Ferréol Salomon
email: ferreol.salomon@gmail.com

Jean-Philippe Goiran
email: jean-philippe.goiran@mom.fr

Simona Pannuzi
email: simona.pannuzi@beniculturali.it

Hatem Djerbi
email: hatem.djerbi@eveha.fr

Carlo Rosa
email: carlorosa62@gmail.com

Jamie Woodward

Long-Term Interactions between the Roman City of Ostia and Its Paleomeander, Tiber Delta, Italy

Keywords: alluvial geoarchaeology, landscape evolution, paleomeander, Ostia, Tiber delta

This study examines the long term interactions between the well-known Roman city of Ostia and a river meander. Located at the mouth of the Tiber River, Ostia was a major harbor city that connected Rome to the Mediterranean Sea. Based on aerial photography and boreholes analysis, the paleodynamics of the Fiume Morto's paleomeander are understood to be linked to the urban evolution of the city of Ostia. Four periods of evolution have been identified as a result of this interdisciplinary work: (1) the foundation of Ostia's urban center, in the 4 th -3 rd century BC, occurred when the meander already existed;

(2) between the 4 th century BC and the 3 rd century AD, human/environmental interactions contributed to the compound growing of the meander which possibly eroded an important Roman road linking Ostia to Rome; (3) from the Imperial period until the meander was cut off in AD 1557-1562, the constricted meander channel at the apex led to the stability of the downstream river channel; (4) the cut off of the paleomeander was completed in 1562, leading to the filling of the paleochannel. These successive phases of channel evolution mark changing fluvial risks from the Roman period to today.

INTRODUCTION

Fluvial risks considered in a long term perspective have been the focus of several studies with the development of geoarchaeology [START_REF] Brown | Alluvial geoarchaeology: floodplain archaeology and environmental change[END_REF][START_REF] Arnaud-Fassetta | La géoarchéologie fluviale[END_REF], and the results of some studies have been used to evaluate present risks in the floodplains [START_REF] Bravard | Un atlas du paléo-environnement de la plaine alluviale du Rhône de la frontière suisse à la mer[END_REF]. It is important to recognize that the current concept of risk takes on different meaning depending on the cultures and the periods considered [START_REF] Kasperson | The social contours of risk[END_REF][START_REF] Arnaud-Fassetta | Fluvial geomorphology and flood-risk management[END_REF][START_REF] Bradford | Risk perception-issues for flood management in Europe[END_REF]. As such, since the 1980's, interdisciplinary studies involving archaeologists, geoarchaeologists and historians have been engaged to examine possible alluvial risks during the Roman period to cities [START_REF] Bravard | Géomorphologie et archéologie dans la région lyonnaise : questions et réponses d'un dialogue interdisciplinaire[END_REF][START_REF] Bravard | Le site de Vienne (38), Saint-Romain-en-Gal (69) et Sainte-Colombe (69): l'évolution de la plaine alluviale du Rhône de l'âge du Fer à la fin de l'Antiquité, proposition d'interprétation[END_REF][START_REF] Allinne | Les villes romaines face aux inondations. La place des données archéologiques dans l'étude des risques fluviaux[END_REF][START_REF] Leveau | Les inondations du Tibre à Rome: politiques publiques et variations climatiques à l'époque romaine[END_REF][START_REF] Arnaud-Fassetta | Fluviatile palaeoenvironments in archaeological context: Geographical position, methodological approach and global change -Hydrological risk issues[END_REF] and deltaic areas [START_REF] Arnaud-Fassetta | Hydroclimatic hazards, vulnerability of societies and fluvial risk in the Rhone Delta (Mediterranean France) from the Greek period to the Early Middle Ages[END_REF]. During the Roman period, the term "risk" did not exist, but the experience of fluvial events and their disruptions in Rome are well recorded in ancient texts [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF]. At the beginning of the 1 st century AD, Romans even considered several plans in the watershed of the Tiber River to prevent or reduce the floods in Rome [START_REF] Leveau | Les inondations du Tibre à Rome: politiques publiques et variations climatiques à l'époque romaine[END_REF].

In this context, the study of the relationship between the Roman site of Ostia, located 20 km downstream of Rome, and the Tiber River is significant to examine the risk of fluvial mobility and the modalities of its inconvenience regarding Ostia over a long period of time.

Founded at the mouth of the Tiber River, Ostia was a crucial city in the Roman world and existed over at least one thousand years (6 th /3 rd century BC to 4 th -5 th century AD), with revivals in the Borgo di Ostia from late antiquity to the present (Figure 1). Today, Ostia is one of the most extensively excavated Roman cities. Historical and archaeological scientific literature attests that Ostia was a harbor city connecting Rome to the Mediterranean Sea and that it controlled access to the Tiber River. Recently, the location of a harbor basin has been confirmed on the left bank of the Tiber at Ostia [START_REF] Heinzelmann | River port, navalia, and harbour temple at Ostia: new results of a DAI-AAR Project[END_REF][START_REF] Goiran | Geoarchaeology confirms location of the ancient harbour basin of Ostia (Italy)[END_REF][START_REF] Hadler | Temple-complex post-dates tsunami deposits found in the ancient harbour basin of Ostia (Rome, Italy)[END_REF], but it is small in size when compared to the basins at Portus [START_REF] Keay | Portus : An Archaeological Survey Of The Portus Of Imperial Rome[END_REF][START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF]. Considering the recent discovery of warehouses facing both sides of the river mouth [START_REF] Keay | High resolution space and ground-based remote sensing and implications for landscape archaeology: the case from Portus, Italy[END_REF], it is possible that the entire Tiber channel flowing through Ostia was a linear harbor. Consequently, we can assume that the river banks and channel mobility would have been considered carefully by the Romans.

This paper looks at the development of the area of Ostia since the Roman period in correlation with the evolution of the paleomeander of Ostia. Specifically, this study will (1) analyze the morphology of the paleomeander in detail using aerial photography, (2) provide the complete sedimentary sequence of the convexity and the cut-off channel, (3) determine the chronology of the evolution of the paleomenader and (4) put this new data into archaeological context. Finally, the changing relationship of river and city through time will provide a solid framework to discuss the evolution of the concept of fluvial risk in Central Italy.

GEOLOGICAL AND ARCHAEOLOGICAL SETTING

The Tiber river drains a watershed of 17,375 km 2 and follows a 405 km course to the Tyrrhenian Sea. From its confluence with the Paglia River to the sea (206 km), the Tiber River shows a meandering morphology. This study focuses on the deltaic plain, ca. 25 km downstream of Rome. The annual mean water discharge at Rome is 240 m 3 /s, with the highest water discharge measured and calculated around 3100 m 3 /s during a flood in 1900 [START_REF] Calenda | Selection of the probabilistic model of extreme floods: The case of the River Tiber in Rome[END_REF]. More ancient data concerning the floods is recorded through water levels [START_REF] Martino | Qui arrivò il Tevere. Le inondazioni del Tevere nelle testimonianze e nei ricordi storici[END_REF] or ancient texts from the Roman period (Le [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF][START_REF] Bersani | Le Piene del Tevere a Roma dal V secolo a.C. all'anno[END_REF]. Before 1950, the Tiber River was still crossed by a few dams, and the suspended matter represented 7.18 x 10 6 t/yr. Now, the suspended matter is only about 1.41 x 10 6 t/yr [START_REF] Iadanza | Sediment transport time series in the Tiber River[END_REF]. The bedload discharge has been measured only once in 1990, with respect to only one medium flood event deposit [START_REF] Bersani | Il trasporto solido del fiume Tevere[END_REF].

