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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
393
DOI: 10.4408/IJEGE.2013-06.B-37
LANDSLIDE AND FLOOD HAZARD
FROM THE LAGO SIRINO, BASILICATA, ITALY
L
uigi
COPPOLA
(*)
, A
ntonio
FILARDI
(**)
& E
dwArd
N. BROMHEAD
(***)
(*)
University of Basilicata - Potenza, Italy
(**)
Mayor of Nemoli and Engineer - Nemoli, Italy
(***)
Kingston University - School of Civil Engineering and Construction - Kingston upon Thames, UK
dam would lead to a flood down the natural channel
to Nemoli and beyond. While the town centre is not
directly threatened, there are numerous houses on the
edge of the town that are. Around the rim of the lake,
which despite its potential for mayhem, is rather at-
tractive, is an array of houses, hotels, restaurants and
bars, as the local community relies on the existence
of the lake and its considerable visual attractions for
a lively tourist business.
This paper describes the geological setting of the
lake, with some accounts of past movements and wa-
ter losses, but in the absence of detailed subsurface
investigations, the short description that follows is
mainly geomorphological in character and no further
prognosis of the behaviour of this system is possible.
K
ey
words
: lake, sinkholes, springs, seepage erosion, landsli-
des, flood hazard
INTRODUCTION
Landslides and floods are endemic throughout Eu-
rope, but the Italian peninsula is particularly suscep-
tible due to its climate, geology and tectonic setting.
It is widely recognised that severe consequences are
associated with landslides that release the contents of
natural lakes or artificial reservoirs, the latter including
those that retain mine waste with a high water content.
While the most well-known and catastrophic ex-
ample of modern times of the release of water from
a reservoir is, of course, the example of the Vaiont
ABSTRACT
The Sirino lake or Lago Sirino in Basilicata, It-
aly, is possibly a disaster waiting to happen, in some
ways similar to the situation at Vaiont in the months
and years leading up to the landslide there in Octo-
ber, 1963, with both precursor movements and expert
opinion dismissed and not acted upon.
The lake is retained in a basin, which is entirely
contained within a fault-bounded and displaced block
of Jurassic-age rocks, thought to be Schisti Selice.
The lake is fed by precipitation on the adjacent moun-
tainside (Monte Sirino), which rises to approximately
2000 m a.s.l. A significant proportion of the precipi-
tation is snow melt. The natural dam retaining the
lake moves from time to time, creating fissures down
which some of the lake disappears. Lake water level
is usually maintained by spring via a stream, and this
water also infiltrates the rock mass, and later issues
downslope in a series of copious springs that feed a
further series of streams. These downslope streams
are exploited for water supply and irrigation. This
lake is approximately circular in plan with a diameter
of about 200 m, and a maximum depth of about 12m.
The maximum lake level is controlled by an overflow
weir and discharge tunnel.
Downslope of the natural dam and the lake which
it impounds, there are numerous small settlements,
and at a distance of 2.5 km and some 370-400 m
lower in elevation, there is the town of Nemoli which
has over 1600 inhabitants. Any breach of the natural
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L. COPPOLA, A. FILARDI & E.N. BROMHEAD
394
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
or the end of the Würm, between about 11,000 and
9,000 years ago. In such landslides, the masses are
structurally chaotic, and even today are currently in a
state of limiting equilibrium, as a result of post-gla-
cial changes in slope morphology caused by river and
other erosion. They have one or more shear surfaces
often at depths greater than 40 m from ground level
and their longitudinal profiles sometimes extend kilo-
metres from their source. Commonplace movements
are slow and of small displacement, but significant
reactivation leads to a devastating collapse of the
slope. Most affected are towns and urban areas on the
summits of the mountains, but settlements below the
source area are at risk of being overwhelmed.
In southern Italy, there are few high-altitude lakes,
largely because of climatic factors, and this the risks of
sudden emptying of such lakes are not widely experi-
enced, in contrast to the processes of vulcanism, earth-
quakes and landslides which are regularly experienced.
However, springs are common at mid slope, especially
in the area under consideration, and the hydrogeology
at Sirino has produced a series of clean, clear spings of
potable water that provides a feed for the lake.
LOCATION AND GEOLOGY
The Lago Sirino or Sirino Lake (Fig. 1) is a small
lake located at the foot of the western slope of Mount
Sirino but at an altitude of 783 m above sea level, and
below the lake the hillside continues to slope steeply.
The lake and the settlements that surround it are part
of the Lucanian Apennines National Park.
