Alfredo Mayoral

Jean-Luc Peiry

Jean-François Berger

Paul Ledger

Bruno Depreux

François-Xavier Simon

Pierre-Yves Milcent

Matthieu Poux

Franck Vautier

Yannick Miras

Simon

John Wiley

Sons Geoarchaeology

J-L Peiry

Geoarchaeology and chronostratigraphy of the Lac du Puy intraurban protohistoric wetland, Corent, France

Keywords: Proto-urbanisation, Storage pits, intra-urban wetland, chrono-stratigraphy, Iron Age

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The prehistoric impact of humans on the environment, especially vegetation and soils, has been widely documented throughout western and central Europe since the Neolithic [START_REF] Dotterweich | The history of human-induced soil erosion: Geomorphic legacies, early descriptions and research, and the development of soil conservation-A global synopsis[END_REF][START_REF] Ellis | Used planet: A global history[END_REF].

Late Holocene climatic oscillations in Western and Mediterranean Europe such as the 4.2 event, late Iron Age-Roman Period climatic optimum, or the Little Ice Age are now well known [START_REF] Magny | Holocene climate variability as reflected by mid-European lake-level fluctuations and its probable impact on prehistoric human settlements[END_REF][START_REF] Martin-Puertas | Arid and humid phases in southern Spain during the last 4000 years: The Zoñar Lake record, Córdoba[END_REF][START_REF] Wanner | Mid-to Late Holocene climate change: an overview[END_REF]. However these events are often considered as minor influences on geomorphological processes when compared to increasing human impacts (e.g. [START_REF] Lavrieux | 6700 yr sedimentary record of climatic and anthropogenic signals in Lake Aydat (French Massif Central)[END_REF]. Rather, the progressive anthropogenic forcing of natural systems is highly correlated with the development of agriculture and urbanization processes [START_REF] Lang | Land degradation in Bronze Age Germany: Archaeological, pedological, and chronometrical evidence from a hilltop settlement on the Frauenberg, Niederbayern[END_REF][START_REF] Notebaert | Characterization and quantification of Holocene colluvial and alluvial sediments in the Valdaine Region (southern France)[END_REF]. The latter has often been considered to be restricted to Mediterranean Europe, but is now known to have begun in Central and Western Europe during the late Bronze Age (LBA) and the Iron Age [START_REF] Milcent | Bourges-Avaricum, un centre proto-urbain celtique du Ve siècle av. J.-C. Les fouilles du quartier Saint-Martin-des-Champs et les découvertes des Etablissements militaires[END_REF][START_REF] Milcent | Résidences aristocratiques et expérience urbaine hallstattiennes en France (VIe-Ve siècle av.J.-C.)[END_REF][START_REF] Fernández-Götz | Paths to complexity. Centralisation and Urbanisation in Iron Age Europe[END_REF]. The environmental impacts of these proto-urbanization processes are still poorly understood at a local scale, as few geoarchaeological and palaeoenvironmental studies have specifically focused on these questions. This is mainly a result of palaeoenvironmental studies typically being pursued in areas suitable for sedimentary record preservation like lakes, floodplains or peatlands. While such studies provide remarkable results at landscape-scale they frequently lack sensitivity to local signals [START_REF] Ledger | Taphonomy or signal sensitivity in palaeoecological investigations of Norse landnam in Vatnahverfi, southern Greenland?[END_REF]. Geoarchaeology has typically been concerned with intrasite studies focusing on specific archaeological challenges, for example, stratigraphic records and postdepositional processes in human structures, analysis of lithic materials or specific deposits like dark earths, and the study of specialist structures such as ramparts or agricultural terraces (Butzer, 2008). However, in suitable contexts, such as intra-urban, or urban-connected wetlands, integrated palaeoenvironmental and geoarchaeological approaches can provide valuable information on human-environment interaction and characterize the environmental impact of the proto-urbanization processes in key periods such as the Iron Age (e.g. Ledger et al., 2015). These human influenced environments, frequently studied in rural areas (e.g. Bernigaud et al., 2014), are a rare and valuable place to develop integrated palaeoenvironmental and geoarchaeological approaches, but are rarely studied in connection with proto-urbanization processes in Western Europe, with some remarkable exceptions [START_REF] Mele | The Terramare and the surrounding hydraulic structures: A geophysical survey of the Santa Rosa site at Poviglio (Bronze Age, northern Italy)[END_REF]. This paper presents the first geoarchaeological results, including context of site formation and post-depositional processes, of a wider palaeoenvironmental study begun in 2014. A highly local approach, using an intra-urban wetland, has been developed to investigate the long-term palaeoenvironmental impacts of proto-urban human settlement episodes of the first millennium B.C.E.

STUDY AREA AND OBJECTIVES

Located in the French Massif Central, 20 km south of Clermont-Ferrand, the Puy de Corent is a volcanic plateau located in a key position controlling the north-south axis of the valley of the River Allier (Fig. 1A andB). The summit (621 m.a.s.l.), located in the southwest of the plateau, is a Pliocene (circa 3 Ma B.P.) monogenetic scoria cone [START_REF] Greffier | Aspects géomorphologiques et stabilité des versants au sud de Clermont-Ferrand[END_REF][START_REF] Nehlig | Les volcans du Massif central[END_REF]. Differential erosion of marls and basalts since the late Pliocene caused the gradual raising of the plateau above the marl lowlands by relief inversion. The central and northwestern sectors are characterized by gentle topography and are situated on a basaltic lava flow deposited over 200 m thick Oligocene sedimentary rocks (limestone and marl with occasional gypsum). The Corent plateau is a major regional archaeological site and several human occupations, from the Middle Neolithic to the Roman period, have been documented. These include two major settlement phases with proto-urban features in the LBA 3 (950-800 B.C.E.) and the late Hallstatt D1 (600-550 B.C.E.) [START_REF] Milcent | Une agglomération de hauteur autour de 600 a.C. en Gaule centrale : Corent (Auvergne)[END_REF], and a vast oppidum (La Tène D1-2, 125-25 B.C.E.) with monumental and planned urban characteristics, which is considered as the possible capital of the Arverni [START_REF] Poux | Corent, Voyage au coeur d'une ville gauloise[END_REF]. Corent is therefore an excellent site to study long-term human-environment interactions, and more specifically the highly-localized palaeoenvironmental impacts of proto-urbanization and urbanization processes during the 1 st millennium B.C.E.

Unfortunately, the plateau is afflicted by severe soil erosion and truncation of deposits, a process which probably began in the Mid-Neolithic, when the first palynological evidence of forest clearance and agriculture appears (Ledger et al., 2015). In some instances this has resulted in a poor preservation of archaeological sediments, especially those preceding the La Tène D period [START_REF] Poux | Corent, Rapport de fouille programmée[END_REF]. The Lac-du-Puy, an ancient pond located in the lowest part of the plateau within the extension zone of the LBA 3 and La Tène settlements, offers a well-preserved intra-urban sedimentary record (Fig. 1C). It appears as a small sub- circular natural depression (of approximately 2 Ha) in the surface of the basaltic rock, similar to others known on neighboring volcanic plateaus also situated above a sedimentary basement. These small basins are quite common in the region (e.g. Lacs de la Pénide, Lac de Pardines) and are usually interpreted as pseudosinkholes (Bureau de Recherches Géologiques et Minières, 2015). The modern pool is small (surface area of c. 300 m 2 ) and mainly fed by occasional runoff and subsurface water flow from the summit of the plateau.

Hydraulic traces and historical maps indicate evidence of recent management, probably in the 19 th century.

However, the palaeobasin represents a much larger area (0.5 Ha) as evidenced by aerial imagery and maps from 1820 (Fig. 1D andF). Small-scale archaeological survey explored this depression for the first time in the early 1990s, confirming the sedimentary nature of accumulation in the center of the depression (approximately 2 m deep) and therefore its palaeoenvironmental interest [START_REF] Guichard | Puy de Corent (Puy-de-Dôme) rapport de prospection et sondages[END_REF]. The Lac-du-Puy was then neglected until 2012 when, under the AYPONA project framework (Paysages et visages d'une agglomération Arverne: approche intégrée et diachronique de l'occupation de l'oppidum de Corent, dir. Y.

Miras & F. Vautier), a sediment coring allowed a palynological analysis. The result of this study (Ledger et al., 2015) confirmed that the Lac-du-Puy offers an exceptional sedimentary record within a protohistoric intraurban context. This excellent palaeoenvironmental potential encouraged a more extensive geoarchaeological survey in the summer of 2015. The objective of this paper is to present the initial results of this survey and to use them to build a robust chronostratigraphic framework as a basis for the forthcoming multi-proxy palaeoenvironmental research of this complex pedosedimentary site. The detailed archaeological analysis of the structures excavated during the geoarchaeological survey of the basin is beyond the scope of this work and will be undertaken in further studies.

