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Regional hydrological thresholds for landslides and floods in the Tiber River Basin (central Italy) P. Reichenbach 7 M. Cardinali 7 P. De Vita 7 F. Guzzetti

Abstract The definition of landslide warning thresholds, based on the analysis of hydrological data, is proposed. In the Tiber River Basin of central Italy historical information on landslides and floods, for the period 1918–1990, was available from a nationwide bibliographical and archive inventory on geohydrological catastrophes. Hydrological data were obtained from mean daily discharge records at various gauging stations within the basin. Several hundred hydrological events, broadly defined as a series of consecutive days having mean daily discharge exceeding a predefined value, were identified. Hydrological parameters obtained from the discharge records were used to rank the events according to their probability to trigger mass movements or inundations and to define regional thresholds for the occurrence of landslides and floods. The proposed approach, not lacking limitations, has conceptual and operational advantages, among which is the possibility of using historical information on geohydrological catastrophes. Key words Landslide 7 Flood 7 Regional warning threshold 7 Hazard assessment

Introduction In many countries landslides and floods generate a yearly loss of property larger than that from any other natural disaster. Casualties due to slope failures and inundations are larger in the developing countries and economic losses are more severe in the industrialized world. In Italy, a nation-wide project aimed at collecting information on landslides and floods for the period 1918–1990 inventoried about 11,000 landslides and 5000 inundations (Guzzetti and others 1994). In the 5-year interval between 1991 and 1995, 44 (out of the 92) Italian provinces were affected by mass movements or inundations, with more Received: 20 November 1996 7 Accepted: 25 June 1997 P. Reichenbach (Y) 7 M. Cardinali 7 P. De Vita F. Guzzetti CNR - IRPI, via Madonna Alta 126, I-06100 Perugia, Italy

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Environmental Geology 35 (2–3) August 1998 7 Q Springer-Verlag

than 100 casualties (Guzzetti and others 1996a). Estimates for 1996, a particularly wet year, indicate that 30 provinces experienced flooding or landsliding, with more than 20 casualties. Direct and indirect damage was not ascertained, but it can be provisionally estimated in the order of several billion dollars. The analysis of the temporal recurrence of natural catastrophes shows that damaging events occur with a frequency higher than the social and economical ability to recover from previous events. Due to the high cost of ground protection, industrialized societies and developing countries are increasingly less eager to invest in structural measures to reduce natural risks. Hence, the new issue is the development of warning systems and land utilization regulations aimed at minimizing the loss of lives and property damage without investing in long-term, costly projects of ground protection (International Decade for Natural Hazards Reduction 1987; United Nation Disasters Relief Organization 1991). Within this framework, the identification of regional thresholds for the initiation of slope movements or widespread flooding may play a renewed role in helping to mitigate the risk. When a storm is expected or while it is passing over a territory, the monitoring of meteorological, hydrological or geotechnical parameters, such as rainfall intensity/duration, river discharge or groundwater pore pressure, coupled with initiation thresholds defined in statistical, deterministic or heuristic fashion, may allow for the identification of potentially hazardous conditions and the issuing of warning or alarm messages to civil protection authorities and the population. Indeed, there are only a few places in the world where rainfall thresholds are part of operational landslide warning systems, such as in the San Francisco Bay region (Keefer and others 1987; Wilson and others 1993; Wilson and Wieczorek 1995), in Hong Kong (Brand 1981; Brand and others 1984; Premchitt and others 1994), and in Japan (Onodera and others 1974).

Thresholds A threshold is defined as the minimum or maximum level of some quantity needed for a process to take place or a state to change (White and others 1996). A minimum threshold defines the lowest level below which a process does not occur. A maximum threshold represents the lev-

el above which a process always occur, i.e., there is a 100% chance of occurrence whenever the threshold is exceeded (Crozier 1996). For rainfall-induced slope failures a threshold may represent the minimum intensity or duration of rain, the minimum level of porewater pressure, the slope angle, the reduction of shear strength or the displacement required for a landslide to take place. Thresholds can also be defined for parameters controlling the occurrence of landslides, such as the antecedent hydrogeological conditions or the (minimum or maximum) soil depth required for failures to take place. Rainfall thresholds Rainfall-induced shallow landslides, mostly soil slips and debris flows (Ellen and Wieczorek 1988), are initiated by a transient loss of shear strength, resulting from the increase in porewater pressure, caused by intense rainfall onto loose surficial soil overlying firmer, less permeable bedrock (Campbell 1975; Wilson 1989). Ideally, the knowledge of the spatial distribution and variability of soil thickness and its properties, coupled with detailed measurements of rainfall, would allow for the temporal and spatial prediction of slope instability conditions during a storm. Unfortunately, knowledge on soil thickness, soil hydraulic and geotechnical properties, subsurface geometry and bedrock interaction is limited, and restricted to a few sites. The physical process controlling how and to what extent rainfall contributes to the increase in porewater pressure (i.e., infiltration) remains poorly understood, particularly on sloping ground. Additionally, antecedent conditions (hydrological and shear strength), locally important in determining where and when a slope will fail, are difficult to ascertain. Despite the promising approaches of deriving information on soil characteristics from topography (Moore and others 1993) and of coupling hydrological and slope stability models to forecast sites of potential instability (Montgomery and Dietrich 1994; Dietrich and others 1995), it remains difficult to define soil characteristics and to identify instability conditions over a territory whose extent is of interest for hazard evaluation and civil protection policy making. An alternative approach to the evaluation of the hazard induced by rainfall-induced shallow landslides consists of the definition of rainfall thresholds based on the statistical analysis of the relationship between rainfall and the occurrence of mass movements. The approach requires accurate rainfall data and detailed information on the occurrence of slope failures. Landslide occurrence is ascertained by compiling an inventory of slope failures. The inventory, completed immediately after the event to minimize information loss, records the location and time of occurrence of failures. The former can be obtained through field mapping and the interpretation of aerial photographs; the latter is determined mostly through interviews. Rainfall data should be collected from a carefully designed and sufficiently dense network of recording rain gauges, of probably no less than 1 gauge for about each 50 km 2. This value may be underestimated in mountainous regions, where topographic effects on rain-

