Claudio Zmarich
email: claudio.zmarich@pd.istc.cnr.it

Barbara Gili Fivela
email: barbara.gili@unisalento.it

Pascal Perrier
email: pascal.perrier@gipsa-lab.grenoble-inp.fr

Christophe Savariaux
email: christophe.savariaux@gipsa-lab.grenoble-inp.fr

Graziano Tisato
email: graziano.tisato@pd.istc.cnr.it

Speech timing organization for the phonological length contrast in Italian consonants

Keywords: speech timing, consonant gemination, kinematic analysis

In Italian, length contrast is exploited in the consonant system. Previous articulatory studies have focused on the temporal organization of gestures in Italian geminates and on the lower lip kinematics of the singleton/geminate distinction, and have showed that the time interval between the nuclei of two successive syllables does not depend on the number of intervening consonants (Öhman"s Vowel-to-Vowel model) . In this paper, data on lip and tongue gestures from four Italian subjects saying "mima" and "mimma" at fast and comfortable rate of delivery are discussed in order to directly test the validity of the Öhman"s model for the gestural organization of Italian geminate consonants.

Introduction

Geminate consonants are at the core of debates that focus on two related issues, namely their underlying phonological structure and their timing in relation with the surrounding vowels. Regarding their phonological structure, there is a wide agreement that in Italian they correspond to a combination of two shorter, strongly connected, segments rather than to a unique and long segment. Consistently, we have found kinematic evidence [START_REF] Gili Fivela | Italian Geminates under Speech Rate and Focalization Changes: Kinematics, Acoustic, and Perception Data[END_REF][START_REF] Zmarich | Consonanti scempie e geminate in Italiano: studio acustico e cinematico dell"articolazione linguale e bilabiale[END_REF] supporting that acoustic time patterns in VCV sequence, in which C is a geminate, are much more similar to corresponding VCV patterns in which C is an heterosyllabic cluster than to VCV patterns in which C is a singleton. This result supports the view that Italian geminates correspond to two identical segments belonging to different syllables [START_REF] Romanu | The representation of geminate consonants: Evidence from the phonological errors of an aphasic patient[END_REF].

As far as the processes underlying their timing is concerned, there are two main classes of phonetic hypotheses:

(1) their longer duration (as compared to singleton, see [START_REF] Bertinetto | Strutture prosodiche dell"italiano[END_REF]) is intrinsically specified and it is associated with a global reorganization of the timing of the surrounding vowels; (2) their longer duration results from more macroscopic motor control strategies, and their impact on the surrounding vowels is a secondary effect of this control. Browman & Goldstein"s c-center model for syllable production [START_REF] Browman | Towards an articulatory phonology[END_REF] belongs to the first class of hypotheses, and it suggests that the initial consonant and the vowel are produced in phase, while the consonant in coda and the vowel are in anti-phase. Öhman"s model [START_REF] Öhman | Numerical Model of Coarticulation[END_REF] suggests that VCV sequences are produced on a V-V basis, which determines the global timing of the sequence, perturbed locally by the production of the consonant. This model is at the basis of Fowler"s model [START_REF] Fowler | Coarticulation and theories of extrinsic timing[END_REF], reframed by Smith [START_REF] Smith | Prosodic patterns in the coordination of vowel and consonant gestures[END_REF] in gestural terms. However, Smith leaves unaffected the timing implications of the Öhman"s model for the VCV sequence. This model would induce that, as compared to singletons, the lengthening of the geminates would be realized within the VCV sequence by an anticipation in the first vowel of the consonant closing gesture and a postponing in the second vowel of the consonant opening gesture. These observations refer also to the first class of hypotheses. On the other hand, the virtual target hypothesis proposed by Löfqvist [START_REF] Löfqvist | Lip kinematics in long and short stop and fricative consonants[END_REF] suggests that the main factor inducing the lengthening of the geminates is a change in the motor commands which specifies a higher virtual target for the main articulator associated with the production of the consonant. In some cases this motor control change is combined with changes in the specified consonant duration [START_REF] Löfqvist | Interarticulator programming: Effects of closure duration on lip and tongue coordination in Japanese[END_REF]. Thus, this hypothesis belongs to the second class. In a former study [START_REF] Zmarich | Consonanti scempie e geminate in Italiano: studio acustico e cinematico dell"articolazione linguale e bilabiale[END_REF][START_REF] Gili-Fivela | Acoustic and kinematic correlates of phonological length contrast in Italian consonants[END_REF], based on the analysis of acoustic and articulatory data, we found for the Italian bilabial geminates (in "mima" vs. "mimma") little support for the virtual target hypotheses (the velocity timing of the closing gesture did not match the predictions of this theory) and some trends in favour of Smith"s hypothesis in the fact that the closing gesture was clearly anticipated in the preceding vowel. However, we found no clear evidence in support of the hypothesis of a constant duration of the V-V sequence across singletons and geminates. Moreover, only data from two subjects were analyzed in that paper, and only a limited number of measures considered to be critical for the evaluation of Smith"s hypothesis were performed.

In this paper, a more extensive and accurate evaluation of Smith"s hypothesis is provided, thanks to the analysis of data from four Italian subjects recorded at both normal and fast speaking rate. In fact, higher speech rate often reduces the phonetic contrast of two structures in phonological opposition, and reduces the number of alternative articulatory strategies

Corpus and method

Corpus

The experimental corpus was composed by the words "mima" (from the verb "to mime") and "mimma" (proper name). Target words were inserted in the carrier sentence richiama mim(m)a malamente "s/he calls back mim(m)a badly"). Speakers were asked to read each sentence aloud at least ten times both at a natural speech rate and, immediately afterwards, at a faster speech rate. The sentences were randomly distributed among a wider corpus of stimuli, involving other consonant opposition for gemination (see [START_REF] Löfqvist | Interarticulator programming: Effects of closure duration on lip and tongue coordination in Japanese[END_REF]). All the subjects were PhD students. A female speaker of a north-eastern variety of Italian (AG), a female speaker (BG) and a male speaker (MP) of a north-western variety, and a female speaker of a variety from mid Italy (FC) were recorded. The audio signal was acquired by means of a DAT recorder, while the kinematic data were collected using the 2D EMA system at GIPSA-lab (formerly ICP) in Grenoble. In this system sensors are all glued in the mid-sagittal plane of the head. Two reference sensors were glued on the nose bridge and the upper incisors, two were placed on the upper and lower lips, and four were glued of the tongue in the range of about 1 cm to 5 cm from the tongue tip. The data discussed here only relate to measurements of Lip Aperture (i.e., the vertical distance between the two lips) to study the consonant gesture and to measurements of tongue dorsum (sensor located at around 3.5 cm from the tongue tip) for studying the vowelto-vowel gesture.

Auditory test

Times Consonant duration is distinctive in (standard) Italian, but various dialects spoken in northern Italy do not use geminates and do not exploit the geminate/singleton contrast, especially at fast rate and/or informal speech. First of all then, we decided to verify, by means of a perception test, whether our speakers had adequately produced geminate consonants. Secondly, we sought to look at acoustic/articulatory correlates of clear geminate vs. singleton contrasts.