MATERIALS AND METHODS

Ten 2 m wide trenches were excavated to survey sedimentary deposits and their stratigraphy across the Lacdu-Puy (Fig 1D). Trenches 1, 2 and 4 served to define the geomorphological context, while trenches 3 and 5-10 delineated the lateral extent of archaeological structures within the depression. The archaeological work involved manual excavation, description and photography of structures. The geomorphological element comprised pedosedimentary description of stratigraphic logs (Table 1) and photography, supported by topographic survey at a centimetric precision (Leica DGPS system 500). Sediment samples were taken for multi-proxy analysis (grain size analysis, geochemistry and micromorphology), which will be exploited in further studies. AMS 14 C dating was undertaken on macrocharcoal, microcharcoal and bulk sediment samples from stratigraphic logs and archaeological structures (see Table 1). For charcoal samples, the sediment was deflocculated in a solution of sodium hexametaphosphate and sieved at 500µm and 100µm. Macro-or microcharcoal were manually concentrated using a binocular microscope. Twelve samples were submitted to Beta Analytic, Florida, with a thirteenth being sent to the Poznan Radiocarbon Laboratory. 14 C dates were calibrated using CALIB V7.04 and IntCal13 calibration curve [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF]. The results of six previous radiocarbon dates, from the 2012 core and used to establish a first chronology (Ledger et al., 2015), were used to assess the potential reservoir effect associated to the bulk sediment and micro-charcoal. Since the locations of most trenches were widely linked to archaeological objectives and their number conditioned by the resources devoted to excavation, information on sedimentary structures and geometry of the sedimentary infilling was incomplete. To overcome this limitation we obtained additional information by using non-destructive techniques such as geophysics. Several methods were implemented within the Lac-du-Puy depression, including electromagnetic induction (EMI) using various devices with different investigation depth (EMP400, EM31, DualEM21S), and also electrical resistivity using Abem Terrameter. Electromagnetic measurements delivered valuable results and are therefore presented, including a map of apparent electrical conductivity undertaken using the EMP400 in Horizontal Coplanar (HCP) geometry (Fig. 1E). An EMI tomography resulting from the development of an experimental protocol [START_REF] Guillemoteau | 1D sequential inversion of portable multi-configuration electromagnetic induction data[END_REF] from DualEM21S provided a magnetic permeability (or magnetic susceptibility) pseudo-section (Fig. 1G).

RESULTS

Archaeological Finds

Older structures are mainly scattered remains of an archaeological soil without pottery fragments and are stratigraphically correlated to Early/Middle Neolithic levels of the 2012 core (Ledger et al., 2015). The , with the exception of 3 pits (within the 37 fully excavated) where a partial overlapping of two structures was observed. Attending to these characteristics the structures can be interpreted as storage pits [START_REF] Sigaut | Les réserves de grains à long terme[END_REF][START_REF] Reynolds | Pit technology in the Iron Age[END_REF][START_REF] Miret I Mestre | Systèmes traditionnels de conservation des aliments dans fosses et silos. Une vision multidisciplinaire[END_REF][START_REF] Miret I Mestre | Trous, Silos et autres[END_REF]. and their systematic proximity to a Stratigraphic Unit (SU) considered as backfill (SU 2, see Table 2), suggests a post-depositional origin related to this SU.

No sherds were found in the bottom of the structures. Moreover the absence of organic macro-remains or charred material in pits, likely a result of taphonomic processes due to frequent water level fluctuations in the basin (Ledger et al., 2015), necessitated the predominant use of bulk samples for dating, which are susceptible to vertisolization processes and possible reservoir effect and complicate interpretation of the results.

Radiocarbon Dates

The AMS radiocarbon database of the Lac-du-Puy is summarized in Table 1: the six first (nº1 to 6) dates come from a core drilled in a natural sequence (Log 8, Fig 2g) and were previously published (Ledger et al., 2015).

Five additional radiocarbon dates were performed in Log 4 (nº7 to 11, see Fig 2f ) to complete the natural chronological framework. Three of these dates (nº9 to 11) are coherent with previous dates and archaeological evidence. Dates nº9 and 10 confirm previous observations (Ledger et al., 2015) of a small reservoir effect on micro-charcoal compared to bulk sediment. Therefore dates from bulk sediment were preferred for chronological interpretation of the natural stratigraphy. The last ones, taken close to the top of the sequence in the SU 2 (nº7 and 8), were rejected because their ages are not compatible with the wellestablished archaeological dating of this unit, probably because of its anthropogenic origin and consequent mixing effects (see [START_REF] Cunliffe | Danebury an Iron Age Hillfort in Hampshire Vol1 the excavations 1969-1978: the site[END_REF][START_REF] Cunliffe | Danebury an Iron Age Hillfort in Hampshire Vol 4 The excavations 1979-1988: the finds[END_REF][START_REF] Pons | Mas Castellar de Pontós (alt Empordá) Un complex arqueològic d'época ibèrica[END_REF]. Therefore, we sought to further investigate the age of these archaeological structures through an in-depth analysis of the stratigraphy and its links with the archaeological data.

Geomorphology And Stratigraphy

The

Palaeo-vertisol type

Very dark grey to black (10 YR 2,5/1) clay, including basaltic particles from fine sand to granules, rarely gravels, and prismatic structure with slickensides-like features. No archaeological material. Very irregular distribution and thickness (0-20 cm) within the depression, lacking lateral continuity. The unit has probably its upper part truncated before SU2 deposition.

Late Bronze Age 3 archaeological structures (950-800 BC)

SU4a, 4a1, 4b 70-115 approx.

Palaeo-gleysol 4a/4a1-center of the depression 4b-peripheral areas

Olive grey (5 Y 4/2) massive heavy clay, few small orange oxydation mottles at the top. Rarely quartz gravels. Homogeneous distribution within the lowest part of the depression, in lateral transition with 4b.

Bioturbation (roots) and irregular palaeotopography with small mounds (4a1) Dark grey to brown (2,5 Y 3/2 to 3/1) clay loam, with few basaltic granules to gravels, scattered pottery fragments (Late Bronze Age) and few small orange oxydation mottles. Replaces laterally 4a in peripheral parts of the depression. Massive to blocky structure. Irregular palaeotopography.

Early/Middle Neolithic scattered remains SU5a, 5b, 5c

115-130 approx.

Palaeo-regosol 5a-center of the depression 5b-peripheral areas 5c-external areas Greyish to greenish brown (10 YR 3/1 to 2,5 Y 3,5/2) sandy clay, with abundant weathered subrounded basaltic granules (white mottles or orange oxydation mottles), few gravels and small blocks, sometimes quartz gravels. Bioturbation (rootlets).

Grayish brown (10 YR 3/1,5) sandy loam, with many subrounded basaltic granules and some angular basaltic gravels. Few rounded quartz gravel.

Very abundant orange oxydation mottles. Replaces laterally 5a.

Dark brown (10 YR 3/1 to 3/2) sandy loam, with many basaltic sub-rounded granules and sparse angular gravels. Some orange oxydation mottles. Replaces laterally 5b.

SU6a

130-145 approx.

Grey clay with scoria

Dark grey (10 YR 4,5/1) clayey silt with many sub-rounded inclusions of scoria (sand to gravels, 10 R 3/3) more or less disaggregated. Appears in discontinuous areas in central and peripheral parts of the depression.

6b 145-?

Scoria breccia with interstitial clay

Dark grey (10 YR 4,5/1) clayey silt, with abundant sub-rounded and heterometric inclusions of scoria (gravels to cobbles, 10 YR 3/3) more or less disaggregated and increasing to the bottom, until forming a scoriaceous breccia with hardened clay matrix. Appears in discontinuous areas, in the lower parts of the bedrock in central parts of the depression. The fill is generally clayey, and ranges in thickness from 30-40 cm at the periphery to 2 m in the deeper sectors of the central part of the basin; the lateral variability is pronounced and sharp due to the undulating bedrock bottom. In the NW sector the natural stratigraphy is well preserved, whereas in the SE sector several successive backfill layers separated by La Tène archaeological levels appear directly in contact with the bedrock. The sedimentary sequence systematically thins from the inner to external parts of the depression (Table 2,Fig 3) and is accompanied by a very gradual lateral change of facies (SU 4 and 5), likely related to the frequency of palaeoflooding and the resulting hydromorphic gradient.

Weathered

Focusing on stratigraphic particularities, results mainly concern the relationship between SU4, SU3 and pits.

First, elevations of the top of SU 4a and 4b are higher around each pit or group of pits (forming small mounds surrounding pits, SU 4a1) than in inter-pits zones, which can only be the result of an irregularity in the palaeosurface that is probably anthropogenic in nature (Fig 2d and2e). Second, there is a complete lack of stratigraphic contact between SU3 and pits filling: SU3 never covers pits and is never cross-cut by pits (Table 2, 2). Finally, SU3, SU4 and the pits are systematically truncated suggesting that a phase of erosion or levelling works occurred before SU2 was deposited (Fig 2c,2d,2e).

DISCUSSION

The archaeological, radiocarbon and stratigraphic data allowed the construction of a robust chronostratigraphic framework for the natural stratigraphy. SU 6b and 6a are not directly dated, but on the basis of their stratigraphic position and their pollen assemblages (Ledger et al., 2015) 1).

With regards to archaeological structures, surveyed trenches and geophysical maps show that a quasi perfect correlation exists between the pits implantation and the spatial extent of the clay basin (Fig 1E). Archeological and ethnographic studies suggest that storage pits are often placed in function of the presence of loose and thick sediment, with a preference for impermeable clays which offer important storage advantages, in spite of the associated humid conditions [START_REF] Sigaut | Les réserves de grains à long terme[END_REF][START_REF] Hill | Storage of barley grain in Iron Age type underground pits[END_REF][START_REF] Reynolds | Pit technology in the Iron Age[END_REF][START_REF] Miret I Mestre | Systèmes traditionnels de conservation des aliments dans fosses et silos. Une vision multidisciplinaire[END_REF]. On the basis of the spatial pattern of the pit distributions, which was visible in surveyed trenches after topsoil stripping (see Fig 2a), we estimate there are likely to be several hundreds, maybe a thousand, across the Lac-du-Puy basin. The narrow arrangement of the pits with rare overlap between them suggests a high degree of synchronicity and their probable excavation in a relatively short length of time, perhaps decades.