The Tiber Delta formation and evolution has been studied using sedimentary data since the 1980s [START_REF] Bellotti | La piana deltizia del Fiume Tevere: analisi di facies ed ipotesi evolutiva dell'ultimo "low stand" glaciale all'attuale[END_REF][START_REF] Bellotti | Physical stratigraphy and sedimentology of the Late Pleistocene-Holocene Tiber Delta depositional sequence[END_REF][START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14C dating[END_REF][START_REF] Milli | The transition from wave-dominated estuary to wave-dominated delta: The Late Quaternary stratigraphic architecture of Tiber River deltaic succession (Italy)[END_REF]. The delta developed from 6000-4000 cal. yr BP onward [START_REF] Amorosi | Late Quaternary depositional architecture of Po and Tevere river deltas (Italy) and worldwide comparison with coeval deltaic successions[END_REF][START_REF] Giraudi | Evoluzione tardo-olocenica del delta del Tevere[END_REF][START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14C dating[END_REF][START_REF] Milli | The transition from wave-dominated estuary to wave-dominated delta: The Late Quaternary stratigraphic architecture of Tiber River deltaic succession (Italy)[END_REF][START_REF] Salomon | Géoarchéologie du delta du Tibre : Evolution géomorphologique holocène et contraintes hydrosédimentaires dans le système Ostie -Portus[END_REF] and is composed of the upper deltaic plain (paleolagoons area), the lower deltaic plain (beach ridges area), and the subaqueous deltaic plain (lobes and prodelta). Paleoenvironmental data comes mainly from the littoral [START_REF] Bellotti | Physical stratigraphy and sedimentology of the Late Pleistocene-Holocene Tiber Delta depositional sequence[END_REF][START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14C dating[END_REF][START_REF] Giraudi | Evoluzione tardo-olocenica del delta del Tevere[END_REF][START_REF] Milli | The transition from wave-dominated estuary to wave-dominated delta: The Late Quaternary stratigraphic architecture of Tiber River deltaic succession (Italy)[END_REF] or the lagoonal systems [START_REF] Di Rita | Holocene environmental instability in the wetland north of the Tiber delta (Rome, Italy): sea-lake-man interactions[END_REF][START_REF] Bellotti | The Tiber river delta plain (central Italy): Coastal evolution and implications for the ancient Ostia Roman settlement[END_REF]Carlo Giraudi, 2011;[START_REF] Vittori | Palaeoenvironmental evolution of the ancient lagoon of Ostia Antica (Tiber delta, Italy)[END_REF], or else from the harbor basins of Portus and Ostia (C. [START_REF] Giraudi | Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Goiran | Palaeoenvironmental reconstruction of the ancient harbors of Rome: Claudius and Trajan's marine harbors on the Tiber delta[END_REF][START_REF] Goiran | Geoarchaeology confirms location of the ancient harbour basin of Ostia (Italy)[END_REF][START_REF] Hadler | Temple-complex post-dates tsunami deposits found in the ancient harbour basin of Ostia (Rome, Italy)[END_REF], but little data available for the Tiber channel during the Holocene.

Many difficulties exist in the attempt to reconstruct the mobility of the river channels over time, thus many hypotheses of paleochannel dynamics have been suggested for the Tiber delta [START_REF] Segre | Considerazioni sul Tevere e sull'Aniene nel Quaternario[END_REF][START_REF] Giraudi | Late Holocene evolution of Tiber river delta and geoarchaeology of Claudius and Trajan Harbor, Rome[END_REF][START_REF] Bellotti | The Tiber river delta plain (central Italy): Coastal evolution and implications for the ancient Ostia Roman settlement[END_REF]. The bedload of the Tiber River has been scarcely observed for the Late Pleistocene or the deltaic transgressive phase [START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14C dating[END_REF], and not clearly since the formation of the prograding Holocene Tiber Delta, ca. 8000-6000 years ago. The paleomeander of Ostia, cut off in 1557-1562 and thereafter called Fiume Morto, offers a good opportunity to study a late-Holocene channel of the Tiber River. The complete stratigraphic sequence has never been reconstructed, and the modalities of paleomeander evolution were avoided or only hypothesized.

Archaeological data concerning the foundation of Ostia [START_REF] Zevi | Les débuts d'Ostie[END_REF] and the evolution of the urban area have been collected since the 19 th century, especially along the river channel [START_REF] Calza | Scavi di Ostia, Topografia generale[END_REF][START_REF] Pavolini | Ostia[END_REF]. Archaeological work also focused on the Fiume Morto [START_REF] Pellegrino | Ricerche archeologiche nel Trastevere Ostiense[END_REF][START_REF] Shepherd | Il "Rilievo Topofotografico di Osita dal Pallone[END_REF] combined with geoarchaeological hand corings but only down to a depth of 3-4 meters [START_REF] Arnoldus-Huyzendveld | Alcune considerazioni sullo sviluppo storico dell'ansa dell Tevere presso Ostia e sul porto-canale[END_REF]. However, the origins of Ostia and its paleomeander are still unclear. Textual tradition links the origin of Ostia to the reign of the king Ancus Martius at the end of the 7 th century BC In contrast, archaeological data relating to the foundation of the Castrum with the cardo and the decumanus dates the origin of Ostia to the 4 th -3 rd century BC [START_REF] Zevi | Les débuts d'Ostie[END_REF]. During the Medieval period Ostia was abandoned, while a relative rejuvenation of the city occurred with the development of the Borgo di Ostia, located close to the paleomeander apex (Figure 2). Finally, according to the contemporary texts, an impressive summer flood initiated the cut off of the meander in 1557, completely separating it in 1562 [START_REF] Bacci | Del Tevere -Libri Tre[END_REF][START_REF] Pannuzi | Il Castello di Giulio II ad Ostia Antica[END_REF].

METHODS

Three sediment cores have been drilled across the palaeochannel of the Fiume Morto using a mechanical rotary coring device and exported to the laboratory of sedimentology of the University of Lyon 2 in France (Core MO1: 12°17'59.16" E, 41°45'32.43" N, 1.79 m above sea level (a.s.l.); Core MO2: 12°18'0.46" E, 41°45'31.22" N, 1.7 m a.s.l.; and Core MO3: 12°17'55.19" E, 41°45'40.02" N, 2 m a.s.l.). The magnetic susceptibility was measured at every centimeter using a Bartington MS2E1 high-resolution surface sensor and enabled with visual recognition to define sedimentary units [START_REF] Dearing | Environmental magnetic susceptibility[END_REF]. The identification of these units using the magnetic susceptibility has been used and validated for the Tiber delta in [START_REF] Salomon | The Canale di Comunicazione Traverso in Portus: the Roman sea harbour under river influence (Tiber delta, Italy). Géomorphologie : relief, processus[END_REF]. Afterwards, the core sequences were sampled systematically regarding the units and the sub-units defined, in order to ensure that each strata was sampled. We extracted 36 samples for MO1, 33 samples for MO2, and 33 samples from MO3. The organic material was measured by loss-on-ignition on a fraction of 10 g of raw, dry sediment placed at 375°C for 16 hours [START_REF] Ball | Loss-on-ignition as an estimate of organic matter and organic carbon in noncalcareous soils[END_REF], in order to give an estimation of the productivity of the overlying water column and the sedimentation rate in fine deposits [START_REF] Doyle | What does percent organic carbon in sediments measure?[END_REF]. The texture diagram (inorganic, organic and shell fractions) was made based on a fraction of 30 g of dry raw sediments. The coarse fraction (> 2 mm), sands

(2 mm at 63 µm) and silts/clays (< 63 µm) were differentiated by weighing the sieve contents. A selection of the most representative samples of each unit was undertaken for particle-size distribution analysis. Laser granulometry was carried out using a Malvern Mastersizer 2000, on clay, silts and sands after treatments with H2O2 for the destruction of the organic matter and KCl for the elimination of the flocculation ions (Ca 2+ ). It provides information on the distribution of grain sizes and enables evaluation of hydrodynamic intensity [START_REF] Folk | Brazos River bar [Texas]; a study in the significance of grain size parameters[END_REF][START_REF] Cailleux | Initiation à l'étude des sables et des galets[END_REF]. Shells fragments were identified and provide information on the paleoenvironments [START_REF] Perès | Nouveau manuel de bionomie benthique de la Mer Méditerranée[END_REF]. Radiocarbon dates were established using the accelerator mass spectrometry (AMS) method and calibrated as described by Reimer et al. [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF] (Table I). Eleven ceramics were studied by S. Zampini (pers. com.), but most of them were too small and damaged to be identified and dated (Table I).

Concerning the photo interpretation, the background of Figure 2 is created with geographical information system software that combines the topographic map of the Tiber Delta (IGM, 1:25,000, 2005) and an aerial photograph taken by balloon in 1911. Main archaeological findings for our purpose have been added to Figure 2 from different kinds of published data [START_REF] Calza | Scavi di Ostia, Topografia generale[END_REF][START_REF] Arnoldus-Huyzendveld | Alcune considerazioni sullo sviluppo storico dell'ansa dell Tevere presso Ostia e sul porto-canale[END_REF][START_REF] Pellegrino | Ricerche archeologiche nel Trastevere Ostiense[END_REF][START_REF] Heinzelmann | Les nécropoles d'Ostie : topographie, développement, architecture, structure sociale. in Descoeudres, Ostie, port et porte de la Rome antique[END_REF][START_REF] Bukowiecki | Ostie, l'eau dans la ville: châteaux d'eau et réseau d'adduction[END_REF][START_REF] Pannuzi | Il Castello di Giulio II ad Ostia Antica[END_REF][START_REF] Goiran | Geoarchaeology confirms location of the ancient harbour basin of Ostia (Italy)[END_REF][START_REF] Keay | High resolution space and ground-based remote sensing and implications for landscape archaeology: the case from Portus, Italy[END_REF]. The aerial photography shows the clearest marks of the morphology of the channel of the paleomeander of Ostia just before the cutting in 1557-1562. Theoretical lobes have been drawn on the basis of the last paleochannel feature.

RESULTS

Aerial Photography Analysis: A Compound Evolution of the Paleomeander

It is possible to recognize several steps of evolution of the Tiber River channel in Figure 2. The aerial photograph taken in 1911 shows a wide Tiber channel curve that erodes part of the archaeological site of Ostia at that time (north of the Castrum / Capitolium) as well as the clearest delineation of the paleochannel of the meander of Ostia just before the cutting in 1557-1562. The amplitude of this last active meander was 1700 m with a radius of 330 m for the main lobe (L1 in figure 2). The sinuosity index has been measured at 3.63 (SI=L(Lengh of the channel) / λ(wavelength) = 4100 m/1130 m) and allows us to identify the last active meander of Ostia amongst the meanders with high sinuosity [START_REF] Rosgen | A classification of natural rivers[END_REF][START_REF] Van Den Berg | Prediction of alluvial channel pattern of perennial rivers[END_REF].

