Effect of waves generated by passing ships on a beach close to the port of Mytilene (Lesvos Island)

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1 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 751 Effect of waves generated by passing ships on a beach close to the port of Mytilene (Lesvos Island) Vagenas M.A., Vousdoukas M., Velegrakis A.F, Zarkadas Th. Department of Marine Sciences, School of Environment, University of the Aegean, University Hill, 81100, Mitilini, Greece, envm01055@env.aegean.gr Abstract This contribution reports the preliminary results of a study on ship generated waves in the nearshore zone. The experiment, which took place in late Summer 2005 at a beach 11 km south of the port of Mitilene (Lesvos), involved nearshore wave measurements through videos. The results showed that ships, particularly the high speed ferry boats, can generate significant nearshore waves the heights of which can significantly exceed those of the wind waves predicted for the same period and thus affect natural beach morphodynamics. Επιπτώσεις διερχόμενων πλοίων σε μια ακτή κοντά στον λιμένα Μυτιλήνης (νήσος Λέσβος). ΠερΙληψη Αναφέρονται τα προκαταρκτικά ευρήματα μίας μελέτης των επιδράσεων που έχουν στην παραλιακή ζώνη κύματα δημιουργούμενα από πλοία. Οι παρατηρήσεις έλαβαν χώρα σε μία παραλία 11 km στα νότια του λιμένα της Μυτιλήνης και έγιναν με την βοήθεια βιντεοκάμερας και βαθμονομημένων σταδιών. Τα προκαταρκτικά αποτελέσματα δείχνουν ότι τα πλοία (και ειδικά τα γρήγορα πλοία της γραμμής) δημιουργούν παραλιακά κύματα με ύψη ανώτερα από αυτά των ανεμογενών κυμάτων της ίδιας περιόδου και έτσι μπορούν να επιδράσουν στην παραλιακή μορφοδυναμική. Keywords Waves, field data, beach erosion, κύματα, δεδομένα πεδίου, διάβρωση ακτής

2 752 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας INTRODUCTION Coasts are subject to natural and anthropogenic forcings which may result in serious environmental problems, such as beach erosion. Beach erosion is particularly acute in Greece, where more than 30% ( 4000 km) of its coastline is currently under erosion (Eurosion, 2003). This erosion is due to both natural (e.g. the sea level rise (Tsimplis and Rixen, 2002) and the lithification of beach sediments (Vousdoukas et al., 2005)) and anthropogenic forcings (e.g. the construction of badly designed coastal developments and protection schemes (Pilkey and Dixon, 1996), fluvial/marine aggregate extraction (Gaillot and Piegay, 1999) and dam construction (Eurosion, 2003). A basic consideration in coastal morphodynamics is that beaches tend to accrete under the summer mild wave conditions, recovering from the erosive winter wave climate (e.g. Aubrey and Ross, 1985). Wind shear is regarded as the main mechanism of wave generation, but others also exist such as ship generated waves (e.g. Dam et al, 2005). These waves may have a detrimental effect on beach morphodynamics, particularly in the case of heavy nearshore traffic. Aegean Sea islands are characterised by increased nearshore shipping traffic, which recently have become heavier in terms of both capacity and speed. In the present contribution the preliminary results of a study of the effects of ship-generated waves on a beach close to the port of Mitilene (Lesvos, NE Mediterranean) are reported. STUDY AREA AND METHODS The beach wave measurements took place in the late summer 2005 at an east-facing beach 11 km south Mitilene port (Fig 1). 3 scaled poles were placed in the shallow nearshore waters, with the deployment pattern forming a triangle (Fig 2). Wave propagation and breaking were videotaped for the cases of normal wind waves, and waves generated by the passenger ferry boats M/V Mitilene and M/V Nissos Myconos. The obtained videos were split into frames, which were then sampled at 5 Hz. In Fig 1. Location map of the study area.