The five dates from the dark lining at the base of the pits provided Early/Middle Neolithic ages, and are clearly too ancient to be consistent with stratigraphic evidence and age of the fill (see Fig 2b,2c,2d,2e and Table 1 date nº15). This is probably because the bases of pits were systematically excavated through SU 5 before reaching bedrock or SU 6a/b, and therefore the bulk or micro-charcoal samples from the dark lining at the base of pits date the age of SU 5. 14 C assays on macro-charcoals from the small excavated structure opened in SU 5 stratigraphically below a pit ( These results make it clear that the pits, in absence of pottery or charred material at the base, cannot be directly dated by radiocarbon on bulk samples. This prevents their dating more precisely than the TPQ and TAQ provided by archaeostratigraphic evidence: pits are excavated through a well-dated LBA 3 archaeological Given the site size and the archaeological importance of this group of pit structures, we attempted to further refine the relative chronology of the pits on the basis of stratigraphic evidence in order to associate them with one of the archaeological occupations of the site. The discontinuous nature of the log-based stratigraphic data and the lack of a unique exposure containing all the archaeological and stratigraphic information required us to construct a stratigraphic synthesis (Fig 2h). This synthesis led us to the chrono-stratigraphic interpretation presented below as the most likely. The well-dated SUs (4a and 3) were used to achieve this, despite the lack of direct stratigraphic relationships between SU 3 and pits, which is interpreted as a consequence of subsequent soil truncation. The very uneven palaeotopography of the top of SU 4a, with small mounds (SU 4a1) around each pit and depressions between them, suggests that these mounds are minor earthworks constructed immediately after the excavation of pits in the clayey SU4 layer (Fig 2c,2d and 2e). This microrelief (SU 4a1) could be interpreted as the truncated remains of a rim or a "neck" around the upper part of the pits (Fig 2c ,2d, 2e and 2h), probably built using the clay extracted during the excavation of the pits (SU 4a). This morphology was observed in approximately 10% of the pits -despite their truncation and postdepositional erosion -and has been documented in storage pits in anthropological studies [START_REF] Sigaut | Les réserves de grains à long terme[END_REF][START_REF] Miret I Mestre | Systèmes traditionnels de conservation des aliments dans fosses et silos. Une vision multidisciplinaire[END_REF] and on archaeological sites such as Danebury Ring (U.K.), which were also systematically truncated [START_REF] Cunliffe | Danebury an Iron Age Hillfort in Hampshire Vol1 the excavations 1969-1978: the site[END_REF][START_REF] Cunliffe | Iron Age Grain Storage Pits -Danebury Ring Hillfort[END_REF][START_REF] Cunliffe | Danebury an Iron Age Hillfort in Hampshire Vol 4 The excavations 1979-1988: the finds[END_REF]. As a consequence the excavation of the pits can, with some certainty, be assumed to have been contemporary with the surface of SU 4a or slightly posterior, but in any case prior to SU 3 which fossilized this palaeotopography after the structures were abandoned (Fig 2h).

SU3 deposition occurred across the basin, but preferentially in the lower areas of SU 4 palaeotopography between pits. Finally, after a general truncation of the top of SU3 and the majority of the pits necks, SU2 deposition occurred across the entire basin (Fig 2h). Therefore, the pits were likely excavated sometime ). On the basis of the existing archaeological chronology for Corent the only two periods of major settlement within this timeframe are a LBA settlement and a Hallstatt settlement [START_REF] Milcent | Les occupations de l'âge du Bronze du plateau de Corent (Auvergne, Puy-de-Dôme) : résultats des campagnes de fouille 2010-2013[END_REF][START_REF] Milcent | Une agglomération de hauteur autour de 600 a.C. en Gaule centrale : Corent (Auvergne)[END_REF].

However the hypothesis of a LBA 3 age is unlikely because of the stratigraphic evidence of pits excavated through the LBA levels and the complete lack of ceramic remains in the base of the pits, while they are frequent in neighbouring Late Bronze Age archaeological levels. For this reason the excavation of the pits is probably related to the Hallstatt D1-2 agglomerated settlement attested on the volcanic plateau at ~600-550 B.C.E. [START_REF] Milcent | Une agglomération de hauteur autour de 600 a.C. en Gaule centrale : Corent (Auvergne)[END_REF], albeit an association with a little occupation of the Hallstatt D3-La Tène A1

(510-425 B.C.E.) is also possible. The absence of ceramic material in the base of the pits is also more coherent with a medium-size and distant occupation (Hallstatt D settlements) than with bigger and material-rich occupations surrounding the basin, such as the LBA 3 and La Tène D2 occupations (Fig 1C). This Hallstatt chronological hypothesis is not surprising considering that this kind of storage techniques was commonplace in this period [START_REF] Gransar | Le stockage alimentaire sur les établissements ruraux de l'âge du Fer en France septentrionale : complémentarité des structures et tendances évolutives. In Les installations agricoles de l'âge du Fer en France septentrionale[END_REF] as attested by examples in Western Europe, especially in or near hillforts (e.g. [START_REF] Pons | Mas Castellar de Pontós (alt Empordá) Un complex arqueològic d'época ibèrica[END_REF][START_REF] Cunliffe | Iron Age Grain Storage Pits -Danebury Ring Hillfort[END_REF]. This unusually large grouping of storage pits in an open area, close to a relatively important Hallstatt proto-urban settlement [START_REF] Milcent | Une agglomération de hauteur autour de 600 a.C. en Gaule centrale : Corent (Auvergne)[END_REF] implies an increased storage capacity suggesting agricultural surplus. This pattern has sometimes been interpreted as a consequence of a short climatic optimum during the First Iron Age, but is more likely linked to socio-economic changes. Increasing centralization, long distance commerce and proto-urbanization is also observed in other sites in central

France [START_REF] Gransar | Le stockage alimentaire sur les établissements ruraux de l'âge du Fer en France septentrionale : complémentarité des structures et tendances évolutives. In Les installations agricoles de l'âge du Fer en France septentrionale[END_REF][START_REF] Milcent | Résidences aristocratiques et expérience urbaine hallstattiennes en France (VIe-Ve siècle av.J.-C.)[END_REF], alongside evidence of contact with Mediterranean world [START_REF] Milcent | Présence de Gaulois du Midi en Auvergne vers 600 avant J.-C. : les fibules méditerranéennes de Corent[END_REF] where similar patterns are noted for the period (Buxô & Pons, 1999;[START_REF] Pons | Mas Castellar de Pontós (alt Empordá) Un complex arqueològic d'época ibèrica[END_REF]. The abundant storage pits at Corent can therefore be interpreted as a marker of the emergence of proto-urban activity outside of the Mediterranean world, aborted towards the end of the first Iron Age.

On the basis of chronostratigraphic evidence, we propose the following reconstruction of the main evolution phases of the basin from the Early Neolithic to Late Antiquity and modern period (Figure 3), that is coherent with the palynological data (Ledger et al., 2015) : (1) In the oldest sedimentary phase, probably during the Early Neolithic, the basin was irregular with some sparse pools where sedimentary inputs concentrated in more or less intense runoff episodes, carrying scoria and clays from the volcanic cone. (2) In the transition and c. This increased erosion could be connected with the Neolithic forest clearing on the plateau suggested in palynological data (Ledger et al., 2015). Scattered archaeological remains at the top of this level are indicative of human activity and soils stability at the end of this phase, probably related to the Middle Neolithic occupation of Corent (fortified settlement).

(3) The period between the middle Neolithic and the Early Bronze Age is difficult to interpret due to the low chronological resolution of this part of the sedimentary sequence. This may be related to a declining sedimentation rate or even a sedimentary hiatus. Palynological data clearly illustrate continuity in the vegetation dynamics for this period, suggesting a slow sedimentation rate rather than a long hiatus (Ledger et al., 2015) This sedimentary hiatus could be related to a contemporary stability and soil genesis phase in the plateau well documented in annual excavations [START_REF] Milcent | Une agglomération de hauteur autour de 600 a.C. en Gaule centrale : Corent (Auvergne)[END_REF]. ( 4 This reconstruction provides an insight to the evolution of socio-environmental interactions from the Early Neolithic to the Late Antiquity in an unusual intra-urban context. These interactions are characterized by different land uses, resources exploitation and impacts in the palustrine system, and appear to be the main driver of changing conditions in the basin. Neolithic sedimentary impacts, linked to a typical pattern of forest clearance and cultivation, are followed by a long period of low-energy hydrosedimentary dynamics. This phase can be interpreted as a system recovery, suggesting a non-linear but nevertheless progressive anthropogenic forcing. Major impacts in the systems evolution occur from as early as the first Iron Age in association with proto-urbanization, represented by the excavation of around 1,000 storage pits in the basin that resulted in major changes on its hydrosedimentary dynamics. These changes caused the shift of the Lac du Puy to an anthroposystem well before the impacts of the Late Iron Age Gallic oppidum.

CONCLUSION

This work presented the initial results of a geoarchaeological survey of the Lac-du-Puy basin, an unusual intraurban wetland with a valuable sedimentary record covering 5000 years of human activity including settlements and proto-urban episodes of the first millennium B.C.E. Despite a complex, discontinuous and sometimes highly disturbed or even absent stratigraphy, the sedimentary archives allowed us to build a robust chronostratigraphic framework and to reconstruct the main evolution phases of this small depression from the Early Neolithic to the Gallo-Roman period. The unexpected find of a large number of Iron Age pits probably used for storage was a major discovery but complicated interpretation, owing to difficulties associated with direct dating and wide time-range provided by the stratigraphy. Nevertheless, the chronostratigraphical approach developed in this work allowed the identification and dating of the effects of post-depositional processes on these Iron Age pit structures and associated sediments. This detailed analysis led us to confidently propose, on the basis of the available data, that these structures are associated with the Early Iron Age Hallstatt D settlements of Corent (600-425 B.C.E.). Moreover, these abundant storage pits are also a local evidence of a short and early phase of socio-economic changes and emerging proto-urbanization processes outside the Mediterranean world, centuries before the well known oppida period at the end of the This geoarchaeological study was also the first attempt to document the human-environment interactions which were likely the main driver of changing pedo-sedimentary and hydrological conditions during the last 5 millennia in the Lac-du-Puy basin. Especially in the first millennium B.C.E., the geoarchaeological data suggest abruptly changing conditions in this wetland in response to changes of use and growing anthropogenic impact linked to storage pits excavation, and more generally proto-urbanization, culminating with its total backfill in the Gallo-roman period.