A detailed observation of this paleomeander reveals morphological specificities, especially a destructive shift of the main lobe to the east. Towards the north and south, secondary lobes have also been observed. The circles in green help to highlight the compound growth of this complex paleomeander. The secondary lobe upstream (L2) has a radius of 210 m and the secondary lobe downstream has a smaller radius of 40 m (L3).

Analysis of Core MO3: Convexity of the Paleomeander of Ostia

Core MO3 was drilled in the convexity of the paleomeander of Ostia. The stratigraphy presents four mains units (Figure 3).

Unit A (13 to 11.67 m below the current Italian mean sea level at Genoa (b.s.l.))

This basal Unit A is composed of yellow laminated silty sand. The coarse fraction is 8% of the total weight of the samples analyzed and contains mostly small psephites and shell fragments. Most of the shells fragments are marine bivalves, but they cannot be identified precisely. However, some Bittium reticulatum living in posidonia or rocky coasts were identified with Lentidium mediterraneum living in sandy to clayey coasts close to a river mouth. The age of this deposit is attributed to the beginning of the Holocene by radiocarbon date performed on a marine shell (10,070 ± 50 14 C yr B.P., 9250-8951 cal. BC, Ly-8799).

Unit B (11.67 to 1.72 m b.s.l.)

A major change in the stratigraphy occurs with Unit B, and especially B1. Coarse sand composes Subunit B1 with pebbles (40 to 70% of the total weight of the samples). Coarsest pebbles have a length of 3.5 to 4 cm (A-axis), a width of 2.5 cm (B-axis), and they are 0.4 cm thick (C-axis). The flatness index is very high, around 7.5 [START_REF] Cailleux | Initiation à l'étude des sables et des galets[END_REF]. Subunits B2 to B6 are characterized by a alternation of very coarse sand (Unit B2), coarse sand (Units B4 and B6) and medium sand deposits (Units B3, B5). These deposits date between the 4 th century BC and the 1 st century BC according to two radiocarbon ages from Units B3 (Bone, 2230 ± 30 14 C yr B.P., 385-204 cal. BC, Ly-8792) and B5 (Charcoal, 2120 ± 30 14 C yr cal. B.P., 344-51 cal. BC, Ly-8793).

Unit C (1.72 to 1.45 m b.s.l.)

Unit C consists of sands with a high content of ceramics and mortar. A piece of mortar is identified in the stratigraphy at -1.72 to -1.70 m. At -1.60 m a Terra sigillata "Africana A" was identified and dated to 90-250 AD (Table I). This sedimentary unit presents bedded gray and yellow silty-clay. Unlike lower units, only 2% of this unit is composed of sand. The coarse fraction disappears and the organic matter content increases up to more than 2% of the total sample analyzed.

Analysis of Core MO1: Last Active Channel of the Ostia Paleomeander

Core MO1 was drilled into the last active channel of the Ostia paleomeander (Figure 4).

l.)

Unit A is a laminated silty-sand deposit similar to Unit A in Core MO3. The samples are composed of 70% sand, 30% silts and clays, and of only 0.5% of coarse fraction. Unidentified shells were found in this unit.

Unit B (14.39 to 10.36 m b.s.l.)

Four subunits compose the sedimentary Unit B. The bottom Subunit B1 and the top layer B4 correspond to high energy deposits of pebbles and coarse sands (1 cm x 1 cm x 0.5 cm for the coarsest pebbles). The corer drilled through two levels of volcanic tuff (-11.36 m to -11.09 m and -10.57 m to -10.37 m) included in Subunit B4. It is difficult to know if the pieces of tuff were broken originally or during the drilling. However, no sedimentary deposits are mixed with these tuffs. These coarse deposits overlap medium sand (Subunit B2 and B3). From the Subunit B4 to E2, the tendency is a decrease of grain-size. Unit D consists of 92% sand at the bottom to 78% at the top. In contrast, the organic matter content grows from 1% to 2%.

Unidentified ceramics and bricks were found in these deposits.

Unit E (4.24 m to 0.82 m b.s.l.)

The decrease of the grain size is confirmed in this unit. Sand fraction is only 48% of Subunit E1 and the silts and clays compose more than the half of the sediments (52%). At the top of Subunit E2, the silts and clays are 99% of the sediment content. The closure of the environment is confirmed by a higher organic matter content (up to 5%). The carbon content of vegetal matter found at -0.76m was too young to be dated (Ly-8040).

Analysis of Core MO2: Concavity of the Ostia Paleomeander

Finally, Core MO2 was drilled within the concavity of the Ostia paleomeander (Figure 5). This unit is composed of an intercalation of coarse sands to pebbles. The coarsest pebbles are 2.5 (A-axis) x 2 (B-axis) x 1 (C-axis) cm, which corresponds to pebbles with a very high flatness index around 4.5 [START_REF] Cailleux | Initiation à l'étude des sables et des galets[END_REF].

Unit C (9.28 m to 6.76 m b.s.l.)

This unit corresponds to an intercalation of fine (96% silts and clays) and sandy (61% sand) deposits. No coarse fraction was observed. Two radiocarbon dates were obtained on a wood fragment (2160 ± 25 14 C yr B.P., Ly 8044; 2035 ± 30 14 C yr B.P., Ly-8780), which date the layer between the 4 th century BC and the 1 st century AD.

Unit D (6.76 m to 0.63 m b.s.l.)

The bottom of Unit D is composed of sand. Above this sandy layer, Unit D incorporates fine particles with 92% silts and clays, 6% sands and 2% coarse fraction. Unlike the lower units, the organic matter increases up to 10%. A piece of wood collected at the top of Unit E was too young to be dated by radiocarbon (Ly-8788).

DISCUSSION

Sedimentological Characteristics and Chronology of the Paleomeander

Photo interpretation draws a relative chronological framework for this study. It is possible to hypothesize a first phase of evolution of the meander toward the east, followed by the growing of two secondary lobes toward the north and toward the south. The last step of evolution of the meander happens with the cut-off of the paleomeander in 1557-1562.

Thus, the core sequences can be better understood in these phases of evolution.

At the base of the three core sequences drilled in the Fiume Morto, the very well sorted yellow laminated silty sand (Units A in Cores MO1, 2 and 3) (Figures 3 to 6) has been interpreted such as coastal sand from the last transgressive period when the coastline was moving up toward the east [START_REF] Bellotti | Late Quaternary landscape evolution of the Tiber River delta plain (Central Italy): new evidence from pollen data, biostratigraphy and 14C dating[END_REF][START_REF] Milli | The transition from wave-dominated estuary to wave-dominated delta: The Late Quaternary stratigraphic architecture of Tiber River deltaic succession (Italy)[END_REF]. Some marine shells have been preserved in this coastal sediment, of which one shell has been dated to 9250-8875 cal.

BC (Ly-8799).

A sharp discontinuity on the top of these sandy sediments is observed. The stratigraphic Unit A is overlapped by pebbles and very coarse deposits from the bedload of the Tiber River (Subunits B1 in Cores MO1, 2 and 3 and also B4 in MO1). For the first time, the bedload of the Tiber river in the delta was recorded and measured, which noted a high energy deltaic river channel. Medians of these deposits fluctuated between 0.58 mm and 1.6 mm with the largest pebbles measuring A-axis=3.5 cm and B-axis=2.5 cm in MO3-Unit B1, 2.5 cm and 2 cm in MO2-Unit B1 and only 1 cm and 1 cm in MO1-Unit B4. Usually, large Mediterranean deltas have sandy river mouths [START_REF] Anwar | Reconstruction of sedimentary environments of Rosetta and Damietta promontories in Egypt, based on textural analysis[END_REF][START_REF] Maillet | Relations sédimentaires récentes et actuelles entre un fleuve et son delta en milieu microtidal: Exemple de l'embouchure du Rhône[END_REF], but it is possible to find pebbles such as those in the mouth of the Rhone Delta [START_REF] Arnaud-Fassetta | Downstream grain-size distribution of surficial bed material and its hydro-geomorphological significance in a large and regulated river: the Rhône River in its delta area (France). Géomorphologie : relief, processus[END_REF]. These pebbles reveal major floods to torrential behaviors of the Tiber River from the late 1 st millennium BC until its cut off [START_REF] Bersani | Tiber River at Rome: summer floods and considerations on the maximum historical discharge[END_REF]. A hypothesis is that these flat pebbles, similar to coastal pebbles, have been reworked and come from an area just upstream of the Tiber delta, where similar material has been observed in the Pleistocene deltaic deposits (Ponte Galeria formation) [START_REF] Milli | Depositional setting and high-frequency sequence stratigraphy of the Middle-Upper Pleistocene to Holocene deposits of the Roman Basin[END_REF].

