frequency for a few selected cases are analysed. Some proposals about the possible combined use of considered techniques are then suggested. 0 2001 Elsevier Science Ltd. All rights reserved
I
and frequency of debris flows and the impossibility of predicting the occurrence of debris flows exceeding a given magnitude in a given Wure period (Davies, 1997). Even bearing in mind these basic limitations, an analysis of the volumes of sediment deposited by debris flows, based on historical data, can contribute to an improved knowledge of intensity and frequency of debris flows. This paper discusses the contribution of historical data to the evaluation of debris flow magnitude in the basins of the Eastern Italian Alps. In case of events consisting of multiple surges, debris flow magnitude refers to the overall volume accumulated in the deposition area.
2 Outline of the study area and data collection The study area is a vast mountainous region in the eastern part of the Italian Alps; from an administrative point of view, it corresponds to the Provinces of Trento and Bolzano and to Veneto and Friuli - Venezia Giulia Regions (Fig. 1).
Introduction
The estimation of debris flow magnitude, i.e. the volume of debris material discharged during a single event, is a basic step toward the assessment of debris flow hazard. A number of methods, including empirical and statistical formulas (e.g. Takei, 1984; Kronfellner-Kraus, 1985; D’Agostino et al., 1996), geomorphological approaches (Hungr et al., 1984; Scheuringer, 1988; Thouret et al., 1995), and combined methods (Spreafico et al., 1999) have been proposed for the estimation of debris flow magnitude. estimation procedures have been Although several developed, the assessment of debris flow magnitude still poses serious problems. Another critical point concerns the uncertainties in analysing, on the basis of the small-size samples usually available, the reiations between magnitude Correspondence
Fig. 1. Location map. Shaded area: mwntaincws belt. 1: Friuli - Venezia Giulia Region; 2: Veneto Region; 3; Province of Bolzano; 4: Province of Trento. M: Moscardo Torrent; B: Rio Bianco; I: Rio Inferno.
to: Lorenzo Marchi
651
658
V D’Agostino
and L. Marchi:
Debris Flow Magnitude
The central and southern parts of the study area, encompassing the Dolomites, are mostly characterised by sedimentary and volcanic rocks. In the inner belt of the alpine range, outcrops of metamorphic rocks prevail, whereas massive cristalline rocks occur in the western part of the considered region. Quaternary deposits are widespread throughout the alpine valleys; they consist of fluvio-glacial deposits, scree. landslide bcylacial and accumulations and alluvial fans. influences the climatic Complex orography characteristics of the Eastern Italian Alps causing high variability in the spatial distribution of precipitation and temperature. As far as the precipitation is concerned, valleys parallel to the Alpine structure are characterised by relatively dry conditions, with annual precipitation of about 500-600 mm, whereas transverse-oriented valleys have a higher precipitation rate (1500-2000 mm); annual amounts of precipitation exceed 3000 mm in some prealpine areas. Seasonal distribution of precipitation is continental, with summer maximum, in the inner part of the alpine range, whereas spring and autumn maxima are observed in the prealpine belt. Landslides and debris flows frequently occur in the studied region, often resulting in high risk because of the heavy urbanisation in valley floors and on alluvial fans and the presence of important transportation routes. Data on debris flow magnitude for basins of the Eastern Italian Alps were collected from scientific and technical (mostly monographs journals, in the grey literature published by local authorities), in technical reports and unpublished documents gathered from the archives of local authorities, and through field surveys carried out by the authors in recent years. Since collected data can be affected by several sources of error, a validation was carried out which included the comparison of different documentary sources, the test of the compatibility of volumes found in the examined documents with the topography of the deposition areas, the recognition of the actual flow process (debris flows versus water floods with bedload). This validation led to discard several cases and to consider a sample of 130 drainage basins. Earliest data date back to the mid-19”’ century; amongst the floods that occurred in the considered period, two major events (September 1882 and November 1966) should be pointed out, which affected vast areas and caused serious damage.
3 Analysis 3.1 Data presentation
Table I presents some basic statistics on the morphometric characteristics of the basins for which quantitative data on debris flow volumes have been collected; the range in drainage basin area is rather wide, however small basins (< 5 km-) prevail, corresponding to about 75 % of the total sample.
Table
in the Eastern Italian Alps
1. Morpbometric
parameters ofstudied
basins.
