In karst environments, heavy rainfall is known to cause multiple geohydrological hazards, including inundations, flash floods, landslides and sinkholes. We studied a period of intense rainfall from 1 to 6 September 2014 in the Gargano Promontory, a karst area in Puglia, southern Italy. In the period, a sequence of torrential rainfall events caused severe damage and claimed two fatalities. The amount and accuracy of the geographical and temporal information varied for the different hazards. The temporal information was most accurate for the inundation caused by a major river, less accurate for flash floods caused by minor torrents and even less accurate for landslides. For sinkholes, only generic information on the period of occurrence of the failures was available. Our analysis revealed that in the promontory, rainfall-driven hazards occurred in response to extreme meteorological conditions and that the karst landscape responded to the torrential rainfall with a threshold behaviour. We exploited the rainfall and the landslide information to design the new ensemble–non-exceedance probability (E-NEP) algorithm for the quantitative evaluation of the possible occurrence of rainfall-induced landslides and of related geohydrological hazards. The ensemble of the metrics produced by the E-NEP algorithm provided better diagnostics than the single metrics often used for landslide forecasting, including rainfall duration, cumulated rainfall and rainfall intensity. We expect that the E-NEP algorithm will be useful for landslide early warning in karst areas and in other similar environments. We acknowledge that further tests are needed to evaluate the algorithm in different meteorological, geological and physiographical settings.
Torrential rainfall is known to cause inundations, flash floods and different types of landslides, including debris flows, soil slides and rockfalls. Less known is that intense rainfall can cause sinkholes, a subtle hazard in many karst environments (Parise and Gunn, 2007; De Waele et al., 2011; Gutierrez et al., 2014; Parise et al., 2015). Here, we describe a series of rainfall events and their ground effects in the period from 1 to 6 September 2014 in the Gargano Promontory, a karst environment and a popular tourist area in the Puglia (Apulia) region, southern Italy. In a 6-day period, a sequence of four heavy rainfall events, separated by periods with little or no rainfall, caused multiple geohydrological hazards in the promontory, including landslides, flash floods, widespread inundation and sinkholes. The death toll amounted to two fatalities, and a number of people were forced to leave their homes or businesses. Urban areas, tourist resorts, roads and rails were inundated and damaged, causing severe economic consequences. We have investigated the spatial–temporal relationships between the rainfall trigger and the geohydrological hazards, and we have designed and tested an algorithm for improved early landslide warning.
The paper is organised as follows. After a brief description of the study area (Sect. 2), in Sect. 3 we present the main meteorological and rainfall characteristic of the heavy rainfall period that has resulted in landslides, flash floods, inundations and sinkholes in the Gargano Promontory, and we investigate the spatial-temporal relationships between the intense rainfall and its ground effects. Next, in Sect. 4 we present a new method to forecast the possible occurrence of rainfall-induced landslides – and possibly the other associated geohydrological hazards – based on the continuous monitoring of local rainfall conditions. This is followed, in Sect. 5, by a discussion of the results obtained, including general considerations on the effects of intense rainfall in karst environments and their possible predictability using the new rainfall-based forecasting method. We conclude (Sect. 6) summarising the lessons learnt.
The study area covers approximately 1600 km
Panel
Meteorological setting for the period 1–6 September 2014 over
central and southern Italy. Images show Meteosat Second Generation (MSG) –
visible (VIS) 0.6
Although not particularly frequent or abundant compared to other areas in southern Italy, different geohydrological hazards have been reported in the Gargano Promontory. In recent historical times, destructive events occurred on 15 July 1972 (Bissanti, 1972) and 29 July 1976, when the city of Manfredonia, to the south of the promontory (Fig. 1), suffered inundations, and on 10–12 September 1982, when the town of San Marco in Lamis was hit by torrential rain. Landslides were reported in 1931, 1935, 1950, 1952, 1962, 1963, 1972, 1996 and 1997 and floods in 1996, 1997, 1998, 2002, 2007 and 2011. The main landslide types are rockfalls, topples and small disrupted rock slides that originate primarily from steep rock slopes. Flash floods and coastal floods occur in response to intense rainfall, but are not very frequent in the historical record. The karst environment favours the formation of sinkholes, i.e. karst forms also known as “dolines” (Ford and Williams, 2007), with a local density of up to 100 dolines per square kilometre (Castiglioni and Sauro, 2000; Simone and Fiore, 2014). Sinkhole features in the promontory range in size from small to very small, extending a few tens of square metres, to large and deep features including the “Dolina Pozzatina” with a depth of 100 m and a perimeter of about 1850 m, and to large polje, including the Sant'Egedio polje, near San Giovanni Rotondo (Fig. 1).