These preliminary interpretations will be followed by additional investigations and analysis such as grain-size analysis, micromorphology or geochemistry, to truly understand the complex trajectory of the humanenvironment interaction in this intra-urban wetland, a privileged mirror of the proto-urbanization processes and the associated anthropogenic forcing on soils during the first millennium B.C.E.

ACKNOWLEDGEMENTS

Figure 1 .

 1 Figure 1. A, B and C: Study area location and approximate extent of the main phases of settlement (C). D: Map of the geoarchaeological survey trenches, with location of the studied stratigraphic logs and the archaeological finds. E: electrical conductivity map and pit distributions (red: low values, blue: high values). F: Early 19 th century map of the Lac-du-Puy (Napoleonic land register). G: EMI tomography (magnetic permeability section) along the A-B transect illustrated in D. Blue: low values, red: high values (adapted from Guillemoteau et al., 2016).

  LBA 3 is represented by an archaeological soil level with abundant basalt pebbles and pottery, located in the central part of the basin (see Fig 1D and Fig 2b). La Tène D1-2 is the best represented period, with abundant structures (mainly pavements) especially in the peripheral south and west areas of the depression (Fig 1D),characterized by the presence of frequent fragments of amphorae type Dressel 1[START_REF] Guichard | Les amphores. Lavendhomme, Guichard. Rodumna, le village gaulois[END_REF]. Gallo-Roman structures (pavements and small buildings foundations) are present in the south of trenches 1 and 6.Modern drainage works are also present in all surveyed trenches. As a major archaeological discovery, 114 protohistoric pits were found covering all the central part of the depression (Fig 1Dand 2a). Excavated in the clayey sediment, these pits have diameters between 70 and 120 cm, a maximal depth around 130 cm, variated shapes (pear-shaped, bottle-shaped, bell-shaped etc.) with a more or less truncated narrower top, and a dark probably organic thin lining in their walls and bottom (Fig 2aand 2c). The sedimentary fill of the pits is a massive and homogeneous dark grey clay presumably the result of deliberate infilling when they were decommissioned. All of the structures are carefully and closely distributed (sometimes only separated by 10 any coalescence(Fig 2a)

Figure 2 .

 2 Figure 2. a) Closely distributed pits in trench 2. b) Pits cross-cutting a LBA archaeological soil in trench 1. c) Pit 25604 and underlying Neolithic structure. d) Log 2.2 including a pit and SU 4a1. e) Stratigraphic survey of the south of trench 1. f) Log 4. g) Log 8. h) Synthetic interpretation of chrono-stratigraphic relationships between pits, natural stratigraphy and well-dated archaeological remains. See Table2for SU codes.

  electrical conductivity map provided information regarding the extent and thickness of clayey sediment in the basin, revealing that it is limited to a sub-circular irregular 100 m radius depression (blue and yellow areas in Fig 1E), and that some scattered spots in the central area have thicker clay accumulation (dark blue areas). The magnetic pseudo-section provides a good indication of the shape of the sedimentary infill and the underlying basaltic bedrock, showing an irregular and dissymmetric bottom of the basin, with a maximum depth reaching 2 m (Fig 1G). The 29 stratigraphic logs distributed in the surveyed trenches (Fig 1D) allowed a detailed characterization of the sedimentary infilling of the depression, which was divided into 12 Stratigraphic Units (Table 2). The more complete sequences were Log 8 (equivalent to the published core extracted in 2012) and Log 4 (Fig 2f and 2g) and hence these were the subject of the highest resolution dating. An additional 15 logs from trench 1 were used to document a 195 m long stratigraphic section which crosses the depression center, including only key archaeological structures for reasons of stratigraphic clarity (Fig 3).

  Fig 2c, 2d, 2e). Furthermore, SU3 is predominately found in low-areas of the very irregular top SU4 topographic surface(Fig 2d, 2e). Third, SU4 and SU3 are both clayey sediment, but SU3 can be differentiated on the basis of its sand and gravel content, vertic characteristics and extremely irregular distribution and thickness. This indicates increased detritism, seasonal shrink-swell processes and probable truncation of the top of SU3 (Table

  soil (950-800 B.C.E., Fig 2b) and are all covered by structures or archaeological soils from La Tène D2b to Gallo-Roman (SU2, 50/40-30 B.C.E., Fig 2c and 2d), except in a single instance where a pit is covered by a La Tène D1B archaeological soil (100/90-80/70 B.C.E., Fig 2e).

  n°3 (819-755 cal. B.C.E.) and n°10 (490-366 cal. B.C.E.

  Middle Neolithic (c. 4500 cal. B.C.E.) detrital inputs leveled the bottom of the basin and were probably associated with an oscillating water table as suggested by the lateral facies change between SU 5a, b

  ) Probably between 600 and 425 cal. B.C.E. a large number of storage pits were excavated in the basin with consequent relief and surface disturbance. (5) After the abandonment and filling of pits, external sedimentary inputs occurred again during ephemeral and episodic flooding, partially or totally fossilizing the previous topography and causing a vertisol-type functioning. These processes can be interpreted as the hydro-sedimentary reaction to the previous anthropogenic disturbance of the basin. (6) Between 100/90 B.C.E. and 50/40 B.C.E. at least two phases of soil levelling, backfill and occupation occurred in the southern part of the basin, whereas SU3 was still in deposition in central parts of the depression. (7) Between 50/40 and 30 B.C.E. (from La Tène D2B to Gallo-Roman period) the whole basin was levelled, backfilled and probably drained, partly truncating and fossilizing the pre-existing vertisol. (8) Finally, from Antiquity to Modern Period, a slow sedimentation gradually covered the antique backfill forming a topsoil with incipient vertic characteristics, and drainage works were performed in undetermined recent period.

Figure 3 .

 3 Figure 3. Main pedo-sedimentary phases of the Lac-du-Puy evolution between early Neolithic and modern period. For SU colors, see Fig 2.

  Iron Age, providing a new insight into the diffusion of proto-urbanization in Western Europe during the first millennium B.C.E.

  This work is under the AYPONA project funded by the Conseil Régional d'Auvergne (France), the Service Régional d'Archéologie d'Auvergne, the Maison des Sciences de l'Homme de Clermont-Ferrand, and the University Lyon 2 (Projet BQR Lyon 2, dir. M. Poux and J-F. Berger). The authors are grateful to LUERN association, which provided logistic assistance, to the full team of Corent annual excavation, and to the landowners Mme. Mioche and M. Rodriguez. We want also to thank Julien Guillemoteau and Mathias Pareilh-Peyrou for the geophysical prospections, and Pierre Boivin, Yann Deberge, Josep Miret i Mestre and Enriqueta Pons for the valuable discussions. Finally, we express sincere thanks to the anonymous reviewers for their remarks and critical reading of the manuscript. 8. REFERENCES B.R.G.M. (2015). Carte Géologique de la France à 1:50000, feuille 765 (Massiac). Bernigaud, N., Berger, J.-F., Bouby, L., Delhon, C., & Latour-Argant, C. (2014). Ancient canals in the valley of Bourgoin-La Verpillière (France, Isère): morphological and geoarchaeological studies of irrigation systems from the Iron Age to the Early Middle Ages (8th century bc-6th century ad). Water History, 6, 73-93. Butzer, K. W. (2008). Challenges for a cross-disciplinary geoarchaeology: The intersection between environmental history and geomorphology. Geomorphology, 101, 402-411. Buxô, R., & Pons, E. (1999). Els productes alimentaris d'origen vegetal a l'adat del ferro de l'Europa occidental : de la producció al consum. Girona: Museo d'Arqueologia de Catalunya.

Table 2 ,

 2 SU2).

	On archaeological structures, two dates were obtained on centimetre size macro-charcoal from two levels of
	the small excavated structure in the SU 5a, under the pit 25604 (see Fig 2c), both provided the same
	Early/Middle Neolithic age (nº12 and 13). The bulk from the base of the pit 25604 (dark lining), partially
	excavated into the summit of the underlying Neolithic structure, provided a similar age (nº14). A bulk
	sediment date on the fill of this structure taken 10 cm above the base (nº15) provided a more recent date

(see

Fig 2c)

. A further four samples (nº 16 to 19), from similar contexts (dark lining at the base of the structures) in other pits also provided Early/Middle Neolithic ages. These dates are surprisingly old, especially when compared with similar settlements in Western Europe with abundant storage pits such as Ensérune (France), Mas Castellar de Pontós (Spain) or Danebury Ring (U.K.)

Table 1 .

 1 Radiocarbon data base from Lac-du-Puy trenches and logs; dates n°1 to 6 were previously published inLedger et al. (2015). Dates have been placed in their stratigraphic context on Fig 2f, 2g and 2h; dates in italic were rejected on a stratigraphic basis.

