SUPPORT OF SATELLITE RADAR TO HAZARD ZONE MAPPINGIN THE ITALIAN ALPS

Belitz, K.(¹), Corsini, A.(4), Mair, V.(2), Strozzi, T(3), Wegmüller, U. (3), Zilger, J. (¹)

(1) Teledata GeoConsult GmbH-srlSiemensstraße 19, 39100 Bozen - Bolzano, Italy, Email: k.belitz@td-geoconsult.com

(2) Amt für Geologie und Baustoffprüfung, Autonome Provinz Bozen-SüdtirolEggentaler Straße 48, 39053 Kardaun - Cardano, Italy, Email: volkmar.mair@provinz.bz.it

(3) Gamma Remote Sensing AGThunstraße 130, 3074 Muri BE, Switzerland, Email: wegmuller@gamma-rs.ch

(4) Dipartimento di Scienze della Terra, Università degli Studi di Modena e Reggio EmiliaLargo S. Eufemia 19, 41100 Modena, Italy, Email: allecors@unimo.it

ABSTRACT

Living in areas of highly active natural processes as the alpine environment demands the careful assessment of naturalhazards and their provision in regional and local development planning processes, providing the chance to substantiallyreduce damage and loss of property and lives. Consequently techniques for the corresponding tasks of mapping andmonitoring natural hazards are subject of ongoing challenging research in this region. Earth observation data andmethodology have proven to be able to provide mapping of the spatial distribution of different environmentalparameters but are still rarely included in operational administrative and planning processes of hazard management.The presented study focussed on the potential integration of Differential SAR Interferometry techniques into currenttasks of landslide hazard management in the Italian Alps including test-cases in the Province of Bolzano-South Tyrol. Aclose cooperation with the responsible Geologic Service was established for detailed regarding of the specific userrequirements.Limitations and obstacles of the potential support from earth observation techniques were considered carefully and thepath of integration of products and services into the users processing chain was analysed in detail under operationalaspects and demonstrated.

1 THE ISSUE

While unstable slopes in alpine environments are a severe permanent threat for the human society and economy in theseareas, the mapping and analysis of all natural hazards and an appropriate management of these risks is of essentialimportance and a strong political issue.As the processes of avalanches, landslides, rockfall, flooding or debris flows are an inseparable part of the naturalenvironment, they cannot be eliminated, but risks can be minimised by regarding these hazards. Thus the completeurban and regional planning has to be based on an analysis and classification of the terrain in terms of risks, to provide abasis to prevent existing buildings and infrastructures from damage or loss. This task is described as hazard assessmentand zone mapping including the definition of appropriate regulations for each zone. Guidelines for the elaboration ofhazard zone maps based upon the legislative background of the Italian state and of the Province have recently been setinto force.In areas of elevated geo-risks additional preventive measures have to be planned and realised. As short-term activity thesafety of inhabitants and visitors has to be guaranteed by recognising potentially hazardous sudden events to initiatesecurity measures timely in advance (closing of roads, evacuation of buildings) supported by appropriate monitoringsystems.Satellite remote sensing data from the optical as well as microwave domain have already proven their applicability toderive parameters relevant to natural hazards and especially differential interferometry (DInSAR) showed to be suitablefor measuring directly geometric surface changes stemming from earthquakes, volcanic activity, excessive groundwateror mineral resources extraction. The application of these techniques for operational usage in hazard assessmentprocesses is still a subject of work.