A perception test was run in order to verify whether our speakers had adequately produced geminate consonants, especially at fast speech rate, and to identify and exclude (from statistics analysis) stimuli that were ambiguously realized with respect to the singleton vs. geminate contrast (i.e. we want to look at correlates of clearly produced and perceived geminate and singleton consonants).

A selection of the stimuli produced by the four subjects was used for the perception test. For each subject, two repetitions realized at a normal speech rate and all repetitions realized at fast rate were selected (for 160 "mima/mimma" stimuli, 96 were selected for the perception test). Five subjects from the northern part of Italy (Torino) and five subjects from the southern one (Lecce-Taranto) took part in the perception test. They listened to audio files containing target words in carrier sentences and judged whether the target words included singleton or geminate consonants. Test results showed that the selection of normal rate "mima/mimma" stimuli was basically correctly recognized (error rate is around 4%), while fast rate stimuli were misidentified in around 22% of cases. Stimuli that were wrongly recognized by at least 4 subjects were considered ambiguous and not taken into account for acoustic and kinematic analysis. The comparison of these "no contrast" cases (mainly observable at a faster speaking rate) with the "clear contrast" was left to a follow-up study.

Measurements

The duration of the consonant and both the preceding and the following vowels was measured by manually segmenting and labelling the acoustic signal in PRAAT. Boundaries within the VC(C)V sequence were inserted thanks to spectrogram inspection, looking for formant and intensity changes (increase towards the vowel and drop towards the consonant). That is, the boundaries between the vowels and the target singleton/geminate consonant were identified and, in case of gemination, no acoustic boundary was placed within the target consonant sequence. Kinematic measurements were performed after semiautomatic segmentation and labelling of the signal [START_REF] Gili-Fivela | Acoustic and kinematic correlates of phonological length contrast in Italian consonants[END_REF]. Figure n. 1 illustrates the acoustic signals (waveforms and sonograms) together with the kinematic signals (Lip Aperture, Tongue Body vertical) relative to the sentences "(richiam)a "mima/mimma ma(rcatamente)". The singleton consonant (up) and geminated consonant (bottom) are aligned to the beginning of the first vowel "i" of ""mi(m)ma) (vertical line on the left), based on Tongue Dorsum (vertical) signal.

Figure 1: Acoustic (waveforms and sonograms) and kinematic (Lip Aperture, Tongue Body vertical) signals relative to ""mima" (up) and ""mimma" sentences (bottom). The two panels are aligned to the beginning of the first vowel "i" in the Tongue Dorsum signal (vertical line on the left)

On the basis of the time variations of the Lip Aperture parameter, of the vertical displacement of the tongue dorsum sensor, and of the derivatives of these two variables, the onsets and offsets of the closing and opening consonant gesture and of the [i-a] gesture were determined. The offset of the gesture toward the first vowel [i] and the onset of the gesture from the second vowel [a] were also determined on the same basis. Thus, the following measures could be provided (see fig. V1 onset -C Mid: between the articulatory onset of the first vowel (/i/) and the center of the consonantal cycle /m(m)/. The articulatory onset for the vowel is defined as the point where the vertical component of the tongue dorsum reaches the highest point coming from a lower position for the last /a/ of "richiama" (i.e. call back). The exact point is calculated by making reference to the point where the istantaneous velocity reduces to 1/10 of maximal velocity for the same gesture. It is also the onset of the articulatory plateau for /i/, identified as the stable trajectory lying between the 2 crossings of the 1/10 of the maximal velocity. The articulatory target of the consonant is defined as the point where the vertical component of Lip Aperture has a unique maximum (that is, the end of the closing gesture coincides with the beginning of the opening gesture). In the case of an articulatory plateau, identified as the stable trajectory lying between the 2 crossings of the 1/10 of the maximal velocity, the mid point of the plateau is taken.

Öhman"s prediction: VCCV should be similar to VCV C Mid -V2 offset: between the center of the consonantal cycle /m(m)/ and the articulatory offset of the second vowel (/a/) (see definitions above).

Öhman"s prediction: VCCV should be similar to VCV V1 onset -V2 offset: between the articulatory onset of the 1 st vowel (/i/) and the articulatory offset of the 2 nd vowel /a/.

Öhman"s prediction: VCCV should be similar to VCV V1 onset -C onset: Between the onset of the articulatory plateau for /i/ and the beginning of the closure gesture for /m(m)/, identified as the point where the instantaneous velocity is 1/10 of the maximal velocity.

Öhman"s prediction: VCCV should be less than VCV C offset -V2 offset: Between the final moment of the opening gesture for the consonant /m(m)/, identified as the point where the velocity drops to 1/10 of the maximal velocity, and the end of the articulatory plateau for /a/, identified as the stable trajectory lying between the 2 crossings of the 1/10 of the maximal velocity.

Öhman"s prediction: VCCV should be less than VCV C onset -C offset: between the beginning of the closure gesture and the end of the opening gesture.

Öhman"s prediction: VCCV should be more than VCV V1 onset -V2 onset: between the articulatory onset of the 1 st vowel (/i/) and the articulatory onset of the 2 nd vowel /a/.

Öhman"s prediction: VCCV should be be similar to VCV V1 onset -V1 offset: the duration of the articulatory plateau for the first vowel.

Öhman"s prediction: VCCV should be similar to VCV V2 onset -V2 offset: the duration of the articulatory plateau for the second vowel.

Öhman"s prediction: VCCV should be similar to VCV C plateau onset -C plateau offset: duration of the articulatory plateau for /m(m)/, identified as the stable trajectory lying between the 2 crossings of the 1/10 of the maximal velocity.

Öhman"s prediction: VCCV should be more than VCV.

Results

Since we excluded a significant number of repetitions on the basis of the perceptual assessment of the singleton/geminates contrast, we did not use the repeated measures ANOVA method and preferred to carry out Univariate ANOVAs separated for rates and subjects, with geminate/singleton as the between-subject factors, and the durations of the first vowel (V1) and the intervocalic consonant (C) as dependent factors for the acoustic analyses, and the timing intervals from articulatory measures as the dependent factors for the kinematic analyses.

Acoustic analyses

For all subjects, C duration in normal rate productions is significantly higher and V1 duration is significantly lower when a geminate rather than a singleton occurs (for

Kinematic analyses

The results are summarized in Table 1 and

Figure 2 :

 2 Figure 2: Blueprint of the reference points on the kinematic trajectories of Tongue Dorsum (vertical) for first and second vowels (black) and Lip Aperture for the consonant (white) in "mim(m)a"

Table 1 .