The meteorological event that brought torrential rainfall in the Gargano area began on 1 September 2014, when a perturbed nucleus originating from northern Europe moved to lower latitudes and impacted the Italian peninsula, starting from the northern and eastern sectors. In the early afternoon of 1 September, the central and southern parts of the Italian peninsula were first affected (Fig. 2a). Between 2 and 3 September, the low-pressure vortex moved towards the Ionian Sea and then to the Balkans and the Hellenic peninsula. The meteorological situation determined an inflow of perturbed masses of air over most of the Adriatic regions (Fig. 2b, c). The anticlockwise circulation affected most of central and southern Italy and persisted until 6 September. Residual perturbed meteorological conditions remained over the southern Italian regions; in particular those facing the Ionian Sea, with isolated minor precipitations until the late morning of 7 September (Fig. 2d).
The perturbed meteorological conditions over Italy resulted in torrential precipitation in the Gargano Promontory, with cumulated rainfall exceeding 500 mm in the 6-day period 1–6 September (Fig. 3). To study the intense rainfall period, we used hourly rainfall records available for 39 rain gauges pertaining to the national network of rain gauges operated in the area by the Italian National Department of Civil Protection and the Puglia Regional Government. Inspection of the rainfall records and of the geographical distribution of the precipitation (Fig. 3) revealed that (i) heavy rainfall persisted for the entire observation period, hitting different parts of the promontory at different times, and that (ii) seven periods could be singled out, including four rainfall (wet) periods and three no-rainfall (dry or nearly dry) periods (Fig. 3). The rainfall periods ranged from 8 to 49 h and were separated by dry periods lasting between 11 and 19 h (Table 1).
Rainfall and hydrological conditions for the period 1–6 September
2014 in the Gargano Promontory.
Characteristics of seven periods in the sequence of rainfall events
that hit the Gargano Promontory between 1 and 6 September 2014. Start and
end times are given in UTC
The first rainfall period (I) lasted 8 h, from 12:00 to 20:00 UTC
Following a dry period of 12 h (IV), rainfall started again on 5
September (V), and this time it was most abundant in the NE part of the
promontory. In this period, the Bosco Umbra and Vico del Gargano rain gauges
measured slightly more and slightly less than 100 mm of rain, respectively,
corresponding to a rainfall intensity exceeding 8.0 mm h
A rank analysis of rainfall measurements for six rain gauges in the 7-year period from April 2009 to April 2016, highlighted the severity of the 6-day rainfall period (Fig. 4). Except for the Monte Sant'Angelo rain gauge, located in the southern part of the promontory, the 1–6 September rainfall period exhibited the highest cumulated rainfall in the observation period. Adopting the classification proposed by Alpert et al. (2002), the rainfall was “torrential” in all the considered rain gauges and, for three of the rain gauges, it was the only torrential event in the (short) record available (Fig. 4).
Rank order analysis of rainfall events in the Gargano Promontory
from April 2009 to April 2016. Coloured bars show cumulated event rainfall
for six rain gauges:
The sequence of intense rainfall events resulted in a number of floods,
flash floods, landslides and sinkholes and caused two flood fatalities at
Peschici and at Carpino (
Map showing location of event landslides, floods and sinkholes triggered by the 1–6 September 2014 intense rainfall event in the Gargano Promontory. WGS84/Pseudo Mercator (EPSG:3857).
