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1Marine Hydrodynamics and Vehicle Control1.1 Anatomy of Sea Level VariabilityAn Example from the AdriaticTides Storm Surges and Seiches Response of the Sea to Planetary-Scale Atmospheric Forcing Seasonal and Year-toYear Variability Interdecadal Variability Sea Level Trend Conclusion


Guidance and Control Systems for Marine VehiclesIntroduction Mathematical Model of Marine Vehicle Dynamics Course- and Track-Keeping Control Systems Roll Stabilization Systems Forward Speed Control Systems Dynamic Positioning Systems Integrated Ship Control Systems

Mirko Orli cUniversity of Zagreb

Zoran Vuki cUniversity of Zagreb

Bruno Borovi cBrodarski Institute

1.3 1.4

Sea Ambient NoiseAn Example from the Middle Adriatic.Sea Noise Sources Properties of Sea Noise Conclusion

Dario MatikaUniversity of Zagreb

Basic Shipboard Instrumentation and Fixed Automatic Stations for Monitoring in the Baltic SeaBasic Shipboard Measurement Systems The IOW Automatic Station Network in the Southern Baltic Sea As a Part of the Marine Monitoring Network (MARNET) of the BSH

Siegfried KruegerBaltic Sea Research Institute

1.1 Anatomy of Sea Level VariabilityAn Example from the AdriaticMirko Orli cOn February, 1 1986 at 01 h EMT an hourly sea level height that surpassed corresponding long-term average by 96 cm was recorded at the Bakar tide-gauge station located on the Croatian coast of the Adriatic Sea (Fig. 1.1). This is one of the largest elevations measured at the station during almost 50 years of continuous operation. As is well known, such episodes bring about the ooding of the north Adriatic coast, with the city of Venice being particularly vulnerable [1]. On the positive side, high (but not too high) sea levels may be benecial for the operation of ports and, in particular, the maneuverability of the large-draught ships in the port of Bakar improves considerably when the sea level is at its maximum [2]. Consequently, an understanding of sea level variability is not only challenging from the scientic point of view, but is highly applicable as wellparticularly having in mind expected rise of sea level during the next century. The tide-gauge station at Bakar is of the stilling-well type: the pen, recording on a cylindrical drum rotated by clockwork, is driven by a oat conned to the well that communicates with the sea through

2001 by CRC Press LLC

FIGURE 1.1 The Adriatic Sea position and topography. Also shown are locations of the meteorological and tidegauge stations.

a narrow pipe. Although there exist a number of more modern instruments for sea level monitoring, e.g., bottom pressure recorders or satellite altimeters, the classical instruments are still indispensable if one is interested, as we are here, in a broad range of processes and therefore needs long time series. The function of stilling well is to damp short-period oscillationsin the case of Bakar, those having periods smaller than about 1 min. In order to determine hourly values, one must lter out oscillations with periods smaller than 2 h. At Bakar, these are mostly related to standing wavesseichesof the Bakar Bay, having a maximum period of about 20 min [3]. After extracting hourly sea levels from a tide-gauge record, one is confronted with the variability extending over a wide frequency range and a number of physical processes that control the variability. This is illustrated by a spectrum computed from 16-year time series collected at Bakar (Fig. 1.2). There are some lines in the spectrum, related to tides. There are also some broad maxima, the most notable being the one at the near-diurnal period. Finally, there is an increase of energy toward the lowest frequencies. The plan for this chapter is to isolate, by using various ltering procedures, different phenomena that control the Bakar sea level variability, and to explore them starting from the smallest periods and progressing toward the largest. After the analysis, a synthesis will be attempted in the concluding section with the aim of explaining the 96 cm elevation observed on February 1, 1986. While the processes considered are to some extent inuenced by the Adriatic and Mediterranean environments in which they 2001 by CRC Press LLC

FIGURE 1.2 Spectrum computed from the hourly sea level heights recorded at Bakar between 1983 and 1999. The spectrum was determined by using the Parzen window, with 63 degrees of freedom.

develop, they become of increasingly general character with an increase in period and, it is hoped, of interest to a broad circle of readers.