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Proc. of FRINGE 2003 Workshop, Frascati, Italy,1 – 5 December 2003 (ESA SP-550, June 2004) 12_belitz

2 OBJECTIVES OF THE STUDY

Main objective of the study presented in this paper was to analyse the applicability of remote sensing data with thefocus on radar satellite data for establishing operationally usable services for mapping and monitoring unstable slopes inalpine environments. The integration in the Geographical Information Systems (GIS) was done in close co-operationwith a selected key-user. Requirements of current administrative and legislative processes and integration into the actualdata processing environment were prioritised. The results in terms of feasibility of the service and its performanceregarding accuracy, speed of delivery and costs and benefits of the service were analysed in detail.The key user committed to the study was the Geologic Service (Geologischer Dienst, Servizio Geologico) of theAutonomous Province of Bolzano-South Tyrol, which is responsible for all geologic mapping tasks regarding geology,tectonics, and associated geo-risks. As intermediate user, the Department of Earth Sciences of the University of Modenaand Reggio Emilia was involved, providing expert know-how and extended experience in mapping of hydro-geologicaland geomorphologic hazards and risk assessment.

3 METHODS APPLIED

Hazard zone mapping is the detection and detailed mapping of the spatial distribution of the hazards including aquantification of the intensity of the impact of the process and the probability of the occurrence of such a hazardousevent at a specific location.In the terrestrial methods used so far, temporally limited or continuous punctual measurements of the surfacedeformation either directly via Differential GPS or laser theodolites or indirectly via borehole instrumentation areassisting, whereas the study objectives were to complement or substitute these with direct information on geometricsurface changes and displacements derived from Differential SAR Interferometry (DInSAR) using ERS and ENVISATdata.

3.1 Actually applied terrestrial methodology in detail

In Italy, the zoning of areas subject to hydro-geological risks has been regulated since 1998 by a specific law (DM180/1998 converted in L 267/1998) and its associated application decree (DPCM 29/9/1998), which were released afterthe “Sarno” mudflow disaster claimed several lives near Naples. In the Autonomous Province of Bolzano-South Tyrol,the Geological, Hydrological and Torrent Control Offices defined guidelines for hydro-geological hazard and riskassessment. These guidelines are widely derived from rules adopted in Switzerland in the last decade for the hydro-geological hazard mapping in cantonal master plans, especially as concerns the basic approach using matrices to assessthe level of hazard on the basis of frequency and intensity (fig. 1) [1] [8], which is applicable for all types of hazards,varying only in process-specific parameters and thresholds. The matrix combination rules between temporal frequencyand intensity parameters as proposed in Switzerland is not exactly that of a direct proportionality, thus not just onefactor versus the other but rather a reasoned combination of them. In particular, frequency is expressed as 4 classes ofreturn period (high < 30 yr; medium 30-100 yr; low 100-300 yr; very low > 300 yr), while intensity can be expressedalternatively by 3 classes of velocity (high > 3m/min; medium 13m/month ÷ 1.8 m/hr; low < 13 m/month), 3 classes ofthickness of material involved (high > 10 m; medium 2 ÷ 10 m; low < 2 m) or by the product of both parameters.

F réq u en c y (F )

Inte

nsi

ty (

I)

H4 = ve ry h igh

H3 = h igh

H 4 H 4 H 4

H2 = m ed ium

H az ard (H )

hig

hm

ediu

mlo

w H 3

H 3 H1

H 2

H 4

H1 = low -res idua l

h igh m e diu m lo w ve ry lo w

H 3

H 3

H 3 H 2

H 2

Fig. 1. Matrix of the Level of Danger of a hazardous process. [1]

3.2 Earth Observation methods

The analysis of satellite radar data should include actual ERS and Envisat data, but due to the gyro failure and followingZero-Gyro-Mode affecting orbital attitude tracking of ERS-2 and Envisat still being in the commissioning phase actual