The consequences of the storm of September 2014 were reported soon after their occurrence, and a first analysis was carried out immediately in its aftermath. The collection of information was obtained searching different sources: (i) field surveys, (ii) technical reports produced by geologists, and (iii) online national, regional and local newspapers.
The collected information allowed the geographical coordinates of each phenomenon, its occurrence date and the type of hazard to be reconstructed.
No geological and geomorphological details were available for the landslides, especially when the information was found in newspapers. A specific landslide catalogue was built and managed in a GIS environment. The catalogue lists the following items: (i) event identification code, (ii) source of information, (iii) landslide location (geographic coordinates, municipality, province), (iv) occurrence date and time (if available), (v) spatial and temporal accuracy and (vi) landslide type.
As concerns floods, the main information consisted of the areas of interest, the reported damage and the extent of the flooded territory. Information on sinkholes included the occurrence site obtained through field surveys (high geographical accuracy) and the occurrence time, which was mostly based upon interviews with local inhabitants (low to medium temporal accuracy).
Flooding was widespread in the Candelaro catchment that bounds to the SW the
Gargano Promontory (Fig. 5). Two hydrological gauging stations, one located
where the Candelaro River crosses State Road SS 272 (W of the Gargano range)
and one where it crosses the Provincial Road SP 60 near to the outlet in the Manfredonia Gulf (Fig. 5), measured very high water levels. The upstream
gauge along the SS 272 measured a first peak of 5.30 m at 02:00 UTC
Examples of geohydrological hazards triggered by the 1–6
September 2014 torrential rainfall in the Gargano Promontory.
Inundations were also severe along the northern coastal area, between Varano Lake and Vieste, and particularly between the towns of Cagnano Varano and Carpino (Fig. 5). Near Varano Lake, large agricultural areas were inundated. Along the northern coast of the promontory, flash floods produced by small torrents occurred mostly on 5–6 September. In the morning of 5 September, the Macchio Torrent overflowed and inundated Vieste, and several touristic sites. Overflowing of minor torrents and ditches was reported in the early hours of 6 September in the towns of Peschici, Vico del Gargano and Rodi Garganico.
The torrential rain caused a number of landslides, mostly shallow landslides (Fig. 6c, e, f). Overall, we collected information on 46 landslides, including 14 earthflows, 14 debris flows, 11 soil slides, 4 rockfalls, 1 mudflow and 2 slope failures of undetermined type. This is a subset of all the event landslides in the Gargano Promontory. Based on the type of failures, we hypothesise that all the landslides were from rapid to extremely rapid. Most of the mapped landslides were in the municipalities of San Marco in Lamis, Ischitella and Cagnano Varano. Landslides were also reported near San Giovanni Rotondo, Rignano Garganico and Rodi Garganico (Fig. 5).
Analysis of the spatial and temporal distribution of the event rainfall and of the triggered event landslides. See text for explanation.
We searched information on the time or period of occurrence of the
landslides. However, for most of the landslides the time or period of
occurrence remains unknown or suffers from very large uncertainty. For only
nine landslides we obtained reasonably accurate information on the period of
occurrence of the slope failures. On 3–4 September, four landslides
occurred near San Marco in Lamis, along SS 272, most probably in the 3 h
period between 23:00 and 02:00 UTC
The torrential rainfall also caused sinkholes. We mapped 10 small sinkholes near the villages of San Marco in Lamis and Monte Sant'Angelo (Fig. 5). Based on their morphology and shape, the sinkholes were classified as collapse or cover-collapse sinkholes (Gutiérrez et al., 2008, 2014). At San Marco in Lamis, four sinkholes affected the lower part of a pre-existing karst depression. The deepest sinkhole was about 6 m deep, 5 m wide and exposed limestone and residual terra rossa deposits that represent the upper part of the epikarst (Williams, 2008) (Fig. 6d). Other sinkholes were less developed and were detected and mapped locally only based upon morphological considerations. Due to the remote areas where the sinkholes occurred, their limited sizes (Fig. 6g) and the difficulty of detecting them, no accurate information is available on the time or period of occurrence of the sinkholes.