 1 3 (for normal speaking rate) and Table2 and 4(for fast speaking rate). In tables 3 and 4 the first column presents the predictions that would be made for the impact of the gemination contrast on the considered duration in the context of Öhman"s model. Subjects" means for time intervals at normal rate (rounded to ms; G = geminates; S = Singletons)

		AG		MP		FC		BG
	Time Interv.	G	S	G	S	G	S	G	S
	V1ons-Cmid 136 114 163 154 195 187 196 179
	Cmid_V2off 158 140 113	92 139 121 140	96
	V1ons-V2off 294 254 276 246 333 308 336 275
	V1ons-Cons	3	16	47	65	70	90	45	81
	Coff-V2off	47	64	1	10	15	38	11	11
	Cons-Coff	245 174 228 171 249 180 281 183
	V1ons-V2ons 259 225 264 235 327 292 324 266
	V1ons-V1off	52	66	97 116 119 142 117 134
	V2ons-V2off	31	29	12	11	7	16	12	9
	CplOns-Cploff	42	17	51	17	46	15	60	14
		AG		MP		FC		BG
	Time Interv.	G	S	G	S	G	S	G	S
	V1ons-Cmid	81	57	91	82 105	96	80 117
	Cmid_V2off 138 110 108	85 124	95 104 100
	V1ons-V2off 219 167 199 167 229 191 184 217
	V1ons-Cons	-3	-9	23	23	34	38	14	41
	Coff-V2off	66	51	32	26	43	22	32	23
	Cons-Coff	157 124 145 119 152 131 139 153
	V1ons-V2ons 189 147 172 152 201 175 163 199
	V1ons-V1off	34	15	45	36	74	66	32	71
	V2ons-V2off	30	19	27	15	28	16	22	18
	CplOns-Cploff	14	6	11	8	13	7	10	12

Table 2 .

 2 Subjects

" means for time intervals at fast rate (rounded to ms; G = geminates; S = Singletons)

Acknowledgements

To CNR for assigning to the first and the third author a Short Mobility Grant (in the years 2005 and 2010, respectively).

These tables show a clear inter-speaker variability. Since only sequences where the singleton/geminates contrast was perceptually clearly noticeable, we don't think that this variability is due to dialectal differences. In addition the results obtained for the two speakers of the north-western variety of Italian are not fully compatible. It suggests that speakers of Italian could have different ways to implement the length contrast. In general, our results confirm the classical finding that geminates are significantly longer than singletons (Cons-Coff). It is important to observe that this contrast is well preserved at fast speaking rate (3 among the 4 speakers) at least in the selected sequences in which the contrast was perceptually considered to be maintained. However, our results are in clear disagreement with the predictions of Öhman"s model: only one subject at normal speaking rate and two subjects at fast speaking rate confirm the hypothesis of a constant [i-a] duration (V1ons-V2off); only one subject at fast speaking rate is compatible with the hypothesis of a postponing of the consonant opening gesture in the second vowel (Coff-V2off). As concerns the third basic assumption underlying the use of Öhman"s model for geminates (V1ons-Cons), i.e. the anticipation of the closing gesture in the preceding vowel, results are twofold: this hypothesis is essentially supported at a normal speaking rate, while all subjects behave in opposition to this prediction at fast speaking rate. Hence, contrary to Smith"s [START_REF] Fowler | Coarticulation and theories of extrinsic timing[END_REF] conclusion, it can be assumed that Italian geminates are not produced in a way compatible with Öhman"s hypothesis.

A constant synchronization of the consonant target with the preceding vowel (V1ons-Cmid) is generally observed, while the duration (Cmid_V2off) is generally depending on the consonant contrast. This is an interesting result supporting the idea of a planning of the gemination within the VCV sequence or the VC sequence rather than within the CV sequence. This result provides also evidence against the c-center model, that would suggest a constant phasing between the consonant and the following vowel.

Discussion and Conclusions

In general, our results confirm the classical finding that geminates are significantly longer than singletons, even at fast speaking rate (3 among the 4 speakers). Our results suggest further that speakers of Italian could have different ways to implement the length contrast independently of the variety of Italian they speak. Smith"s results [START_REF] Smith | Prosodic patterns in the coordination of vowel and consonant gestures[END_REF] appear not to be confirmed both at the normal and fast speech rate. Our data show that the timing of maximum constriction with respect to tongue movement was not always constant across geminate\singleton consonants in the consonant-to-vowel gesture, and the vowelto-vowel interval varied significantly. On the other hand, Smith"s hypothesis concerning the anticipation of the geminate consonantal gesture in the preceding vowel is confirmed (consistent with Öhman"s model); however, no significant delay in the following vowel is observed. At the fast speech rate, geminates and singletons show fewer significant differences than at normal rate.


Thierry Hamon

Julien Derivière

Adeline Nazarenko

Ogmios: a scalable NLP platform for annotating large web document collections

Introduction

Search engines like Google or Yahoo offer access to billions of textual web pages. These tools are very popular and seem to be sufficient for a large number of general user queries on the Internet. However, some other queries are more complex, requiring specific knowledge or processing strategies: no really satisfactory solution exists for these requests. There is thus a need for more specific search engines dedicated to specialised domain or users.

Considering the case of text mining in Microbiology for example, it is clear than one needs more than existing search engines given the specificity and the reliability of the information that is sought by scientists. Even if recent developments in biology and biomedicine are reported in large bibliographical databases (e.g. Flybase, specialised on Drosophilia Menogaster or Medline), such databases and the associated searching functionalities are not sufficient to satisfy biologists' specific information needs, such as finding information on gene interactions in order to progressively figure out a whole interaction network. We previously argued that looking for this kind of relational information requires a domain-specific linguistic analysis and parsing of the documents [START_REF] Nazarenko | Event-based Information Extraction for the biomedical domain: the Caderige project[END_REF].

The ALVIS project aims at developing an open source search engine, with extended semantic search facilities. Compared to state of the art search engines (like Google, the most popular one), the ALVIS search engine is domain specific. It relies on a specialised crawler, which selects the web pages on terminological grounds. Indexing exploits various types of linguistic and domain specific annotation (cf. figure 1)). Through a dedicated user interface, the ALVIS search engine processes the query more accurately, taking into account the topic and the context of search to refine both the query and the document analysis. This paper focuses on the design and the development of the text processing platform, Ogmios, which has been developed in the ALVIS project. The challenges were to handle rather large domain specific collections of documents (typical specialised collections gather hundreds of thousands of documents, rather than hundreds of millions of documents), to analyze documents from the web using a single platform, how heterogeneous they may be, to enrich documents with domain-specific semantic information to allow semantic querying. The present paper shows how the three constraints of genericity, domain semantic awareness and performance can be handled all together. The Ogmios platform is a generic one. It is instantiated using existing NLP modules and resources, which can be tuned to specific domains. The figure 1 shows the role of the NLP annotation and resource acquisition in the whole IR process. For processing texts in the biological domain, we exploited a specific named entity dictionaries and terminologies and we adapted a generic syntactic analyzer.

Section 2 gives an overview of the existing platforms designed for document annotation. Sections 3 and 4 describe the global architecture of the platform and its various NLP modules. Section 5 describes the performance of our system on a collection of crawled documents relative to Microbiology.

