F4. Study of sediment dynamics.
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.
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.
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).
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
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.
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.
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 
, 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.