Basin area
Main channel
Main channel
(km’)
length (km)
slope (%)
Median
2.44
2.5
38
Minimum
0.07
0.4
13
Maximum
32.7
14.8
82
Lower quartile
1.04
1.79
28
Upper quartile
5.16
4.28
48
Figure 2 shows a scatterplot of debris flow magnitude versus drainage basin area; when more than one event has been recorded in the same basin, only the largest value was plotted. An upper limit can be outlined, which approximately corresponds to an unit value of 70000 m’km-‘; this value confirms the findings of a previous study (Marchi and Tecca, 1996) and expresses the maxima that were attained in the considered region on the occasion of high intensity storms in basins where large amounts of sediment were available. The upper envelope does not show a clear tendency to a reduction of volumes per unit area for increasing basin size. A particular case is represented by two basins in which the mobilisation of large landslides resulted in multiple surges debris flows, which lasted for several days and discharged huge amounts of sediments (Fig. 2). le6
0
loo!
01
10
10.0
Basin area (km’) Fig. 2. Scatterplot
of debrisflow
magnitude
versus drainage basin area.
Since also small magnitude debris flows can prove be very hazardous, e.g. when they affect railways and motorways, attention was paid also to the lower limit of debris flow volumes. Minimum values of 1000 m3 are often observed and only in two cases lower values were reported. These volumes represent a lower level of perception for the personnel involved in torrent control and in watershed management more than a physical limit of debris flow magnitude. Debris flows of lower magnitude actually occur, but they are reported only for a very few basins carefully surveyed because of their dangerousness. Even taking into consideration all the debris flows occurred in each basin and not only the largest event, volumes in the range of 300 800 m’ would be reported only for a very few streams. The increase of magnitude M (m’) with basin area A (km2) is very limited for the lower envelope. This latter can be expressed by the following equation:
V.D’Agostinoand L. Marchi: Debris Flow Magnitudein the Eastern Italian Alps M = 1000. .4°.3
(1)
Structural measures for the reduction of debris flow hazard (e.g. debris storage areas) should be based on debris flow volumes corresponding to high intensity events. However, when local topographic conditions or financial constraints limit the implementation of control works, the lower envelope drawn in Fig. 2 can help defining minimal values of magnitude for which attenuation measures can prove to be effective. The two envelope lines drawn in Fig. 2 are merely intended to outline the volume range of debris flows in Northeastern Italy and do not represent statistical relations between debris flow magnitude and basin area. 3.2 Predictive
equations
An analysis aimed at assessing the relationships between debris flow magnitude and morphometric and geolithologic characteristics of the basins was carried out for a sample of basins lying in the Provinces of Trento and Bolzano (Fig. I). The analysis of historical records in the archives of the Forest Offices of these Provinces made it possible to extract the largest debris flows occurred over a long time period (about 100 years). On these bases, debris flow volumes may be deemed representative of high intensity, centennial frequency events. A previous analysis conducted by D’Agostino (1996) and D’Agostino et. al. (1996) on the debris flow events occurred in the eastern part of the Province of Trento, proposed a relation to assess the magnitude of the total sediment volume yielded. The relation assumes, as independent variables, the catchment area A (km’), the mean gradient of the stream S (%) and a dimensionless geological index (GI). The latter parameter expresses the erodibility of the lithology feeding the channel network. Its value is computed weighting the score associated to each geolithological class (Tab. 2) in proportion to the area of the basin covered by the class. Local fracturation and alteration conditions of the rock are also accounted for refining GI estimates. Table 2. Lithological
classes and geological
index
(GI)
values.
Gf value Quaternary deposits Schists and phyllites Marls. marly-limestone, siltstones. Volcaniclastic rocks Dolomite and limestone rocks Massive igneous and metamorphic
etc.
rocks *
* For basins entirely or almost entirely consisting ofthis a cautionary value of0.5 instead ofO.0 is advised.
The D’Agostino
5 4 3 2 1 0 lithology.
et al. (1996) equation results:
M=45.,&9.S’.5.G/
(2)
Equation 2 was obtained by means of a multiple regression, imposing to minimise the mean square error S,” 20000
1
10000~
a
l
,
o-,-t
.
l
l
0 EVI reduced
being y the EVI reduced variate and MA (m’ km’) the unit volume. For return periods of 100 and 150 years, Eq. 5 provides values of 63200 and 69000 m’ km-‘, which are very close to the upper envelope of Fig. 2. A further frequency analysis has been carried out for the Moscardo Torrent by using the partial duration series of annual exceedence (Chow et al., 1988). According to this procedure, a threshold value of magnitude is selected, above which all the data are considered for building the series. The threshold value is such to obtain the dimension of the sample equal to the number of years of the record. In this way, in a very active stream like the Moscardo (all the debris flow observations are independent), secondary or tertiary debris flow in a year may be taken into consideration and substitute years of the series without debris flows or with very low magnitude events. The application of this approach to the Moscardo Torrent reveals the capability to represent the pattern of high frequency-low magnitude events (Fig. 5). Notwithstanding the small size of the tested sample discourages to fit the data for EVI distribution presented in Fig. 5 or to evaluate the opportunity to adopt other families of probability distribution, the procedure seems to support an evaluation of the low magnitude - high frequency relations in basins with an unlimited sediment supply. On the contrary, this procedure is less reliable for assessing the magnitude of
5
l
0
MlA=14500.y-3500
661
1 variate,
2 y (-)
Fig. 5. Moscardo Torrent: plot ofdebris flow volumes versus reduced variate ofthe Extreme Value type I distribution.
