Des courants de turbidité hyperpycnaux dans la tête du canyon du Var ? Données hydrologiques et observations de terrain
The Var River is 120 km long upon entry into the western Mediterranean Sea and drains a 2820 km(2) basin (Fig. 1). A steep submarine canyon connects directly to the river mouth (Fig. 2). The submarine canyon is sinuous and shows typical badland features such as high slopes resulting from erosion (Fig. 3). The average water discharge 152-53 m(3) s(-1); Fig. 4) can be multiplied by tens during spring or fall ''flash floods'', when suspended sediment concentration can reach many tells of kg m(-3). The rating coefficient b corresponding to instantaneous discharges is 1.534 (Eq. (1) and Fig. 5, curve 1), according to data published by Laurent (1971), but might be larger if suspended sediment concentration related to floods occurring after a dry period (Fig. 5, curve 2) is taken into account. Nevertheless, concentrations predicted by (2) using a larger value of b (1.65 b less than or equal to 1.7) are not consistent with data published by Laurent (1971). For this reason, we used relationship (1) in the paper that follows, and our results can thus be considered as the minimum estimate of hyperpycnal plume production. A definitive choice between relationships (1) and (2) in Figure 5 requires additional measurements of suspended sediment lend, especially during peak flood discharges. The b value corresponding to monthly discharges is estimated to be 1.7 to 1.75 (which leads to mean annual suspended sediment concentrations in the range 0.68-0.83 kg m(-3) Relationship (1) suggests that the critical discharge needed to produce a turbid hyperpycnal plume during floods is of the order of 1250 m(3) s(-1), depending on the salinity and temperature of the sea water near the river mouth (Fig. 6). A hyperpycnal plume is a turbidity current generated at a river mouth when thedensity of the river plume exceeds the density of ambient sea water due to sediment in suspension. The plume plunges and becomes an auto-maintained turbidity current. The threshold is between 620 and 750 m(3) s(-1) using relationship (2) in Figure (5). Using relationship (1), short duration (<1 day) hyperpycnal plumes may be produced every four years (Fig. 7a) and hyperpycnal plumes lasting more than one day may be produced every 21 years (Fig. 7b). Using curve 2 in Figure 5, the return periods are 2 and 5 years for short duration and 1-day-long plumes, respectively. The Var river is thus a river that can produce relatively frequent hyperpycnal plumes. as might be inferred from the study of Mulder and Syvitski (1995), who showed that a majority of rivers with low-to-medium average discharge (< 460 m(3) s(-1)) can produce such plumes. The calculated return period of these events at the mouth of the Var river could be greater at times of active delta construction, e.g. during ice ages, if no important change in precipitation occurred (Mulder and Syvitski, 1996). The November 1994 flood (Fig. 8) could have produced a hyperpycnal event almost 20 hours in duration, and could have carried 18 x 10(6) t of sediment towards the deep basin. This represents 11-14 times the total average yearly suspended sediment load (1.32 x 10(6) to 1.63 x 10(6) t for b values related to monthly discharges = 1.7 and 1.75, respectively). It is also the range of volume of sediment estimated by Habib (1994) in connection with the failure at Nice Airport in 1979 (8 x 10(6) m(3)). The November 1994 plume could have deposited at least 12 x 10(6) m(3) of sediment, irrespective of any erosion that might have occurred along the travel path. This should correspond to a turbidite layer many millimetres to many centimetres thick in the upper and median fan valley. We calculate center line values of the sedimentation rate in the Var deep sea fan using the ID model for deposition from a spreading plume at a river mouth published by Syvitski and Lewis (1992; Eq. (2) and (3)) using a width at the river mouth. w(o) = 250 m. Values are circulated for a total suspended toad = 1.32 x 10(6) t yr(-1) (values into brackets provide values for a total suspended load = 1.63 x 10(6) t yr(-1) as a comparison), We use five removal coefficients (lambda) corresponding to five grain sizes (from clay to fine sand) and an average lambda = 6.8 day(-1) (medium slit) to describe the whole sediment. Results show that (Fig. 9a), the sediment inventory at the river mouth varies from 0.03 (0.04) to 0.43 (0.53) kg m(-2) d(-1) for clay and 0.12 (0.14) to 1.65 (2.O6) kg m(-2) d(-1) for fine sand in August and May (lowest and highest average monthly discharge, respectively). Using a different removal rate for every grain size (Fig. 9b, see Bursik, 1995). fine sand cannot be deposited further than 13 kin from the river mouth, whereas clay can be deposited up to several tens of kilometres. The corresponding mean monthly center line values of theoretical accumulation rates at the river mouth using a mean grain size (lambda = 6.8 d(-1)) (Fig. 10b) are 140 (175.2) cm yr(-1) in May and 10 (11.8) cm yr(-1) in August for an initial flow thickness, H-o = 0.3 in August and 0.58 m in May. For a total suspended load = 1.32 x 10(6), the mean annual value varies from 48.7 to 99.7 cm yr(-1) depending on H-o. The mean annual center line sedimentation rate is 53.8 (66) cm yr(-1) for H-o = 0.3-0.58 m (Fig. 10). Using 5 grain size . 