The maximum depths of these paleochannels recorded by the cores are the following: 11.70 m b.s.l. (Core MO3), 14.40 m b.s.l. (Core MO1) and 12.20 m b.s.l (Core MO2). However, it has to be taken into consideration that the ancient sea level was 80 cm below the current level during the 4 th -5 th century AD at Portus [START_REF] Goiran | Découverte d'un niveau marin biologique sur les quais de Portus: le port antique de Rome[END_REF] and probably even lower in previous centuries.

Core MO3 was drilled within the convexity of the main lobe (Figures 3 &6). It therefore contains sediments related to the migration of the Ostia paleomeander towards the east. Fluvial deposits have been separated into three main units. First, Unit B is composed of six subunits dated to between the 4 th and the 1 st century BC (Ly-8792; Ly-8793). The common facies of the six subunits is the fluvial dark gray coarse sediments. They are interrupted by Subunit B3 with medium sand and a gray silty clay layer. Units B2 to B6 correspond to fluvial point bar deposits, in other words the building of the meander convexity. Unit C corresponds to medium to fine sand deposits. A pottery sherd of terra sigillata "Africana A" was found in this layer and provides a limiting age (terminus post quem) to the deposit. This ceramic dates the layer to around 90-250 AD. This layer reveals the beginning of Roman activity on the river bank soon after the point bar deposits accumulated on the inside of the meander channel. At the top of the core, Unit D is a bedded gray-yellow silty clay layer interpreted as a floodplain deposit. The bottom of this unit (1.35 m b.s.l.) is dated between AD 1212 and 1280 (Ly-8781).

Cores MO1 and MO2 are located within the concavity of the southern secondary lobe of the paleomeander (Figures 4, 5 and6). Core MO1 is in the middle of the last active channel dated to AD 1557, just before the cutting. Unit B contains four subunits composed of medium sand to pebbles. Two layers of volcanic tuff fragments have been found in Subunit B4 at 11.36-11.10 m and 10.57-10.37 m. Fragments of terrestrial shells were collected in this subunit and suggest that this layer with fluvial deposits and tuff was partially formed by the erosion of a close riverbank. Subunits B2 and 3 are interpreted either as fluvial deposits or as old coastal deposits eroded and trapped below the tuff layers and the coarse bedload. From Unit C to Unit E, we can observe a decrease in the grain size. We observe very coarse deposits in Unit C, medium and silty sand in Unit D and clayey silt and silty clay in Unit E. A piece of bone found in Unit C has been dated to AD 1455-1635 (Lyon-8041). We consider Unit C as the last bedload of the Tiber river before the meander cut off, and Units D and E as the subsequent channel infill from AD 1557 onwards. As previously expressed, Unit B in Core MO2 represents bedload. This unit is covered by different types of facies (sandy and silty clay / Unit C) dated by a fragment of bone to 4 th century BC -1 st century AD. Silt and clay might be deposited decantation in a period of low energy (Seasonal? Over several years?). Flocculation could have been active with the intrusion of salt water in the Tiber river channel [START_REF] Capelli | Intrusione salina nel delta del fiume Tevere. Evoluzione del fenomeno nei primi anni del terzo millennio[END_REF]. The sandy layers can be attributed either to fluvial deposits or to erosion of the river banks' coastal sands. Unit D is considered a post-cut off deposit related to the last bedload at the bottom (sandy layer), and to an oxbow lake existing until the end of the 19 th century AD, after which it was finally reclaimed [START_REF] Amenduni | Sulle opere di bonificazione della plaga litoranea dell'Agro Romano che comprende le paludi e gli stagni di Ostia, Porto, Maccarese e delle terre vallive di Stracciacappa, Baccano, Pantano e Lago dei Tartari[END_REF]. A pollen analysis on Core MO-2 supports this interpretation [START_REF] Pepe | Late Holocene pollen record from Fiume Morto (Dead River), a palaeomeander of Tiber River near Ancient Ostia (central Italy)[END_REF].

Sedimentological Data in their Archaeological and Paleoclimatic Contexts

Four main phases of evolution of the active meander of Ostia can be observed (Figure 7). The first phase corresponds to the origin of the paleomeander. The oldest sedimentary data comes from the base of Core MO3 in the convexity of the main lobe of the Ostia paleomeander. Unit B is dated to the 4 th -1 st century BC by radiocarbon dating. It is possible to show that the meander of Ostia was already established when the Castrum was built in the 4 th -3 rd century BC. Combined archaeological and sedimentological data bring to light the same period of time. However, these data do not give evidence for the existence of Ostia during the reign of the king of Rome, Ancus Martius (ca. 642-617 BC). Neither does it give evidence for the location of the Tiber River during that period. The main lobe channel shifts toward the east from at least 385-205 BC until the 1 st century AD.

The second phase corresponds to the formation of the secondary lobes. The southern secondary lobe was already in formation in the 1 st century AD. Core MO2 reveals the sedimentation at the maximum southern extent of the Fiume Morto dated between the mid-4 th century BC and the mid 1 st century AD (terminus post quem). The chronology provided by excavated archaeological remains is comparable concerning the date of the maximum erosion of the southern secondary lobe. The construction of the aqueduct at the end of the 2 nd / beginning of the 3 rd century AD is the terminus ante quem for the paleomeander mobility toward the south, because its outline roughly follows the curve of the last channel of the meander before its cut off in AD 1557-1562.

During the third phase, archaeological evidence and sedimentological data are consistent to support a lateral stability of the southern secondary lobe channel from the 1 st / 3 rd century AD to the cut-off of the meander in AD 1557-1562. The cut-off of the paleomeander was initiated by a summer flash-flood in AD 1557 that also broke the Pons Aemilius (also called Ponte Rotto) in Rome. This exceptional meteorological event is recorded in texts from South France to Sicily [START_REF] Bacci | Del Tevere -Libri Tre[END_REF]. Most of the 16 th century is characterized in the Tiber River by rare hydrological events with sudden major summer floods (AD 1530& AD 1557;[START_REF] Bersani | Tiber River at Rome: summer floods and considerations on the maximum historical discharge[END_REF]. These records tell us about particular conditions occurring within the Tiber watershed during some decades of the Little Ice Age [START_REF] Camuffo | The analysis of two bi-millenary series: Tiber and Po River Floods[END_REF][START_REF] Camuffo | The Little Ice Age in Italy from documentary proxies and early instrumental records[END_REF]. Afterwards, the paleomeander was slowly filled up by sediments and finally reclaimed at the end of the 19 th century / beginning of the 20 th century (fourth phase).

Erosion of the Via Ostiensis?

When the Castrum of Ostia was established in the 4 th -3 rd century BC, two main streets of the urban area of Ostia were delineated: the cardo (north-south), and the decumanus (west-east). The cardo at the north of the Capitolium of Ostia was eroded after the cut-off of the meander in AD 1557-1562, but at this point the city of Ostia had already been abandoned for about a millennium. However, the Via Ostiensis, continuing off the decumanus, could have been eroded during antiquity, when the economic activity of Ostia was flourishing. A notable problem here is the possibility that the Via Ostiensis already avoided the secondary lobes of the meander during its construction. The chronology of the deposits based on radiocarbon dates is too extended in time to solve this problem. The shifting of the Tiber to the east and the formation of the secondary lobes could have happened in the 4 th century -3 rd century BC and then been followed by the construction of the road. However, the discovery of tuff in the bedload of the paleo-Tiber channel just below the theoretical position of the Via Ostiensis (Core MO1) could support the hypothesis of an effective erosion of the road by the Tiber River (Figures 2 &6). The excavations undertaken across the Via Ostiensis near Ostia recorded a wide distribution of pieces of tuff [START_REF] Calza | Scavi di Ostia, Topografia generale[END_REF]. More investigation needs to be done to definitively solve this question. Similarly, the question arises concerning an aqueduct built in the 1 st century AD, which was connected to a reservoir included within the city wall of Ostia. Was the first aqueduct following the hypothetical Via Ostiensis eroded by the paleomeander? Unfortunately, no outline of this early aqueduct has been found near Ostia. Instead there are the remains of a "second" aqueduct dated to the 2 nd -3 rd century AD with a route following the curve of the southern secondary lobe [START_REF] Bukowiecki | Ostie, l'eau dans la ville: châteaux d'eau et réseau d'adduction[END_REF].

