Action F4. Study of sediment dynamics.
Partner: IMEDEA.

Study and modelling of the effect that Posidonia fields have on three-dimensional currents and on the transport of sediments on the turbulent layer of the sea floor and its influence on the evolution and conservation of beaches.

Figure 1.- Aerial photograph of the beach.

The aim of this action is to analyse diverse aspects of the interaction between the sediment dynamics of shallow water and the presence of Posidonia fields. The study covers such aspects as the influence of the field on the limit layer of the sea floor and the relation between natural and artificial evolution of the beaches and conservation of the P. fields.

Figure 2. chronological evolution of a beach.

Development and focus:
The work carried out in Action F.4 has focused mainly on the study of the most relevant aspects of the relation between Posidonia oceanica fields, hydrodynamics and their effects on the re-suspension and transport of sediment. This Action is part of a series of scientific tasks included in the F Module of this project, the aim of which is to provide scientific knowledge and advice to other actions carried out within the project. In particular, IMEDEA scientists participating in this action have collaborated providing suggestions with respect to some aspects of the installation of anchoring points, and they provided this help by working both within the project and in the workshop held to this effect (Action A3). They also provided information (text and figures) to be included in the educational and publicizing material (Action E1). Besides this, IMEDEA scientists have also carried out educational and publicizing actions on the project during the forums in which they participated (Semana de la Ciencia de Baleares ---Balearic Science Week---, Fira de la Ciència, divulgación del Centro Baleares Europa ---Science Fair of the Balearic-European Centre---, Congreso de la Societat d’Historia Natural de les Balears---Congress of the Natural History of the Balearics---). Last, the report on Action F4 includes a series of recommendations which appear in the Management Plans for protected areas (Action A4).
Tasks carried out according to Action F4 are two-pronged: one line of work was observational and the other aimed at modelling. Experimental work was carried out in the Bay of Alcudia and in Cala Millor. These areas have very active sediment dynamics and, in the case of Alcudia, the backbeach there alternates well-preserved areas with other areas highly affected by the hand of man due to the fact that dumping of mechanically transported sand has taken place.
During the tasks carried out an analysis of the following was done, using aerial photographs: the chronological evolution of the beach, the shallowest waters in which Posidonia oceanica is growing, and the geo-morphological changes caused by the artificial dumping of mechanically transported sand. The results were interpreted taking into account the hydrodynamic conditions (currents and waves) existing in the area .

Figure 3: Concentration of Posidonia along the coast.

As for the modelling, several different analysis of numeric data were carried out applying a model of the currents that uses de-structured mesh of finite elements. During tests the coefficient of sea-floor roughness was varied in order to simulate the effect of P. fields and the results obtained were compared to those obtained from sandy floors.

The analysis were carried out at Es Port de Cabrera and the Bay of Palma.

Figure 4: Simulation of currents at Port de Cabrera (surface).
Figure 5: Simulation of currents at Port de Cabrera (sea floor).

Relation between beach line and Posidonia oceanica:
When studying the variations in beach profile, it is assumed that there exists a balanced profile which the beach will reach when exposed to certain wave patterns during a sufficient length of time.
It is believed that when the beach is in this state of balance it dissipates the energy of the waves affecting it without significant change of its profile taking place.
A is a parameter related to the average size of the sediment grain and the speed at which it settles and M is related to the concavity of the beach line.
To carry out this task a comparison was made between the profiles obtained by topographic means at Alcudia Bay (March 2003) and those of other beaches.
The values of A obtained for beaches (Magalluf and Santa Ponça) where the limit of growth of P. fields was in shallow waters (approx. 3 m) differs from theoretical values for existing granule measures. These beaches present a sharper incline than expected.
On the other hand, at beaches of the Bay of Alcudia (Muro-Can Picafort), where fields are found under deeper waters (approx. 9 m), the inclination of the sea floor is more similar to the theoretical profile.
This increase in the inclination of the beach could be due to the fact that the energy of the waves is further dissipated by the leaves of plants situated under shallow waters. The effect would be similar to that which it has been observed that reefs structures produce at beaches elsewhere. Should this inter-relation be confirmed at other beaches of the Archipelago, a relation between field conservation and the extension of a beach could be established as the loss of vegetation would cause a re-distribution of sediment and reduce the area occupied by dry beach.

Figura 6: Esquema de la relació perfil platja-Posidonia oceanica.

Scientific Activities.

As for the development of scientific activities, the works carried out during Action F4 were two-pronged: one line of work was observational and the other aimed at modelling, both obviously closely related (Fig. 7). Field work was carried out at Es Port of Cabrera, Alcudia Bay and Cala Millor. In the case of Es Port of Cabrera the studies were done in an area of regulated anchoring points similar to those which are being installed thanks to the LIFE-Posidonia project. The work focused on the study of the time the water resided in the anchoring area and the implications thereof in terms of conservation of the fields situated there. The studies combined acoustic sampling, installation of current-measuring metres, biological sampling and numeric modelling of the currents and tracers. The results of these studies have been included in several national and international scientific publications [1,2,3] and the conclusions have been incorporated in the recommendations suggested for the management of new anchoring areas.

Figure 7

A second set of tasks centred on the study of morpho-dynamics of beaches holding extensive fields of Posidonia oceánica. Specifically this subject matter was studied at the beaches of Can Picafort (Alcudia) and Cala Millor. These tasks were based on previous observations carried out at the Magalluf beach [ 4]. The beaches were selected because they present active sediment dynamics and there exist both relatively well-preserved areas (Muro beach) as well as areas which have been strongly affected by man and in which mechanically transported sand has been dumped [5] , which factor could affect the growth rhythm of Posidonia oceanica. The studies included, among others, tasks such as: the analysis of the chronological evolution of the beach and of the shallowest water boundary in which Posidonia oceánica fields grow, and these analysis were carried out on the basis of topographic restitution of aerial photographs; the study of sediment dynamics as affected by the different types of waves; and the analysis of the influence of the topography generated by the growth of Posidonia oceanica on the propagation of wave patterns, etc. Again, in this type of analysis experimental work has been combined with numeric modelling although, in contrast to the previous case in which the emphasis was on the relation to waves, in this case most of the work was on wave modelling as the action of waves is a determining element in sediment dynamics in shallow water.

Results and conclusions:
The most relevant results in terms of field protection were included in the final memoir of Action F4, collected in the annex F4, in which is compiled a large proportion of the scientific information published on this matter in recent years. The memoir is intended for coastal managers and attempts to give a general perspective on all the aspects of coastal dynamics that should be taken into consideration in order to protect the Posidonia oceanica fields. Some sections of the memoir have been more fully developed as there exists more profound knowledge on some matters, whereas in other sections it becomes evident that there is a need for further efforts in research.