According to S
cAndonE
(1972), the lake lies in a
small basin excavated in glacial debris deposits com-
posed of purplish red silt-clay mixed with calcareous
dam (which retained a storage reservoir for the Piave
hydroelectric scheme) and the mountain landslide
that pushed much of the reservoir contents over the
dam without breaching it, other examples, even in
Italy have occurred. For example, the 1972 collapse
of an earth embankment retaining tailings from a
fluorite mine in the Val di Stava caused significant
loss of life and material damage.
Historically, lakes retained behind valley-block-
ing landslides have overtopped and eroded the natural
dam away, for example the landslide at Borta (c
AvAL
-
Lin
& M
ArtiniS
, 1974), and so a number of such land-
slide dams have been provided with control works to
enable the normal river discharge to flow safely over
the dam. A modern example is the Val Pola landslide,
and historical examples is at Alleghe (d
E
B
iASio
et
alii, 2000). Other seiches causing dam overtopping
include the case of the reservoir at Fedaia, caused by
a landslide of ice and debris from the Marmolada Gla-
cier. As at Vaiont, the dam survived the event. S
chuS
-
tEr
(1983) describes numerous examples of landslide
dams; S
chuStEr
(2000) describes many instances of
dams built on landslides. The head drop across a land-
slide dam is commonly utilised for hydroelectric gen-
eration, with examples in the Dolomites (Venetian Pre
Alps) given by c
oppoLA
& B
roMhEAd
(2008).
Natural lakes sometimes release large volumes
of water. This is common in the case of glacial lakes,
which may be dammed by ice or moraine. In October
1994, a partial collapse of a moraine along the edge of
the Luggye Lake in the Bhutan Himalaya released a
glacial outburst flood (n
AyAr
, 2009) that swept away
a farming community with their livestock, crops, and
homes. The death toll of 21 would have been much
higher in a more densely populated region.
Occasionally, lakes are impounded in the hollows
of an existing landslide. A major example is given by
h
Ancox
& p
Errin
(1994): their example of the Green
Lake landslide exhibits several of the phenomena
above: it slid into an existing lake, it also formed a
landslide dam, and a lake also formed on top of the
26 km
3
of landslide deposits. Landslide ponds or lakes
are at risk of being released through movements of the
landslide, particularly where such movements open up
pathways via joints and fissures in the landslide mass.
The Lago Sirino is believed to be one such lake.
It is believed that the lake has formed on top of a
landslide that occurred at the end of the last ice age,
Fig. 1 - Location map of the Italian Peninsula, indicating
the Basilicata Regio in the south together with the
location of the Lago Sirino and the town of Nemoli
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LANDSLIDE AND FLOOD HAZARD FROM THE LAGO SIRINO, BASILICATA, ITALY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
395
passes, and on which there are two crossroads. This
ridge separates the lake from the head of a Vee-shaped
and therefore stream-cut valley, down which the over-
flow spillway channel runs, and the overflow from
the lake passes underneath the road in a small tunnel.
Some distance down the valley, small springs emerge,
which are used inter alia to feed a fish farm. Given
that the width of the road and the two intersections are
modern constructs, the original ridge must have been
smaller still, in which case it is tempting to ascribe the
whole lake to an unrecorded damming of the course
of a stream at some time in the past. Moreover, the
and siliceous breccias which rest on the Lagonegro
siliceous shale unit. The western side of M. Sirino is
composed of Mesozoic and Cenozoic soils of differ-
ent paleogeographic domains structurally affected by
the Apennine tectonic phase I (Pliocene). The soils be-
long to the oldest unit (Lagonegro) (S
cAndonE
, 1967,
1973), from the oldest to the most recent we have (see
Fig. 2. which shows the structural geology):
- Monte Facito Formation - Lower-middle Triassic
(MF);
- Limestone with flint - Upper Triassic (CS);
- Siliceous shale - Upper Triassic - Upper Jurassic
(SS);
- Flysch Galestrino - Upper Jurassic - Lower Creta-
ceous (G).
(Letters in this list are a key to the structural geology
map).
The most recent rocks outcropping on the site be-
long to the Liguride unit and are represented by part of
the Crete Nere (CN) Formation which is Cretaceous
- Middle Eocene (B
AnArdi
et alii, 1988).
All these units are strongly tectonized, often un-
structured with clastic fragments e.g. the siliceous
shale (SS) and flysch Galestrino (G).