	Nº	Localization	Depth(cm)/position	Lab. code	Material	14C yr. BP	Cal BC/AD (2σ)
	1	Core/Log L8	54-57 (SU3-top)	Beta-379416	Microcharcoal	2240 ± 30	390-205 BC
	2	Core/Log L8	54-57 (SU3-top)	Beta-379417	Bulk sed.	1990 ± 30	48 BC-AD 71
	3	Core/Log L8	71-72 (SU4-top)	Beta-377232	Bulk sed.	2590 ± 30	819-755 BC
	4	Core/Log L8	71-73 (SU4-top)	Beta-379418	Pollen	2750 ± 30	944-823 BC
	5	Core/Log L8	87-88 (SU4-middle)	Beta-375785	Bulk sed.	3510 ± 30	1916-1749 BC
	6	Core/Log L8	102-103 (SU4-bottom)	Beta-379419	Bulk sed.	3330 ± 30	1688-1528 BC
	7	Log L4	43-47 (SU2-bottom)	Beta-434187	Microcharcoal	5550 ± 40	4457-4339 BC
	8	Log L4	43-47 (SU2-bottom)	Beta-430610	Bulk sed.	1430 ± 30	575-657 AD
	9	Log L4	59-63 (SU3-bottom)	Beta-434188	Microcharcoal	2390 ± 30	542-397 BC
	10	Log L4	59-63 (SU3-bottom)	Beta-430611	Bulk sed.	2340 ± 30	490-366 BC
	11	Log L4	87-91 (SU4-bottom)	Beta-430612	Bulk sed.	3130 ± 30	1457-1300 BC
	12	Excav. Struct.	Base	Beta-418695	Macrocharcoal	5650 ± 30	4546-4394 BC
	13	Excav. Struct.	Middle	Beta-418694	Macrocharcoal	5650 ± 30	4546-4394 BC
	14	Pit 25604	Lining-Base	Poz-74925	Microcharcoal	5520 ± 40	4453-4327 BC
	15	Pit 25604	Fill	Beta-425790	Bulk sed.	3430 ± 30	1876-1643 BC
	16	Pit 25677	Lining-Base	Beta-425426	Bulk sed.	6130 ± 30	5208-4992 BC
	17	Pit 25703	Lining-Base	Beta-425787	Bulk sed.	5240 ± 30	4226-3971 BC
	18	Pit 25699	Lining-Base	Beta-425788	Bulk sed.	4780 ± 30	3641-3519 BC
	19	Pit 25610	Lining-Base	Beta-425789	Bulk sed.	5640 ± 30	4541-4372 BC

Table 2 .

 2 Synthetic natural and archaeological stratigraphy of the Lac-du-Puy excavation area. Depths in cm are indicative. ) distributed all over the excavation area with constant thickness around 15 cm. Scattered basaltic particles from granules to coarse gravels. Many amphorae fragments (Dressel 1). Blocky/prismatic structure, sometimes crumbly. Frequent brown-orange oxidation mottles. ) clay loam with common heterometric basaltic inclusions from granules to small blocks, macrocharcoals and scattered pottery (La Tène D). Granular to blocky, sometimes prismatic structure. Present mainly in peripheral areas of the depression (south and west).

	Stratigraphic Unit (SU)	Depth (cm)	Pedo/ Lithofacies	Description
	SU1	0-30	Modern topsoil/Plow zone	Dark brown (10 YR 3/2) clay loam distributed over all the excavation area with constant thickness around 30 cm. Scattered basaltic fragments from granules to coarse gravels. Frequent pottery fragments (mainly amphorae type Dressel 1). Blocky to prismatic structure, sometimes crumbly. Surficial cracks in summer dry periods.
			La Tène D2B to Gallo-Roman (50/40-30 BC) archaeological structures
	SU2	30-45	General anthropogenic backfill Dark brown clay loam (10 YR 3/2La Tène D2A (80/70-50/40 BC)archaeological structures
	SU A1-A2	Variable	Local anthropogenic backfill Mainly dark brown (10 YR 3/1 to 2/1La Tène D1B (100/90-80/70 BC) archaeological structures
	SU3	45-70 approx.

  50-40 cal. B.C.E. Finally, SU 2 is dated by archaeological artefacts (ceramic) between 50 and 30 B.C.E. The pedo-sedimentary characteristics and the generally low sedimentation rate of the sequence suggest that it is a succession of more or less cumulic hydromorphic palaeo-soils, including archaeological levels and phases of low sedimentation (or maybe hiatuses), rather than a classical lacustrine sedimentary sequence (Table

	A c
	c e
	p
	t e
	d
	V e
	r
	s i o
	n
	, they likely date from
	the Early Neolithic (c. 5000 B.C.E.) . Units 5a, b and c are dated to the Early/Middle Neolithic transition by two
	radiocarbon dates from the archaeological structure (Table 1 dates nº12 and 13, 4545-4394 cal. B.C.E.). Units

4a and b cover at least the period from approximately the Early Bronze Age (c. 1900-1600 cal. B.C.E.) to the end of the LBA (c. 800 cal. B.C.E.). SU 3 deposition takes place from between 490-366 cal. B.C.E. to between

  Table 1 date nº12 and nº13, Fig 2c) provided the same Neolithic age as the pits. However this structure cannot be contemporaneous with the pits, as the only date obtained from the pit fill provides a much younger age (Table 1 date n°15, Fig2c). This date is also inconsistent with stratigraphic evidence (Fig2b), but could be explained by the fact that this kind of structure was usually backfilled when a new neighbouring structure was excavated. Sediments from the new pit were then likely used to fill the abandoned pit, and logically provide a bulk age sensibly similar to the obtained dates from SU 4 sediments in logs 4 and 8 (seeFig 2c, 2f, and 2g).

  . In the Early Bronze Age (after c. 1900-1600 cal. B.C.E.) a new sedimentary phase began, associated with low-energy inputs and a permanently high water table

(gleysol development). This phase was interrupted c. 800 cal. B.C.E., where an archaeological soil in the depression shows human occupation associated with the Corent LBA 3 settlement. Between c. 800 cal. B.C.E. and 5 th c. B.C.E. (dates 3 and 9-10 on Table

2

) sedimentation ceased.


Shaun Ferdous

Sreyash Sarkar

Frederic Marty

Patrick Poulichet

William Cesar

Elyes Nefzaoui
email: elyes.nefzaoui@esiee.fr

Tarik Bourouina

Sensitivity optimization of micro-machined thermo-resistive flow-rate sensors on silicon substrates

Keywords: Flow-rate, sensor, Thermal sensor, MEMS, Micro-machined, Water, Silicon, Sensitivity

We report on an optimized micro-machined thermal flow-rate sensor as part of an autonomous multi-parameter sensing device for water network monitoring. The sensor has been optimized under the following constraints: low power consumption and high sensitivity, while employing a large thermal conductivity substrate, namely silicon. The resulting device consists of a platinum resistive heater deposited on a thin silicon pillar ~ 100 µm high and 5 µm wide in the middle of a nearly 100 µm wide cavity. Operated under the anemometric scheme, the reported sensor shows a larger sensitivity in the velocity range up to 1 m/s compared to different sensors based on similar high conductivity substrates such as bulk silicon or silicon membrane with a power consumption of 44 mW. Obtained performances are assessed with both CFD simulation and experimental characterization.

Introduction

An accurate measurement of fluid-flow is of paramount importance in different fields of science and technology such as-gas chromatography [START_REF] Kaanta | Novel device for calibration-free flow rate measurements in micro gas chromatographic systems[END_REF], environmental monitoring [START_REF] Bartlett | Modeling of Occupant-Generated CO2 Dynamics in Naturally Ventilated Classrooms[END_REF][START_REF] Zhu | 2-D Micromachined Thermal Wind Sensors #x[END_REF], weather forecasting [START_REF] Shen | A FCOB packaged thermal wind sensor with compensation Microsyst[END_REF], bio-sensing [START_REF] Renaudin | Integrated active mixing and biosensing using surface acoustic waves (SAW) and surface plasmon resonance ( SPR ) on a common substrate Lab[END_REF], medical [START_REF] Billat | Monolithic integration of micro-channel on disposable flow sensors for medical applications Sens[END_REF] and biomedical applications [7][START_REF] Hsiai | Micro Sensors: Linking Real-Time Oscillatory Shear Stress with Vascular Inflammatory Responses[END_REF][START_REF] Soundararajan | MEMS shear stress sensors for microcirculation Sens[END_REF], aircraft monitoring [START_REF]Anon Advanced flow measurement and active flow control of aircraft with MEMS[END_REF] and industrial process control [START_REF] Bruschi | Design Issues for Low Power Integrated Thermal Flow Sensors with Ultra-Wide Dynamic Range and Low Insertion Loss[END_REF]. Flow-rate measurement is crucial for water network management as well. Water is a precious natural resource and therefore is a global priority. Consequently, there is a significant need for an effective measurement system for water networks monitoring in order to reduce wastage and to ensure apposite water quality. Measurement of water flow-rate is necessary not only to survey water consumption but also to estimate and even localize the water loss due to leakage of distribution networks. To obtain a complete and monolithic measurement system for water networks, other parameters need to be considered along with the flow-rate such as pressure, temperature and electrical conductivity. Pressure and flow-rate sensors can provide information about the leakage of the distribution system; whereas the conductivity sensor can indicate the overall water ionic contamination level. The temperature of water is also important since the water conductivity depends on temperature as well. It is therefore tempting to co-integrate all the sensors mentioned above on the same chip so as to provide a monolithic multi-parameter sensing solution, which puts a strong constraint on using silicon as a substrate material * . Nowadays, several micro-machined flow-rate measurement systems are available to measure the flowrate in different fluid media [START_REF] Glaninger | Wide range semiconductor flow sensors Sens[END_REF][START_REF] Ju | Microfluidic flow meter and viscometer utilizing flow-induced vibration on an optic fiber cantilever 2011 16th International Solid-State Sensors[END_REF]. The flow-rate range of such systems range from nL/min to L/min [START_REF] Dijkstra | Miniaturized thermal flow sensor with planar-integrated sensor structures on semicircular surface channels Sens[END_REF][START_REF] Meng | A biocompatible Parylene thermal flow sensing array Sens[END_REF]. A majority of devices reported in the literature are based on thermal operation principles. Thermal flow-rate sensors can be divided into three categories based on their operation principle-anemometric (Hot-wire), calorimetric and time-of-flight (TOF) [START_REF] Kuo | Micromachined Thermal Flow Sensors-A Review[END_REF]. The anemometric flow-rate sensor measures the effect of the flowing fluid on a heating resistance, the hot wire for instance. The velocity of the corresponding fluid can be deduced by measuring the amount of heat extracted by the fluid from the hot surface of the sensor. The temperature variation of a hot-wire submitted to a fluid flow, indeed depends on the fluid velocity. Measuring this temperature variation enables the calculation of the fluid velocity. An anemometric flow-rate sensor can be operated in one of the following three modes: constant current, constant temperature or constant power [START_REF] Chiu | Low power consumption design of micro-machined thermal sensor for portable spirometer[END_REF][START_REF] Nguyen | Micromachined flow sensors-a review Flow[END_REF]. A calorimetric flow-rate sensor measures the asymmetry of the temperature profile caused by the fluid flow around a heating resistor. In general, at least two temperature sensors are placed upstream and downstream the heating resistor for this purpose [START_REF] Zhu | 2-D Micromachined Thermal Wind Sensors #x[END_REF][START_REF] Kuo | Micromachined Thermal Flow Sensors-A Review[END_REF]. A TOF flow-rate sensor usually measures the transit time of a thermal pulse over a known distance. This kind of sensor involves two or more resistors. One of them is used as a heater; the others are placed downstream and used as temperature sensors. A short thermal pulse generated by the heater travels through the fluid medium and is sensed by the temperature sensors. The pulse travel time from the source to the sensor enables the calculation of the fluid velocity [START_REF] Van Kuijk | Multi-parameter detection in fluid flows Sens[END_REF][START_REF] Berthet | Time-of-flight thermal flowrate sensor for lab-on-chip applications Lab[END_REF].