fall are difficult to ascertain. Additionally, the temporal resolution of rainfall measurements must be adequate. Daily precipitation records do not capture peak rainfall intensity, a fundamental measure where convective precipitations of short duration and high intensity trigger diffused mass movements, and are inadequate for the definition of accurate, site-specific rainfall thresholds. Use of daily precipitation records is restricted to the definition of regional thresholds, where more accurate data are not available (Glade 1998). Where information on landslides and rainfall is available, plots can be prepared and threshold curves can be fitted as lower bounds for the occurrence of slope failures. The most commonly investigated rainfall parameters are: total (“cumulative”) rainfall; antecedent (“pre-event”) rainfall; rainfall intensity and duration. Various combinations of these parameters have been attempted. Thresholds were defined for: rainfall intensity (Brand and others 1984); the duration/intensity ratio (Caine 1980; Cancelli and Nova 1985); the duration above a predefined intensity level (Wieczorek and Sarmiento 1983); the cumulative rainfall in a given time (Govi and others 1985; Page and others 1993); the antecedent rainfall/daily rainfall ratio (Kim and others 1991); the event rainfall/yearly average rainfall ratio (Govi and Sorzana 1980); and the daily rainfall/antecedent excess rainfall ratio (Crozier 1986). Most of the proposed thresholds perform well in the region where they were developed but can not be exported to neighboring areas. Also, their temporal accuracy remains largely untested (Crozier 1996). Possibly, the best known rainfall threshold is that proposed by Caine (1980), who collected a worldwide set of rainfall and landslide data and proposed a function relating rainfall intensity and duration. Similar thresholds have been proposed for the San Francisco Bay region (Cannon and Ellen 1985; Wieczorek 1987), for Carinthia (Moser and Hohensim 1983), for the Southern Alps (Cancelli and Nova 1985; Ceriani and others 1992; Guzzetti and others 1992; Polloni and others 1992; Crosta 1990, Crosta 1994), for the pre-Alpine regions of northern Italy (Crosta 1994) and for the Piedmont region (Polloni and others 1996). Caine’s approach, not considering the antecedent conditions, is not suited for deep-seated landslides or failures triggered by low-intensity rainfall events, and where complex, leaky aquifers are present within the slope. The main operational advantage of rainfall thresholds, that justify their definition and application in warning systems, refers to the fact that rainfall is relatively simple and inexpensive to measure over large areas. Indeed, in places rainfall is being forecast with increasing accuracy. If rainfall data (measured or forecasted) are sufficiently dense in space, thresholds may allow for a good (fine) spatial resolution. Additionally, where data on soil properties and porewater pressure at shallow depth are available, rainfall thresholds can be linked to site-specific geotechnical models, largely improving the temporal prediction of shallow landslides (Keefer and others 1987). Drawbacks of rainfall thresholds are both conceptual and


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operational. Conceptually, rainfall is not the primary cause of slope failures. Landslides are caused by the buildup of high porewater pressure into the ground (Campbell 1975; Wilson 1989). Groundwater conditions responsible for slope failures are related to rainfall through infiltration, soil characteristics, antecedent moisture content, and rainfall history. These environmental variables (“preparatory factors”) are poorly known over large areas and exhibit considerable spatial and temporal variability. The assumption that the higher the rainfall intensity, the larger is the probability of failures is simplistic and not always acceptable. During the July 1987 storm in Valtellina (southern Italian Alps), an event with 50–100-year return period, soil slips and debris flows were more abundant on the southern side of the valley, which received one-third more rain than the northern one, but did not occur in the area where the highest intensity and cumulative precipitation were recorded. Due to the local geological setting, soil cover was absent and shallow failures did not develop where rainfall was more intense (Crosta and others 1990; Guzzetti and others 1992). In general, for the initiation of failures, the geological and morphological setting as well as the antecedent hydrological conditions play an important role until a regional (“maximum”) threshold of rainfall intensity is exceeded, and diffused landsliding occur independently of the antecedent conditions (Brand 1981). The maximum regional threshold may vary seasonally (Govi and Sorzana 1980; Govi and others 1985) as well as in response to changes in the environmental conditions (soil cover, land use). Operational limitations for the definition of rainfall thresholds refer mostly to the availability of data of adequate quality, resolution and recording length. During an event, a dense network of recording rain gauges is required, and immediately after the event a detailed inventory of landslides must be compiled. At least one event must be investigated in detail. Indeed, a single event may be so intense (“extreme” i.e., with a return period exceeding 100 years) as not to be representative of the local instability conditions. Thresholds based on extreme events can underestimate the probability of failures. Hence, a long record of rainfall measurements and many events resulting from different meteorological conditions, should be analyzed to define reliable rainfall thresholds. Unfortunatelly, information on an adequate number of events is seldom available. In Italy, as in many other countries, detailed rainfall data, from dense networks of continuously recording stations, are available only locally and for the last few years (20–30 years at most). Additionally, accurate landslide inventories are prepared mostly for major events. This largely limits the possibility of using historical data in defining accurate (site-specific) rainfall thresholds. Daily records of rainfall measurements are available for longer periods (up to 120 years in Italy), allowing for the evaluation of regional thresholds. Hydrological thresholds An alternative approach to the definition of thresholds for the (temporal) occurrence of landslides is based on 148


