engleski

ESEAS-RI in Croatia: a step towards sea level service

Systematic monitoring of sea-level fluctuations in Croatia started in 1950s, when three long-term stations (Rovinj, Split, Dubrovnik) were installed, joining the tide gauge at Bakar which was mounted already in 1929. All of these gauges have been operational till nowadays, having little or no gaps in the records, even during the war activities in the early 1990s. In addition, new stations at Sućuraj, Zadar and Ploče were installed in the last two decades, becoming members of the sea level monitoring network. However, the gauges had no digital recording till 2003, when all of Croatian tide gauges were upgraded through the ESEAS-RI and Jadran projects. All of these tide gauges are float type in stilling well, with weekly chart records, being digitized and archived as hourly values before 2003. Although the data quality is regularly checked, a number of possible systematic and random errors had been occasionally detected in the observing system, such as clock errors, charts shifts in time and height, digitisation errors, problems with ink diffusion and recording system, etc. These errors were eliminated when digital equipment was installed, consisting of A/D converters mounted on old devices, remote data acquisition system via GSM lines and operational software. Since June 2003 the sea-level data have been downloaded on daily basis, checked and stored as 1 min sea level values, enabling the analyses of high-frequency sea-level oscillations. The choice of 1 min sampling interval instead of a greater recording interval was fully justified soon, only two weeks after the upgrade. Namely, a travelling air-pressure disturbance struck the Middle Adriatic in the morning hours of 27 June 2003, and caused flooding in some areas and a severe damage of shell plants. It was shown by 2D numerical model that the disturbance excited high-frequency sea-level oscillations (0.01-0.1 min-1), with the respective sea-level and current amplitudes surpassing 1 m and 1 m/s in some areas. The disturbance was captured by MedGLOSS station having 2 min sampling interval of air-pressure, being a cosine-like wave with amplitude and period of about 3 hPa and 80 min, respectively. It moved towards ESE at a speed of 22 m/s and was resonantly coupled with the gravity wave in the sea 50 m deep. This mechanism is called Proudman or open-sea resonance and, since several bays in the area have funnel-shaped form and are opened to the west, the forced wave was further amplified due to the imposed topographic constraint. In addition, the forcing atmospheric wave and its counterpart in the sea encountered many bays and harbours while travelling over the Middle Adriatic. Having broadband spectral characteristics, they excited normal modes of the coastal basins through the mechanism called harbour or coastal resonance. Harbour resonance was particularly pronounced in Stari Grad Bay, where the disturbance excited seiches in the harbour (6.1 min) and bay (10.6 min). The seiches flooded a great part of the city, with the maximum amplitude of 120 cm, being three times larger than tides in the area. All of these mechanisms have been reproduced by a 2D numerical model, however none of them could have been verified if the tide gauges had not been upgraded thanks to the ESEAS-RI and Jadran projects.

sea level; tide gauge upgrade; resonance

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