data pairs for interferometric processing were ordered but could not be used in this study yet. Thus the analysisregarding the application presented in this paper includes only historical ERS data. Further analysis of actual Envisatdata is foreseen for the year 2004, as operations hopefully are moving further towards normality.For the SAR processing of the selected ERS RAW data products, the Modular SAR Processor (MSP) of GammaRemote Sensing was used [9, 10]. In standard mode the processing was done to deskewed (zero-Doppler) geometry andincluding Doppler Centroid estimation, autofocusing and radiometric calibration for the slant range distance, theprocessor gain, the projected ground area size and the antenna diagram. For actual ERS-2 Zero-Gyro-Mode data muchlarger than usual and especially temporally varying Doppler Centroids were observed while the Doppler function couldnot be assumed constant across the image, too. In addition, the processing had to be adequate for a subsequentgeneration of interferograms for specific pairs. Many repeat-pass pairs were completely uncorrelated because of toonon-overlapping Doppler spectra. As a further obstacle until September 2002 no information on the Doppler functionwas available from ESA to support data selection, and the information afterwards had an uncertainty of 500 Hz, makingthe usefulness of a data pair for interferometric analysis questionable. For obtaining more reliable results on the DopplerCentroids, the MSP was applied with an individual estimation of the scene from the data itself. For interferometric pairsthe processing was then done to non-deskewed geometry using a common Doppler function and reduced azimuthbandwidths.For the interferometric processing the Interferometric SAR Processor (ISP) of Gamma Remote Sensing was used. SLCimages were registered with sub-pixel accuracy to the same reference geometry and then the interferograms calculated.In this step common band filtering was applied to maximise the fringe visibility. A 5 look multi-looking in azimuth wasused in the interferogram calculation. In addition, registered 5-look intensity images were calculated. Based on the orbitparameters and the average interferogram fringe frequency an initial baseline estimate was calculated, the flat-earthphase trend subtracted from the interferogram and the degree of coherence estimated. The complex valuedinterferograms were filtered for phase noise with an adaptive filter.

4 TEST-SITES

The study was carried out within two test-sites requested by the key-user: Nals in the valley of river Etsch and Corvarain Badia, both located in the Autonomous Province of Bolzano-South Tyrol in the Italian Alps.The test-site near the village of Nals / Nalles in the Etsch-valley, lies half way between Meran and Bozen on thesouthern mountain slopes of the valley. The test-site is part of a long colluvial slope, extending below a steep wall ofoutcropping rocks from Meran-Marling / Merano-Marlengo in the Northwest to Eppan / Appiano near Bozen in theSoutheast. Landslides and debris flow events are occurring regularly since autumn 2000, and implementation ofappropriate monitoring measures is a pressing issue.As the test-site is quite small in size and mostly covered by woods, the application of DInSAR techniques is challengingin this kind of terrain, but of very high significance for Alpine environments and typical hazard situations.An extended monitoring system including terrain movement survey, precipitation gauges and discharge gauges as wellas geophones and a video-camera for immediate debris-flow observations has been installed by the key-user in 2001,mainly for alarm and subsequent evacuation of housing areas located down in the valley. The recorded data was ideallysuited to compare and validate the differential interferometric results.As the activity of the slope started only late 2000 and data from ERS-2 ZGM from 2001 could not be taken applyinginterferometry, only Envisat data of 2003 are applicable, but due to the aforementioned limited data availability thisanalysis has to be postponed to the year 2004.

Thus the data processing and hazard mapping is yet preliminary and based only upon the results of the Corvara test-site.The Corvara landslide is an active slow moving rotational earth slide - earth flow, located uphill of the village ofCorvara in Badia, one of the main tourists centres in the Alta Badia valley in the Dolomites [2]. Present day movementsof the Corvara landslide cause National Road 244 and other infrastructures to be damaged on a yearly basis. Themovements also give rise to more serious risk scenarios for some buildings located in front the toe of the landslide. Forthese reasons, the landslide has been under observation since 1997 with various field devices that enable slopemovements to be monitored for hazard assessment purposes [3]. Differential GPS measurements on a network of 47benchmarks has shown that horizontal movements at the surface of the landslide have ranged from a few centimetres tomore than one metre between September 2001 and September 2002. Over the same period, vertical movements rangedfrom a few centimetres to about ten centimetres, with the maximum displacement rate being recorded in the track zoneand in the uppermost part of the accumulation lobe of the landslide. Borehole systems, such as inclinometers and TDRcables, have recorded similar rates of movement, with the depths of the major active shear surfaces ranging from 48 mto about 10 m. From these data, it is estimated that the active component of the landslide has a volume of about50 million m3 [4, 6]. The Corvara landslide has also been included in a pilot study of the Province of Bolzano-South

Tyrol aimed to landslide hazard assessment [5]. A panoramic view of the landslide and the village of Corvara is givenin figure 2.

