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Relation between the type of wave exposure and seagrass losses (Cymodocea nodosa) in the south of Gran Canaria (Canary Islands - Spain)

Portillo E.

Oceanological and Hydrobiological Studies

2014

Vol. 43, No. 1

29--40

Effects of different types of wave events on Cymodocea nodosa seagrass meadows were observed and investigated by quantitative and qualitative evaluation of material washed ashore a few days after the events. The studied seagrass meadows are located on the south coast of the island of Gran Canaria (Canary Islands - Spain) and they are protected from frequent swells arriving from the North Atlantic. However, sporadic phenomena associated with winter storms occasionally hit this coastline, causing the loss of entire plants (fresh leaves with rhizomes and roots attached). An unusual type of southern swells generated in the South Atlantic also reaches the Islands in spring and summer. A clear relation was observed between the wave events (southern swells and storm waves) and the material cast ashore over the following days, with differences in composition (fresh vs. decaying leaves) depending on the type of event. After southern swells, detached portions of C. nodosa consisted mostly of decaying leaves shed after senescence. These old swells cause frictional drag with moderate oscillations over a wider range at greater depths, removing only decaying leaves from the seagrass meadows and favoring the natural clean-up process.

Hydrodynamics of offshore structures under waves and current: fundamental theory, formulation of the problem and a case study

Mendes A. C.

Vibrations in Physical Systems

2002

Vol. 20

34--57

This lecture is devoted firstly to a statement of the physical properties of gravity waves that is followed by a review on Linear Theory of waves of small amplitude, together with the related basic equations. This material includes the calculation of water pressure and water-particle kinematics, and the establishment of mass, momentum and energy balances. On a second stage it will be given a comprehensive material concerning the methodologies applied to calculate the wave forces. In the present case we will be involved with the Morison model and the corresponding hydrodynamic coefficients, when predicting the wave forces exerted upon tubular structures. Finally, a case study will be presented, relating both theoretical and experimental results obtained with a physical model of an offshore jacket platform.