Essais de quantification du débit des résurgences sous-marines autour du Piton de la Fournaise (la Réunion, Océan Indien)

On étudie, par des mesures en plongée sous-marine, les émergences en mer des écoulements souterrains dans le Piton de la Fournaise (île de la Réunion) dans le cadre de recherches sur les relations entre circulations souterraines, flux de chaleur et phénomènes magmatiques. On établit:- que l'ensemble des émergences se situe au niveau du rivage, et non en profondeur- que grâce à des mesures approchées des phénomènes de mélange des eaux, on peut estimer le débit moyen des résurgences en mer à 0,4 m3s-1 par kilomètre de côte sur la façade sud de l'île. Ce chiffre est cohérent avec des estimations indirectes obtenues à partir du bilan hydrique.The « Piton de la Fournaise » on the island of la Réunion receives as much as 6 to 10 m y-l of rainfall, which almost immediately infiltrates due to the highly pervious nature of the basalt (microfissures, scoria). Very few springs or rivers drain the systeml however, numerous marine outlets have been observed by infrared thermographic surveys. Some of these outlets can also be observed on SPOT images, although no infrared charmel is available on SPOT.In an attempt to study the role of underground flow in the thermal balance of the volcano and its relation with magmatic phenomena, we have tried to observe the nature of these outlets by underwater exploration and to evaluate their flow rate by salinity and temperature mesurements.Longitudinal and transverse salinity profiles were measured by divers using a salinity-conductivity meter with automatic correction of temperature, transported in a waterproof container, on those plumes that had been observed on airborne images or by helicopter surveys.All the observed plumes are indeed mixing zones of seawater and freshwater, with salinity ranging from 29.9 to 35.1 %o (ocean salinity is35.2%o at la Réunion). Their temperature is in general lower than that of the sea, which explains why they are visible on infrared irnages.It was found, however, that all outlets are systematically situated very close to the shore line, with an elevation between + 1 and - 2 m of sea level. In none of the observations were outlets found that could originate deeper down in the sea. Most observable oulets are located at the base of basalt flows, in natural « tunnels » under such lava flows. This is consistent with the existence of an abrupt interface between seawater and freshwater on the island, which has been observed in a fewboreholes, and which forces the freshwater flow upwards towards the shore line, even if the nature of the flow is very discontinuous in the basalt.Based on the salinity profiles, we have attempted to estimate the flow rate. We focus here on a particular outlet at Vincendo. We liken the development and mixing of the plume to what occurs in an estuary with low flow. It has been observed that three mechanisms control the mixing in an estuary: wind which creates currents and pushes freshwater toward the edges; tidal effects and waves creating currents and turbulent mixing due to rugosity of the sides and bottom; density difrerences between seawater and freshwater, the latter floating on top of the former. Three cases are generally considered:a - Seas without tide: the interface is stratified;b - Seas with small tides: stratification and mixing occur sirnultaneously;c - Seas with large tides: no stratification and regular mixing in the vertical dimension.The observed salinity profiles at Vincendo clearly indicate that we are in the second case at la Réunion, where the tide amplitude is small (0.7 m).Two dimensionless numbers are used in estuaries: the Richardson and Froude numbers (see expression in text) (FISHER et al., 1979). It has been observed that the transition from case a to c corresponds to Richardson numbers in the range 0.08 to 0.8. Assuming that salinity profiles are consistent with case b, i.e. a Richadson number in the range 0.25 - 0.80, we find that the freshwater flowrate should be in the range 0.020 - 0.260 m3. s-1.A second independent estimation can be obtained by observing that the average concentration gradient in the 7,000 m2 of the obseved mixing zone (30 x 40 m) is on the order of 0.3 kg. m-3. m-1. Selecting (from FISCHER et al., 1979) a turbulent dispersion coefficient for coastal watcrs over the scale of several thousand m2 of 2 to 5 x 10-3 m2. s-1, we can estimate the vertical dispersive flux over the mixing zone and, by mass balance, we obtain another estimate of the flux in the range 0.150 - 0.400 m2. s-1.We conclude that the flowrate is on the order of 0.150 m3.s-1, with a plausible range of 0.100 - 0.300 m3.s-1.These results were extrapolated to the entire South and East shores ofthe island by assuming that the flow rate of an outlet was proportional to its area as observed on infrared surveys. We obtain an average flux of 0,4 m3.s-1 km-l for the southern flank of the volcano. This ligure is consistent with a global estimate (0,6 m3.s-1km-l) obtained by a surface hydrologic balance over this part of the volcano. The difference can represent diffuse outlets into the sea

Modélisation 3D des transports de sel et de chaleur au cours des 248 Ma d’évolution du bassin de Paris : implications diagénétiques