the analysis of river discharge measurements. Runoff is the result of a complex interaction between rainfall, infiltration, percolation to the groundwater table, overland flow and channel flow occurring along the drainage network and on the slopes (Freeze and Cherry 1979). Although not lacking uncertainties, particularly during flooding events, runoff measures the temporal and spatial response of a catchment to a meteorological event (a “rainstorm”) as well as the catchment hydrological antecedent conditions. In many areas, intense or prolonged rainfall can trigger diffused landsliding and cause extensive inundation. Hence, it should be possible to relate the variation in water levels at a gauging station in a river basin to the occurrence of landsliding and flooding in the catchment. Water levels, or the corresponding discharge measurements, can be used to define hydrological thresholds for the occurrence of geohydrological catastrophes. Requirements for such analysis are basic, namely: an inventory of landslides and inundations; and a record of runoff measurements. The latter, although not as common as rainfall measurements, are available at several localities with the required accuracy (i.e., average daily measurements) and for a long period of time (more than few decades). Historical information on past catastrophic events can be compared with historical discharge measurements to define statistical thresholds. This represents a major advantage over the definition of rainfall thresholds. Additionally, runoff is a better measure than daily rainfall to estimate regional thresholds for the occurrence of landslides. The approach does not lack limitations due mostly to the quality and representativeness of the runoff record. Hydrological thresholds can be defined only for catchments where runoff reflects the short-term hydrological behavior of the basin, particularly during extreme events. Hence, the approach can not be applied 1) to ephemeral streams, where discharge is episodic and peak water levels are difficult to measure; 2) where highly permeable rocks (i.e., limestone) crop out, altering the short-term hydrological response of the basin; and 3) where rivers are regulated or where the natural water flow is artificially controlled. Other limitations refer to the assumption that all environmental variables controlling river discharge remain constant during the time frame of the analysis. Modification in land-use practice (i.e., clear-cutting, reforestation, etc.) or the build up of levees and reservoirs change the runoff regime, limiting (and locally preventing) the use of discharge measurements for the definition of hydrological thresholds for landslide occurrence. Despite these limitations, the definition of regional hydrological thresholds for the occurrence of mass movements and inundations was attempted in the Tiber River Basin of central Italy. In the following, we present the results of such an evaluation, discussing advantages and drawbacks.

1918–1990 (the AVI Project), produced a catalogue of historical landslides and inundations in Italy (Guzzetti and others 1994). The inventory was completed through the The Tiber River basin extends for 17,169 square kilometsystematic review of newspapers, the interview of expert ers in central Italy and represents the second largest river witnesses, and the inspection of technical and scientific basin in Italy (Fig. 1). The area has a long history of hyreports and papers. For 22 provinces in cental Italy, 3521 drological catastrophes. Reports on landslides and inunlandslides and 2365 inundations were inventoried and dations go back to Etruscan and Roman periods. The provisionally mapped at the 1 : 100,000 scale. About onefirst documented information on slope movements in the half of the events (1495 landslides and 1051 floods) were hillsides of Todi and Orvieto dates back to the fourteenth located within the Tiber River Basin (Fig. 1). The 1495 century (Guzzetti and others 1996b). The oldest reported landslides occurred at 936 sites, and the 1051 inundations flood occurred in Rome in 381 B.C. Since then tens of caaffected 388 different localities (Guzzetti and others tastrophic events were reported. 1996a). There are 22 sites that were inundated more than 10 times in 72 years (1918–1990). Historical information on landslides and floods In the Tiber River Basin the spatial recurrence of floods In the early 1990s, a nation-wide project aimed at collectis twice that of landslides. This reflects both the morphoing information on landslides and floods for the period logical setting and a recording problem. Inundations occur along the drainage network and are (spatially) less variable than landslides that can occur on any slope, albeit with different probabilities. The historical catalogue is affected by recording problems related to the human perception of hydrological catastrophes. Floods are generally perceived as more catastrophic events than landslides. Landslides may occur days or even weeks after the onset of a meteorological event. Slope failures may not be perceived as directly related to the triggering event and are locally underestimated in number. For these reasons the historical record reflects only the minimum number of events that actually occurred. Of the 786 municipalities whose territory lie within the Tiber River basin, 251 were affected at least once by landslides and 148 by inundations. On average, each municipality experienced 6 landslides and 7 inundations. Townships more frequently affected by landslides were: Rome (150), Orvieto (88), Perugia (64), and Todi (41). Inundations were recurrent in Rome (180), Deruta (66), Torgiano (40), and Orte (38). The considerably high density of events in large municipalities (i.e., Rome, Perugia, Orvieto, etc.) reflects a known bias in the historical catalogue toward densely populated areas (Guzzetti and others 1994). For about one-third of all landslides (560) and 95% of all inundations (1005) the date of the event is known, making the information suitable for the definition of hydrological thresholds. A preliminary analysis of all dated events revealed a predictable seasonality. Inundations and landslides are more frequent in the winter season (Fig. 2A). Inundations are abundant in December, when precipitation is higher. Landslides are frequent in January and February, when rainstorms last longer and the antecedent conditions are heavier, increasing soil moisture. The frequency of catastrophic events exhibits a correlation with the general climatic trend (Fig. 2B). Landslides and inundations were found to be rare during World Fig. 1 War II and the post-war period, reflecting the incompletTiber River Basin location map of 1495 landslides (gray square) ness of the catalogue rather than a peculiar climatological and 1051 inundations (black dot) inventoried by the AVI condition (a dry period). project for the period 1918–1990 (Guzzetti and others 1996a). The reliability of archive catalogues is difficult to ascerLandslides occurred at 936 sites. Inundations affected 388 tain, and depends largely on the quality, availability, localities. For 560 landslides and 1005 inundations the date of occurrence is known completeness, and redundancy of the historical sources