3 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 753 Fig. 2. Experimental layout. each of these frames, the wave heights at each of the poles were then estimated. RESULTS In Figure 3, the wave height time series are shown for two of the poles, the inshore Pole 3 (water depth of 1.2 m) and the offshore Pole 2 (water depth of 2 m) (the other offshore pole gave similar results to Pole 2). The wind wave climate during the experiment was extremely mild, with wind wave heights rarely exceeding 5 cm; in contrast, the ship generated waves were significant. M/V Mitilene, which is an old generation ferry boat (operational speed of 17 knots), generated waves which reached nearshore heights of 25 cm; these waves were transmitted for 2 min and constituted a wave group. In comparison, the high speed ferry boat M/V Nissos Myconos (operational speed in excess of 26 knots) generated nearshore waves with heights of 50 cm, which shoaled on the beach for more than 10 min; these waves formed 4 wave groups (with a wave group period of 100 sec) of which the third included a solitary wave with a height of more than 70 cm and a run-up of 3 m from the coastline. The M/V Mitilene wave spectra show two peaks in the frequencies of 0.22 and 0.46sec -1 (corresponding to periods of 4.7 and 2.2 sec, respectively) (Fig. 4a). The latter appears to be the most dominant frequency, but its dominance is weakened on the inshore pole s spectra, a pattern that is not in agreement to the expected non-linear interaction between the two frequencies. In comparison, the energy density of the M/V Nissos Myconos waves was almost an order of magnitude higher than that of the M/V Mitilene waves (Fig. 4b). In this case, there were 3 peaks centred around the frequencies of 0.16, 0.25 and 0.39 sec -1 (corresponding to periods of 6.3, 4 and 2.6 sec, respectively). The most dominant period was 6.3 sec, but as the waves shoaled its dominance weakened and on the shallower pole s spectra the middle frequency peak (0.25 sec -1, period of 4 sec) became dominant. It is interesting to note that the energy peak centred around the period of 2.6 s corresponded to the wave periods of the third wave group which contained also the

4 754 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας Fig. 3. Wave height time series. Figures 3a and 3d refer to wind waves at the offshore Pole 2 (water depth of 2 m) and the inshore Pole 3 (water depth 1.2 m), figures 3b and 3e to ship generated waves from M/V Mitilene at Poles 2 and 3 respectively and Figures 3c and 3f to ship generated waves from M/V Nissos Mykonos respectively 70 cm high freak wave; this may imply that this wave created a significant energy flux to the beach. Finally, an energy cascade appeared at the high frequencies (Fig. 4), implying inertia force dominance and energy transfer from low to high frequencies. DISCUSSION Previous works (e.g. Sorensen, 1969, cited in Dam et al, 2005) have showed that ship generated wave height decays exponentially with distance from the sailing line, and the value of the exponent depends on the sailing speed, implying that waves may be significantly damped when they reach the shore. However, the measured results showed that both ferry boats and especially the high speed M/V Nissos Myconos can generate nearshore waves, which in some cases can be remarkably energetic for the period of the experiment. Wind wave hindcasting, based on hourly wind speed and direction data for the experimental period (late summer) showed that the mean offshore wave directions are mainly from the SE (not affecting our study area), and the extreme observed conditions referred to NE waves with offshore heights and periods not exceeding 70 cm and 4 sec, respectively. The use of a wave

5 8ο Πανελλήνιο Συμποσιο Ωκεανογραφίας & Αλιείας 755 Fig. 4 Wave energy density spectra for the case of (a) M/V Mitilene and (b) M/V Nissos Mykonos. The (blue and red lines) refer to the waves at the offshore and inshore pole, respectively. The energy cascade area is shown by the black line with a 5/3 slope. shoaling/refraction software (Ebersole et al., 1986) showed that such offshore waves result in nearshore wave heights which do not exceed 20 cm at the breaking zone. Thus, it appears that ship waves, although short in duration, may affect significantly the summer beach recovery patterns, as they are not only steep (and thus erosive), but they may also form an important constituent of the wave energy fluxes to the beach. This may have significant implications for the island beaches of the Aegean Sea, particularly for those that are affected by heavy nearshore high speed ferry boat traffic. CONCLUSIONS The preliminary results of the study showed that ship wakes can generate waves which can reach significant nearshore heights. These heights, which depend on the vessel s speed, may exceed significantly those of the late summer wind waves, be a significant component of the wave energy flux to the beach and, thus, affect its morphodynamics. ACKNOWLEDMENTS The authors gratefully acknowledge the provision of meteorological data by Prof. K. Kalambokidis (Geography Dept., University of the Aegean) and the financial support by the Greek Ministry of Education through the Programme Iraklitos. REFERENCES AUBREY, D. G. and ROSS, R. M., Τhe quantitative description of beach cycles. Marine Geology 69, DAM, K.,T., TANIMOTO, K., NGUYEN, B.,T., and AKAGAWA, Y., 2005.Numerical study of propagation of ship waves on a sloping coast, Ocean Engineering, IN PRESS. EBERSOLE, B.A., CIALONE, M.A. and PRATER, M.D., RCPWAVE A Linear Wave Propagation Model for Engineering Use. Regional Processes Numerical Modelling System, Report No 1. Department of the Army. Eurosion, Τrends in Coastal Erosion in Europe. Final Report of the Project Coastal erosion Evaluation of the need for action

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