Background

Several text engineering architectures have been proposed to manage text processing over the last decade [START_REF] Cunningham | Software Infrastructure for Language Resources: a Taxonomy of Previous Work and a Requirements Analysis[END_REF] without being in the context of the information retrieval or the linguistic enrichment of very large corpora from the web. Thus, some architectures like GATE [START_REF] Bontcheva | Evolving GATE to meet new challenges in language engineering[END_REF], UIMA [START_REF] Ferrucci | UIMA: an architecture approach to unstructured information processing in a corporate research environment[END_REF] or Textpresso system [START_REF] Müller | Textpresso: an ontologybased information retrieval and extraction system for biological literature[END_REF], aim at linguistically annotating and exploring mean sized corpus for the information extraction. LinguaStream [START_REF] Widlöcher | La plate-forme LinguaStream : un outil d'exploration linguistique sur corpus[END_REF] is rather helpful in minig corpora and carrying out experiments with complex linguistic processing.

Document analysis of those linguistic platform relies on existing NLP tools. These tools are reused in module which wrapped themm and insure conformity of the intput/output streams. Defining a exchange format is crucial to insure communication between the modules and the integration of the results in external applications. Various exchange formats have been proposed. They are generally based on SGML and more recently on XML. For instance, the exchange formats of GATE, CPSL (Common Pattern Specific Language) and UIMA, CAS (Common Analysis Structure) are issued form the annocation format of TIPSTER [START_REF] Grishman | Tipster architecture design document version 2.3[END_REF]. Those format proposed stand-off annotations to be more flexible.

Given our constraints (generic, reduced time processing, and esay tuning to a specialised domain), those annoatation platforms are not well adapted to the context of the specialised information retrieval, and especially to process very large corpora. The Textpresso system [START_REF] Müller | Textpresso: an ontologybased information retrieval and extraction system for biological literature[END_REF] pursues the same purpose: developping a generic architecture to processus specialised corpora. It has been specifically developed to mine biological documents, abstracts as well as articles. It is designed as a curation system extracting gene-gene interaction that is also used as a search engine. It has been evaluated on a mean collection composed of 16,000 abstracts and 3,000 full text articles related to Caenorhabditis elegans.

Generally, very few information are available to évaluate performance of the systems on corpora. Our first test shows that GATE is not suited to process large collections of documents. GATE has been designed as a powerful environment for conception and development of NLP applications in information extraction. Scalability is not central in its design, and information extraction deals with small sets of documents. However, we have observed that problems appear on small sets of documents. Based on an external linguistic annotation platform, namely GATE, the KIM platform [START_REF] Popov | KIM -a semantic platform for information extraction and retrieval[END_REF] can be considered as a "meta-platform". It is designed for ontology population, semantic indexing and information retrieval. KIM has been integrated in massive semantic annotation projects such as the SWAN clusters1 and SEKT.2 The authors identify scalability as a critical parameter for two reasons: ( 1) it has to be able to process large amounts of data, in order to build and train statistical models for Information Extraction;

(2) it has to support its own use as an online public service. However, no information is provided to evaluate its scalability. Document collections could be processed in UIMA thanks to the Collection Processing Engine, which proposes amoung others performance monitoring and parallelization.

Those linguistic annoation platforms answer partly to our constraints. They are rather mining environment than platform designed to annotate very large collection of documents issus from the web. In that respect, we choose to propose a NLP architecture able to analyze large amounts of documents, and focus on the efficiency of the processing. 1

A modular and tunable platform

In the development of Ogmios, we focused on tool integration. Our initial goal was to exploit existing NLP tools rather than developing new ones but integrating heterogeneous tools and nevertheless achieve good performance in document annotation was challenging. We developped NLP systems onmy when no other solution was available.

And we preferably chose GPL or free lilcence software.

Ogmios platform was designed to test various combinations of annotations in order to identify which ones have a significant impact on information retrieval, information ex-traction or even extraction rule learning. In that respect, the platform can be viewed as a modular software architecture that can be configured to achieve various tasks.

Specific constraints

The reuse of NLP tools imposes specific constraints regarding software engineering and processing domain-specific documents requires tuning resources to better fit the data.

From the software engineering point of view, the constraints mainly concern the input/output formats of the integrated NLP tools. Each tool has its own input and output format. Linking together several tools requires defining an interchange format. This engineering point of view is important for testing various combinations of annotations.

The second type of constraints is the cost linguistic analysis in terms of processing time.

The main pitfall is the deep syntactic dependency parsing which is time consuming) and which lead us to design a distributed architecture.

A domain specific annotation platform also requires lexical and ontological resources or the tuning of NLP tools such as the Part-of-Speech tagger or parser. For instance, we have argued in [START_REF] Nazarenko | Event-based Information Extraction for the biomedical domain: the Caderige project[END_REF]) that identification of gene interaction requires gene name tagging, which relates to traditional named entity recognition, term recognition and a reliable syntactic analysis.

General architecture

The different processing steps are traditionally separated in modules [START_REF] Bontcheva | Evolving GATE to meet new challenges in language engineering[END_REF]. Each module carries out a specific processing step: named entity recognition, word segmentation, POS tagging, parsing, semantic tagging or anaphora resolution. It wraps an NLP tool to ensure the conformity of the input/output format with the DTD. Annotations are recorded in an XML stand-off format to deal with the heterogeneity of NLP tools input/output (the DTD is fully described in [START_REF] Nazarenko | The ALVIS Format for Linguistically Annotated Documents[END_REF]). The modularity of the architecture simplifies the substitution of a tool by another.

Tuning to a specific field is insured by the exploitation of specialised resources by each module. For instance, a targeted species or gene list can be added to the biology-specific named entity recognizer to process Medline abstracts. In the ALVIS project, the problem of acquiring automatically these specialised resources from a training corpus is also addressed (see Figure 2 We assume that input web documents are already downloaded, cleaned, encoded into the UTF-8 character set, and formatted in XML [START_REF] Nazarenko | The ALVIS Format for Linguistically Annotated Documents[END_REF]. Documents are first tokenized to define offsets to ensure the homogeneity of the various annotations. Then, documents are processed through several modules: named entity recognition, word and sentence segmentation, lemmatization, part-of-speech tagging, term tagging, parsing, semantic tagging and anaphora resolution.

Although this architecture is quite traditional, few points should be highlighted:

-Tokenization computes a first basic non-linguistic segmentation of the document, which is used for further reference. The tokens are the basic textual units in the text processing line. Tokenization serves no other purpose but to provide a starting point for segmentation. This level of annotation follows the recommendations of the TC37SC4/TEI workgroup, even if we refer to the character offset rather than pointer mark-up (TEI element ptr) in the textual signal to mark the token boundaries. To simplify further processing, we distinguish different types of tokens: alphabetical tokens, numerical tokens, separating tokens and symbolic tokens.

-Named Entity tagging takes place very early in the NLP line because unrecognized named entities hinder most NLP steps, in many sublanguages; -Terminological tagging is used as such but is also considered as an aid for syntactic parsing. As this latter step is time consuming, we exploit the fact that terminological analysis simplifies the parsing cost.

For each document, the NLP modules are called sequentially. The outputs of the modules are stored in memory until the end of the processing. XML output is recorded at the end of the document processing.