The Tiber River project


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Fig. 2A, B Tiber River Basin frequency plots of landslides and inundations inventoried by the AVI project for the period 1921–1990, compared with the regional climatic trend, expressed by mean daily discharge at the Ripetta gauging station, Rome (heavy thick line). A Average, monthly frequency of landslides (white) and inundations (black). Dashed line is rainfall expressed as mean daily discharge equivalent. B Frequency of hydrological events with information on landslides (white) and inundations (black) in 3-year intervals

(Ibsen and Brunsden 1996). The drawback to any historical analysis is the lack of spatial completeness, resolution and precision, and an undefined overestimation of events which caused damage to man-made structures as opposed to an underestimation of events, even large ones, which took place in unpopulated areas (Casale and others 1994; Guzzetti and others 1994; Ibsen and Brunsden 1996). In the Tiber River Basin, due to the physiographical setting, sites affected by inundations are easily (and more precisely) localized than those affected by mass movements. Newspapers and chronicles most commonly report information on “widespread” or “diffused” landsliding (i.e., numerous landslides) without any detail on the exact location of failures. Hence, a landslide, unless it involved a town or a major facility, was difficult to locate with accuracy. Regardless of this limitation, if the date of occurrence was known, the landslide was used to define the threshold. An attempt was made to test the spatial reliability of the AVI inventory by comparing the distribution of recorded historical landslides with the spatial distribution of more than 20,000 slope failures mapped by photo-interpretation in the Umbria and Marche regions of central Italy (Guzzetti and Cardinali 1990; Antonini and others 1993). The test showed that the density of recorded historical events falling directly on landslide deposits, or within a distance of 500 meters, is twice the density of the events that lay at a greater distance. In other words, 70% of all recorded historical landslides in the Umbria and Marche regions lay on, or within a distance of 500 m to the nearest mapped landslide body. This is probably the best that one can expect from inventories completed with different techniques. Hydrological records Mean daily discharge measurements for 85 gauging stations in central Italy are available, in table format, from the Servizio Idrografico Nazionale. Data were digitized by the University of Perugia and stored into a computer database available on the Internet (http://wwwdb.gndci.pg. cnz.it). Of these gauging stations, 27 are located within the Tiber River Basin. Table 1 lists the Santa Lucia, Bet-

Table 1 Hydrological and lithological characteristics of 7 gauging stations in the Tiber River Basin that exhibit the longest record of measurement. The P.te Nuovo and Ripetta gauging stations (bold) were selected to define regional hydrological thresholds Gauging station

a b c d e f g

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S. Lucia P.te Felcino P.te Bettona Torgiano P.te Nuovo Macchiagrossa Ripetta

Basin

Tevere Tevere Topino Chiascio Tevere Nera Tevere

Record length (years)

Catchment area (km 2)

Permeable (%)

mean (m 3 s P1)

max (m 3 s P1)

37 37 39 46 55 26 64

934 2033 1220 1956 4147 4020 16 545

16 4 56 32 16 85 32

14 27 11 23 48 84 231

432 697 323 708 1011 354 2730


Daily discharge

Largest event (date) 14/12/34 25/12/59 15/12/37 15/12/37 15/12/37 15/12/37 17/12/37

corresponding to days of low water flow. The remaining 50% of the total discharge occurred on a limited number of days (about 20%), characterized by sustained, or high flow. Hence, hydrological events possibly associated with the occurrence of landslides and inundations are limited to 20% of the record of daily discharge measurements. Where predominately permeable rocks crop out (mostly limestone) the hydrological behavior is different. Due to the infiltration capacity and the storage effect of limestone, about half of the total discharge occurred on 35% of the days, characterized by sustained water flow (cf. Macchiagrossa Fig. 4).