Hypothesis Regarding the Compound Evolution of the Ostia Paleomeander

Two questions are therefore pending: (1) Why does the meander's growth stop in its migration to the east? and (2) why did the southern secondary lobe show lateral stability between antiquity and AD 1557? The stability of such a lobe is surprising over almost one

millennium. An answer can be found in the theoretical knowledge concerning meander dynamics [START_REF] Jin | A new technique for modelling river morphology[END_REF]. The meander could have been blocked by cohesive siltyclayey deposits remaining from the lagoon of Ostia (limit delineated on Figures 1 &2) and by the presence of a huge structure composed of volcanic tuff, a possible harbor installation, dating back to the early Republican period [START_REF] Pavolini | Ostia[END_REF]. This structure still remained in the channel in AD 1557 despite the high energy of the Tiber River. These anthropic and natural obstacles could have led to the formation of secondary lobes. Later on, the effect of these obstacles could have had consequences on the resonance of the energy upstream, accelerating the migration of the upstream channel of the meander of Ostia and stabilizing the channel downstream of the obstacles. In support of this hypothesis, the outlines of paleochannels on the two parts of the neck are not symmetrical (Figure 2). The upstream curve of the paleomeander is broader than the downstream curve in the city of Ostia and the northern secondary lobe has a larger radius. Similar mechanics are observed in flume studies [START_REF] Jin | A new technique for modelling river morphology[END_REF].

Additionally, a possible reduction of the water energy for medium floods is conceivable since the construction of Portus in AD 42-45. Today, one-fifth of the Tiber River discharge is flowing through Fiumicino. Other canals were excavated and in use after the construction of Portus [START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF][START_REF] Keay | Portus : An Archaeological Survey Of The Portus Of Imperial Rome[END_REF][START_REF] Salomon | A harbourcanal at Portus: a geoarchaeological approach to the Canale Romano: Tiber delta, Italy[END_REF] and could have led to the decrease of the energy of the Tiber river downstream in Ostia (Figure 1).

The City of Ostia Facing River Mobility Risks during the Roman Period

From 414 BC onward, floods of the Tiber River are recorded in ancient texts for the lower Tiber valley (Le [START_REF] Gall | Le Tibre, fleuve de Rome dans l'antiquité[END_REF][START_REF] Aldrete | Floods of the Tiber in ancient Rome[END_REF]. These records are not systematic and depend on the observations of the ancient authors as well as the preservation of the texts through time. However, floods became a major problem in Rome at the beginning of the 1 st century AD, forcing the Senate to consider solutions [START_REF] Miller | Tacitus: Annals Book I[END_REF][START_REF] Leveau | Les inondations du Tibre à Rome: politiques publiques et variations climatiques à l'époque romaine[END_REF]. Flood management through flood-relief canals is also underlined in some inscriptions from the middle of the 1 st century AD and early 2 nd century AD found at Portus [START_REF] Keay | Portus : An Archaeological Survey Of The Portus Of Imperial Rome[END_REF][START_REF] Salomon | Géoarchéologie du delta du Tibre : Evolution géomorphologique holocène et contraintes hydrosédimentaires dans le système Ostie -Portus[END_REF]. Overbank flooding is the main inconvenience reported during this period in Rome, rather than river mobility disruption. However, a shift in the course of a river was observed at least elsewhere during the Roman period. Ancient jurists took into account these fluvial disruptions through their consequences on land use and possession [START_REF] Campbell | Rivers and the Power of Ancient Rome[END_REF]. The present study exposes the fact that the city of Ostia faced the problem of erosion by relocating infrastructure: locally, by moving roads and the aqueduct to the south, or regionally, by the foundation of Portus, a huge harbor system indirectly linked to the fluvial system by canals flowing to the north and to the south of the harbor basins [START_REF] Salomon | The Canale di Comunicazione Traverso in Portus: the Roman sea harbour under river influence (Tiber delta, Italy). Géomorphologie : relief, processus[END_REF][START_REF] Salomon | A harbourcanal at Portus: a geoarchaeological approach to the Canale Romano: Tiber delta, Italy[END_REF]. Some of these canals prevent lateral mobility by the construction of built riverbanks such as the Canale Romano, the Canale Traverso and the Fossa Traiana, now called Fiumicino [START_REF] Testaguzza | Portus: illustrazione dei Porti di Claudio e Traiano[END_REF][START_REF] Keay | Portus : An Archaeological Survey Of The Portus Of Imperial Rome[END_REF]. They may have been conceived and used for ship unloading, but these lateral structures did in fact also prevent lateral mobility.

By comparison, it is clear that the Republican structure in the north of the paleomeander played its part on the evolution of morphology of the paleomeander together with the cohesive deposits of the palaeolagoon of Ostia, and participated in crushing the apex of the meander. However, at the moment, we cannot assume that this structure was built on purpose to prevent river bank erosion.

CONCLUSION

The study of the Tiber River paleomeander in relation to the city of Ostia provides new data to investigate long term interaction between human settlements and fluvial dynamics and brings new evidence for the understanding of fluvial risk management across time. This transdisciplinary approach clarifies the multiscale parameters involved in the evolution of the Ostia paleomeander and identifies the successive redistribution of fluvial erosion along the riverbanks. During the Roman period, the combination of natural geomorphological processes and human pressures led to the formation of a compound meander near Ostia. This new river course constrained the development of Ostia to the east.

As such, the Romans responded to this by displacing their infrastructure both locally (aqueduct and Via Ostiensis) and regionally (construction of Portus and excavation of the canals).

According to our hypothesis, clayey deposits in the riverbank and/or the massive Republican structure at the apex of the meander caused the formation of secondary lobes that subsequently redistributed the fluvial erosion upstream. From late Antiquity to the cutoff of the meander, the southern part of the meander of Ostia was stable and the urban area of the Borgo and the Castle of Giulio 2 remained safe even though they were located on the concave riverbank. A large flood occurring in AD 1557 initiated the cut-off of the meander and stopped definitively the lateral mobility of the paleomeander. Since then, the new river channel considerably eroded the archaeological site of Ostia at the north of the Capitolium -Castrum.

This active fluvial erosion in northern Ostia began during the Rinascimento, with the revival of Roman antiquity in Italy. However, the current acceptation of the concepts of cultural heritage and fluvial risk did not apply at that time. The two concepts originated in the 19 th century [START_REF] Segarra Lagunes | Il Tevere e Roma: Storia di una simbiosi[END_REF]Combe, 2007). Fluvial risk management started in Rome when it became the capital of unified Italy (1871) and after several major floods of the Tiber occurring in the middle of the 19 th century. In this context, embankments were built in Rome along the Tiber River (muraglioni) and, five decades later, embankments were built in the Tiber Delta [START_REF] Segarra Lagunes | Il Tevere e Roma: Storia di una simbiosi[END_REF]. Since the beginning of the 19 th century, the archaeological site of Ostia has been studied by archaeologists [START_REF] Fea | Relazione: di un viaggio ad ostia e alla villa di Plinio detta Laurentino[END_REF][START_REF] Fea | Il tevere navigabile oggidi come ne suoi piu antichi secoli e la citta d'Ostia ivi edificata dal re Anco Marcio emporio di Roma da risorgere a nuova vita. Rome: Nella Stamperia della Reverneda Camera Apostolica[END_REF][START_REF] Canina | Sulla stazione delle navi di Ostia: sul porto di Claudio con le fosse indicate nella iscrizione scoperta l'anno 1836 e sul porto interno di Traiano e la fossa distinta col nome di questo imperatore[END_REF][START_REF] Vaglieri | Notizie di Scavi[END_REF], and Carlo Fea, Director General of Antiquities, helped to control the random excavations and the trade of antiquities in Rome and Ostia [START_REF] Ridley | The pope's archaeologist: the life and times of Carlo Fea[END_REF]. Today, for Western societies, the concept of risk management extends not only to population and economically and socially viable infrastructures, but also toward cultural heritage preservation. Coastal erosion and its effects on archaeological sites have been recently taken into account in Italy [START_REF] Lollino | The interplay of erosion, instability processes and cultural heritage at San Nicola Island (Tremiti Archipelago, Southern Italy)[END_REF][START_REF] Gerolamo | Erosione costiera in siti di interesse archeologico[END_REF]. This work contributes to the consideration of fluvial damage to archaeological contexts.

More generally, the geoarchaeolocial approach can complete the picture of the risks prevailing for a river system and it can also identify the risks occurring in extreme conditions experienced at some point in the past [START_REF] Thorndycraft | Palaeofloods, historical data & climatic variability: Applications in flood risk assessment[END_REF][START_REF] Bravard | Le risque d'inondation dans le bassin du Haut Rhône : quelques concepts revisités dans une perspective géohistorique[END_REF][START_REF] Kidder | The Alluvial Geoarchaeology of the Sanyangzhuang Site on the Yellow River Floodplain, Henan Province, China[END_REF]. The big pebbles measured in the bedload of the paleomeander of Ostia and the destructive summer flash-flood of AD 1557 are evidence of exceptional fluvial events of the Tiber River. This question is particularly relevant to the beginning of the 21 st century in which we are experiencing a climatic change and an increase of natural hazards. morphology and the locations of boreholes MO1, 2 and 3. The limit of the lagoonal deposits near the paleomeander is drawn using the paleolagoon shape from Amenduni [START_REF] Amenduni | Sulle opere di bonificazione della plaga litoranea dell'Agro Romano che comprende le paludi e gli stagni di Ostia, Porto, Maccarese e delle terre vallive di Stracciacappa, Baccano, Pantano e Lago dei Tartari[END_REF] and sedimentary data (De [START_REF] Angelis | Geo-pedogenesi delle terre sul delta del Tevere[END_REF].