4 Conclusions Historical data on debris flow magnitude have been collected and analysed in basins of the Eastern Italian Alps. A threefold procedure, encompassing empirical equations, geomorphic methods and statistical analysis of magnitudefrequency relations appears as a promising approach to the estimation of debris flow magnitude in alpine basins. The critical application of different estimation methods can contribute to attenuate the intrinsic approximations of using a single category of methods, resulting in a more reliable evaluation of debris flow magnitude. As an example, a scatterplot of magnitude versus basin area, like the one presented in Fig. 2, can be used to test the plausibility of the results arising from field estimates of sediment potential: this check is a part of a comprehensive procedure proposed by Spreatico et al. (1999) for estimating the sediment potential both in debris flow basins and in mountain bedload streams. Historical data play an important role in each of the three considered methods (Table 3): this confirms the importance of continuing collecting
662
V D’Agostino
quantitative data on debris flow means of standardised procedures. Table 3. Use ofhistorical
magnitude,
data in the estimation
Method
and L. Marchi:
Scatterplots
ofmagnitude
versus
possibly
by
of debris flow magnitude.
Use of historical
simple basin variables: statistical and empirical equations.
Debris Flow Magnitude
data
Basis for regional equation
developmentand comparison with magnitudepredictedby
in the Eastern Italian Alps
scenarios of magnitude-intensity seems to be a suitable prospect (Cojean et al., 1999; Laigle and Marchi, 2000). In delineating such scenarios, predictive equations and
geomorphic field evaluation can usefully integrate the analysis of time series, especially as far as high magnitudes are concerned.
available equations. Geomorphic mohilisahle Analysis frequency
estimation
of
Comparison
with field estimates
inventory. The study was funded by the National Research Council ofltaly (Special Project GNDCI, U.O. I .29 and 2.7, Paper no. 2132) and by the
material.
of magnituderelations.
Acknowledgements The authors thank Mario Cerato, Roberto Coali. Rudolf Pollinger and Bruno Bertotto for the support in the historical data
Required for building
the time
series
Empirical and statistical predictive equations are essentially intended to provide an approximate assessment of debris flow magnitude; their use should be restricted to the physical context (geologic, geomorphologic, climatic) where they have been developed. It has been recently pointed out (Meunier et al., 2000) that empirical methods for computing the magnitude of debris flows produce very different results depending on the specific formula which is employed. Consequently, researchers should carefully describe the physical context in which they have been developed, in order to define their field of applicability. A number of geological, geomorphological, hydrological and land use factors affect debris flow magnitude and cannot easily be taken into account by predictive equations. Characteristics and intrinsic limitations of predictive equations for debris flow magnitude have been considered in developing Eq. 4: basin parameters chosen as independent variables are simple and easy to determine from standard topographic and geologic maps. Although somewhat sub.jective, geomorphic field estimates of debris flow volumes give an important contribution to the assessment of debris flow magnitude. These methods would greatly benefit from new field data
which link debris flow magnitude to the volumes of debris eroded from sediment sources along channel bed and banks. Since a number of equations for the assessment of debris flow volume already exist, whereas geomorphic techniques still have limited application in the Italian Alps, it can be argued that collecting new field data and using them for improving field geomorphic methods is a more urgent and suitable task than the development of more empirical and statistical equations. As it has been pointed out above, the small sample size of the time series usually available in the alpine basins makes it hardly possible a precise assessment of the return period corresponding to a given debris flow magnitude. The assessment of magnitude and frequency of past debris flows through field observations and dendrogeochronological methods (Strunk, 1992; Bovis and Jakob, 1999) is limited to debris fans covered by an old, undisturbed forest: such conditions are seldom met in urbanised alluvial fans which pose the greatest problems associated to debris flow hazard in alpine basins. When available data do not allow a more precise probabilistic analysis, to define semi-quantitative
EU Prqject: DAMOCLES for local end-users, Contract
Debrisfall assessment in mountain UE EVGI-CT-1999-00007.
catchment
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