SUMMARY
La Réunion Island in the southwest Indian Ocean is seasonally affected by austral swells among which some extreme events may have strong impacts on coastal infrastructures. The very limited ...number of sensors available on and around the island and in the whole SW Indian Ocean impedes any direct monitoring of the swell activity. In this study, we analyse direct observations of the ocean swell by combining terrestrial measurements of the microseismic noise with in situ oceanographic observations issued from two pressure gauges and an Acoustic Doppler Current Profiler (ADCP), together with swell numerical modelling. The reliability of the terrestrial seismic station to characterize the ocean activity in both the primary and secondary microseisms peaks (PM and SM, respectively), and also in the long period secondary microseismic peak (LPSM) for the case of La Réunion Island is presented and discussed here. By computing the hourly RMS of the PM and LP(SM) amplitudes, we establish a transfer function between the PM and (LP)SM amplitude and the maximum wave height, which appears to be valid for any PM and LPSM amplitudes >0.15 μm and >1.0 μm, respectively. The correlation coefficient between the PM amplitude and the wave height is >0.92. It suggests that the PM amplitude can be used as a robust proxy for the swell height and may help calibrating the wave heights from other independent observable. For some swell events, we observe LPSM that correlate well (>0.91) with the local wave height suggesting a generation by coastal swell reflection. From polarization and spectral analyses, directions and periods of swells are also well retrieved from seismic data. Finally, continuous measure of the SM amplitude shows that it can be used as precursor information for distant swells that may hit La Réunion Island a few days after their generation in the southern Indian Ocean.
Ocean wave activity excites seismic waves that propagate through the solid earth, known as microseisms, which, once recorded on oceanic islands, can be used to analyse the swell. Here, we analyse the ...microseismic noise recorded in different period ranges by the permanent seismic station RER on La Réunion Island and by a temporary network of 10 broad-band seismic stations deployed on the island to analyse extreme swell events. We perform a comparative analysis of cyclonic and austral swell events by analysing not only the primary (PM, ∼10–20 s period) and secondary (SM, ∼3–10 s) microseisms but also the long-period secondary microseisms (LPSMs, ∼ 7–10 s), which may result from the interaction between incident ocean waves and the reflected waves off the coast. We compare the microseismic observations with buoy data when available and with hindcasts from numerical ocean wave models. We show that each cyclone is characterized by its own individual signature in the SM, which depends not only on its distance and intensity but also on its dynamics and trajectory. Thus, the SM contains relevant information for cyclone detection and monitoring. Analysing the PM and the LPSM, and comparing it to direct buoy observations and/or wave numerical models allows characterizing the local impact of the swell with the island in terms of amplitude, period, and sometimes, direction of propagation, making possible to use a seismic station as an ocean wave gauge. The microseisms, which link the atmosphere, the ocean and the solid Earth, can thus provide valuable observations on extreme swell events, in addition to oceanic and meteorological data.
While seagrass meadows are considered as vulnerable or declining habitats worldwide, facing many natural and anthropogenic pressures, the opposite trend is suggested by this study in Reunion Island ...(Indian Ocean). Located at the benthos-pelagos interface, seagrass beds are critical coastal habitats and can be used as relevant health indicators for larger marine ecosystems or land-sea continuum. In order to determine which are the factors driving seagrass ecosystems health it is essential to quantify their seascape pattern fluctuations. The long-term (over 65 years) and seasonal scale variability was assessed in the monospecific Syringodium isoetifolium seagrass bed seascape at the Ermitage/La Saline fringing reef using aerial photographs and field measurements. Both long-term and short-term scales have been informative and both types of monitoring appear as useful tools for seagrass ecosystem management. Strong variations in seagrass coverage were observed in the 16 rasters analyzed from years 1950–2015, the magnitude order was however similar to the one observed at the recent seasonal scale (up to 2016 m2 gained or 4863 m2 lost over few months at site scale). Seascape pattern analysis revealed that physical factors (swell events, cyclones) had a major impact on the ocean-exposed site with varying impact degree depending on frequency, duration and intensity. Biotic (herbivory) or anthropogenic (grubbing, nutrient inputs) factors were also identified to influence the structural shape, fragmentation, or disappearance of seagrass beds. Further work is required to better quantify the effect of each single factor, a difficult task due to their combined expression. At the reef scale, these results showed a positive correlation between seagrass beds and inner reef flat coverage suggesting that common factors drive these highly resilient ecosystems.
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•Combined study of seasonal and mutltidecadal changes in seascape patterns are useful to identify environmental and anthropogenic factors impacting seagrass beds.•Multidecadal analysis of seagrass beds seascape patterns identified long term trends due to anthropogenic disturbances.•Seasonal fluctuations of seagrass seascape patterns may be more important than responses to cyclones.•Monospecific seagrass beds of Syringodium isoetifolium in Reunion island subject to natural and anthropogenic stresses are highly resilient.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Hydraulic and thermal numerical model of Aquifer Thermal Energy Storage.•Results are combined with a realistic time-varying heating network demand.•Energy mix of the heating system over time is ...derived.•This is applied to the Dogger aquifer and the Paris region.