In the present work, we consider anemometric thermal flowrate sensors operated under constant current. Several versions are designed, characterized and compared: first, a flow-rate sensor, fabricated on bulk silicon. This device exhibits a small sensitivity and a large power consumption. This is mainly due to a large conductive heat leakage through the substrate. We have recently shown that the power consumption and sensitivity of a thermal flow-rate sensor are strongly dependent on the used substrate material [START_REF] Shaun | Design of micro-fabricated thermal flow-rate sensor for water network monitoring[END_REF]. For better performances in terms of power consumption and sensitivity, low thermal conductivity substrates are required. Due to the excellent linearity of its temperature-dependent resistivity, platinum is the most used material to fabricate the heating/sensing element of a thermal flow-rate sensor [START_REF] Meng | A biocompatible Parylene thermal flow sensing array Sens[END_REF][START_REF] Mailly | Anemometer with hot platinum thin film Sens[END_REF][START_REF] Fürjes | Thermal characterisation of a direction dependent flow sensor Sens[END_REF] while sometimes a mixture of platinum and titanium is also used [START_REF] Bailey | Turbulence measurements using a nanoscale thermal anemometry probe[END_REF]. Most of the micromachined thermal flow-rate sensors are fabricated on silica glass [START_REF] Norlin | A chemical micro analysis system for the measurement of pressure, flow rate, temperature, conductivity, UV-absorption and fluorescence[END_REF][START_REF] Wilding | Manipulation and flow of biological fluids in straight channels micromachined in silicon[END_REF][START_REF] Roh | Sensitivity enhancement of a silicon micro-machined thermal flow sensor Sens[END_REF] for maximum thermal efficiency and due to its low thermal conductivity. Sometimes, for nanofluidic or biomedical applications [START_REF] Wu | MEMS flow sensors for nano-fluidic applications Sens[END_REF][START_REF] Yu | MEMS Thermal Sensors to Detect Changes in Heat Transfer in the Pre-Atherosclerotic Regions of Fat-Fed New Zealand White Rabbits[END_REF] for example, silicon, which is not the optimal choice from thermal considerations, is used. Hence, when the use of silicon as a substrate material is a constraint, other solutions are required so as to optimize heat transfer in the device. Therefore, modification of the material's geometry is the only option to achieve low thermal conductance and maximize the device sensitivity. First, a bulk silicon substrate device is considered as a reference. Then, a second version of the sensor, which is based on a silicon membrane is inspected, but this version renders unsatisfactory results. Therefore, further optimization is required. A suspended resistor can be considered and have already been suggested by Neda et al. as early as 1995 [START_REF] Neda | A Polysilicon Flow Sensor For Gas Flowmeters[END_REF] and as of late by various works [START_REF] Berthet | Time-of-flight thermal flowrate sensor for lab-on-chip applications Lab[END_REF][START_REF] Jack | Two-Dimensional Micromachined Flow Sensor Array for Fluid Mechanics Studies[END_REF]. However, such geometric configuration increases the device fragility, particularly if wastewater is targeted as a possible operation medium. An intermediate configuration is then proposed in the present paper: the heater resistor is fabricated on a thin silicon microwall at the middle of a large cavity etched on bulk silicon. Such configuration would reduce the conduction heat loss while ensuring an acceptable robustness of the heater/sensor structure. The numerical optimization and design, as well as the experimental characterization of this improved micro-machined thermal flowrate sensors, are reported in the following sections. Obtained performances are also compared to those of the previously designed and fabricated devices.

The manuscript is organized as follows: first, modelling and fabrication processes of the different devices are described. Then, the experimental setup and numerical methods used are presented. Finally, the main numerical and experimental results regarding the sensors' responses to the fluid flow rate are reported and discussed. A particular attention is paid to the comparison of the new pillar-based device and previous prototypes, with respect to their sensitivity to the fluid velocity and their power consumption.

Methods

Design and Fabrication

The multi-parameter sensor has a MEMS chip size 1 x 1 cm in which the lateral size of each sensor is no more than a few millimetres.

The first prototype of the multi-parameter sensing module is presented in Figure 1. The sensor chip is placed on a PCB, which contains the CMOS chip and other electronics for wireless data transmission as shown in Figure 1(b). Finally, it is inserted into a hollow plastic pole (Figure 1(c)) which will be employed in the water distribution network to measure the multiple parameters. In Figure 2, the detailed description of different sensing parts of the fabricated chip is illustrated. The chip wire bonding to a PCB, which is designed to build the sensors' electrical connections for lab testing, is shown in Figure 2(a). The entire PCB used for laboratory characterization is shown in Figure 3. The whole PCB can be divided into 2 parts-base and body. The entire PCB length is 7.2 cm, where the base length is 0.89 cm. The width of the base and the body are 3.82 cm and 1.18 cm respectively. Two sets of header holes are created at the PCB base in order to build the electrical connection between the PCB and the electronic circuit. A 1 x 1 cm square shaped space is created at the tip of the PCB body to place the sensor chip. An adhesive is used to glue the chip on the PCB. The electrical connection between the chip's connection pads and the PCB's wires is done by gold wire bonding. The electrical connections between the chip and the PCB are protected hermetically by epoxy resin to make it aqua-resistive. It is noticeable that only the silicon and silicon membrane based sensors contain three platinum resistors while the pillar device contains only one. The arrangement of three platinum resistors allows us to implement any one of the three conventional operation schemes i.e., anemometric, calorimetric and Time-of-Flight (TOF) of a thermal flow-rate sensor. In case of the calorimetric operation principle, the central resistor can be used as a heater and two up and downstream resistors can be used as temperature sensors. Besides, in TOF mode, the left resistor is a thermal pulse generator where the other resistors are temperature sensors depending on the velocity range. Shorter distance between the pulse generator and the sensing resistor exhibits high sensitivity to low velocity, whereas the longer distance enhances the sensor's sensitivity to high velocity [START_REF] Zhu | 2-D Micromachined Thermal Wind Sensors #x[END_REF][START_REF] Ashauer | Thermal flow sensor for liquids and gases based on combinations of two principles Sens[END_REF]. The pillar-based device's heater is also made of platinum but with different geometric dimensions. Platinum is chosen for its high TCR (Temperature Coefficient Resistance) and excellent linearity over a wide temperature range. The TCR value of our sputter-deposited platinum is 2.218×10 -3 C -1 . Bulk silicon and silicon membrane configurations contain three platinum resistors to enable both calorimetric and TOF operating modes, which have not been considered in this report. The distance between the resistors is 20 m. The length and width of each platinum resistor are 106 m and 10 m respectively, and the thickness is 340 nm. The cavity depth of the pillar-based device is 100115 m while the width and length are 100 m and 200 m, respectively. The resistor length of this last configuration is equal to the length of the cavity while it is only 5-μm wide. This width is obtained by a parametric numerical optimization of the device sensitivity with respect to its geometric dimensions. Therefore, the resistor width is reduced by half compared to the two other configurations. It can be reduced more, but this would increase the complexity of the fabrication process and makes the resistor more fragile. For the same reason, a fully suspended resistor is not considered. More details on this optimization process are presented in section 3.1.1.