10a) provides identical sedimentation rate values at river mouth but larger values with distance from the river mouth because of the use of a constant men monthly grain size distribution (Fig. 10a). Note that the center line values calculated using lambda = 6.8 d(-1) (Fig. 10b), can be considered to be more calistic but are still slightly underestimated close to the river moth and slightly overestimated in more distal areas for two reasons: (i) the peak of sediment concentration that occurs just before the peak of discharge during the rising limb of the flood hydrogram could not be measured (Laurent, 1971); and (ii) during high-discharge periods origin and size of particles carried to the river mouth during mean-discharge periods and tend to settle at a different distance from the river mouth (see Fig. 9). For example, using an average gram size for sedimented particles corresponding to coarse silt (lambda = 12.3 d(-1)), the center line value of sedimentation rate at river mouth is 88.8 (109.1) cm yr(-1). Sedimentation rate values are lower using a model of a semi-circular plume in which sediment rate decreases as a square function of the distance measured from the river mouth Eq. (4) and (5), the average yearly sedimentation rate at the river mouth varies from 6.3 (8.3) to 10.6 (13.6) cm yr(-1) (using 8.8 x 10(5) m(3) (into brackets, 1.08 x 10(6) m(3)) for the yearly sediment load). These calculated values represent the sediment that should theoretically accumulate at the river mouth if Ilo erosion occurred. Since we have shown that mass wasting events and hyperpycnal plumes are frequently triggered, only a very small part of the sediment that should accumulate according to Figure 10 remains at the river mouth over a long period of time. Using the values of sedimentation rate measured by Habib using C-14 dating (1994) (4.8 to 5.9 mm yr(-1)) close to Nice Airport and the results provided by Eq. (4) and (5), we calculate that only 3 to 9% using the hemipelagic plume model (< 2% using center line values provided by the ID sedimentation model) remain effectively at the river mouth; the remainder is either not deposited (hyperpycnal plume), or transported downslope through different mass wasting events after a short time of deposition. According to Mulder (1992), failure of underconsolidated sediments on the upper slope can occur as soon as the sediment column reaches 0.5 to 2 m above a potential failure surface. Comparison of this value with the sedimentation rates calculated above suggests that failure by loading could happen with return period in the range of ten years. This return period may be amplified by failures triggered by changes in the slope gradient due to sediment accumulation (the slope at the head of the canyon is locally steeper than 20 degrees) and by excess pore pressure when the accumulation rate is large but river discharge insufficient to produce hyperpycnal plumes (Mulder et al., 1996). However, because hyperpycnal plumes are produced during period of maximum discharge, the volume of sediment involved in hyperpycnal plumes is larger than the volume of sediment transported by shallow failures produced at the shelf break. The mass wasting processes that can be produced according to the intensity of the flood are summarized in Figure 11. Hyperpycnal plumes generated during the greatest floods may be responsible for the deposition of fine turbidites at the top of the sedimentary ridge. Core KS-6 suggests a frequency between 700 and 1000 years during the Holocene. Considering that the maximum flood discharge for the Var River (17,200 m(3) s(-1)) corresponds to a return period ranging between 1000 and 2000 years (i.e. the range of climatic and geomorphological changes), then we can consider that: (i) the flood that took place in 1994 was an event with a return period < 200 years; and (ii) hyperpycnal plumes depositing a turbidite that might be preserved and detected in the long-term geological record are related to floods with a peak discharge > 10,000 m(3) s(-1). Nevertheless, this hypothesis remains speculative, since no real sedimentological evidence exists to differentiate turbidites due to hyperpycnal plumes from those generated by ignitive failures. A final remark relates to the event that occurred in 1979. Since discharge values as high as 1200 m(3) s(-1) were measured at the river mouth during the period preceding the triggering of the slide and since the event involved a fall flood occurring after a dry summer, a hyperpycnal plume might have been triggered and have played a role in the sedimentary mass wasting processes that took place later. For the Var River, hyperpycnal plumes are probably one of the most important transport mechanisms according to the volume of sediment transferred, as shown by the 1994 flood. This Study confirms that the Var River is one of the best sites in the world for the study of slope sediment transfer processes, including: (1) - hyperpycnal plumes with a direct sediment transport from the continent to the basin; (2) - high-frequency surficial failures in underconsolidated sediments due to excess pore pressure; and (3) - lower-frequency failures affecting a large volume of sediment and generated by external forcing. Although mechanisms (1) and (2) include unitary events involving relatively low volumes of sediment, they may represent a large contribution to the construction of the Var delta because of their higher frequency in comparison of externally-forced failure events.
Auteurs du document :
Mulder, T, Savoye, Bruno, Syvitski, Jpm, Parize, O
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Thème (issu du Text Mining) :
Oceanolica Acta (0399-1784) (Gauthier-Villars), 1997 , Vol. 20 , N. 4 , P. 607-626