Aquifer Thermal Energy Storage (ATES) is a promising solution for reducing the time mismatch between energy production and demand in urban environments, and recent successful experiences suggest that technical issues can be overcome. The Paris area is a priori a favorable region, since there is locally a surplus of heat production during the summer, an appropriate geological reservoir and both existing and projected district heating networks. This article focuses on a remaining issue: estimating the geothermal contribution to the energy mix of a district heating network over time when using an ATES. This result would then enable estimating the fuel cost savings obtained by avoiding the consumption of expensive energies during the winter retrieval. This work considers an ATES made of two reversible wells reaching the Dogger aquifer and providing energy to a new low-temperature district heating network heating 7500 housing-equivalents. Non-geothermal energy sources with fluctuating prices over time are used for winter peak demand and for summer heat storage. The temperature of brine unloading at the hot and cold wells is simulated and the adequacy of this geothermal system to meet the load is studied in order to evaluate the time dependent energy mix of the network. Results suggest that in average over the 30 years of operation, the ATES delivers 54GWh per year to the heating system, i.e. a power of 9.5MW during the 34 unloading winter weeks. The geothermal energy share in the energy mix is 70%, higher than the 50% possible with a conventional geothermal doublet. The ratio of energy delivered by the ATES divided by energy spent for storage reaches 143%, and is only slightly reduced to 137% when the cold storage is located on an existing cold plume created by past geothermal energy operations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Productivity, nutrient input, nutrient uptake, and release rates were determined for a coral-dominated reef flat at La Réunion, France, to assess the influence of groundwater nitrogen on carbon and ...nutrient budgets. Water samples were collected offshore in the ocean, at the reef crest and back reef for nutrients, picoplankton, pH, and total alkalinity. Volume transport of ocean water across the reef flat was measured using both current meters and drogues. Groundwater advected onto the reef flat and mixed with incoming ocean water. Metabolic rates for the reef community were determined to be: gross primary production = 1,000 mmol C m
−2
d
−1
, community respiration = 960 mmol C m
−2
d
−1
, and community calcification = 210 mmol C m
−2
d
−1
. Across the reef flat, silicate behaved conservatively, there was net uptake of phosphate (0.06 mmol P m
−2
d
−1
) and net release of nitrate, ammonia, dissolved and particulate organic nitrogen (total 7.0 mmol N m
−2
d
−1
). Groundwater nitrate contributed 37% of the increase in nitrate plus ammonia. The first-order mass transfer coefficient of phosphate was 3.3 m d
−1
, and for nitrate plus ammonia, 5.9 m d
−1
. Gross N and P uptake from estimates of mass transfer and uptake of particles were 0.37 mmol P m
−2
d
−1
and 7.2 mmol N m
−2
d
−1
, respectively giving an N:P uptake ratio of 20:1. Thus, the elevation of nitrogen across the reef flat maintains a high N:P flux, enhancing algal growth downstream of the transect. We conclude that net community production (40 mmol C m
−2
d
−1
) was sustained by net uptake of phosphate from the ocean and net uptake of new nitrogen from groundwater.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
This study investigated sediment deposits and suspended particulate matter in three reef areas of Reunion Island characterized by varying benthic communities and bioerosion processes, as a result of ...different eutrophication levels. The sediments were sampled using two different methods, hand-collecting directly on the reef bottom and sampling by traps. This approach allowed a comparison between the composition of reefal bottom sediments and suspended particles. In locations where algal communities prevailed over corals as a result of nutrification, a shift from coral to coralline algae dominated sediments could be observed. In addition, a decrease in sediment production and a prevalence of very fine sand to mud sized grains over medium to fine sands existed in the nutrient-enriched areas. This grain-size difference probably is caused by a decrease in grazing activity in the enriched areas. High proportions of coralline algal debris and sponge spicules were specifically found in the sediments within areas receiving high nutrient input. These two diagnostic components were abundant in both the suspended particulate matter and in the fine fractions (<
0.40 mm) of the bottom sediments. The implications for the interpretation of the fossil record of these bioerosion and sedimentation processes in recent reefal environments are discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Phosphate uptake (P-uptake) into coral reef communities has been hypothesized to be mass-transfer limited. One method of demonstrating mass-transfer limitation of P-uptake is to show dependence of ...P-uptake on water velocity. Water velocity across reef flats varies with tides and swell; thus, we measured P-uptake over the entire reef flat on eight different days, representing a range in water velocities. P-uptake was calculated from changes in P concentration of the water column. Changes in P concentration were measured by water sampling at six sites along a 300-m cross-reef transect while simultaneously measuring water velocity. To smooth the variability in phosphate concentrations, peristaltic pumps were used to get time-integrated water samples for 4–6 h at each site. Water velocities were measured in the middle of the transect using an acoustic Doppler current profiler and were averaged to match the time-integrated water sampling. Depth-averaged cross-reef water velocities were 0.031 ± 0.013 m s