  Unit D (1.45 m b.s.l. to 1.80 m a.s.l.)

  Unit A (14.69 to 14.39 m b.s.

  Unit C (10.36 to 8.62 m b.s.l.) Unit C incorporates 95% sands (mainly coarse to very coarse sand), 2% silt and clay and 3% coarse fraction. A bone fragment has been dated to AD 1455-1635 (355 ± 25 14 C yr B.P. / Ly-8041). Unit D (8.62 m to 4.24 m b.s.l.)

  Unit A (13.55 m to 12.19 m b.s.l.) This yellow laminated silty sand unit is similar to Units A in Cores MO1 and MO3. Most of the shells are unidentified, but shells living in sandy coast (Cerastoderma edule, Mactra sp., Neverita Josephina) and near a river mouth (Zonites nitidus) were observed. Unit B (12.19 m to 9.28 m b.s.l.)

FIGURE CAPTIONS Figure 1 .

 CAPTIONS1 FIGURE CAPTIONS

Figure 2 .

 2 Figure 2. Morphometric characteristics of the paleomeander, archaeological data, and core

Figure 3 .

 3 Figure 3. Stratigraphic descriptions of Core MO3 (convexity of the Ostia paleomeander).

Figure 4 .

 4 Figure 4. Stratigraphic descriptions of Core MO1 (channel of the Ostia paleomeander).

Figure 5 .

 5 Figure 5. Stratigraphic descriptions of of Core MO2 (concavity of the Ostia paleomeander).

Figure 6 .

 6 Figure 6. Interpretative cross-section of the Ostia meander paleochannel.

Figure 7 .I

 7 Figure 7. Four maps showing the evolution of the paleomeander in relation to the city of

o

  Figure 1

ACKNOWLEDGMENTS

We would like to express our gratitude to the Soprintendenza Speciale per i Beni Archeologici di Roma, Donna Livia Aldobrandini, for access to the lands for corings. We would also like to thank Sabrina Zampini for the analysis of the pot shards found in the core

Core M01 -Analysis

/. /a 0 a • Studied cores location


Leon Constantin Fopa
email: fopal@imag.fr

Fabrice Jouanot
email: fabrice.jouanot@imag.fr

Alexandre Termier
email: alexandre.termier@imag.fr

Maurice Tchuente
email: maurice.tchuente@lirima.org

Oleg Iegorov
email: oleg.iegorov@st.com

Benchmarking of triple stores scalability for MPSoC trace analysis

A Multi Processor System-on-Chip (MPSoC) is a complex embedded system used in consumer electronic devices, such as smartphones, tablets and set-top boxes. In order to cope with the complexity of MPSoC architectures, software developers rely on post-mortem trace analysis for application debugging or optimization. The traces are explored to localize expected and unexpected programs behaviors. However, the low semantic value of low-level trace events make the trace exploration difficult. We propose to perform trace exploration through an ontology which adds semantics to events and provides a declarative language for querying data. Because traces can be huge, such an ontology contains a large number of instances stored as RDF triples. Because analysts need fast results on classical computer, an efficient system for query answering is preferred. Therefore, saturating, loading and querying those triples pose a scalability challenge to state-of-the-art knowledge base repositories (KBR). In this paper, we have conducted a benchmark of 7 KBRs: Jena, Sesame-native, Sesame-memory, tdb, sdb, rdf-3x and vertical-mdb, to test their scalability in a nondistributed environment close to analyst environment. We used these KBRs to analyze real traces through VIDECOM, an ontology we designed for trace analysis of applications on MPSoC. Results show that vertical-mdb has a loading rate 3 times faster than the others. It is the only KBR able to saturate the biggest trace of our dataset without exceeding system memory and to run complex queries on it in an acceptable time. Other approaches failed, due to memory limitation or inefficient join implementation.

INTRODUCTION

Multi Processors Systems-on-Chip (MPSoC) are small chips containing multiple components like processors, memory units, buses, Graphical Processor Unit (GPU), input/output ports. They are widely used in our everyday life through mobile phones, washing machines, automotive control, flight control and set-top boxes. Developing embedded software on MPSoC is difficult because of the inherent parallelism of these chips. Indeed, industrial studies on quality control of embedded softwares indicate high defect densities of 13 major bugs per 1000 lines of code [START_REF] Roychoudhury | Embedded Systems And Software Validation[END_REF]. In multimedia applications, such inefficient code can cause, for example, frozen images or desynchronized images and sound.

The main task in embedded software debugging or optimization is to track bugs or inefficient code manifestations in order to correct them. Inefficient codes and bugs related to parallelism manifest themselves mostly at runtime. Developers, therefore, rely on post-mortem trace analysis methods to debug embedded software [START_REF] Ball | The Concept Of Dynamic Analysis[END_REF]. The basic idea is to run the program against specific tests and to explore its execution trace, in order to compare the observed program behavior with expected behavior. The semantics of the trace events, such as relations and constraints between them, are known by the developers, but are not explicit in the trace. Therefore, characterizing program behaviors in a trace is a challenge.

Interpreting events as program behavior is quite similar to data interpretation in the semantic web. The key idea of the semantic web is to propose logical assertions that relate a resource to some concepts in predefined ontologies [START_REF] Abiteboul | Web Data Management[END_REF]. Thus, by using a domain ontology for trace analysis, trace exploration can be done through declarative queries whose results will be closer to developer expectations. Because a trace can consist of several million of events for only few minutes of execution, such an ontology will contain a large number of instances stored as RDF triples, which will definitely pose scalability challenges to knowledge base repositories (KBR).

In this paper, we present VIDECOM, an ontology that we have designed for trace analysis of applications on MPSoC. We present a benchmark of 7 KBRs to test their scalability when they are used to saturate, load and query RDF triples obtained from real trace events mapped to classes and properties of VIDECOM. Because the analyst environment is mainly non-distributed, and because we focus on query answering efficiency on a saturated knowledge base, such KBRs have been chosen mainly for their support of query engine and storing system on a non-distributed environment, but not for their inference capabilities.

The rest of the paper is organized as follows. The VIDE-COM ontology is presented in Section 2. In Section 3 we present data storage mechanisms in KBR and the performance criteria for our comparative study. In Section 4 we present results of our experiments. We present some related work in Section 5. In Section 6 we conclude the paper and propose some future work.

THE VIDECOM ONTOLOGY

One important contribution of this paper is the VIDE-COM ontology. VIDECOM is based on a deductive triple store composed of two parts. The first part is a domain ontology built on RDFS triple patterns extended with rules expressing domain knowledge. The second part is a populated ontology consisting of triples coming from trace events and the saturation mechanism.

Domain ontology. In this section, we briefly present some classes and properties of VIDECOM. We also present how developers can enrich VIDECOM using their knowledge about expected and unexpected behaviors.

Trace captures events that occurred during execution, such as interrupts, task running and context switches. Each event carries basic information like the start time, the duration, the task or the interrupt executed, the processor, the function called and arguments used. Table 1 shows an example of 8 trace events. The first event starts at timestamp 3771 and ends at timestamp 3781. It corresponds to a sys read operation executed by the task ts record on cpu 0 with argument 0x46d. VIDECOM is based on a lightweight ontology of 608 classes and 238 properties. Figure 1 shows some of these classes and properties. The main class Event represents different types of events, such as TASK RUNNING and CON-TEXT SWITCH. Properties eventStartAt and eventEndAt identify the start and the end timestamps of the event. The property isExecutedOn indicates the processor on which the event occurred, and the property runningTask indicates the task executed. The property requestComponent represents the software component (interrupt, system call or function call) requested by the event. The order between events is provided by eventPrecedeInTrace and eventPrecedeInCPU that indicate the order in trace and on each CPU. Moreover, eventPrecedeOccurrence indicates the order between Some classes represent program behaviors concepts. The class FUNCTIONALITY represents expected behaviors and ANOMALY represents unexpected ones. To locate those behaviors, VIDECOM uses the class SLICE that represents a portion of a trace. A SLICE is bounded by two events using sliceHasStartEvent and sliceHasEndEvent properties. A SLICE can be related to a FUNCTIONALITY (respectively an ANOMALY ) with the property sliceIsRelatedTo-Functionality (respectively sliceIsRelatedToAnomaly).

In VIDECOM, RDF triple patterns can be enriched with Datalog rules of the form φ ⇒ ψ, where φ = φ1 ∧ ... ∧ φn is a conjunction of atoms (triple patterns) called premise, and ψ = {ψ1, ..., ψm} is a conjunction of atoms called conclusion. An atom is a triple with variables at subject, property or object positions. The variable identifiers start with ?. An atom holds if there are triples that matched the triple form of the atom. Therefore, the inference rule works as follows.

If the conjunction of atoms of the premise holds, then the set of triples from the conclusion is added to the triple store. For example, let's consider the following inference rule ?e,runningTask,?t ⇒ ?e,rdf:type,TASK RUNNING , if there is a triple in the triple store with runningTask in the property position and whatever in the subject (?e) and the object (?t) positions, then a triple is built and added to the triple store based on the atom of the conclusion with the corresponding value of variable ?e.