The linguistic analysis of the documents are distributed according to the client/serveur model. The server manages the documents distribution by sending them to the clients and gathering the analysed documents coming from the clients. Each client performs the whole linguistic annotation described at the figure 2.

Description of the NLP modules

This section describes the different NLP modules. Il also explains what is the expected impact of each linguistic annotation step on IR or IE performance.

Named Entity tagging

The Named Entity tagging module aims at annotating semantic units, with syntactic and semantic types. Each text sequence corresponding to a named entity is tagged with a unique tag corresponding to its semantic value (for example a "gene" type for gene names, "species" type for species names, etc.). We use the TagEN Named Entity tagger [START_REF] Berroyer | TagEN, un analyseur d"entités nommées : conception, développement et évaluation[END_REF] , which is based on a set of linguistic resources and grammars. Named entity tagging has a direct impact on search performance when the query contains one or two named entities, as those semantic units are have a high discriminative power.

Word and sentence Segmentation

This module identifies sentence and word boundaries. We use simple regular expressions, based on the algorithm proposed in [START_REF] Grefenstette | What is a word, what is a sentence? problems of tokenization[END_REF]. Part of the segmentation has been implicitly performed during the Named Entity tagging to solve some ambiguities such as the abbreviation dot in the sequence "B. subtilis", which could be understood as a full stop if it were not analyzed beforehand.

Morpho-syntactic tagging

This module aims at associating a part of speech (POS) tag to each word. It assumes that the word and sentence segmentation has been performed. We are using a probabilistic Part-Of-Speech tagger: TreeTagger [START_REF] Schmid | Probabilistic Part-of-Speech Tagging Using Decision Trees[END_REF]. The POS tags are not used as such for IR but POS tagging facilitates the rest of the linguistic processing.

Lemmatization

This module associates its lemma, i.e. its canonical form, to each word. The experiments presented in [START_REF] Moreau | Revisiter le couplage traitement automatique des langues et recherche d'information[END_REF] show that this morphological normalization increases the performance of search engines. If the word cannot be lemmatized (for instance a number or a foreign word), the information is omitted. This module assumes that word segmentation and morpho-syntactic information are provided. Even if it is a distinct module, we currently exploit the TreeTagger output which provides lemma as well as POS tags.

Terminology tagging

This module aims at recognizing the domain specific phrases in a document, like gene expression or spore coat cell. These phrases considered as the most relevant terminological items. They can be provided through terminological resources such as the Gene Ontology [START_REF] Consortium | Gene Ontology: tool for the unification of biology[END_REF], the MeSH (Mesh, 1998)(MeSH) or more widely UMLS (National library of medicine, 2003). They can also be acquired through corpus analysis (see Figure 1). Providing a given terminology tunes the term tagging to the corresponding domain. Previous annotation levels as lemmatization and word segmentation but also named entities are required. The goal in identifying domain specific phrases in the documents is the same as for the named entitiy recognition, i.e. to identify the relevant semantic units. Even if previous experiments (see [START_REF] Lewis | An Evaluation of Phrasal and Clustered Representations on a Text Categorization Task[END_REF] among others) have shown a little impact of the phrases on IR performance, we argue that terminology should have a more significant impact on specialised search engines, as a terminology is relevant for a specific domain. In addition to that, a normalization procedure can associate a canonical form to any phrase occurrence (e.g. gene expression, expression of gene, gene expressed?). This normalization step is similar to the lemmatization one for words. Gathering associated variants under a single form modifies the phrase frequencies and thus affects IR.

Parsing

The parsing module aims at exhibiting the graph of the syntactic dependency relations between the words of the sentence. Parsing is a time and resource-consuming NLP, especially when compared to other NLP tasks like named entity recognition or partof-speech tagging. As mentioned above, the syntactic analysis is especially important for the tasks that involve relations between entities (either information extraction or relational queries such as X's speeches as opposed to speeches on or relative to X). However, this technology is not yet fully compatible with Information Retrieval or Extraction.

Even if processing time is a critical point for syntactic parsing, we argue that it may enhance the semantic access to web documents. On the one hand, it is usually not necessary to parse the entire documents. A good filtering procedure may select the more relevant sections to parse. We still have to develop a method for pre-filtering the textual segments that are worth parsing as proposed in [START_REF] Nédellec | Sentence Filtering for Information Extraction in Genomics, a Classification Problem[END_REF]. On the other hand, as we will show in Section 3.2, a good recognition of the terms can reduce significantly the number of possible parses and consequently the parsing processing time.

In Ogmios, the word level of annotation is required in the parser input. Depending on the choice of the parser, the morpho-syntactic level may be needed. The Link Grammar Parser [START_REF] Sleator | Parsing English with a link grammar[END_REF] is integrated.

Semantic type tagging and anaphora resolution

The last modules are currently under test and should be integrated in the next release of the platform. The semantic type tagging associates to the previously identified semantic units tags referring to ontological concepts. This allows a semantic querying of the document base.

The anaphora resolution module establishes coreference links between the anaphoric pronoun occurrences and the antecedents they refer to. Even if solving anaphora has a small impact on the frequency counts and therefore on IE, it increases IE recall: for instance it inhibits Y may stand for X inhibits Y and must be interpreted as such in a extraction engine dealing with gene interactions.

Performance analysis

We carried out an experiment on two collections ofweb documents. The first one gathers 55,329 web documents from the biological domain (henceforth BIO). Most documents have an XML size between 1KB and 100KB. The size of the biggest document is about 5.7 MB. Figure 3 shows the distribution of the input document size (both axes are on a log scale). The sceond document collection is composed of 48 422 news related to the search engines (henceforth SEN). All the documents have a size between 1 and 150 KB. All the documents went through all NLP modules, up to the term tagging (as mentioned before, the goal is not to parse the whole documents but only some filtered part of them). To annotate the BIO collection, we used a 375,000 term list, issued from the MeSH and Gene Ontology have been used, while for the SEN collection, the list was composed of 17,341 terms automatically extracted. In both cases, we exploited a 400,000 named entity list, including species and gene names for BIO, and names of person, software and company for SEN.

We used 20 machines to annotate these documents. Most of these machines were standard Personal Computers with 1GB of RAM and 2.9 or 3.1 GHz processor. We also used a computer with 8GB of RAM and two 2.8GHz Xeon (dual-core) processors. Their operating system were either Debian Linux or Mandrake Linux. The server and three NLP clients were running on the 8GB/biprocessor. Only one NLP client was running on each standard Personal Computer.

Even if a real benchmark requires several tests to evaluate the performance, we consider this performance as an interesting indication of the platform processing time. Timers are run between each function call in order to measure how long each step is (user-timewise). We used the functions provided in the Time::Hires Perl package. All the time results are recorded in the annotated XML documents. The documents of the BIO collection have been annotated in 35 hours, while The annotation of SEN was completed in 3 hours and 15 minutes.