Fig. 3 Tiber River Basin lithological map with the location of 7 water discharge gauging stations: a Santa Lucia, b Ponte Felcino, c Bettona, d Torgiano, e Ponte Nuovo, f Macchiagrossa, and g Ripetta. See Table 1 for station characteristics. Thin lines are major rivers, heavy lines are major drainage divides. Legend: 1 recent alluvial deposits, 2 volcanic rocks, 3 limestone and marl, 4 flysch deposits

tona, Ponte Felcino, Torgiano, Ponte Nuovo, Macchiagrossa and Ripetta gauging stations that exhibit the longest record of measurements (626 years; Fig. 3). Successively, the Ponte Nuovo and Ripetta gauging stations were selected to define regional hydrological thresholds because of their particularly long record (655 years) and the availability of information on historical catastrophic events. For each of the 7 gauging stations listed in Table 1, the frequency distributions of mean daily discharge and daily discharge volume were computed (Fig. 4). No attempt was made to consider different rating curves. Inspection of the cumulative curves revealed that, at each gauging station, about half of the total discharge occurred during approximately 80% of the entire measurement period,

Hydrological events To establish hydrological thresholds based on historical data, “hydrological events” were defined as a series of consecutive days having a mean daily discharge exceeding a predefined value, set equal to 80% of the cumulative frequency distribution of mean daily discharge at each gauging station. This cut-off value corresponds to about half of the total volume of water discharged by the Tiber River. Table 2 lists the cut-off values and the number of events that were identified at each gauging station. Hydrological events were identified by means of a computer program that searched the record of daily runoff, selected a cut-off value, and systematically recognized the events. Events lasting for more than 7 days were visually inspected to check for the presence of relative maxima within the sequence, i.e., for the occurrence of multiple events. For each event a set of hydrological parameters were automatically measured or estimated by the computer program, namely: 1. The highest value (“maximum”) of mean daily discharge (in m 3sec –1); 2. The cumulative (total) flood volume (in millions cubic meters), estimated as the sum of the daily flood volume computed by multiplying the mean daily discharge by the number of seconds in the day; 3. The event duration (in days); and 4. The event intensity (in m 3sec –1km –2), computed, at each gauging station, dividing the average value of the event daily discharge by the catchment area. The selected hydrological parameters reveal different aspects of each hydrological event. The maximum mean daily discharge (a measure of peak water flow) and the estimated flood volume (a measure of the total amount of water) allow for ranking the events according to their magnitude. The duration and intensity provide information on the geographical extent of the event, i.e., it being localized or spead over the entire basin. Hydrological parameters, being derived from the empirical analysis of the same data set, are all positively correlated. In general, severe events lasted for several days, extended over the entire basin, and were characterized by large flood volumes and moderate to high daily runoff. The average values of the (measured or estimated) hydrological parameters at each gauging station exhibit a relationship with catchment area. The correlation is positive and linear for flood volume, positive and nonlinear


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same time. These regional events exhibit peculiar hydrological characteristics. At the Ripetta gauging station they last for 7 days or more, mean daily discharge exceeds 800 m 3 sec –1, flood volume is greater than 300!10 6 m 3, and intensity exceeds 0.031 m 3 s –1 km –2. For each hydrological event, the number and location of all landslides and inundations known to have occurred were counted, and a set of “event maps” were prepared (Fig. 6). Catastrophic events (for which landslides and infor maximum mean daily discharge, and negative and undations are reported) exhibit larger hydrological panonlinear for the event intensity (Fig. 5). Estimated flood rameters (maximum mean daily discharge, flood volume, volume is not influenced by catchment lithology, justify- event duration and intensity) than the events for which ing the selection of cut-off values based on the percentnothing is reported in the historical catalogue. In general, age of total flood volume. On the other hand, both maxi- long-lasting and severe hydrological events were assomum mean daily discharge and the event intensity vary ciated with a greater occurrence of landslides or inundawith basin lithology and morphology. For the latter, two tions (Fig. 7). Landslides are relatively more frequent sets of curves, for mostly permeable (limestone) and than inundations when the event intensity is low, that is, mostly impermeable (flysch) basins can be identified for long-lasting events characterized by a constant, re(Fig. 5C). Differences in the hydraulic parameters for the gionally distributed rainfall. Ripetta gauging station are due to the catchment area Inspection of the cumulative frequency distributions of (much larger than that of its subbasins), the lithological catastrophic events reveals that, albeit with low frequency setting (the basin encompasses permeable and impermea- (^20–30%), landslides and inundations occurred also ble rocks), and the presence of dams. when the hydrological conditions in the catchment were The majority of the events that were identified at each not severe. Indeed, landslides and inundations were regauging station was limited to a portion of the basin. ported when maximum mean daily discharge, estimated Only a limited number of events (177) were identified in flood volume and event intensity had low (or very low) all gauging stations and affected the whole basin at the values. This stresses the role of (short- and long-term)

Fig. 4 Tiber River Basin cumulative frequency distribution plots of mean daily discharge (solid line) and of daily discharge volume (dashed line) for 4 gauging stations. The Ponte Felcino and Macchiagrossa gauging stations represent mostly impermeable (flysch terrain) and mostly permeable (limestone) catchments, respectively (see Fig. 3). The Ponte Nuovo and Ripetta gauging stations were selected to define regional hydrological thresholds