The phase of compression in the genesis of the
Apennines (d
ogLioni
et alii, 1996; p
AtAccA
& S
cAn
-
donE
, 2001), is still causing shear deformations along
strike-slip faults with the oldest oriented NE-SW and
along NW-SE thrust faults (c
oppoLA
, 1993).
ORIGINS OF THE LAKE
It is widely and perhaps correctly believed that
the lake is a natural feature, although one that has
been somewhat landscaped into today’s form. It is
fed by a series of springs, the discharge from which
flows into the lake via a small channel which en-
larges into a sort of canal before entering the lake at
about its deepest point. For this paper, the Authors
have also adopted the conventional explanation that
the lake is a natural feature.
Local folklore has a fanciful (and religious) ex-
planation for the origins of the lake, in which the lake
formed as divine retribution for peasants working in
their farms instead of attending church. An interesting
point about the myth is that it is unequivocal that there
was a time when the lake did not exist. The south east-
ern end of the lake is dammed by a very small ridge,
along which the main road through the settlement
Fig. 2 - Structural-geological map of the area around Lago
Sirino and the town of Nemoli. Line of section
Marked AA with arrows showing view direction
Legend: Solid lines: large displacement transcur-
rent fault associated with the recent stress field,
with arrows showing left or right lateral move-
ment. Dashed line: position of fault not apparent at
surface. Tags denote normal fault, Vee tags denote
overthrust.fault. Dip angles shown conventionally
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L. COPPOLA, A. FILARDI & E.N. BROMHEAD
396
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
located in the debris slope just upstream of the reser-
voir and to the NNW. Water exits the lake through a
variety of processes including evaporation, seepage,
some outflow through a tunnel and spillway at very
high water levels, and more worryingly water losses
through occasional ephemeral sinkholes (Fig. 4) that
for centuries have opened and subsequently closed in
the bottom of the lake. When a sinkhole opens, the
lake level falls rapidly. These sinkholes form due to
tension in the rock mass, and are an indication of the
precarious state of stability of the whole slope. An ex-
ample of one of these sinkholes was reported to have
occurred on the 24
th
July 2011 (L
ovoi
, 2011), although
in the photograph accompanying that article this sink-
hole occurred above lake level in the bank. More seri-
ous is the case where they occur in the bed of the lake,
and evidence of such submerged sinkholes was seen
by the Authors in July 2012.
Three other springs are situated downslope of
the lake in the direction of a left-lateral NW-SE
strike-slip fault which skirts the southern limit of the
lake consists of siliceous shales of the formation of
red siliceous shales. They are perennial (or continu-
ous) springs with flow rates that are fairly constant
over time, although when one of the sinkholes opens
in the lake floor, the discharge from the springs in-
creases correspondingly. Discharge from the springs
is collected and used for water supply purposes down
valley: at one time the discharge from these springs
also drove a water mill.
spring-fed stream that enters the lake turns by more
than 90° at a very imposing ridge that also supports
the road at the northern end of the lake.
This issue could be resolved by putting a borehole
in the road, or perhaps sampling the lake-bottom sedi-
ments. However, even if this speculation were to be
proved true, then the situation regarding hazard would
remain unchanged, although the lake would lose its
status as a natural feature. One of the Authors (EB) is
convinced that the lake is at least in part man-made,
and if in the UK, would be covered by the Reser-
voirs Safety Act of 1975 (the latest in a series of laws
starting in 1930) which would certainly cover it as
Lago Sirino’s volume is significantly greater than the
25,000 m
3
qualifying volume.
HAZARD SITUATION
The principal perceived hazard is that the reser-
voir contents could be released, travelling down one
of the two main ravines in the general direction of the
town of Nemoli. The lake elevation is c 783 m asl, and
the town, which is about 2.5 km distant, lies at about
421m asl (Fig. 3). As the plan of the lake is roughly
elliptical, with a length (NW-SE) of 325 m and width
(NE-SW) of about 260 m, and its maximum depth is
around 12 m in the central part, on average, therefore,
the lake when full contains 1,014,000 m
3
of water or
less. It undergoes large variations of the water table
during the year due to the imbalance between the
influx, from a set of local springs. These springs are
Fig. 3 - Cross section AA: M. Sirino - Lago Sirino - Nemoli
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LANDSLIDE AND FLOOD HAZARD FROM THE LAGO SIRINO, BASILICATA, ITALY
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
397
the slope in the vicinity of the reservoir as described
by c
ArrArA
et alii (1985) that affected the source and
at the same time, the North marginal part of the lake.