Table 1 shows the total number of entities found in the BIO collection. 106 million words and 4.72 million sentences were processed; 4.53 million named entities and 13.9 million domain specific phrases were identified. Each document contains, on average, 1,913 words, 85 sentences, 82 named entities and 251 domain specific phrases. 147 documents contained no words at all; they therefore underwent the tokenization step only. One of our NLP clients processed a 414,995 word document.

Table 2 shows the average processing time for each document of BIO. Each document has been processed in 37 seconds on average. Due to the exploited resource, the most time-consuming steps are the term tagging (56% of the overall processing time) and the named entity recognition (16% of the overall processing time). The mean time processing of the SEN is 2 seconds.

The whole BIO document collection, except two documents, has been analysed. Thanks to the distribution of the processing, the problems occuring on a specific document had no consequence on the whole process. Clients in charge of the analysis of these documents have been simply restarted.

The performance we get on this collection show the robustness of the NLP platform, and its ability to analyse large and heterogeneous collection of documents in a reasonable time. We have proven the efficiency of the overall process for semantic crawlers and its accuracy for a precise indexing of web documents.

Conclusion

We have presented in this paper a platform that has been designed to enrich specialised domain documents with linguistic annotations. While developments and experiments have been performed on biomedical texts, we assume that this architecture is generic enough to process other specialised documents. The platform is designed as a framework using existing NLP tools which can be substituted by others if necessary. Several NLP modules have been integrated: named entity tagging, word and sentence segmentation, POS tagging, lemmatization, term tagging, and syntactic parsing. Semantic type tagging and anaphora resolution are currently being under stress.

We also focused on the system performance, since this point is crucial for most Internet applications. We have experimented a distributed design of the platform, by splitting the corpus in equal parts: this strategy dramatically increased the overall performance (see [START_REF] Ravichandran | The Terascale Challenge[END_REF]. We have also shown that Ogmios is a robust NLP platform with respect to the high heterogeneity of the document sizes and types.

These first experiments show that a deep analysis of web documents is possible. Besides the necessary improvement the Ogmios platform, our next goal is to assess the impact of NLP on IR performance. Our hypothesis is that this impact should be higher in the case of a specialised search engines than for a generic IR framework, on which the IR-NLP cooperation has mainly been tested until now. Specific experiments are currently carried out in the ALVIS project to test the potential resulting enhanced functionalities on a microbiological search engine.

Figure 1 .

 1 Figure 1. Role of NLP in the ALVIS semantic search engine

  [START_REF] Nazarenko | Event-based Information Extraction for the biomedical domain: the Caderige project[END_REF]) but this question falls out of the scope of the present paper.

Figure 2

 2 Figure2gives an overview of the architecture. The various modules composing the NLP line are represented as boxes. The description of these modules is given in section 4. The arrows represent the data processing flow. Intermediary levels of annotations can be produced if the complete NLP line is not used. For instance, anaphora resolution is seldom activated.

Figure 3 .

 3 Figure 3. Distribution of document size

Table 1 .

 1 Average and total numbers of linguistic units

		Average number of units	Total number of units
		by document	in the document collection
	Tokens	5,021.9	277,846,470
	Named entities	81.88	4,530,368
	Words	1,912.65	105,821,243
	Sentences	85.41	4,726,003
	Part-of-speech tags lemma	1,883.5	104,208,536
	Terme	250.76	13,874,089

Table 2 .

 2 Average time for one document processing (in seconds)

		Average time processing Percentage
	loading XML input doc.	0.38	1.02
	Tokenization	0.7	1.88
	Named entity recognition	6.12	16.42
	Word segmentation	5.19	13.92
	Sentence segmentation	0.18	0.48
	part-of-speech tagging lemmatization	1.84	4.94
	Terms tagging	20.83	55.89
	rendering XML output doc.	2.03	5.45
	Total	37.27	100

Acknowledgements

This work is supported by the EU 6th Framework Program in the IST Priority under the ALVIS project. The material benefits from interactions with the ALVIS partners, especially with INRA-MIG.


Marc Olivier

Olivier Sedan

Bernard Monod
email: b.monod@brgm.fr

Contribution of physical modelling to landslide hazard mapping: case of the French Basque coast

Keywords: Physical modelling, probabilistic approach, landslide hazard, landslide zoning, Pays Basque coast

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Introduction

Mechanical approaches represent a solution for quantitative landslide susceptibility and hazard modelling. These methods consist in, (1) generalizing at regional scale geometrical and geotechnical parameters which contribute to slope stability regarding two types of landslide (i.e. rotational and translational), (2) applying triggering factors to the studied area (water table level and/or seismic stress for example), and (3) calculating a safety factor for each point of the area.

However, as the quantity and the quality of data are generally very heterogeneous at a regional scale, it is necessary to take into account the uncertainty in the analysis. In this perspective, a new hazard modelling method, using a probabilistic approach, is developed and integrated in a software called ALICE®.

This method is experimented for landslide hazard mapping on the French Basque coast. Situated in the South West of France, this region presents high human and economic stakes. Indeed, the Pays Basque littoral has a population density of 504 people/km², whereas it is of 81 people/km² in average in the South West littoral of France [START_REF] Mouhali | Littoral aquitain, diagnostic de territoire[END_REF]. Numerous instability problems affect the 42 km long Basque coast and contribute for a great part to the littoral erosion [START_REF] Mallet | Synthèse des études réalisées sur les instabilités de la côte basque entre 2001 et[END_REF]. Landslides are concentrated in a 12 km long coastal zone between Biarritz and Saint-Jean-De-Luz (Fig. 1) while the rest of the coast is more subject to rock fall phenomenon.

A ground movement (landslides and rock falls) hazard map at a scale of 1/20 000 already exists on the Basque coast [START_REF] Nédellec | Evaluation et cartographie de l'aléa mouvements de terrain sur la côte basque[END_REF], made by an "expertise" analysis. The works presented in this paper aim at designing a more precise landslide hazard map (1/10 000) with a more objective method.

This paper provides a short presentation of the method, developed through the software ALICE® and the different steps which led to a new landslide hazard map of the French Basque coast area. A new landslide physical modelling software: ALICE® Principle ALICE®, which stands for Assessment of Landslides Induced by Climatic Events, is a software designed to support landslide hazard mapping [START_REF] Sedan | Logiciel ALICE version 7 -Guide Utilisateur[END_REF].

The model is based on a mechanical and geotechnical approach for which the main physical characteristics of the medium are quantified and used by a mathematical model calculating a safety factor [START_REF] Aleotti | Landslide hazard assessment: summary review and new perspectives[END_REF]. In these models, the spatial variability of the parameters (e.g. mechanical characteristics) has to be known and is handled through GIS software.

The probabilistic approach used in the software allows to take into account uncertainties by giving probabilistic distributions to some of the model parameters (Stillwater, 2007).

Modelling

The software uses the Morgenstern andPrice method (1965, 1967), which is a finite slope stability model based on the equilibrium calculation between slices subdividing the landslide volume. This method is used on regularly spaced topographic 2D profiles which cover the whole studied area. These profiles are automatically generated by the software using four input raster maps: a Digital Elevation Model (DEM), a slope map, a flow direction map and a flow accumulation map.