CMOS chip

All considered sensors are based on resistive read-out. All of them, except the pressure sensor, are obtained by metal micro-patterning with the combination of titanium, platinum and gold. Further cointegration of the pressure sensor along with the other sensors on the same chip requires additional steps, which include patterning of polysilicon strain gauges and backside etching of the silicon bulk. The latter step is also used to produce thin silicon membranes, not only for the pressure sensor but also for the flowrate sensor schematically shown in Figure 4(b). The membrane acts either as a mechanically flexible structure or as a thermal insulating layer depending on the considered sensor. A glass substrate is used to support the silicon membrane. The schematic cross-section view of the multi-parameter sensor chip with the silicon membrane flow-rate sensor is presented in Figure 5. In the case of pillar structure device, a 100 m deep cavity is created by anisotropic dry etching. The platinum resistor is patterned on the top of the pillar position at the cavity centre. The fabrication sequence is presented in detail in Figure 6(a, b, c, d ande) and Figure 6(f) shows a SEM picture of the pillar device based sensitive element of the sensor which is fabricated in the new version of the multi-parameter chip.

Experimental setup

The experiment is conducted to extract the sensor's responses with respect to the fluid velocity. The setup is constructed using a rigid pipe with a diameter of 25 mm. The length of the pipe is 10 times its diameter in order to reduce edge effects and to ensure an established flow regime around the sensor. A schematic of the experimental setup is presented in Figure 7. A variable speed pump, submerged in a 20litre water reservoir, is used to generate water flow at different flow rates. The velocity is measured by following the volumetric measurement process using the same setup (Figure 7). The required time to fill a certain volume of water is measured at different voltage supply levels. Then, the flow rate is calculated. Since the pipe diameter is known, the average velocity is calculated for each flow rate and pump power supply. LabView software interface is used for data acquisition. The sensor is operated according to hotwire (anemometric) operation scheme under constant current mode. For this purpose, a constant current source is built using an adjustable three-terminal positive-voltage regulator IC device (LM317). To increase the accuracy of measured resistance data, a 4-probe measurement scheme is used so as to eliminate the parasitic wiring resistance effect which may vary due to local heating.

The temperature dependence of the platinum resistance is used to assess the water velocity and flow rate. The setup, indeed measures a change in the resistance value due to a temperature variation induced by the fluid flow. The temperature variation is then obtained using the equation, 𝑅(𝑇) = 𝑅(𝑇 0 )[(1 + 𝛼Δ𝑇]; where, R(T) is the resistance value at temperature T, R(T0) the reference value of the resistance at the initial temperature T0, Δ𝑇 = 𝑇 -𝑇 0 , is the temperature difference and 𝛼 is the TCR. The resistance temperature drop with respect to zero velocity situation is then calculated. It is related to the fluid velocity by a calibration process. Consequently, a measurement of the resistance value and hence that of the temperature, enables access to the fluid velocity.

Numerical analysis

The different device configurations presented in Figure 4 are considered for numerical simulations. A parametric study is performed with respect to several parameters such as-shape, geometric dimensions, the substrate material and power consumption. Computational Fluid Dynamics (CFD) simulations are carried out based on Finite Element Method (FEM), using COMSOL Multiphysics. Material properties are imported from COMSOL material library and Conjugate Heat Transfer module is used to account for heat transfer both in the solid and fluid medium. Since the Conjugate Heat Transfer physics is a combination of conductive heat transfer in solid and fluids, and fluid flow (Laminar). Boundary conditions are defined for these two physics. For heat transfer-the left side of the fluidic domain is set to a constant temperature, and a continuity of heat flow is ensured at the other boundaries (Figure 8(a)). The platinum resistor is defined as the heat source with an energy dissipation rate of 10 mW. On the other hand, for the laminar flow physics, the velocity inlet (𝑣 = 𝑣 𝑓𝑙𝑢𝑖𝑑 = constant) and outlet (Δ𝑃 = 0) boundary conditions are applied to the left and right side of the water domain respectively, while the top and bottom edges are defined as symmetry axes. All the exterior edges of the sensor body are defined as walls with the 'no-slip condition'. The water medium includes the interior parts of the cavity. A 450 nm thick silicon dioxide insulation layer is introduced between the substrate and the heater in all configurations. For the membrane sensor, the platinum resistor is mounted on a silicon membrane where the backside cavity depth is around 430 m. The medium inside the cavity is defined as vacuum. A parametric study is conducted to optimize the aspect ratio of the cavity and the pillar structure. The cavity height and width range from 50 to 300 m and from 100 to 1000 m respectively. The pillar height and width are equal to the cavity height and the platinum resistor width, respectively. Because of the almost parabolic velocity profile inside the considered pipes, the sensor is placed on the pipe axis in order to be exposed to the maximum velocity. The sensing element is perpendicular to the main flow direction, thus aligned with the cross-sectional diameter of the pipe. A very large fluidic domain is chosen compared to the sensor size to avoid edge effects.

Results and Discussion

This section reports obtained numerical and experimental results. The numerical parametric study enables the determination of the optimal sensor design. Then, the optimal device is fabricated, experimentally characterized and compared to previous designs.

Numerical results

The sensor's sensitivity to the resistor's width.

As mentioned in section 2.1, a parametric numerical study is performed to optimize the geometry of the sensing element, thereby optimizing the resistor width of the pillar structure flow-rate sensor. We report in this section on the device sensitivity with respect to the heater resistor width Rw. The flow-rate sensor's response for a given velocity strongly depends on the heating resistor width. The latter, indeed affects Joule self-heating and the cooling effect of the fluid flow. We show that a decrease in Rw increases the sensor's sensitivity. Simulations are done at 10 mW power supply under a wide velocity range from 10 -6 m/s to 10 m/s. The length and thickness of the resistor are fixed at 100 m and 340 nm, respectively, while Rw is varied from 1 µm to 10 µm. For a better assessment of the sensors' response with respect to the velocity, the normalized temperature value of the device is plotted as a function of velocity. The normalized temperature is defined as the ratio between ΔT and Δ𝑇 𝑚𝑎𝑥 , where ΔT is the temperature difference between the heater resistor and room temperature at a given velocity and Δ𝑇 𝑚𝑎𝑥 is the same temperature difference at zero velocity. The variation of this non-dimensional temperature provides valuable information about the sensor's response to velocity. Figure 9. Pillar device normalized temperature and sensitivity plot as a function of the velocity. Here, the primary Yaxis shows the thermal variation, hence the sensitivity of the pillar device with respect to the velocity at different resistor's widths under 10 mW power supply within the velocity range from 10 -6 m/s to 10 m/s. And the secondary Y-axis expresses the sensitivity to the heater resistor width under the same power supply and velocity range.

The normalized temperature (NT) due to water flow is plotted with respect to the primary Y-axis and the NT sensitivity to Rw is plotted with respect to the secondary Y-axis. The effect of the resistor width on the device sensitivity to velocity is evaluated considering the 10 µm width as a reference, where the resultant normalized temperatures for 1 µm, 2 µm and 5 µm wide resistors are compared with the reference resistor response. This sensitivity is calculated as,

𝑆 𝑅 𝑊 = 𝑁𝑇 𝑅 𝑊 -𝑁𝑇 𝑅 𝑊𝑟𝑒𝑓 | 𝑣 ∆𝑅 𝑊
Where, 𝑆 𝑅 𝑊 is the device sensitivity to the resistor width, 𝑁𝑇 𝑅 𝑊 -𝑁𝑇 𝑅 𝑊𝑟𝑒𝑓 | 𝑣 is the normalized temperature difference between a specific resistor and the reference resistor at a specific 𝑣; where 𝑣 ∈ [10 -6 , 10] m/s and ∆𝑅 𝑊 = 𝑅 𝑊𝑟𝑒𝑓 -𝑅 𝑊 is the width difference between a given resistor and the reference resistor.

The best response is observed for the 1 m wide resistor. On the other hand, the sensor with the broadest resistor (10 µm) rapidly reaches room temperature at high velocity which means a small turndown ratio compared to the other resistors. In addition to the sensitivity improvement due to Rw reduction observed in Figure 9, we observe a decrease of sensitivity in all cases at high velocities. This is due to the low temperature variation at high velocities since large velocities induce large convective cooling.

In spite of the better performances expected with resistors having smaller widths, the technological fabrication process of 1 µm and 2 µm resistors was found to be significantly more complex. Consequently, the pillar device is fabricated with a 5 m wide resistor. All experimental results presented and discussed in the following paragraphs were obtained with this device.

Comparison of the three devices: on bulk silicon, silicon membrane, and silicon pillar

A numerical study is now conducted to compare the pillar device response, with a 5-m wide resistor, with the previously fabricated and tested devices, namely the bulk silicon and the silicon membrane device. Simulations are also done at 10 mW power supply under the same velocity range from 10 -6 m/s to 10 m/s. Results are shown in Figure 10, where normalized temperature values are plotted as a function of velocity for the three sensors. For the sake of comparison, NT variations are preferred to absolute temperature values. Bulk silicon and silicon membrane devices exhibit almost similar NT variations with respect to velocity. On the other hand, the pillar structure device shows higher sensitivity within the considered velocity range than the two other devices. This is mainly due to a lower heat leakage to the substrate and a much larger Joule self-heating. The black circle in Figure 10 indicates a discrepancy of the sensor's response for a certain velocity range. A monotonic decreasing temperature is indeed, expected when the flow velocity increases. This discrepancy is due to a reverse flow which appears at this velocity range. The reverse flow velocity field is shown in Figure 11.