TidesAs is obvious from Fig. 1.2, tidal lines in the Bakar sea level spectrum are numerous. Yet, the greatest amplitudes are related to the three near-diurnal periods (25.82, 24.07, and 23.93 h) and to the four nearsemidiurnal periods (12.66, 12.42, 12.00, and 11.97 h). A usual approach is to t the sum of the so-called harmonic terms, characterized by the above-mentioned periods, to the sea level time series, and to determine corresponding amplitudes and phasesa procedure known as harmonic analysis. Harmonic synthesis then enables tidal signal to be isolated from the other contributions to the original time series. The tides thus obtained for the two-month interval encompassing the episode of February 1, 1986 are shown in Fig. 1.3. They are of a mixed type, semidiurnal at the new and full moon, diurnal at the rstor last-quarter moon. On February 1, 1986 the tides culminated at 01 h EMT and contributed 20 cm to the sea level maximum. Empirical and theoretical investigations of tides of the world oceans and seas are summarized in several excellent review articles [4, 5] and books [6, 7] published on the subject. As for the Adriatic, Galilei [8] noticed that its tides were large in comparison with the tides occurring elsewhere in the Mediterranean (although they are much smaller than the tides observed in some other basins). Harmonic analysis, performed for various Adriatic ports by Kesslitz [9], as well as the rst maps depicting both the corange 2001 by CRC Press LLC

FIGURE 1.3 Tides recorded at Bakar in January and February 1986. Indicated are phases of the moon (NMnew moon, FQMrst-quarter moon, FMfull moon, LQMlast-quarter moon). In this, as well as in all subsequent gures, the arrow points to 01 h EMT on February 1, 1986.

and cotidal lines in the Adriatic, constructed by Polli [10], corroborated the early observations. Airy [11] was the rst to interpret the relatively large tides of the Adriatic in terms of the resonant excitation of its normal modes by the open Mediterranean tides. The interpretation was substantiated by the numerical models of the Adriatic, inaugurated by Sterneck [12], in which periodical forcing was imposed at the open boundary, in the Otranto Strait, and the tides were computed for the basin interior. It has received further support from the ne-resolution, two-dimensional modeling of the whole Mediterranean, which showed that the Adriatic takes tidal energy from the eastern basin through the Otranto Strait and that this energy is dissipated by bottom friction [13].

Storm Surges and SeichesPeriods close to, but somewhat greater than tidal, characterize processes related to the synoptic-scale atmospheric disturbances. According to meteorological analyses [14], the upper limit of the periods may be placed at 10 d. Consequently, sea level time series, from which the tidal signal had been subtracted, has been subjected to a high-pass digital lter having a cutoff period of 10 d. The resulting sea levels are depicted in Fig. 1.4, along with the air pressure and wind data simultaneously collected along the east Adriatic coast. Maximum sea level height, amounting to 57 cm, occurred on February 1, 1986 at 01 h EMT. It was related to the low air pressure and strong southerly wind blowing over the Adriatic, and they, in turn, were due to a cyclone that approached the Adriatic on January 30, 1986 [15]. Two days later a front swept over the sea [15] bringing about a sudden decrease of the wind speed (Fig. 1.4). This resulted in the lowering of sea level and generation of persistent free oscillations having close-to-diurnal periods. The initial sea level rise, controlled by the meteorological agents, is termed storm surge, while the subsequent free oscillations represent Adriatic-wide seiches. Some in-depth overviews of the storm surge [1618] and seiche [19] research are available in the literature. The rst comparison of the Adriatic sea level with the air pressure and wind was completed by Bui [20]. Later on, Kesslitz clearly distinguished between the storm surges and seiches [21, 22]. cc Early spectral analyses of the sea level time series enabled 21.7 and 10.8 h to be pinpointed as periods of the rst- and second-mode Adriatic seiches [23]. Maxima may be noticed at the same periods in Fig. 1.2, along with another maximum at about 7 h. In a recent study [24] decay of the seiches has been studied and attributed partly to the bottom-friction control in the Adriatic and partly to the energy loss through 2001 by CRC Press LLC

FIGURE 1.4 Storm surges and seiches recorded at Bakar in January and February 1986. Also shown are air pressure and wind data simultaneously collected at various stations distributed along the east Adriatic coast.

Otranto Strait. Numerical modeling of the Adriatic seiches was pioneered by Sterneck [12] and it took more than 50 years to extend it to the storm surges [25]. In both instances, nodal line was assumed at the model open boundary, in the Otranto Strait. This assumption could be relaxed when the seiche [26] and storm surge [27] modeling was extended to the whole Mediterranean. The former model reproduced the observed seiche periods with unprecedented accuracy. The latter showed that the difference between computed and observed sea level heights m