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52 45 38 44 40 16 31 0.032 0.030 0.020 0.026 0.024 0.029 0.018 0.070 0.060 0.035 0.046 0.043 0.035 0.027 0.385 0.175 0.120 0.135 0.171 0.057 0.083 75 70 66 77 68 26 54 30 60 25 50 100 120 300 99 190 58 130 270 154 610 103 432 94 697 110 323 106 708 106 1011 133 354 103 2730 3 5 2 4 9 10 26 19 37 12 26 61 57 237 123 187 94 226 548 700 2174 18 31 32 45 84 36 214 453 501 363 424 642 252 800 S. Lucia 30 P. Felcino 60 Bettona 25 Torgiano 50 P. Nuovo 100 Macchiagrossa 120 Ripetta 300 a b c d e f g

mean min cv max mean min cv max mean min cv max (10 6 m 3) (10 6 m 3) (10 6 m 3) (%) (m 3 s P1) (m 3 s P1) (m 3 s P1) (%) (m 3 s P1 km P1) (m 3 s P1 km P2) (m 3 s P1 km P2) (%)

Event intensity Maximum daily discharge Total flood volume

Cut-off Total Events value events with (m 3 s P1) information Gauging Station

Table 2 Characteristics of hydrological events inventoried at the 7 gauging stations in the Tiber River Basin

Fig. 5A–C Tiber River Basin plot of the correlation between average event parameters and catchment area: A estimated flood volume; B maximum mean daily discharge; C event intensity. Diamonds represent largely impermeable and circles mostly permeable catchments. Open symbols indicate events for which no information is reported in the historical catalogue. Close symbols indicate events for which information on landslides or inundations is reported. Thick lines fit close symbols and thin lines fit open symbols

antecedent conditions, but it also suggests a degree of unpredictability of hydrological catastrophes. Additionally, the historical database shows that, when hydrological conditions are not severe, landslides are more frequent than inundations, confirming that the spatial variability of local environmental factors that trigger mass movements cannot be fully captured by measuring daily runoff. At the Ponte Nuovo gauging station (e on Fig. 3) for the 65 years between 1925 and 1990 (with gaps between 1943 and 1945, due to World War II, and between 1981 and


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Fig. 6 Tiber River Basin event maps showing the location of sites affected by landslides (gray square) and floods (black dots)

1987), 642 hydrological events were identified with an average of about 11.7 events per year (Table 2). For 84 events (13%) the historical catalogue reports information on landslides and/or inundations. This is an average of 1.5 catastrophic event per year. For the majority of these events (52%) only information on inundations is reported. For the remaining events, 27% have historical information on slope failures, and 21% on both landslides and inundations. The latter can be ranked as particularly damaging events. About half (45%) of all hydrological events that lasted 10 days or more caused landsliding or inundations somewhere in the basin. The frequency of catastrophic events decreases for shorter events. About 90% of the events for which nothing is reported exhibit a maximum mean daily discharge of less than about 450 m 3 sec –1, an estimated flood volume of less than 125 millions m 3, and an event intensity of less than 0.06 m 3 sec –1 km –2. For the same level of runoff, 25% of inundations and 40% of landslides are reported. For a similar flood volume 40% of landslides and 40% of inundations, and for the same event intensity 50% of landslides and 30% of inundations are reported in the historical catalogue. Similar observations can be made for the Ripetta gauging station, located at the basin outlet (g on Fig. 3). Between 1921 and 1985 (64 years, with a gap in 1984) 800 hydrological events were identified, with an average of about 12.5 events per year (Table 2). For 214 (27%) of them the historical catalogue reports information on landslides and/or inundations, with an average of 3.3 catastrophic events per year. The most damaging events, for which both landslides and inundations are reported, account for 26% of the events for which historical information are reported. Of the remaing events, 43% report information on landslides and 31% on inundations occurring some154


where in the basin. Approximately 90% of events for which nothing is reported exhibit a maximum mean daily discharge of less than 950 m 3 sec –1, an estimated flood volume of less than 450 millions m 3, and an event intensity of less than 0.035 m 3 sec –1 km –2. For the same level of runoff, 48% of inundations and 65% of landslides are reported; for the same flood volume 70% of landslides and 60% of inundations, and for the same event intensity 65% of landslides and 45% of inundations are known. Regional hydrological thresholds The analysis of the frequency distributions of the 3 hydrological parameters estimated for the events inventoried at the Ponte Nuovo and Ripetta gauging stations, can be used to define regional hydrological warning thresholds. The cumulative frequency distributions of Fig. 7 are statistical, historically based thresholds for the occurrence of landslides or inundations. Indeed, any percentile in the cumulative curves can be selected as a warning or alarm threshold for the occurrence of slope failures or floods. Since the parameters used to classify the events reflect different hydrological characteristics, an attempt was made to combine them to improve the reliability of the regional thresholds. Various combinations were attempted, and the best results were obtained by coupling the event maximum mean daily discharge with the estimated flood volume, and the event intensity. The relationship between the event estimated flood volume and the corresponding maximum mean daily discharge at the Ponte Nuovo (Fig. 8A) and the Ripetta (Fig. 9A) gauging stations, reveals that the most damaging events, i.e., those for which both landslides and inundations were reported, are located in the upper-right section of the diagram, where hydrological conditions (water level and flood volume) are more severe. Horizontal dashed lines, at about 450 m 3 sec –1 for Ponte Nuovo and about 950 m 3 sec –1 for Ripetta, represent the 90% thresholds for the events for which no information is reported in the historical catalogue (Fig. 7). In other words, only 10% of the events for which no information is available