We consider safe rules, that means that all variables in the conclusion are also in the premise, even the so-called blank node that is interpreted here as a constant for the safety property. RDFS semantics are captured by inference rules listed in the W3C recommendation1 . Rules expressing domain knowledge corresponding to program behaviors are also captured using the same type of rules.

Populated ontology. Instances of VIDECOM classes and properties are built from events and stored as RDF triples subject, property, object in a triple store. Table 2 shows 10 RDF triples that represent the basic information contained in event1 from table 1. id subject, property, object 1 event1, eventStartAt, "3792" 2 event1, eventEndAt, "3873" 3 event1, isExecutedOn, cpu0 4 event1, runningTask, ts record 5 event1, requestComponent, sys write 6 event1, eventPrecedeInTrace, event2 7 event1, eventPrecedeInCPU, event4 8 event1, eventPrecedeOccurrence, event7 9 event1, hasDuration, "81" 10 event1, hasDurationToNextOccurrence, "1000"

Table 2: Set of RDF triples representing the basic information contained in event1 from table 1.

The saturation ensures the completeness as well as the soundness of the query answering. It is done through an inference engine called reasoner. The reasoner implements a forward-chaining algorithm that applies all users and rdfs inference rules to populate the triple store with new facts. The saturated triple store is loaded in a KBR for querying purpose. In the next section we will briefly describe stateof-the-art KBRs.

KBR DESCRIPTION

In this section, we will briefly present several state-ofthe-art KBRs. We classify the KBRs by the data storage mechanism they use to store the RDF triples.

Data storage mechanism

Various data storage layouts are presented in [START_REF] David | A Survey Of RDF Storage Approaches[END_REF]. They distinguished native and non-native storage.

Native storage

This solution provides a way to store RDF triples in a model similar to the graph model. These solutions can be classified as persistent disk-based and main memory-based.

The persistent disk-based storage of RDF triples uses proprietary file format in many cases. Among the existing solutions we can mention tdb, Sesame-native and rdf-3x. tdb2 uses a file system and stores triples in B+ Tree data structures. Sesame-native uses dedicated on-disk data structures for storage [START_REF] Sesame | [END_REF]. rdf-3x stores all the triples in a compressed clustered B+ Tree and uses an exhaustive index for all permutations of subject-property-object triples [START_REF] Neumann | The RDF-3X Engine For Scalable Management of RDF Data[END_REF].

The main memory-based storage of RDF triples allocates a certain amount of the available main memory to store the whole RDF graph structure. Jena [START_REF] Mcbride | Jena: Implementing The RDF Model and Syntax Specification[END_REF] and Sesame-memory [START_REF] Broekstra | Sesame: An architecture for storing and querying rdf data and schema information. Spinning the Semantic Web: Bringing the World Wide Web to Its Full Potential[END_REF] fall into this category.

Non native storage

The non-native storage solution makes use of Relational Database Management Systems (RDBMS) to store RDF triples. Storage of RDF triples in RDBMS exists in three models: triple table, property table and vertical partitioning.

In the triple table approach RDF triples are stored under the form subject, property, object in one large table with a three-columns schema corresponding to subject, property and object. Usual RDBMS indexes are built on each column to optimize access. sdb3 is an example of this solution.

The property table technique improves triples organization by allowing multiple triple patterns referencing the same subject to be retrieved with less join. In this model, triples are physically stored in a representation close to traditional relational schema in order to speed up the queries over the triple stores. In this approach each named table includes a subject and several fixed properties. The main idea is to discover clusters of subjects that appear frequently with the same set of properties. 4store uses this approach [START_REF] Harris | 4store: The Design And Implementation Of A Clustered RDF Store[END_REF].

Abadi et al suggested the vertical partitioning as an alternative to the property table. They illustrated the approach in swStore [START_REF] Abadi | SW-Store: A Vertically Partitioned DBMS For Semantic Web Data Management[END_REF]. In this approach the triple table is divided into n two-columns tables, one table for each property in the data. In each of these tables, the first column contains the subject and the second column contains the object value related to this subject. Tables are stored by using a columnoriented RDBMS as a collection of columns rather than a collection of rows.

Inference support

Not all KBRs provide an RDFS reasoner. In those that we cited above only Jena and Sesame provide reasoners. The Jena reasoner implements a configurable subset of RDFS inference rules using the RETE algorithm [START_REF] Forgy | Rete: A Fast Algorithm For The Many Pattern/many Object Pattern Match Problem[END_REF] for forward chaining. We retained Jena reasoner because it is the only one that allows the implementation of user defined inference rules.

Performance criteria

We consider the following performances criteria to test scalability of KBR: the saturation time which is the time spent to saturate the triple store, the loading time which is the time spent to load the saturated triple store into the repository, and the query response time which is the time spent to answer a query.

We consider various characteristics of queries. We first consider the selectivity, because a high selective query must efficiently return a small portion of the entire triple store as answer. Next we consider the k-complexity defined as the number of atoms with k variables in the query. A 1complexity atom has one variable and a 2-complexity atom has variables in two positions. We do not consider the case where a variable appears at the property position because it concerns very infrequent category of queries. Moreover a conjunction between two atoms with at least a variable in common in different places will indicate a join. We next consider whether or not the query uses sorting operators on the result. Indeed, due to the temporal nature of events, results may need to be sorted to facilitate their exploitation.

RESULTS AND DISCUSSIONS

In this section, we present a use case on STMicroelectronics MPSoC. This use case corresponds to an analysis of a real video recorder program called ts record. We present experimental settings and results of our comparative study.

Experimental settings

We performed our experiments on 6 traces corresponding to different execution times of ts record. Table 3 presents details on traces, such as, the execution duration, the number of runtime events recorded and the disk size of the trace. Section A of the Appendix provides more details on the ts record use case, and details on two users inference rules related to the ts record use case that we used to enrich VIDECOM.

During the saturation, we ignored rdfs rules rdfs1, rdfs4a, rdfs4b, rdfs12 and rdfs13, because they produced useless triples for trace analysis, such as rdfs:Resource and rdfs:Literal.

We consider 8 test queries from real analyst needs related to ts record use case. Table 4 shows their characteristics. The query Q0 is a conjunction of two atoms of 1-complexity and 2-complexity. More details on those queries are provided in the Appendix (Table 7).

Q0 Q1 Q2 Q3 Q4 Q5 Q6 Q7 Selectivity H L L H H H H L 1-complexity 1 0 0 2 1 0 1 1 2-complexity 1 1 1 1 2 3 7 7 join 0 0 0 0 1 0 2 2 Filter 0 0 0 1 0 1 1 1

Order by x

Table 4: List of characteristics of our 8 test queries.

We performed our experiments on a machine with a 2.27 GHz Intel Xeon CPU and 64 GB of RAM. We chose the following non-commercial KBR for our comparative study: jena and tdb, sesame-memory and sesame-native sdb, and vertical-mdb. More details on their configuration are provided in the Appendix (Section B).

The reader can find the VIDECOM ontology, non-saturated and saturated public datasets 4 , user inference rules, 8 tests queries and experimental results, on the website hosted at http://videcom.imag.fr.

Experimental results

The saturation time

Table 5 shows the saturation time for each trace, the number of triples after saturation and the disk size of the ontology stored in a N-Triple5 format. The main memory was insufficient for Jena reasoner to saturate T5. We used a naive SQL-based implementation of forward-chaining algorithm in vertical-mdb. We saturated T5 in 2 days 11 h 35 m 8 sec, but many optimizations are possible in our implementation to have better performances.

We observed that, one trace event produces 25 RDF triples, and that saturated triple store disk size is 68 times larger than corresponding trace size. This result illustrates the difference of magnitude between the number of trace events and RDF triples, and the limitation of resource for saturation at this scale.

The loading time

Figure 2(a) shows the loading time for each repository and saturated triple store size. Indexes construction is included in the loading time. Because vertical-mdb uses batch import operations provided by MonetDB to copy data from file to tables and constructs index after data are loaded, it is the most efficient comparing to others condifered KBRs. It is 2 orders of magnitude faster than sdb, which builds indexes before loading data and, thus, frequently updates its indexes. Figure 2(a) also shows that sesame-memory and sesame-native cannot load 95 million triples. As it took sdb more than 2 hours to load 51 million triples (see Figure 2(a)), We were not able to load 95 million triples with sdb. In conclusion, results show that vertical-mdb can load 95 million triples in 2 minutes. We also observed that loading all the 95 million triples of T5 with Jena filled all the main memory and an additional 6 GB space from the swap. Another fact is that being non-persistent, main-memory based repositories load data for each working session.

Query response time

We executed our queries on each KBR and for each trace from T0 to T5. We ran each query 10 times and we collected the mean time as query response time.

Selectivity: Figure 2(b) shows response time for Q0. All KBR answered within 10 seconds. Thanks to their B+Tree based indexes rdf-3x and tdb are faster than the others. Jena scales poorly on 95 million triples because of swap-in and swap-out needed to get free space in main memory. In the case of Q1 depicted in Figure 2(c), all KBR performance dropped by one order of magnitude but rdf-3x dropped by 2 orders of magnitude. However, tdb remains the fastest, which indicates that its indexes are well adapted for both high and low selectivity queries.