The main water flow is heated by the heating resistance after a first pass. When the reverse flow carries back this hot water to the heater resistor, the latter experiences a temperature increase. As a result, a velocity increase induces a heater temperature increase rather than the opposite. At 0.3 m/s and 0.4 m/s the pillar flow-rate sensor shows this inconsistent response. A similar reverse flow is observed for bulk silicon and membrane based sensors. However, in these cases, reverse flow fluid is not hot enough to increase the heater temperature because of lower initial Joule self-heating and larger thermal leakage through the substrate. The reverse flow issue at the heater resistor vicinity is fixed by modifying the height and the cavity edge curvature. CFD simulations enable the optimization of these parameters to suppress reverse flow. The re-circulation effect is observed noticeably only for the pillar device among all of the silicon based sensors in the numerical study. The experimental measurement shows a monotonic decrement of the pillar heater temperature within the velocity range from 0 m/s to 0.91 m/s (Figure 14) and the recirculation effect is not observed in practice. The 2D flow assumption in the numerical model may explain the difference between simulation and experimental results where, flow is 3D.

Experimental results

The main purpose of this section is to present the experimental responses of the different sensors to the fluid flow. This will be tackled in the last subsection while the two first subsections will tackle static, i.e with no fluid flow, experiments results to discuss the main governing phenomenon for a thermal flow-rate sensor: Joule self-heating. The resistor's Joule self-heating is an important parameter for a thermal flow-rate sensor. The maximum temperature of the heater resistor under no-flow condition defines the turn-down ratio and the sensitivity of the sensor with respect to velocity. Moreover, the relation between the initial temperature and the power supply indicates the device power efficiency. A large temperature increase at low power consumption leads to a low power consuming thermal flow-rate sensor. Therefore, to determine the considered sensors' efficiency, both Joule self-heating and power consumption tests are studied under zero velocity condition. Results are presented in the following section.

Joule self-heating and Power consumption.

Information on Joule self-heating at the resistor level under the no-flow condition helps to anticipate the sensor's response in terms of the velocity and the minimal operating current. The sensor is submerged under the non-flowing water and supply current is varied from 10 mA to 30 mA with a step of 5 mA. Results of Joule self-heating tests are presented in Figure 12(a). We observe that the pillar sensor shows a maximum temperature increase, which is 2.57 °C at 30 mA current supply. Besides, the bulk silicon and the silicon membrane devices show small temperature increase and the maximum temperature values for these two sensors do not surpass 0.8 °C for maximum current supply. Although the three sensors are fabricated on the same substrate material, pillar configuration flow-rate sensor exhibits a significantly larger temperature increase than the others due to an optimized geometry. The power consumption of the three devices is also calculated under the same current supply range. It is illustrated in Figure 12(b). The pillar device consumption is relatively high, up to 43.78 mW for a 30mA current supply. On the other hand, bulk silicon and silicon membrane sensors are three times less power consuming than the pillar configuration at the same current supply. This is due to the high initial resistance value of the pillar device, which is 43.6 Ω. According to Ohm's law, power is proportional to resistance. Therefore, the larger the resistance value, the larger the power consumption. The pillar device heater resistor's width is reduced by two times compared to bulk silicon and membrane resistors, which causes a larger resistance value at room temperature. Consequently, the pillar device is expected to have a power consumption two times larger than the other device. This is not what is observed in Figure 12(b). At the same current supply, a larger resistance value leads to a larger Joule self-heating, hence corresponds to a larger equilibrium temperature. Joule self-heating is then amplified by two phenomena: i) the temperature increase due to Joule heating induces an increase in the resistance values, which results in an additional second-order Joule heating and a temperature increase; ii) The pillar device resistor contact with the substrate due to the pillar structure which reduces heat leakage through the substrate, increases the device equilibrium temperature. This double amplification of Joule self-heating for the pillar device, induces a larger increase of the resistance value, hence a larger power consumption. This amplification can be deduced from the variation of the sensors' resistance values reported in Table 1. For a current supply between 0 and 30 mA, we observe a relative variation of the pillar device resistance of 11.5 % versus a variation of 1 % for the other two devices. The relative resistance variation, hence the Joule selfheating, is larger by one order of magnitude for the pillar device. The tabulated resistance variation values are related to the temperature variation obtained from the Joule self-heating test that is presented in Figure 12(a). As mentioned above, the velocity is calculated according to the cooling magnitude of an anemometric flow-rate sensor. Therefore, we experimentally measure the sensor's response within a velocity range from 0 ms -1 to 0.91 ms -1 . The flow direction is parallel to the resistor's width. A schematic diagram of the flow direction with respect to the resistor is illustrated in Figure 13. We observe in Figure 14, a larger sensitivity for the pillar sensor as expected from numerical results. The relative temperature variation with respect to the velocity of the pillar flow-rate sensor exhibits a monotonic decrease over almost the entire considered fluid velocity domain, whereas, the silicon bulk and membrane based sensors show almost no temperature variations for velocities larger than 0.3 m/s. The device time constant, defined as the time needed to reach 63% of thermal equilibrium temperature for a given velocity, for pillar, silicon membrane and bulk silicon flow-rate sensors are 1.7 s, 1.9 s and 2.1 s respectively. Experimental results obtained under fluid flow are consistent with preliminary Joule self-heating tests and simulation results.

Conclusions

We demonstrated that the performance of the thermal flow-rate sensor largely depends on the thermal conductance of the substrate material, which has a great impact on the device sensitivity and power consumption. When there is a limitation regarding the selection of the substrate material, low thermal conductance can be achieved in spite of using high thermal conductivity materials by geometric optimization. In addition, the resistor dimensions are also a governing parameter of the sensor's sensitivity. We demonstrated the successful co-integration of a thermal flow-rate sensor with other MEMS sensors requiring silicon as a substrate material. Unfortunately, the relatively large thermal conductivity of silicon reduces the performance of the thermal flow-rate sensor. Consequently, the main challenge was to achieve a sensitive flow-rate sensor on a silicon substrate. This was successfully achieved through microstructuration of the silicon substrate using a pillar-like structure and reducing the substrate overall thermal conductance. In the present work, the flow rate sensor is operated independently from the other sensors of the chip. Due to a large number of sensing elements on a very small footprint, cross-interactions between the different sensors is possible and might have an effect on the overall chip performances. The investigation of these cross interactions is the next step of the present project.

Figure 1 .

 1 Multi-parameter sensor for monitoring water networks-(a) the MEMS chip co-integrating 4 physical sensors (in addition to 5 chemical sensors); (b) capsule board with MEMS and CMOS chip, (c) sensor head.

Figure 2 .

 2 Figure 2. Details of (a) fabricated silicon multi-parameter sensor chip including (b) flow-rate sensor, (c) temperature sensor, (d) 2 conductivity sensors and (e) 2 pressure sensors.

Figure 3 .

 3 Figure 3. PCB for conducting experiments with the sensor chip.

Figure 4 .

 4 The 3 different configurations considered for the flow-rate sensor aiming to study the impact of thermal conductance on the device sensitivity with respect to the velocity-(a) silicon, (b) silicon membrane and (c) silicon pillar structure.

Figure 5 .

 5 Figure 5. Cross-section view of the multi-parameter sensor chip containing the membrane structure flow-rate sensor.

Figure 6 .

 6 Schematic of the fabrication sequence-(a) front-side resistors patterning of the multi-parameter sensor chip, (b) patterning the hard mask on the backside for back-side DRIE process in order to create the pressure sensor's membrane, (c) Hard mask removal for silicon-glass anodic bonding, (d) patterning of the front-side in order to create the cavity for the pillar structure flow-rate sensor and etching the front-side deposited layter, (e) the cavity after the front-side DRIE and (f) the SEM photo of the fabricated pillar structure flow-rate sensor.

Figure 7 .

 7 Figure 7. Schematic of the experimental setup.

Figure 8 .

 8 Simulation geometry : (a) the whole geometry and (b) zoomed picture of the pillar device

Figure 10 .

 10 Figure 10. Numerical results of the corresponding three sensors response to the velocity at 10 mW.

Figure 11 .

 11 (a) Reverse flow at the resistor vicinity illustrating the anomalous behaviour within the velocity range highlighted by black circles in Figure 10; (b) normal velocity profile for other fluid velocities. Only the fluid domain at the heater resistor vicinity is shown here (in blue) while the white domain corresponds to the sensor substrate. The reverse flow is due to the finite size of the sensor chip (not shown here) and the flow separation around it.

Figure 12 .

 12 (a) Joule self-heating temperature of the heater resistors at different supply current under the no-flow condition, (b) power consumption of the three corresponding sensors.

Figure 13 .

 13 Figure 13. Water flow direction in parallel with respect to the resistor width. Here, Is denotes supply current, S + and S -is the voltage reading ports and GND stands for ground.

Figure 14 .

 14 Figure 14. Pillar, silicon membrane and bulk silicon flow-rate sensor measured response as a function of velocity at 30 mA current supply.

Table 1 .

 1 Resistance variation with respect to the current supply.

	Sensor	Resistance at room temperature (Ω)	Resistance variation range (Ω)
	Pillar	43.6	43.6 ~ 48.65
	Silicon membrane	15.07	15.07 ~ 15.27
	Bulk silicon	15.78	15.78 ~ 15.97

3.2.2 The sensor's response to the velocity.

Acknowledgement

Authors would like to thank Hugo Regina for his contributions to the present project. This work received funding from the European Union's H2020 Programme for research, technological development and demonstration under grant agreement No644852. Part of this work received the support of the National Research Agency (ANR) in the frame of the EquipEx project SENSE-CITY of the programme d'Investissements d'Avenir (PIA), involving IFSTTAR and ESIEE Paris as founding members of the consortium. Fabrication of the chips is done in the cleanroom facilities of ESIEE Paris, whose technical staff is deeply acknowledged.