−1
(mean ± SD), while the root-mean-square water velocities, accounting for oscillatory flow, averaged 3.3 times higher, 0.101 ± 0.021 m s
−1
(mean ± SD). Phosphate decreased along all transects. The first-order rate constant for P-uptake (S) was 8.5 ± 2.4 m d
−1
(mean ± SD) and increased linearly with root-mean-square water velocity. The Stanton number derived from oscillatory flow, the ratio of the first-order rate constant for P-uptake to the root-mean-square water velocity (
S
/
U
rms
), was (9.4 ± 1.2) × 10
−4
(mean ± SD). P-uptake ranged from 0.2 to 1.1 mmol P m
−2
d
−1
, demonstrating that P-uptake is variable on short time scales and is directly related to P concentration and water velocity.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A hydrodynamic-transport coupled model is used to understand the transport of fine suspended sediments in the Southwest Lagoon of New Caledonia. The hydrodynamic model is briefly presented and the ...circulation due to the tide and to averaged trade wind forcings is analyzed. The transport model for fine suspended sediment is described. Parameters involved in this model (settling velocity, critical shear stresses, erosion rate coefficient, Schmidt number) are discussed and a calibration procedure is proposed. Using the resultant parameters, the erosion and deposition areas predicted by the sediment-transport model are in very good agreement with the distribution of the percentage of mud at the seabed. The sensitivity of the model to the different sedimentary parameters is studied, and the influences of the tide and wind on deposition and erosion are discussed. The influence of the wind is dominant in seabed exchange processes in shallow areas and produces large erosion rates where the water depth is less than 20 m. The tide controls the particulate transport, vertical mixing, and deposition rates in the areas where the influence of the wind is weak.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Background: If conservative therapy fails, the standard treatment for chronic ankle instability is surgical reconstruction of the lateral ligaments. For the last seventy years, the tenodesis ...principles have been used for reconstruction. Recently however, surgical reconstructions-respecting the intact joint anatomy-have been developed, thus called “anatomical reconstruction principles”. Methods: This study focused on the investigation of the range of motion of the ankle and the subtalar joint following anatomical reconstruction surgery. Three different types of anatomical reconstruction procedures were compared: Direct ligament repair, tendon graft and carbon-fiber implant. Results: All procedures restored the original range of motion of the subtalar joint, except for the plantarflexed/dorsiflexed positions. As for the talocrural joint, the tendon graft and the carbon fiber implant left a small laxity for movements of inversion/eversion and internal/external rotation. The direct repair procedure achieved a more accurate result and restored the physiologic kinematics almost completely. During each procedure the insertion points and the direction of the original ligaments were maintained. However, the different results for the procedure of direct ligament repair compared to the other two anatomical reconstruction procedures showed that this condition alone is not sufficient to perfectly restore the kinematics of the talocrural and subtalar joints. It is important to note that none of the procedures caused a restriction of the range of motion. Conclusions: The maintenance of the range of hindfoot motion decreases the risk of osteoarthritis as well as chronic pain or problems for the patient to walk on uneven surface. Therefore, we believe that standard therapy for chronic instability of the ankle should include direct surgical reconstruction of the ligaments. If this direct procedure cannot be performed because of poor quality of the ligaments an alternative anatomical reconstruction procedure should be considered.
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NUK, OILJ, SAZU, UKNU, UL, UM, UPUK
During the development of the Multi-Strip Multi-Gap Resistive Plate Chambers (MMRPCs) for the time-of-flight (ToF) upgrade of the FOPI detector system, we have designed different versions of the ...front-end electronics (FEE). The signals from a MMRPC are read out on each side of the anode strips by an amplifier followed by a leading-edge discriminator. They are digitized in a time-to-amplitude converter (TAC) followed by a charge-to-digital converter (QDC). These counters are designed to provide a ToF resolution below sigma t les 60 ps requiring an amplifier-diecriminator stage with an intrinsic electronic resolution below sigma t(FEE) les 20 ps. We describe the design steps of the FEE-card from a 4-channel version (FEE1) for the R&D phase to the final 16-channel card (FEE5) for the readout of 4800 channels of the total detector array. Through these design steps we have kept the following key characteristics: low noise sigma n les 25 muV RMS (to the amplifier input), high gain (200) and a high bandwidth (1 GHz).
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