On the use of long-term observation of water level and temperature along the shore for a better understanding of the dynamics: example of Toulon area, France

Abstract

A dense network of instruments has been deployed within harbours along the Mediterranean coast, in the Toulon Metropole area, between the Hyères islands and the Sanary Bay in the framework of the observation network HTM-NET. Each station is equipped with two piezometric sensors, the first immersed and the second emerged, which allows the calculation of the water level. Both piezometric sensors are also equipped with a temperature sensor. Water level and temperature data are analyzed and discussed, also considering meteorological data provided by Météo-France stations. The tide gauges provide information about tide harmonic components, extreme water level and seiching. Moreover, significant differences are observed between sheltered zones in enclosed bays and offshore zones, such as between the back of the Bay of Toulon and at the Port-Cros Island. Differences in water level up to 0.10m are indeed observed under windy conditions, of same order as the tidal range (order of 0.20m) or the annual level variability due to the volumetric expansion (order of 0.10m). Water level variations, up to about 1m, are found to be mainly due to atmospheric effects, with a more or less isostatic behavior according to the weather events. In addition, seiching with an amplitude of few centimeters is observed within the Little Bay of Toulon, for east wind conditions. The near-surface water temperature is measured at the submerged piezometer location (depth of immersion range 0.10m -1.80m according to the station and to the water level). The analysis of the temperature associated with the weather conditions allows to detail a strong variability of the upwelling intensity under Mistral wind conditions in summer, leading to more or less pronounced temperature drops according to the shore configuration. The Bay of Toulon is more prone to the generation of upwellings than the neighboring bays. During winter, water exchanges between the Little Bay of Toulon and offshore are also clearly observed during windy conditions. The HTM-NET long-term observation network thus provides useful insights to increase our knowledge of the hydrodynamics and mass fluxes, and therefore enhances our modeling capacity and risk assessment at the scale of a bay.

High resolution seafloor thermometry for internal wave and upwelling monitoring using Distributed Acoustic Sensing

“…Sea surface temperature estimates at 1.2 km-resolution provided by the operational model F2-MARS3D-MENOR1200 83 , 84 also confirmed this upwelling event, which started on the 27/07/2019 at the northwestern-most tip of the Gulf of Lions and propagated towards the SE, in consistency with a Mistral-wind origin (see Supplementary Video S1 , where the black square corresponds to the approximate location of the thermistor chain). As the studied shelf region is located in an well-known Mistral wind-driven upwelling area 85 – 87 , strong cooling of coastal waters in summer in the shelf off Provence are expected which could explain the observed temperature decreases of a few degrees in the neighborhood of the thermistor chain, at shallow ( 20 m) depths. For comparison, more than 5 K decreases were reported during an upwelling event at 20 m depth in 2017, west of the Cassidaigne canyon, which only lies 30 km away from Cap Vieux 88 .…”

Field Study of Wind Tide in Semi-Enclosed Shallow Basins

“…Two stations are deployed in the N-W and the S-E parts of the lagoon, corresponding to the axis of the prevailing winds in the area. A similar experimental setup is used in the Berre lagoon thanks to the HTM-NET network (Rey et al (2020)). Two stations are deployed in the N-W and S-E parts of the lagoon since February 2019.…”

Wind tides and surface friction coefficient in semi-enclosed shallow lagoons

“…Water level data in Berre lagoon are provided by HTM-NET network (https://htmnet.mio.osupytheas.fr), a network of instruments providing long term air and water pressure and temperature measurements along the French south-east Mediterranean coast (Rey et al, 2020). The data used for the present study was recorded from March 2019 to August 2020 with a similar setup as for Vaccarès, i.e.…”

“…The sea water density is calculated using the UNESCO seawater equation of state (UNESCO, 1979), using measured temperature and constant salinity (37.5 PSU). Considering the natural variability of lagoon salinity (15 to 45 PSU), this implies a water level computational bias lower than 0.2 cm, which corresponds to the precision of the pressure measurements (Rey et al, 2020).…”