To help investigate the effect of the changing spatial and temporal
distribution of the rainfall on the location of the landslides, floods and
sinkholes, we prepared Fig. 7, which portrays, for each period in the sequence
of rainfall events (Sect. 3.2), maps showing the spatial distributions of
the mean rainfall intensity (in mm h
An inspection of Fig. 7 reveals that the total cumulated rainfall, exceeding
100 mm in large parts of the promontory, was the result of separate rainfall
events that hit different parts of the promontory at different times. The
first rainy period (I) was more widespread but characterised by an overall
moderate cumulated rainfall not exceeding 50 mm and rainfall intensity not
exceeding 6.2 mm h
In the central part of the promontory, landslides were also reported
(cluster C) during the second dry period (IV). Given the poor temporal
accuracy of these landslides (i.e. between 14:00 and 21:00 UTC
We used the rainfall records and hazard information available for the Gargano event to design and test an algorithm for the possible operational forecasting of rainfall-induced landslides and other geohydrological hazards, including flash floods and sinkholes
The ensemble–non-exceedance probability (E-NEP) algorithm exploits a
standard rainfall record obtained by a rain gauge to trace in time the
probability of possible landslide occurrence and of related
geohydrological hazards. For the purpose, for each time
Figure 8 portrays the logical schema for the E-NEP algorithm, which consists
of two nested loops. First, the maximum length of the considered rainfall period,
Logical scheme for the E-NEP algorithm.
Figure 9 exemplifies the application of the E-NEP algorithm to a specific
rainfall record, at a given time
We applied the E-NEP algorithm to the 13-day period between 31 August and
12 September,
which encompasses the entire series of rainfall events that hit the
Gargano Promontory. We applied the algorithm to synthetic hourly rainfall
records reconstructed for the locations of the four spatial-temporal
landslide clusters identified in the study area (Fig. 7). To reconstruct the
synthetic rainfall records, we interpolated the hourly rainfall measurements
obtained by 39 rain gauges in the Gargano Promontory and the surrounding
regions at the landslide locations. For that purpose, we used a standard
inverse weighted distance (IDW) spatial interpolator (Shepard, 1968) to
obtain hourly rainfall values on a regular 500 m
For our analysis, and for each landslide cluster,
Results of the analysis of the four landslide
clusters are shown in Fig. 10, in which the single plots show, from top to bottom, the temporal
evolution of (i) the measured and the cumulated rainfall, (ii) the
NEP
Exemplification of the E-NEP algorithm used to provide a
non-exceedance probability (NEP) of possible landslide occurrence. Panels
Results of the application of the E-NEP algorithm to synthetic
rainfall records reconstructed for the locations of four landslide
clusters.
For cluster A, encompassing landslides occurred along State Road SS 272 and
SP 48 near San Marco in Lamis; a short rainfall burst hit the landslide area
at 12:00 UTC
Following the landslide occurrence, all the NEP values remained high for 12 h. When the rainfall stopped, at 14:00 UTC
We observe that landslides in cluster A occurred when the NEP
Inspection of the other plots in Fig. 10 reveals significant similarities in the temporal evolution of the metrics computed by the E-NEP algorithm for the other three landslide clusters, when compared to the same metrics computed for cluster A. Specifically, (i) all landslides occurred when the NEPmax was close to its maximum value, and immediately before landslide occurrence (ii) NEP50 was close to NEPmax, (iii) the range NEP25-NEP75 was narrow, (iv) there was a sudden increase of all NEP percentiles (Fig. 10a2) except NEP10 (Fig. 10c2) and (v) the landslides occurred after the DNEPmax had started to rise following a previous sudden drop. We consider these observations diagnostic of the rainfall conditions that have resulted in landslides (and other geohydrological hazards) in the Gargano Promontory in the period 1–6 September 2014.