Apart from some very small slides NNW of the lake,
the main landslide is mapped as extending into the town
of Nemoli at its toe, and extending upslope in two (or
possibly three) separate source areas comparatively close
to the lake. Indeed, the two main head zones are situated
in valley features that appear to be associated with the
lake, and in the case of the westernmost head, to within
some tens of metres of the lake. This landslide system
has a history of movement, especially in the area of its
NW part, where within the last decade, shearing move-
ments of around 15 m were experienced. In the spring of
2005, the hilly area adjacent to the W side of the lake has
been involved in a massive translational movement cut-
ting the municipal roadway, carrying it downstream and
isolating the farms to the west of the lake.
Specific slope instability risks to the lake from
this landslide system (marked on Fig. 2 as the main
slide area, and labelled d
f
) are the potential for it to
move such that it:
(a) unloads the toe of the slope holding up the lake, or
(b) chokes off the lower springs, permitting piezome-
tric heads in the slope to rise.
In both cases, destabilization of the landslide de-
bris short of outright collapse could increase the fre-
quency of sinkhole occurrences in the lake basin. The
shape of this deposit of landslide debris indicates in
general terms the fall line for landsliding, pointing in
the direction of the town centre.
PRECURSOR MOVEMENTS
Perhaps more worrying is the thought that the wide
range of deformation types could be a series of precur-
sor movements for a much larger failure. These events
are, of course, self-evident after the event, but they are
rarely identified a priori. When specifically sought out
SEEPAGE EROSION
The mechanism of seepage erosion has been re-
viewed usefully by c
roStA
& d
i
p
riSco
(1999), who ob-
serve that instabilities induced by seepage erosion have
been reported in a wide variety of geological settings and
in a range of different geomaterials. This runs counter
to the commonly-held (but incorrect) belief that seep-
age erosion is a phenomenon solely of loose coarse silts
and fine sands, although these materials are of course,
highly susceptible. Hence there is no guarantee that the
rock types beneath the Lago Sirino are not susceptible,
At Lago Sirino, the problem of seepage erosion is com-
pounded by the postulated movements of the landslide
block, and the opening of sinkholes and associated pas-
sages through the rock mass that have the potential to
greatly accelerate the process under the pertinent condi-
tions. Moreover, where exposed in a road cutting imme-
diately south of the lake, and therefore at a lower eleva-
tion, the rocks are observed to be strongly jointed and
fissured, with fissure flow likely to dominate the perme-
ability of the rock mass. Any seepage erosion induced
increase in flow could mobilize the landslide debris in
the slopes below the Lago Sirino and above Nemoli.
It is fortunate that the precipitation catchment of
the lake is small, and that it is fed mainly from springs
with generally constant discharge, and the maximum
lake level is held by the outlet works so that the low
embankment at the SE corner of the lake cannot be
overtopped, and that the hydraulic gradients will not
therefore be surcharged by an unexpected rise in water
levels in the lake.
LANDSLIDE MOVEMENTS ABOVE NE-
MOLI
In the mapping (Fig. 2) landslide deposits are
denoted in a light colour, and these include both cur-
rently active landslides (d
f
) and ancient slope deposits
(d
v
). Many landslides in historic times have affected
Fig. 4 - Composite photo panorama of the lake viewed from the SW (in vicinity of overflow structure). Pipes in foreground are
used to feed a fish farm
background image
L. COPPOLA, A. FILARDI & E.N. BROMHEAD
398
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
by instrumentation or remote observation, it is found
that precursor movements are related to pre-failure
ground strains (K
ALAughEr
et alii, 2000) in materials
that have different stress-strain characteristics and dif-
ferent degrees of mobilization of their shear strength.
Particularly as failure is approached, these differences
in strain lead to differential movements, and in some
cases, to the release of minor failures, or at the very least
to cracking and fissuring of the ground.
While under constant conditions, the frequency of
precursor events increases as a large failure develops
(F
uKuzono
, 1990), in a landscape that experiences
seismicity, a single shaking event can instantly cause
collapse if the stability reserve is small.
CONCLUSIONS
The Lago Sirino is an unusual feature in Basili-
cata, and exhibits many of the signs indicative of both
local and overall instability. The geological setting is
one that provides little security against instability and
consequently engenders little confidence in the mind
of the experienced professional applied geologist. It
merits proper investigation and careful treatment as a
potentially hazardous landscape feature.
As there have been a suite of minor instabilities
to date, the case of the Lago Sirino must be taken as
having possibly already indicated the probability of
a future large-scale failure by this range of precur-
sor events.
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