4 RESULTS

In the aforementioned research framework of the Corvara landslide, the methodology based on terrestrial methods asdescribed above in detail has been successfully applied adopting a high frequency class (< 30 yr) and a low to mediumintensity class (depending on the landslide portion under analysis) derived by the product of a low or medium class forvelocity (in the accumulation lobe and in the track-source zones respectively) and a high class for thickness of materialinvolved (on the whole landslide). The resulting hazard level ranged from high to very high. The map is shown in fig. 3.

Fig. 2. Test-site Corvara landslide. The unstable area and Fig. 3. Hazard Zone Map of Corvara landslide. the village of Corvara are clearly visible. Zones in Red, Blue and Yellow.

The results of the Differential Interferometric SAR analysis were geocoded and integrated as additional data layer intothe Geographic Information System (see below).The following figures show the interferometric results of surface displacements from the period summer - autumn 1997in look direction of the side-looking radar satellite. Areas, where no colour is shown are characterised by a lowcoherence. Surface changes lead to a loss of coherence and high surface displacement velocities is one possibleexplanation. This interpretation may be valid for the track-zone of the slide as GPS values prove fast movements there.

Fig. 4. Quantitative results of DInSAR in look direction, Aug-Sept 1997.

For using the DInSAR results in the common hazard mapping system together with terrestrial data analysis results, thedata was modelled from surface displacements in radar look direction to surface movements along the slope-gradientusing a digital elevation model (DEM) and in a first approach assuming that the movement always follows themaximum slope gradient. Then the resulting values had to be classified regarding the thresholds defined by the guide-lines of the Autonomous Province of Bolzano-South Tyrol applicable for hazard zones in general and landslides inparticular as given above. It has to be pointed out clearly, that for hazard mapping a vertical geometric resolution in themm-range is not relevant, but the reliability of results reclassified in the given classes is very important.While return periods are estimated from geomorphologic records and a calculation for the time since 1990 can besupported by longer time series of ERS-1 and ERS-2 radar data analysis, the intensity of the hazardous process isdescribed by the matrix of velocity and geometry. The scheme to be applied is shown in figure 5.

Fig. 5. Scheme applied for assessment of hazards.

The integration of DInSAR displacement calculations as average annual velocity values in the Geographic InformationSystem (GIS) of the Geologic Service with the existing landslide database of the project IFFI (Inventario FenomeniFranosi in Italia) is shown in figure 6.

Fig. 6. Geographic Information System customised for landslide hazard assessment.

Level of Danger

Velocity Geometry

Intensity

Return period

5 CONCLUSIONS

The path of integration of satellite radar data analysis results into Hazard Zone Mapping for landslides is demonstratedand proven using historical ERS data.The study confirms the maturity of radar interferometry technique for addressing mapping and monitoring tasks ofunstable slopes and is in support of the interest of the Geologic Service of the Autonomous Province of Bolzano-SouthTyrol involved in this study to integrate the results in its current operational tasks.The main advantages to be outlined refer to the spatially extending information collection and the monitoringcapabilities, which regarding the hazard zone mapping has also relevance, as it allows to estimate the behaviour of alandslide by evaluating historic time series and correlating them with information on changes, either anthropogenic (e.g.road construction) or natural ones (e.g. weather conditions). It can be a valuable additional, maybe even primaryinformation source to increase safety and to obtain in-depth information of the mechanics of landslide processes.Limits of the method are the applicability only for slopes in terms of a minimum size and a maximum velocity andsteady movement of the landslide, but primarily the issue of continuous data availability is the most important issuewhich has to be solved as longer time series of SAR acquisitions (ten years and more) are essential for achieving a realoperational status.Unfortunately, the lack of appropriate actual radar data from ERS and Envisat satellites affected this study and theobjectives could not be fully achieved yet, but the user acknowledges the preliminary results and the successfulintegration into the authorities information systems and administrative processes and plans to go on with Envisat dataexpected for the year 2004. A pre-operational project on permafrost phenomena is in preparation.