Fig. 7 Tiber River Basin frequency distributions plots of the events hydrological parameters for the Ponte Nuovo and Ripetta gauging stations. Solid dark lines represent the events for which no information on landslides or inundations is reported in the historical catalogue. Dashed lines represent the events for which landslides are known to have occurred. Dashed dotted lines represent the events for which inundations are known to have occurred

exceeds the threshold. Likewise, the vertical dashed lines represent the 90% thresholds for the event estimated flood volume. In this case, only 10% of the events that exceeded the given thresholds did not generate landslides or inundations. Similar considerations can be made for the relationship between the event maximum mean daily discharge and the corresponding intensity (Figs. 8B and 9B). Vertical dashed lines, at about 0.06 m 3sec –1 km –2 for Ponte Nuovo and about 0.035 m 3 sec –1 km –2 for Ripetta represent the 90% thresholds for the events with no reported historical information.


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Fig. 8A, B Tiber River Basin regional hydrological thresholds for the Ponte Nuovo gauging station: A relationship between maximum mean daily disharge and total estimated flood volume; B relationship between maximum mean daily discharge and the event intensity. Filled squares indicate that both landslides and inundations are reported. Filled triangles when landslides only are reported, and filled circles when inundations only are reported. Open symbols indicate events for which no information on landslides or inundations is reported in the historical catalogue. Dashed lines are 90% thresholds for total estimated flood volume and maximum mean daily disharge (see Fig. 7). Thick and thin solid lines are “upper” and “lower” regional hydrological thresholds, respectively

Fig. 9A, B Tiber River Basin regional hydrological thresholds for the Ripetta gauging station: A relationship between maximum mean daily disharge and total estimated flood volume; B relationship between maximum mean daily disharge and the event intensity. Filled squares indicate that both landslides and inundations are reported. Filled triangles when landslides are reported and filled circles when inundations are reported. Open symbols are events for which no information on landslides or inundations is reported in the historical catalogue. Dashed lines are 90% thresholds for total estimated flood volume and maximum mean daily disharge (see Fig. 7). Thick and thin solid lines are “upper” and “lower” regional hydrological thresholds, respectively

The solid lines in Figs. 8 and 9 are regional warning thresholds. They separate hydrological conditions that, from the historical record, are more (or less) prone to the occurrence of landslides or inundations. The “upper” thresholds (thick lines) correspond to regional “maximum” thresholds, whereas the “lower” thresholds (thin lines) are higher than the “minimum” regional thresholds. Although drawn heuristically, the thresholds fit a preliminary subdivision of hydrological events based on a multivariate regression model prepared using as predictor variables, the events hydrological parameters; and as the dependent variable, the occurrence of landslides or inundations. At Ponte Nuovo (Fig. 8) the most damaging events (black squares) are all located in the higher probability zone (above the “upper” threshold), where water level is higher and flood volume is larger. A few open dots are also present in the same area. They represent events for which

no information on landslides or inundations is reported. A detailed review showed that these events occurred in the decade 1940–1950 when, due to World War II and the post-war reconstruction, the historical catalogue is poor. These events can be considered as possible “false negatives.” An attempt was made to test the efficency of the Ponte Nuovo discharge/volume thresholds (Fig. 8A). The regional thresholds, defined considering catastrophic events which occurred upstream from the Ponte Nuovo gauge, were compared with the historical data on landslides and inundations available for the entire Tiber River Basin (symbols of Fig. 9A). As expected some of the most damaging events fell below the warning threshold, but the overall fit of the regional threshold remained satisfactory. At the Ripetta gauging station the most damaging events (black squares) exhibit a large scatter and hydrological thresholds are somewhat less well defined (Fig. 9A and


9B). In addition to possible inconsistencies in the historical catalogue, the loose constraint and the larger scatter may be explained by the large extent of the catchment and the corresponding large lagtime, the lithological and morphological complexity of the basin, and a lower spatial resolution of the hydrological threshold in the lowermost part of the basin. Despite these limitations thresholds for the Ripetta gauging station are still acceptable for regional warnings. As previously discussed, landslides and inundations were reported also when hydrological conditions were not severe. The combination of maximum mean daily discharge with estimated flood volume and the event intensity confirms that the probability of occurrence of landslides or inundations in such circumstances (i.e., in the lower-left part of the diagrams, below the lower threshold) is low, though greater than zero.