The analysis of the rainfall records and the geohydrological hazard information available for the Gargano Promontory rainfall events between 1 and 6 September 2014 (Sect. 3) and their application to test the ensemble–non-exceedance probability (E-NEP) algorithm (Sect. 4) allows for general and specific considerations.
We first observe that landslides in the four examined clusters occurred for
different levels of the cumulated event rainfall,
As discussed in Sect. 4.2, a number of potentially diagnostic observations
drawn from the ensemble of metrics produced by the E-NEP algorithm were
common to all the examined landslide clusters, including the facts that (i) all
the landslides occurred when NEP
We conclude that the ensemble of the metrics produced by the E-NEP algorithm
provides better diagnostic results than the single metrics often used for
landslide forecasting, including rainfall duration, cumulated event
rainfall and rainfall intensity (Guzzetti et al., 2007). This is visually
portrayed in Fig. 10, where the temporal trend of the cumulated rainfall is
less diagnostic than the corresponding trends of the NEP
We maintain that the E-NEP algorithm is potentially useful for near-real-time landslide warning, but we acknowledge that more investigations are required to test the algorithm in different meteorological, geological and physiographical settings. The sequences of closely spaced rainfall events in the Gargano Promontory covered a long period (6 days), and this made it particularly well suited for the design and testing of the E-NEP algorithm. The sequence of rainfall events was also the result of a relatively simple meteorological setting. More tests are needed to evaluate the performance of the E-NEP algorithm for shorter and longer rainfall periods, and in different and more complex meteorological conditions.
We stress that the E-NEP algorithm was designed to attempt to forecast rainfall conditions for the possible occurrence of landslides that react rapidly to a rainfall input. These are typically shallow landslides, including soil slides and debris flows. E-NEP was not designed to attempt to evaluate other landslides that react slowly or very slowly to a rainfall input, including, for example, deep-seated landslides, shallow landslides in stiff clay. Also, E-NEP was not designed to attempt to predict landslides caused by meteorological triggers other than intense rainfall, including, for example, rain-on-snow events or rapid snow-melt events. However, we expect that E-NEP can be adapted to forecast shallow landslides caused by intense rainfall even in specific, local conditions (e.g. in areas burnt by wildfires; Cannon et al., 2010; De Graff et al., 2013; Moody et al., 2013), provided that sufficient information is available to apply the method proposed by Brunetti et al. (2010) and Peruccacci et al. (2012).
Analysis of the rainfall conditions that have resulted in landslides, flash floods, inundation and sinkholes in the investigated period in the Gargano Promontory revealed that the geohydrological hazards occurred in response to extreme rainfall conditions. This is confirmed by the fact that (i) rainfall was torrential (Alpert et al., 2002) (Fig. 4) and (ii) the geohazards – and particularly the landslides – occurred when all the NEP percentiles were close to the possible maximum value of the non-exceedance probability of possible landslide occurrence (Fig. 10), which represent very severe rainfall conditions. The available record of historical landslides and floods indicates that these are not very frequent or abundant compared to other areas in southern Italy. We conclude that in the Gargano Promontory meteorologically driven hazards occur in response to extreme (i.e. rare) meteorological conditions. For rainfall-driven hazards, the landscape in the Gargano Promontory exhibits a threshold behaviour that can be modelled conceptually by a Heaviside step function (Abramowitz and Stegun, 1972). For light to heavy rainfall events (Alpert et al., 2002) geohydrological hazards do not occur or are rare and minor, whereas for heavy–torrential to torrential rainfall events they are abundant and particularly disruptive. We attribute the threshold-based behaviour to the karst environment that dominates the landscape in the promontory.
In the karst environment of the promontory, rainfall infiltration is efficient even for high-intensity rainfall rates. This limits the occurrence of landslides, except for very intense (i.e. extreme) rainfall events. On the other hand, the arrival of a great amount of rainfall in a setting typically characterised by water infiltrating within the rock mass through the network of conduits and joints highly facilitates the formation of flash floods, particularly in small catchments, as has been frequently registered also in other parts of Puglia (Parise, 2003; Mossa, 2007). Further, karst aquifers have very poor retention capacity. These and other characteristics allow the flash floods to be identified as one of the main hazards in karst terrains (Fleury et al., 2013; Gutierrez et al., 2014; Parise et al., 2015).
