Essais de quantification du débit des résurgences sous-marines autour du Piton de la Fournaise (la Réunion, Océan Indien) Estimation of the freshwater discharge into the sea around the Piton de la Fournaise volcano (Reunion lsland, Indian Ocean)

Résumé

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.

Mots-clés:

• Ile de la Réunion,
• volcan,
• basalte,
• résurgences en mer,
• images infrarouge,
• hydrologie

Abstract

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.

Keywords:

• Ia Réunion,
• volcano,
• basalt,
• freshwater,
• discharge,
• infrared images,
• hydrology