ACKNOWLEDGEMENTS

The authors wish to thank ESA-ESRIN for the financial support to the project within the Data User Program DUP(contract no. 15646/01/I-LG). See also [7].

REFERENCES

1. Heinimann H.R., Holtenstein K., Kienholz H., Krummenhacher B. & Mani P., 1998. Methoden zur Analyse undBewertung von Naturgefahren. Umwelt-Materialien Nr. 85, Naturgefahren, BUWAL, 248 pp. Bern.

2. Corsini A., Panizza M., Pasuto A., Silvano S., Siorpaes C. & Soldati M., 1998. Indagini preliminari per ladefinizione della pericolosità da frana nella conca di Corvara in Badia (Dolomiti). Mem. Soc. Geol. It., 53. 207-224.

3. Corsini A., Pasuto A. & Soldati M., 1999. Geomorphological Investigation and Management of the CorvaraLandslide (Dolomites, Italy). Transactions, Japanese Geomorphological Union. 20(3). 169-186.

4. Corsini A., Pasuto A., Soldati M., Zannoni A., 2002. Technical guidelines for the definition of a conceptual andgeotechnical model of the Corvara landslide. In: D. Delaye, F. Levoy & O. Maquaire (eds.), Geomorphology: fromexpert opinion to modelling. Proceedings of the Symposium held in Strasbourg, France, on April 26-27, 2002.CERG Editions. 37-41. Strasbourg.

5. Corsini A., Panizza M, Pasuto A., Silvano S. & Soldati M., 2001. Piano del Rischio Idrogeologico del Comune diCorvara in Badia - Parte Frane. Progetto pilota: I piani dei Comuni di Corvara, Fortezza, Postal e Santa Cristina.Ufficio Geologia e Prove Materiali, Provincia Autonoma di Bolzano Alto Adige. Bolzano.

6. Corsini A., Pasuto A., Soldati M. & Zannoni A., 2003. Field monitoring of the Corvara landslide (Dolomites, Italy)and its relevance for hazard assessment. Geomorphology. accepted for publication.

7. Paganini M., Palazzo F., Arino O., Manunta P., Ferretti A., Gontier E., Wunderle S., Pasquali P., Strozzi T., ZilgerJ., & Westen C.v., 2003. SLAM, the development of an EO Service to support the legal obligations of Swiss andItalian Geological Risk Services in landslide risk forecasting and prevention. Proceedings of the InternationalGeoscience and Remote Sensing Symposium - IGARSS 03, Toulouse, France, July 21 - 25, 2003. Toulouse.

8. Raetzo H., Lateltin O. & Tripet J.P., 2002. Hazard assessment in Switzerland – Codes of Practice for massmovements. Bulletin of Engineering Geology and the Environment. 61. 263-268. Heidelberg.

9. Wegmüller U. & Strozzi T., 1998. SAR interferometric and differential interferometric processing chain.Proceedings of the International Geoscience and Remote Sensing Symposium - IGARSS 1998. Seattle.

10. Wegmüller U. & Werner C.L., 1997. GAMMA SAR processor and interferometry software. Proceedings 3rd ERSScientific Symposium, Florence, Italy, 17 - 20 March 1997. Firenze.