Author: IRPI

Fausto Guzzetti, Silvia Peruccacci and Mauro Rossi

CNR, Istituto di Ricerca per la Protezione Idrogeologica, Perugia, Italy

Knowing the temporal frequency of landslides is important information to determine landslide hazard. Information on landslide frequency can be obtained from archive inventories compiled from: (i) data captured from the literature, (ii) through inquire to public organisations and private consultants, (iii) by searching chronicles, journals, technical and scientific reports, and (iv) by interviewing experts. From archive inventories one can obtain catalogues of historical landslide events, which list the date of the event, the exact or approximate location of individual or multiple landslides, and the damage caused by the slope failures. In the framework of the E2-C2 project, we report an attempt to analyse catalogues of historical landslide events, including the relationships between the occurrences of landslides and the rainfall conditions, and to determine landslide temporal hazard for three provinces of the Emilia Romagna Region in Italy.

HISTORICAL LANDSLIDE Catalogues
The Geological Survey of the Emilia Romagna Region compiled catalogues of historical landslide events for three selected areas in the Region. The first catalogue lists 1197 historical landslide events in the Reggio-Emilia Province in the period 523-year from 1472 to 1994 (A). The second catalogue lists 1784 historical landslide events in the Modena Province in the 589-year period from 1414 to 2002 (B). The third catalogue was compiled for the Reno River basin, corresponding mostly to the Bologna Province, and lists 1487 landslide events in the 527-year period from 1470 to 1996 (C).
For the three catalogues, and for the periods for which rainfall records were available, we prepared an ensemble of 3 graphs. In the ensembles, the upper histogram shows rainfall history. Thin dark-blue bars show monthly rainfall (right y-axis), and thick light-blue bars show annual rainfall (left y-axis). The central plot shows history of landslide events. The monthly number of inventoried landslides is shown with dots (left y-axis), and the cumulative number of landslides is shown with a line (right y-axis). The number and total number of historical landslides are normalized for area. Dots are coloured based on deviation from mean annual precipitation (MAP). A linear fit to the cumulative line is shown for the portion of the catalogue deemed substantially complete (given the methods used to compile the catalogues). The slope of the best-fit line is a measure of the rate of the historical landslide events in the investigated area. The lower graph shows yearly deviation from the long-term MAP value, computed for the studied period. Wet years are shown in red, and dry years are shown in blue.

PRELIMINARY ANALYSES
We performed various analyses on the available catalogues of historical landslide events. The first analysis consisted in comparing the cumulative number of historical landslide events for the three catalogues. Results are shown in the graph below, which shows the yearly cumulative number of landslide events, normalized by the extent of the study area. Inspection of the graph reveals a different degree of completeness of the historical catalogues. The (apparent) rate of the historical landslide events has increased significantly since the beginning of the record, largely as a result of variations in the completeness of the catalogues. The more remote the period considered, the larger the number of events that probably remained unrecorded.
The second analysis consisted in comparing the rate of occurrence of landslide events. Rates of landslide occurrence were obtained separately for each catalogue by dividing the total number of events in the considered period by the length of the period. Only the more recent periods of the catalogues were considered. Normalized rates were computed by dividing the obtained rates by the extent of the study areas. Results are summarized in the table below. Inspection of the table reveals that normalized rates are in the range from 5.0×10-3 to 2.1×10-2, and that the largest normalized rates were observed in the Modena Province. We attribute low normalized rates obtained to incompleteness of the catalogues.
In the third analysis we compared the historical records of landslide events with the corresponding rainfall records. One expects to observe a larger number of landslides in “wet” years, and a reduced number of landslides in “dry” years. Inspection of the graphs indicates that this is the case for some of the areas and for some periods (e.g., Reggio Emilia, from 1959 to 1960 and from 1975 to 1980), but a common trend cannot be easily identified.

TEMPORAL probability of landslide events

Different probabilistic models have been proposed to determine the temporal occurrence of landslides. The proposed models adopt specific distributions and exploit records of historical landslide events. Completeness of the historical records is important for the determination of the temporal occurrence of slope failures. For the Reggio-Emilia, Modena and Bologna Provinces two records of historical landslide events are available. The first record is the portion of the National catalogue of historical landslide (and flood) events in Italy (available at http:/sici.irpi.cnr.it) that covers the three considered Provinces. We obtained the second record merging the three individual catalogues available for the Reggio-Emilia, Modena and Bologna Provinces described above. The two obtained records list a different number of historical landslide events. We attribute the difference to the different completeness of the catalogues.
We used the two historical records to prepare and compare models for the temporal occurrence of landslides. Adopting a simple Poisson model for the occurrence of landslide events, and exploiting the two historical records, we estimated the temporal probability of landslide events in the 152 municipalities in the Reggio-Emilia, Modena and Bologna Provinces. For simplicity, we selected the part of the two historical records considered more complete, i.e. the 39-year period from 1955 to 1993. Figure 1 shows the exceedance probability of landslide events obtained from the national historical record in the three considered Provinces. Figure 2 shows estimates of the exceedance probability of landslide events obtained from the detailed historical record compiled for the three Provinces by the Geological Survey of the Emilia Romagna Region.

REGGIO-EMILIA, MODENA AND BOLOGNA PROVINCES
PROBABILITY OF LANDSLIDE EVENTS OBTAINED FROM A NATIONAL HISTORICAL RECORD

Figure 1

1 year

5 years

10 years

20 years

EXCEEDANCE PROBABILITY

0.0 - 0.1

0.1 - 0.2

0.2 - 0.3

0.3 - 0.4

REGGIO-EMILIA, MODENA AND BOLOGNA PROVINCES
PROBABILITY OF LANDSLIDE EVENTS OBTAINED FROM DETAILED HISTORICAL RECORD

Figure 2

1 year

5 years

10 years

20 years

0.4 - 0.5

0.5 - 0.6

0.6 - 0.7

0.7 - 0.8

0.8 - 0.9

0.9 - 1.0

Inspection of Figure 1 and Figure 2 allows for a comparison of the temporal forecasts of landslide events. The mapping unit of reference (i.e., the municipality), and the type of historical information (i.e., the occurrence of landslide events) are the same for the two forecasts. This facilitates comparison.
The two historical records used to estimate the temporal probability of landslide events span the same 39-year period, but differ for their completeness. We attribute the geographical differences between the two temporal forecasts shown in the maps to the different completeness of the available historical records.

© CNR - IRPI, 2006