Un modèle de bassin 3D a été développé sur le bassin de Paris, reconstituant ses 248 Ma d’histoire géologique depuis le Trias jusqu’à l’actuel. Cette modélisation s’appuie sur une base de données stratigraphique et lithologique détaillée constituée d’environ 1100 forages pétroliers. Ce modèle, d’échelle régionale, couvre un domaine de 700 000 km2, plus vaste que l’extension actuelle du bassin, afin de prendre en compte l’évolution paléogéographique de la plaque européenne. Cette histoire géologique est simulée à l’aide du modèle numérique NEWBAS de l’Ecole des Mines de Paris. Le modèle simule la sédimentation, l’érosion, la compaction, les écoulements de fluides et les processus de transport de solutés et de chaleur. L’objet du présent article est de montrer l’intérêt d’une telle modélisation pour l’estimation et la quantification de l’importance des circulations de fluides dans les processus géologiques. Les études sur les ciments diagénétiques des réservoirs Dogger et Keuper du bassin de Paris ont souvent conduit leurs auteurs à invoquer des circulations de fluides régionales. Ces études, qui fournissent des estimations de paléotempératures et de paléosalinités, apportent des contraintes à la modélisation, mais en retour la modélisation peut apporter un calage dans le temps de ces événements et une estimation des processus pertinents. La reconstitution des transports de chaleur et de sel proposée dans cet article permet ainsi de cerner l’influence de l’hydrodynamique sur ces processus. L’histoire thermique et saline du bassin est présentée à différentes étapes sur une coupe NW-SE représentative d’une ligne d’écoulement actuelle également valable au cours du Tertiaire. On montre l’importance de la paléotopographie pour expliquer les fortes salinités dans les réservoirs et le rôle de la faille de Bray pour l’évolution de la salinité dans le Dogger. Le basculement et l’érosion de la base tertiaire crée un écoulement gravitaire qui se substitue au régime d’écoulement en compaction, permettant ainsi la migration de saumures depuis la formation salifère à l’est du bassin vers les réservoirs du Keuper à l’ouest. La recharge des aquifères aux affleurements et la mise en charge des systèmes permet une migration ascendante des eaux salées depuis le Keuper vers le Dogger en considérant une perméabilité plus importante au niveau de la faille de Bray. Bien que dominé par la composante conductive, le transport de chaleur est également influencé par l’hydrodynamique avec un effet de refroidissement convectif possible lors de la mise en charge des aquifères à la fin de l’érosion tertiaire, pouvant expliquer une partie de l’excès de température déduit des inclusions fluides du Keuper entre l’état thermique à la fin du dépôt de la craie et l’actuel. D’après nos simulations, la base du Tertiaire est la période la plus compatible avec les observations diagénétiques, pour des raisons thermiques (maximum d’enfouissement et effet de refroidissement convectif) et chimique (topographie favorable aux migrations de saumures dans le Keuper et le Dogger)

Dynamics of Wetting Fronts in Porous Media

We propose a new phenomenological approach for describing the dynamics of wetting front propagation in porous media. Unlike traditional models, the proposed approach is based on dynamic nature of the relation between capillary pressure and medium saturation. We choose a modified phase-field model of solidification as a particular case of such dynamic relation. We show that in the traveling wave regime the results obtained from our approach reproduce those derived from the standard model of flow in porous media. In more general case, the proposed approach reveals the dependence of front dynamics upon the flow regime.Comment: 4 pages, 2 figures, revte

Impact of transient groundwater storage on the discharge of Himalayan rivers

International audienceIn the course of the transfer of precipitation into rivers, water is temporarily stored in reservoirs with different residence times such as soils, groundwater, snow and glaciers. In the central Himalaya, the water budget is thought to be primarily controlled by monsoon rainfall, snow and glacier melt, and secondarily by evapotranspiration. An additional contribution from deep groundwater has been deduced from the chemistry of Himalayan rivers, but its importance in the annual water budget remains to be evaluated. Here we analyse records of daily precipitation and discharge within twelve catchments in Nepal over about 30 years. We observe annual hysteresis loops--that is, a time lag between precipitation and discharge--in both glaciated and unglaciated catchments and independent of the geological setting. We infer that water is stored temporarily in a reservoir with characteristic response time of about 45 days, suggesting a diffusivity typical of fractured basement aquifers. We estimate this transient storage capacity at about 28km3 for the three main Nepal catchments; snow and glacier melt contribute around 14km3yr-1, about 10% of the annual river discharge. We conclude that groundwater storage in a fractured basement influences significantly the Himalayan river discharge cycle

Modelling Flow and Nitrate Fate at Catchment Scale in Brittany (France).

Abstract not availableJRC.H-Institute for environment and sustainability (Ispra