Introduction

Italy is one of the most landslide risk prone countries and, in particular the Umbria Region, located in central Italy, is almost yearly affected by landslide

and flood events at different spatial and temporal scales. Because of the high social impact of the hydrogeological risk, a more accurate and reliable methodology for the landslide risk assessment and management is of great interest.

Following recent laws and regulations concerning extreme natural events management, within the Italian national hydrometeorological early warning

system, 21 regional offices (called “Decentralized Functional Centres” - CFD) are supposed to be fully operative in few years. Nowadays 13 of them are

officially operating. Functional Centres are in charge of acquiring and collecting both real time and quasi-static data, and each CFD provides technical tools able to support decisions when significant flood/landslide events occur. The alert system for the hydrogeological risk is based on rainfall thresholds

set of three different increasing critical levels according to the expected ground effects.

In order to improve the performance of the alert system for the landslide risk, Umbria Region CFD, in cooperation with the Research Institute for Geo-

Hydrological Protection (IRPI-CNR), developed and tested a continuous physically based soil water balance model (Figg. 2, 3), addressed to the estima-tion of soil moisture conditions over the whole regional territory. Acting on the pore water pressure, soil moisture modulates the strength-stress ratio in

soils and so is a significant precondition for the triggering of landslides; maybe as important as thresholds based on accumulated rain values. In fact, it

was found that the triggering of several large landslide events occurred in the region is strongly influenced by the initial soil moisture (Fig. 4).

On this basis, an high risk landslide in Umbria Region, the Torgiovannetto Landslide, is now monitored using a pre-alert system based on real time ob-served and predicted rainfall along with real time soil moisture information provided by the soil water balance model; thus furnishing site-specific

thresholds useful for civil protection activities. This landslide is a very large rock slide involving a 140.000 m2 wedge, threatening county and state

roads. An extensimeter network is used as alert system, based on velocity thresholds. In particular, the opening of the tension cracks was found strictly linked to rainfall depth and soil moisture conditions. Moreover, through a multiple regression analysis, the quantitative opening of the tension cracks fol-

lowing predicted rainfall and soil moisture conditions can be predicted.

observed rain and temperature

input file

in - site predicted rainand temperature

Meteo Area -evaluation of the in-situ

predicted rainfallquantitative values

observed and predicted > 72h input

file

in site - calculation ofsoil moisture values

calculationof the 3 observedand predicted cumulated rain

values : 24, 36, 48h

comparation with the rainfall thresholds,

corrected by the soilmoisture index

Fig. 2 – The structure of Soil Water Balance model.

Fig. 3 – Comparison between the observed and simulated saturation de-gree by the Soil Water Balance model for a 15 cm depth.

Fig. 8 – from the top) Percentage trend values of soil saturation: observed and 72 hours fore-cast data are shown; previous 7 days and 72 hours forecast ac-cumulated rain; regression lines of the values of the ordi-nary criticity rainfall (24, 36, 48 hours) thresholds vs. per-centage of saturation of soil: cumulate rain-fall values observed , 24 and 72 hours fore-cast.

References

Brocca, L., Melone, F., Moramarco, T. (2008). On the estimation of antecedent wetness condition in rainfall-runoff modeling. Hydrological Processes, 22, 629-642.

Ponziani, F., Pandolfo, C., Stelluti, M., Berni, N., Brocca, L., Moramarco, T. (2010). Soil moisture and rainfall thresholds assessment for hydrogeological risk prevention in a regional operational warning centre. Landslides, submitted.

The pre—alert system

The University of Florence—Earth Sciences Department is in charge for the alert procedure based upon the overcoming of velocity of cracks opening de-tected by the extensimeter network (Fig. 1), using three levels of criticity: nor-mality, pre-alert and alert. The alert is declared when at least two extensime-ters exceed their velocity thresholds (1mm/day or 0.5 mm/day, depending on their position). The system developed by the CFD, aimed to the support of the monitoring of the rockslide, performs the following tasks (Fig. 7): - the calculation of the accumulate rain measured by the in-situ meteorologi-cal station, and the predicted ones for the three following days using the COSMO-ME local scale prevision models; - the calculation of the soil moisture content using the balance model above described; - the comparation between the rain dataset and the rainfall thresholds dy-namically adjusted by a correction factor linked to the soil moisture. The system result is consituited by the real time evaluation of rainfall and soil moisture conditions in the rockfall site and the estimate for three following days; this is used as a pre-alert threshold in case of intense meteorological events, supporting the alert procedure based on the extensimetric network.