Pedological and geological characteristics are taken into account thanks to the altitude maps of the interfaces between each soil layer, the highest limit corresponding to the topographic surface (DEM).

Variability and uncertainty of the geotechnical parameters are introduced in the software by means of probabilistic distributions (normal, uniform, triangular or trapezoid). A distribution is attributed to each soil characteristic (cohesion (c), angle of friction (ϕ) and unit weight (γ)) and for each soil layer.

The safety factor calculation also needs the landslide type (rotational or translational) and its length. These parameters are defined for the whole studied area.

Water table level is set by determining two piezometric maps: one for the maximal water table level and the other for the minimal. The water table is then automatically generated between these piezometric levels by setting a filling ratio.

Software output

On each profile of the studied area, the software calculates safety factors for several landslide positions (Fig. 2). The probability of having the safety factor below than one represents the probability of occurrence of the landslide for a given triggering scenario (i.e. landslide geometry and water table level). The dispersion of the distribution gives the uncertainty of the result.

The calculated probability of occurrence is then attributed to all the pixels of the result raster located on the profile and intersecting the landslide geometry. Thus a pixel receives as many probabilities as landslide positions it is included in. Finally, a map is created, displaying the highest calculated probability of occurrence for each pixel of the studied area.

Application to the Basque Coast

General context

Geology

The French Basque coast displays folded rock units that regionally dip towards the north. Thus, from north-east near Biarritz to south-west near Saint-Jean-de-Luz, geological formations are mainly composed of tertiary sandstone, marly-calcareous and marl formations until Bidart city, and cretaceous flysch with flint, calcareous flysch until Saint-jean-de-Luz city (Fig. 3). A major fault zone is located north of Bidart city, at the base of the tertiary strata. It is responsible for the outcropping of triassic gypsiferous shale and the grinding of rocks around.

The bedrock displays two facies along the coastal cliff: fresh rocks at the base and weathered rocks, called alterite, to the top (few meters from the surface). The alterites are well developed over the eocene units and over the santonian-coniacian flysches.

Plio-quaternary Onesse and Arengosse formations, together with sand formations were deposited over the eocene alterites. Alluvium and colluvium deposits fill in the troughs and the valleys.

The coastal instabilities are concentrated into the alterites and the plio-quaternary formations which present poor mechanical characteristics.

A simplified geological model has been developed, grouping together the coast formations into three "geotechnical units": Plio-quaternary, Alterites and Bedrock (Peter-Borie, 2008, Fig. 3). Hundreds of borehole reports were gathered and interpreted in order to interpolate the altitude of each interface, allowing the creation of maps for the base and the top of each unit.

Hydrogeology

Water circulations are very complex within the Basque coast reliefs and suffer from a lack of information and studies [START_REF] Winckel | Etat des connaissances hydrologiques et hydrogéologiques de la côte basque[END_REF]). However, some punctual studies showed that water level was close to the surface at the time of various landslide triggering [START_REF] Capedeville | Géologie et hydrogéologie de la falaise de la côte des basques et de son arrière-pays. Schéma de principe pour le drainage[END_REF][START_REF] Peter-Borie | Les massifs rocheux du Crétacé supérieur du Labourd occidental : processus d'altération et instabilités littorales[END_REF].

Geotechnical parameters

Despite the numerous geotechnical studies carried out on Basque coast instabilities, few laboratory tests were made on the materials. Thus some of them had to be determined thanks to expert statements. Ranges of values were then established for each parameter and for each soil unit (right columns, Fig. 3). For the safety factor calculations, triangular distributions of these values, which can be compared to discretized Gaussian functions, were chosen because this type of simple distribution is well adapted when few data are available.

First modelling scenario

Based on the documents available on the studied area and on a short field visit, a first triggering scenario is established: a constant 2m deep water table and 25m long landslides are used as input parameters in the software.

The results of this first scenario are presented for the Arretgia beach area (Fig. 4a). A high probability means that the landslide will occur if the triggering scenario is achieved, even if the material has the highest values within the geotechnical characteristic distributions. On the other hand, low probabilities point out stable areas, even for materials with very poor characteristics.

This result map can be used directly in order to evaluate quantitatively the landslide hazard. For a wider accessibility, to integrate public policy documents for example, a "classical" zoning map, with hazard levels (low, moderate, high), is recommended.

a b

Thus a preliminary zoning of the studied area is realised, based on the software results, satellite images and 5m spaced contour lines created from the DEM (Fig. 4b). Four hazard levels are defined from the probabilities of occurrence: Very Low (0%); Low (< 10%); Moderate (10% -30%); High (> 30%).

Field checking

A second field visit was carried out in order to face the modelling results and the hazard zoning with the ground truth. The whole studied area was observed and commented. In general, a good correlation exists between the modelling results and the reality. However, several areas seem to be under-evaluated by the modelling regarding the existing instability evidences. In some areas, the modelling does not correspond at all to the field observations. Three sample areas are detailed in this paper: (1) the Erromardie beach presents several zones with various probabilities of occurrence which correspond to the stability context of the area, (2) the Parlementia sector presents some instability evidences which don't fit with the modelling low probabilities and (3) the "Côte des Basques" modelling shows no landslide hazard whereas instabilities are visible nearly all along the area. Erromardie and Parlementia instabilities take place into the alterites of the flysches whereas the Côte des Basques ones are located into marly-calcareous formations.

Erromardie

Erromardie beach, situated in the city of Saint-Jean-De-Luz, presents many instabilities, from 10 to 40 meter long. The result of the modelling shows many zones with high probabilities of landslide occurrence but also some with lower values (Fig. 5) and this lateral variation is well observed on the field.

Figure 5 Results of the modelling (probabilities of occurrence) and preliminary hazard zoning map on the Erromardie beach area. The blue arrows show the locations of the photographed banks of Figure 6.

The high probabilities correspond to the sea bank showing either recurrent instabilities (Fig. 6b), or recent landslides (Fig. 6a,6c) while the low values match with zones without bank (the main sand beach), where the bank slope is less steep or where the bedrock is closer to the surface. Thus the preliminary hazard zoning designed from the modelling results is validated by the ground truth. However on the field, the sea bank showed signs of instabilities such as tension cracks on the road or on the materials of the bank. Moreover, the slopes situated under the coast houses are comforted by geotextiles and draining blanket in order to avoid infiltration of water within the soil. One of the draining blankets was in construction at the time of the visit (Fig. 8). The need for such soil protections implies a history of instabilities threatening the houses on this site. Here, the preliminary zoning is wrong and have to be modified.

This difference between modelling and reality was encountered several times along the coast. A hypothesis for the bad modelling of these areas is that the landslide size of 25m chosen in input is too short. Indeed, many landslides encountered along the coast were longer than 35m.

Côte des Basques

The cliffs of the Côte des Basques area are well known for their frequent instability problems and many studies and stabilization works have already been carried out to secure them. Levelling works are still in progress on a part of this area, and the access of the beach underneath is forbidden (Fig. 9). However, the modelling only shows a few low probability areas which are not even matching with the most active landslide zone (Fig. 10).