Sorting: Figure 2(d) shows the performance of Q2. We observed that performance of vertical-mdb did not change, unlike the others which lost at least one order of magnitude in response time. The reason can be the efficient implementation of sorting in MonetDB. We observed that the performance of rdf-3x dropped by 2 orders of magnitude, which indicates that the implementation of its sorting operators is not efficient. Figures 2(e) and 2(f) show the performance of Q3 and Q4. Using inferred classes, Q4 returns the same result as Q3. We observed that using inferred classes leads to fast query response time. The reason can be that inferred classes have higher selectivity. Interval of trace: Figure 2(g) shows the result when querying the same interval of time in all traces. We observed that vertical-mdb has better performance. The reason is that vertical-mdb identifies numerical object values; therefore, indexes built on columns containing numbers are more efficient than indexes built on strings like others do.

k-complexity: Figure 2(h) shows results for Q5. rdf-3x did not provide results due to an internal error in the query parser. Conjunctions are implemented in RDBMS as joins. In the case of sdb it consists of self-joins on the unique table Triple Table, and in the case of vertical-mdb it consists of joins between multiple tables. sdb failed to produce results; we suppose the reason being the inefficiency of self-join on large triple tables. vertical-mdb has acceptable response time (2 minutes for 95 million triples) unlike tdb which needed 5 minutes for 38 million triples. Figure 2(i) shows the performance of Q7. vertical-mdb has better response time (2 minutes for 46 million triples). Other KBRs performed poorly over 13 million triples. Jena took 8 minutes, sesame-memory took 48 minutes, tdb took 2 hours and sesame-native took 3 hours.

Discussion: Results show that the saturation with Jena is efficient but depends on the available memory. We also found that tdb indexes are efficient at large scale, and that vertical-mdb join implementation is efficient. Due to its fast loading speed vertical-mdb loads 95 million triples in 2 minutes and supports RDFS rule reasoning without memory limitation. The inefficiency in the saturation mechanism for vertical-mdb is mainly due to unoptimized code and far better performance should be expected. vertical-mdb is the only one that exhibited low running times accross all queries. It is also the only system that could handle Q7, a complex but realistic query. Considering the type of queries the analysts are interested in, the constraints of their practice, a vertical database system is the best solution in this context. Because an efficient inference engine is not required in the solution we chose based on a saturated triple dataset, some KBRs have been discarded. Nonetheless, KBRs such as, OWLIM and Virtuoso should be considered for future works considering distribution of the dataset and parallel processing.

RELATED WORK

Several RDF benchmarks were previously developed. We can cite, the Lehigh University Benchmark (LUBM) [START_REF] Guo | LUBM: A Benchmark For OWL Knowledge Base Systems[END_REF], the Berlin SPARQL Benchmark (BSBM) [START_REF] Bizer | Benchmarking the performance of storage systems that expose sparql endpoints[END_REF], and DBpedia [START_REF] Morsey | Dbpedia sparql benchmark-performance assessment with real queries on real data[END_REF]. Those benchmark handle large synthetic or real datasets (300 million of triples for DBpedia). They are mainly focused on the loading time and the query response time on various KBRs, such as, Jena, TDB, SDB, Sesame, virtuoso and OWLIM. Unlike those benchmarks, our benchmark is also focused on the saturation time of triples, because our datasets are deductive triple stores and need inference rule to be provide sound and complete query answers.

CONCLUSION AND FUTURE WORK

In this paper we presented a benchmark of triple stores scalability for MPSoC trace analysis. We presented VIDE-COM, an ontology for trace analysis of application on MP-SoC and we made a comparative study to test the scalability of 7 state-of-the-art KBRs. These experiments have shown that even with relatively simple traces and a large server, existing KBR have difficulties to scale up. Among the tested KBR, vertical-mdb is the only one that exhibited low running times across all queries. Given that execution traces are likely to grow much larger than the traces of these experiments, we can conclude that solutions based on a vertical approach will be required to handle them, and will have to be improved.

For our future work we are interested in giving answers to query over large traces in a fixed time. We plan to develop efficient approaches to speedup saturation, and we are interested in different strategies for query parallelization. Following this track we plan to consider OWLIM and Virtuoso KBRs for comparison with the vertical database approach.

APPENDIX A. THE TS RECORD USE CASE

The program ts record contains three tasks which can be scheduled on different CPUs of the MPSoC. The first task t1 collects streaming data and stores them in small IP buffers. Every 100 milliseconds, the second task t2 copies data from IP buffers to main memory. Finally, every 5 seconds, the last task t3 copies them to a USB disk. The period between each task is important to avoid errors which can cause data loss. Based on this domain description, Table 6 presents 2 user inference rules that correspond to the behavior of task t2. For simplicity we present only two rules, but more rules can be added to VIDECOM. The number of user inference rules influence the saturation time. The rule R1 instantiates the FUNCTIONALITY subclass sysWriteNormal when two occurrences of events corresponding to t2 are separated by a period equal to 100 milliseconds. The second rule R2 instantiates the ANOMALY subclass sysWriteBlocked if the period is greater than 100 milliseconds.

B. KNOWLEDGE BASE SYSTEMS FOR EX-PERIMENTS

We chose the following non-commercial KBRs for our comparative study. jena and tdb (version 2.11.2) with their default configuration. We set the java heap size to 60 GB. We also chose sesame-memory and sesame-native (version 2.7.7), we configured sesame-native to support all the combination of subject-property-object indexes as spoc, posc, and opsc. We set the java heap size to 60 GB. We chose sdb (version 1.3.4) backed on postgresql (version 9.3), and we configured sdb with the default "layout2" indexing storage. We set the java heap size to 60 GB.

Unfortunately 4Store has not been maintained for 5 years and we were not able to install it in our setup configuration.

The column-store-based approach swStore implementation was not available. We implemented the approach as described in [START_REF] Abadi | SW-Store: A Vertically Partitioned DBMS For Semantic Web Data Management[END_REF], but used MonetDB6 (version 11.17.9) as a backend instead of C-Store because C-Store is no longer maintained. We called our implementation vertical-mdb.

Figure 1 :

 1 Figure 1: Some classes and properties of VIDECOM

Figure 2 :

 2 Figure 2: Comparison of saturated triple store loading time and query response time for each KBR.

Table 1 :

 1 Illustration of 8 events from a real trace.

	id Start End Operation Task	CPU Arg
	0	3771 3781 sys read	ts record	0	0x46d
	1	3792 3873 sys write	ts record	0	0x11b
	2	3879	switch to	sshd	1
	3	3884 3885 Interrupt	mdtp 1	1
	4	4260	switch to	kworker	1	0
	5	4502 4602 sys poll	ts record	0	0xc1c
	6	4605 4647 sys read	ts record	0	0x11d
	7	4792 4873 sys write	ts record	0	0xe1e

Table 6

 6

	of the Appendix provides

Table 3 :

 3 List of traces and their corresponding execution time, number of runtime events and number of triples before saturation.

Table 5 :

 5 Saturation time, number of triples and disk size of each trace after saturation.

	Traces Saturation time # of triples Size (MB)
	T0	08 m 01 sec	13 051 370	2 185
	T1	18 m 28 sec	25 981 602	4 347
	T2	22 m 43 sec	38 790 013	6 508
	T3	1 h 12 m 18 sec	46 494 600	7 818
	T4	1 h 41 m 01 sec	51 607 411	8 670
	T5	x	95 309 610	16 404

http://www.w3.org/TR/2014/REC-rdf11-mt-20140225/#rdfs-entailment

http://jena.apache.org/documentation/tdb/architecture.html

http://jena.apache.org/documentation/sdb/

Because of the privacy of STMicrolectronics data, we provided datasets obtained from execution traces of video decoding on GStreamer (http://gstreamer.freedesktop.org/) an open source framework for multimedia

applications 5 http://www.w3.org/2001/sw/RDFCore/ntriples/

https://www.monetdb.org/

ACKNOWLEDGEMENT

This work was supported by French FUI project SoC-Trace.

R1

?e1, runningTask, ts record ∧ ?e1, requestComponent, sys write ∧ ?e1, eventPrecedeOccurrence, ?e2 ∧ ?e1, hasDurationToNextOccurrence, ?period ∧ (?period = 100) ⇒ :s, sliceHasStartEvent ?e1 , :s, sliceHasEndEvent, ?e2 , s:, sliceIsRelatedToFunctionality, sysWriteNormal R2 ?e1, runningTask, ts record ∧ ?e1, requestComponent, sys write ∧ ?e1, eventPrecedeOccurrence, ?e2 ∧ ?e1, hasDurationToNextOccurrence, ?period ∧ (?period > 100) ⇒ :s, sliceHasStartEvent ?e1 , :s, sliceHasEndEvent, ?e2 , s:, sliceIsRelatedToAnomaly, sysWriteBlocked