In the sinkholes, the presence of residual soils varies largely, depending on the location, size and depth of the sinkholes. Where the infiltration is reduced, partial or total inundation of the sinkholes occurs. These local inundations are difficult to detect because they last only for short periods and because they often go unnoticed in the rural, scarcely populated landscape of the promontory.
The torrential rainfall has triggered sinkholes in the Gargano Promontory (Fig. 6d, g). Accurate information on the time or period of occurrence of the sinkholes is not available, and even the simple detection of the sinkholes was hampered by their small size and the remote location of the events. However, sinkholes represent a subtle and serious hazard in the promontory and in other karst areas (Parise and Gunn, 2007; Gutierrez et al., 2014; Parise et al., 2015), and establishing methods and procedures for their possible forecasting is of primary interest in karst environments. Based on the analysis of the 1–6 September 2014 Gargano rainfall period, we confirm that in the promontory, and in similar karst areas, torrential rainfall can trigger sinkholes, and we hypothesise that approaches based on the near-real-time monitoring of rainfall (e.g. the E-NEP algorithm) can be used to forecast the possible occurrence of rainfall-induced sinkholes. We acknowledge that an analysis of a larger number of events is required to test this hypothesis.
We studied a period of torrential rain between 1 and 6 September 2014 in the Gargano Promontory, Puglia, southern Italy, which caused a variety of geohydrological hazards, including landslides, flash floods, inundations and sinkholes. We obtained information on the location of the events through field surveys and the analysis of anecdotal information obtained from various sources. The temporal information varied among the hazards. For inundations the time or period of occurrence were known from gauge data (Fig. 3h), and for flash floods from anecdotal sources (Fig. 6a, b). For landslides, the period of occurrence was inferred from anecdotal sources for only nine (out of 46) slope failures and with significant uncertainty. No information on the time or period of occurrence was available for the sinkholes. We conclude that the ability to obtain accurate temporal information for the different hazards, which is important for establishing and validating early warning systems, depended on the extent and the location of the different hazards. The temporal information was most accurate for flooding along the Candelaro River (Fig. 3h), followed by flash floods and landslides (Fig. 3), and was not available for the sinkholes.
We used the rainfall and the landslide information available to us to design and test the new ensemble–non-exceedance probability (E-NEP) algorithm for the quantitative evaluation of the probability of possible occurrence of rainfall-induced landslides and of related geohydrological hazards (e.g. flash floods, sinkholes). For the investigated rainfall events, the ensemble of the metrics produced by the E-NEP algorithm provided better diagnostics than the single metrics often used for landslide forecasting, including rainfall duration, cumulated rainfall and rainfall intensity (Guzzetti et al., 2007; Brunetti et al., 2010; Peruccacci et al., 2012). We maintain that the E-NEP algorithm is potentially useful for landslide early warning, but we acknowledge that more work is needed to test the algorithm in different meteorological, geological and physiographical settings.
Our analysis revealed that in the Gargano Promontory meteorologically driven hazards occur in response to extreme (i.e. rare) meteorological conditions, and the karst landscape responds to torrential rainfall with a threshold behaviour. For light to heavy rainfall events (Alpert et al., 2002) landslides, floods and sinkholes do not occur or are rare and minor, whereas for heavy–torrential to torrential rainfall events they are abundant and particularly disruptive, as for the case for the 1–6 September 2014 event. We maintain that this information is useful for landslide early warning systems (and for other geohydrological hazards).
The authors declare that they have no conflict of interest.
Work performed in the framework of projects supported by the Italian National Department for Civil Protection (DPC), and the Puglia (Apulia) Regional Government (PRG). Maria Elena Martinotti and Massimo Melillo were supported by two grants of DPC. Luca Pisano was supported by a grant of PRG. Edited by: A. Günther Reviewed by: J. De Waele and one anonymous referee