© BRGM © BRGM

Thus, strong mechanical characteristics are assigned to this formation for the software stability calculations (Fig. 3). However, a decompression fringe corresponding to the first meters of rock below the surface displays highly weathered materials [START_REF] Capedeville | Géologie et hydrogéologie de la falaise de la côte des basques et de son arrière-pays. Schéma de principe pour le drainage[END_REF]. Geotechnical tests on these materials gave much lower characteristics. Thus the modelling is widely overestimating the stability of this area.

Moreover, as the banks are at least 50 meter long, the input landslide size is not adapted to this area.

Second modelling scenario

Following the field visit and the observations made, a second modelling scenario was established in order to get closer to the reality:

-Input landslide size is raised up to 40m to be more representative of the instabilities existing in the studied area. -Water table level is created using a filling ratio of 90% between the topographic and the sea levels (Fig. 11). Doing so, the water gets closer to the surface while reaching the bottom of the sea bank. This behaviour is close to the ground truth as many smalls water springs were observed in the bottom part of the sea banks. -The Côte des Basques sector is treated separately from the rest of the coast. The mechanical characteristic values of the Marly-calcareous formations are lowered, corresponding to weathered rock characteristics. For the stability calculation a landslide depth limit of 10m is defined based on the estimated thickness of the decompression fringe.

Figure 11 Cross section along the Basque coast showing the topographic surface, the geological model, sea and water table levels. In this example, water table depth is determined as being 10% of the difference between the topographic and the sea levels.

The results of this second modelling are much more representative of the actual instabilities of the Basque coast than the first, as it can be seen on the Parlementia and Côte des Basques areas (Fig 12). However, some localized sectors still do not match the field observations. It happens mainly in high geological heterogeneities areas (e.g. major fault disturbances, Fig. 3) which are not taken into account in the simplified geological model.

Final landslide hazard zoning

Based on the new modelling and the field observations, the preliminary landslide hazard map is modified to represent accurately the hazard level on the French Basque coast. In the zones where the modelling is not relevant, a classical expertise approach is used. The final map for the city of Bidart is presented (Fig. 13) A "Very High" hazard level is added for the zoning. It corresponds to zones showing recurent landslide events and can be associated to sectors where the triggering scenario is regularly achieved.

Finally, a regression zone of 20 m in average is delimited behind the sea bank. The mapping of these areas is based on the results of the software and the study of the historical coast erosion (Peter-Borie, 2008).

As the map was designed at a scale of 1/5000, it has to be used at a maximal scale of 1/10000.

Conclusions and perspectives

A landslide hazard map has been designed on the French Basque coast, based mainly on the results of a physical modelling software.

The ALICE® software provides a useful tool for landslide hazard zoning. It creates a probability of occurrence map for given landslide type and water table level. This map allows a fast spotting of the most sensible areas and a hazard quantification of these areas. However, it still cannot be dissociated from a ground truth approach as shown in the example of the French Basque coast. The main purpose of developing such method is to be able to combine quantitative hazard data with quantitative vulnerability data in order to have a quantitative assessment of the landslide risk.

The difficulties which could be encountered when using this method will probably come from the quantity and quality of data needed. Very few areas are endowed with a 3D geological model and enough geotechnical characteristics values.

For future works on the Basque coast landslide hazard map, the hydrological modelling should be improved. It could be done using the GARDENIA® rain/flow model [START_REF] Thiéry | Logiciel GARDÉNIA version 6.0 -Guide d'utilisation[END_REF] which simulates the water cycle from rain on a watershed to a phreatic level at a given point or a flow at an exit point. This model involves periodic rainfall and piezometric measurements as well as weather data (e.g. temperature and moisture of the air) in order to highlight the periodic variations of the water table in the coast reliefs.

For the software perspectives, it is planned to generalized the probabilistic approach on other parameters than geotechnical characteristics, like landslide geometry, soil layer thickness or water table level. ALICE® should also be combined with a landslide material propagation software in order to create a complete landslide modelling tool.

The DO-SMS project is financially supported by the STC SUDOE through the Interreg IV B funding.

The geological model and some other numerical data on the French Basque coast were provided by the OCA organisation.

Figure 1

 1 Figure 1 Situation of the studied area (in red) which represents the portion of the coast the most affected by landslide phenomena.

Figure 2

 2 Figure 2 Illustration of the way the software calculates safety factors for several landslide positions (red circles) along a topographic profile on the Basque coast. The pink circle represents the landslide position with the highest probability of occurrence along the profile. The graph in the top right hand corner shows the distribution of calculated safety factors for the pink landslide. (For a detailed description of the cross section, see Fig. 11).

Figure 3

 3 Figure 3 Vertical representation of the geological formations out cropping along the Basque coast and geo-mechanical model associated. The north-dipping rock units are represented in the same order as they are exposed along the coast, from the SW (bottom) to the NE (top). The thickness of the formations represented is proportional to the outcropping thickness along the coast. Main faults are represented in red. Units for the geotechnical parameters are: γ: kN.m -3 ; c:kPa; ϕ:°.

Figure 4

 4 Figure 4 Results of the first modelling scenario on the Erretgia Beach area (a) and corresponding preliminary hazard zoning (b).

Figure 6

 6 Figure 6 Different instability areas on Erromardie beach. The red arrows point at recent landslides.

Figure 7

 7 Figure 7 Results of the modelling on the Parlementia area and locations of movement evidences and soil protections observed. The red rectangle represents the localisation of Fig. 8.

Figure 8

 8 Figure 8 Construction of a draining blanket over slid materials.

Figure 9

 9 Figure 9 Forbidden area due to frequent landslide occurrence on the South of the Côte des Basques (see localisation on fig. 10).

Figure 10

 10 Figure 10 Results of the modelling on the Côte des Basques cliffs. The most active area is highlighted by the red lining. The blue rectangle represents the localisation of Fig. 9. The problem was identified as coming from the simplified geological model used with the software. Indeed, in this model, the Marly-calcareous rocks of the Cote des Basques cliffs are classified as Bedrock unit.

Figure 12 Figure 13

 1213 Figure 12 Results of the second modelling for (a) the Parlementia beach and (b) the Côte des Basques cliffs. The displayed probabilities of occurrence correspond much better to the field observations compared to figures 7 and 10.

© ortholittorale 2000

Proceedings of the Second World Landslide Forum -3-7 October 2011, Rome

© BRGM © BRGM © BRGMProceedings of the Second World Landslide Forum -3-7 October 2011, Rome

Acknowledgments

The authors would like to thank Christian Mathon who accepted to share is strong knowledge on the French Basque coast and who provided many useful advices all along the study. We express our gratitude to José Darrozes for both leading the DO-SMS project and reviewing this article providing very useful suggestions. We also wish to thank Sandrine Aubie for accompanying us on our first field visit on the Basque coast and Anais Hoareau for her precious help in the handling of the numerical data. Finally, the authors express their gratitude to all the partners of the DO-SMS project who showed their interest in our work and shared their point of view and experience on difficult parts of our researches.