AN INTEGRATED APPROACH FOR THE REAL – TIME MONITORING OF A HIGH RISK LANDSLIDE

F. Ponziani(1), N. Berni(1), C. Pandolfo(1), M. Stelluti (1) and L. Brocca(2)

(2) Research Institute for Geo-Hydrological Protection, National Research Council, Perugia, Italy

(1)Umbria Region Functional Centre, Foligno (PG), Italy

The landslide site

The Torgiovannetto landslide originated from a dismissed stone quarry near the town of Assisi, in Central Italy. The strati-grapy is constituited by stratified limestone with intercalation of thin weak clay layers: the dip of the front of the quarry is a-bout 30 - 38°, similar of that of the limestones bedding planes. A huge sliding rock wedge in the upper part of the quarry has formed, delimited by a big tension crack up to one meter wide and over 100 m long, shoving slow movements after heavy raifalls, relating the raising of the pore pressure to the reduction of the shear strength on the stratification planes. The rock wedge threatens two roads at the base of the slope: the landslide is monitored by a meteorological sation, an e-xensimetric network and recently by a high dynamic accelerometer. The dataset up to now collected demonstrated the cle-ar influence of the rainfall on the rock slide movements.

Fig. 1 – Location of the Torgiovannetto landslide; tension craks – view of the protective barrier at the base of the quarry; extensimeter network

Fig. 7 – flow chart of the pre-alert system

The seismic monitoring

A 130 dB, 4 channels last generation Kinemetrics Basalt accelerograph has been installed at the base of the quarry, with the aim of support the alert procedure in case of dangerous movements of the rock wedge. The accelerograph, equipped with a Episensor triaxial sensor and using threshold and STA-LTA trigger, is recording a wide dataset of data related to the landslide movements; another goal of the data processing is to find a power correlation low between vibration data and rock movements.

Based on the previous study

(Ponziani et al., 2010) indicat-

ing the significant effect of ini-

tial soil moisture conditions on

landslide triggering, the rela-

tionship between rainfall, mod-

eled soil moisture and exten-

simeter opening was investi-

gated. It was found that also

for Torgiovannetto Landslide ex-

tensimeter opening is strictly

linked to rainfall and initial soil

moisture conditions.

Sep07 Dec07 Apr08 Jul08 Oct08 Jan09 May09 Aug09 Nov090

10

20

30

Exte

nsim

ete

ro

pe

nin

g (

mm

)

Sep07 Dec07 Apr08 Jul08 Oct08 Jan09 May09 Aug09 Nov090

0.5

1

sa

tura

tion

de

gre

e

Sep07 Dec07 Apr08 Jul08 Oct08 Jan09 May09 Aug09 Nov090

50

100

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mo

nth

ly c

um

ula

ted

ra

infa

ll (m

m)

MULTIPLE REGRESSION ANALYSISMULTIPLE REGRESSION ANALYSIS

SDPdH totest

A multiple regression analysis to estimate the quantitative opening of the

tension cracks, dHest, was done:

0 0.2 0.4 0.6 0.8 10

2

4

6

8

10

12

14

Antecedent saturation degree

Exte

nsim

ete

r o

pen

ing

(m

m)

color = total rainfall (mm)

15

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0 2 4 6 8 10 12 140

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4

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Observed extensimeter opening (mm)

Esti

mate

d e

xte

nsim

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r o

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ing

(m

m)

Results showed that extensimeter opening can be estimated with fairly good accuracy; in fact

a correlation coefficient equal to 0.72 was obtained.

Figure 4 – Cumulated rainfall values over 24 (a), 36 (b) and 48 (c) hours versus the simulated saturation degree for the landslide events reported in the AVI database and considering the period 1991-2001.

Based on the analysis of the widespread landslide events available within the Italian AVI Land-

slide database, a linear relation between the rainfall thresholds and the initial soil moisture con-

ditions was found (Fig. 4) with correlation coefficients up to 0.60 showing the key role of initial

soil moisture conditions on landslide triggering. Therefore, the correlation established between

the maxima cumulative rainfall values and the soil moisture prior to the triggering of landslides

allows to dynamically adjust the rainfall thresholds which is of paramount interest for warning

activities. Indeed, these results should provide a significant contribution to the real time land-

slides risk assessment for the regional territory, decreasing the uncertainties tied to the applica-

tion of the rainfall thresholds only.

Moreover, the obtained results confirm the capability of the hydrological monitoring for

setting up early warning systems for real slopes. In fact, collected data can be used both for the

effective calibration of physically based models of infiltration and for establishing correlations

for empirical models of landslides triggering.

Fig. 5 – correlation be-tween rainfall (left) and ex-tensimeter opening (mm/day) (right)

Fig. 6 – web site of the Torgiovannetto meteo—extensimeter network