plied where runoff is episodic and peak water levels are difficult to measure, where catchment are underlain by highly permeable rocks, or where the natural water flow is artificially controlled. Further limitations involve to the assumption that all environmental variables controlling runoff remain constant. Significant changes in runoff due to the buildup of reservoirs, the change in land-use practice (clear-cutting, reforestation, etc.), or intense urbanization may limit (or prevent) the definition and reliability of hydrological thresholds. In the Tiber River Basin dams are present (Corbara Lake) or are under construction. In the period 1930–1960 reforestation was performed at several sites; in the last 30 years uncultivated and abandoned areas have increased over cultivated land and built-up areas have expanded. The contrasting effects of these changes on runoff, although largely unknown, do not appear to be sufficient to undermine the reliability of the proposed thresholds. The spatial resolution of hydrological thresholds is lower Conclusions (or much lower) than that of warning thresholds based on the analysis of rainfall data, the monitoring of pieThe evaluation carried out in the Tiber River Basin zometric levels or the measurement of ground displaceproved that hydrological conditions prone to the occurments. A better catalogue of historical information on rence of landslides and inundations can be ascertained landslides and inundations will certainly improve the through the analysis of runoff measurements coupled spatial resolution and reliability of hydrological threshwith historical information on catastrophic events. Exam- olds. In the Tiber River Basin, the Ponte Nuovo gauging ination of runoff records at several gauging stations alstation represents the best compromise between basin lowed for the identification of hydrological events that size and the volume of available information. For smaller were ranked according to a set of hydrological parametsubbasins the available historical information is insuffiers: 1) maximum mean daily discharge, 2) estimated cient to define thresholds. For larger areas (cf. Ripetta) flood volume, and 3) event intensity. These parameters, the basin is probably too large and complex to allow for expressing different hydrological characteristics, allowed the definition of reliable thresholds. for the statistical definition of regional thresholds for the A substantial improvement of the historical catalogue occurrence of landslides and inundations. Such threshwould require a large research effort, in order to increase olds can be used to issue regional warnings and may be the number of events inventoried and to refine the locahelpful in planning civil protection policies. tion of landslides and inundations. Unfortunately, collecBenefits and drawbacks of the approach have been distion and validation of historical information on hydrologcussed. The definition of hydrological thresholds has con- ical catastrophes is time consuming, expensive, error ceptual and operational advantages over other types of prone and subject to large uncertainties. The historical thresholds. Ideally, they better conform to the rather catalogue could be extended back further into the past, complex physical processes involved in the occurrence of but this will not improve the definition of thresholds inundations and rainfall-induced slope failures. Operawhich are constrained by the availability of runoff meastionally, they maximize the information content of histor- urements that cannot be extended in the past. ical data on natural catastrophes, and they prove effective For alert or alarm purposes and civil protection policy where meteorological (rainfall) and geotechnical data are making, regional hydrological thresholds cannot substinot available with the adequate spatial or temporal reso- tute for other warning thresholds. Rather they complelution. Limitations are due to the availability, quality and ment the other approaches where appropriate informaaccuracy of the historical records, as well as to geomortion is not available. It would be desirable to compare phological and physiographical constraints. Being defined the effectiveness and reliability of thresholds based on through the analysis of the frequency distribution of rudifferent approaches. For the Tiber River Basin an atnoff measurements, hydrological thresholds endure contempt was made to correlate hydrological thresholds with ceptual and operational limitations typical of statistical rainfall. Detailed rainfall data for the basin were not modelling. They cannot be exported to neighboring areas available to us, and rainfall characterstics were estimated and they suffer, in an unknown way, from the inadequa- from daily records. Inspection of rainstorms that trigcy of the basic data, whose reliability, accuracy and com- gered regional events revealed that landslides and inunpleteness are largely untested. The approach works only dations are more frequent when the runoff coefficent for catchments where the drainage network reflects the (i.e., the percentage of rainfall transformed into runoff) hydrological behavior of the territory. It cannot be apexceeds 0.5. Larger runoff coefficents produced wide-


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spread inundation and diffused landsliding. The largest hydrological event recorded in the basin during this century occurred on 15 December 1937 and produced inundations and landslides (Fig. 6). At the Ponte Nuovo and Ripetta gauging stations mean daily discharge exceeded 1000 and 2700 m 3 sec –1, respectively (Table 1). During this catastrophic event the runoff coefficent (at Ponte Nuovo) reached 0.6, accounting for the widespred flooding and the mass movements that, most probably, were more abundant than what was reported in the historical catalogue. In central Italy slope failures occur more frequently in landslide prone areas (Guzzetti and others 1996b). Where the territory is classified as unstable, landslides occur in response to hydrological conditions (rainfall intensity and duration, soil saturation, infiltration, overland flow, etc.) that are probably less severe than those needed to trigger landslides in stable areas. As a consequence, regional hydrological thresholds may over- or underestimate the local instability conditions. Ideally different thresholds should be prepared for different lithological and geomorphological environments. At present the information contained in the historical catalogue does not allow for such a detailed analysis, but research is under way to test the feasibility of the approach. Lastly, it should be pointed out that the (statistical and operational) performance of the proposed (“lower” and “upper”) thresholds is different. The “upper” (90% probability) threshold is close to the regional “maximum” threshold and can be used to issue alarms and for taking civil protection actions. The “lower” threshold requires a more complex interpretation. There is a 50% probability that a catastrophic event will occur when the threshold is exceeded. This is a rather poor statistical performance. Yet, since a community may not be willing to accept a 50% chance of being inundated or affected by a landslide, the threshold may prove valuable in a regional warning system. Indeed, the effectiveness of the “lower” threshold for civil protection actions can be improved by preparing hazard maps, i.e., by zoning the territory into areas that are prone to landslides or inundations. Acknowledgements We are indebted to the people who carried out the strenuous work of collecting historical information on landslides and floods in Central Italy, and to Prof. Manciola (University of Perugia) who made available the database of daily discharge measurements. The reviewers comments greatly improved the paper. The research was supported by CNR-IRPI and CNR-GNDCI grants. The paper is CNR-GNDCI report no 1640.

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